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[[Image:105582main GlobalWarming 2060 lg.jpg|right|thumb|Annual average global warming by the year 2060 simulated and plotted as color differences using EdGCM|339px]]   
[[Image:105582main GlobalWarming 2060 lg.jpg|right|thumb|Annual average global warming by the year 2060 simulated and plotted as color differences using EdGCM|339px]]   


'''Global warming''' is the increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation.  
'''Global warming''' is the increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation. There is strong evidence that significant global warming is occurring; this evidence comes from direct measurements of rising surface air temperatures and subsurface ocean temperatures and from phenomena such as increases in average global sea levels, retreating glaciers, and changes to many physical and biological systems. It is likely that most of the warming in recent decades is attributable to human activity, particularly the burning of [[fossil fuel]]s and [[deforestation]].


Global average air temperature near the Earth's surface rose 0.74 ± 0.18&nbsp;°[[Celsius|C]] (1.33 ± 0.32&nbsp;°F) from 1906 to 2005. The prevailing scientific view,<ref>See [http://tigger.uic.edu/~pdoran/012009_Doran_final.pdf Doran (2009)] for information on a poll of research-active [[climatology|climate scientists]], other researchers and the public regarding the scientific consensus on global warming. ''Eos'' 90(3): 21-22.</ref> as represented by the science academies of the major industrialized nations<ref name = "academies">http://nationalacademies.org/onpi/06072005.pdf</ref> and the [http://www.ipcc.ch/ Intergovernmental Panel on Climate Change] (IPCC),<ref name=grida7>{{cite web | url=http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Print_SPM.pdf | format=[[Portable Document Format|PDF]] | title=Summary for Policymakers | work=Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change | accessdate=2007-02-02 | date=[[2007-02-05]] | publisher=[[Intergovernmental Panel on Climate Change]]}}</ref> it is very likely that most of the temperature increase since the mid-20th century has been caused by increases in atmospheric greenhouse gas [[concentration]]s produced by human activity.
Global average air temperature near the Earth's surface rose by 0.74 ± 0.18&nbsp;°[[Celsius|C]] (1.33 ± 0.32&nbsp;°F) from 1906 to 2005. The prevailing scientific view,


Climate models project that average global surface temperatures are likely to increase by 1.1 to 6.4 °C (2.0 to 11.5 °F) by the end of the century, relative to 1980–1999.<ref name=grida7/> The range of values reflects the use of differing assumptions of future [[greenhouse gas]] emissions and results of models that differ in their sensitivity to increases in greenhouse gases.<ref name=grida7/>  
<ref name = Doran>See [http://tigger.uic.edu/~pdoran/012009_Doran_final.pdf Doran (2009)] 'Examining the Scientific Consensus
on Climate Change' for information on a poll of research-active [[climatology|climate scientists]], other researchers and the public regarding the scientific consensus on global warming ''Eos'' 90: 21-2</ref> as represented by the science academies of the major industrialized nations<ref name = "academies">[http://nationalacademies.org/onpi/06072005.pdf Joint science academies’ statement: Global response to climate change]
*"There will always be uncertainty in understanding a system as complex as the world’s climate. However there is now strong evidence that significant global warming is occurring. The evidence comes from direct measurements of rising surface air temperatures and subsurface ocean temperatures and from phenomena such as increases in average global sea levels, retreating glaciers, and changes to many physical and biological systems. It is likely that most of the warming in recent decades can be attributed to human activities (IPCC 2001). This warming has already led to changes in the Earth's climate."</ref>
and the ''[http://www.ipcc.ch/ Intergovernmental Panel on Climate Change]'',<ref name=grida7>{{cite web | url=http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spm.html|title=Summary for Policymakers|work=Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change|date=2007}}
*"Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations...Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns" </ref> it is very likely that most of the temperature increase since the mid-20th century has been caused by increases in atmospheric greenhouse gas concentrations produced by human activity. Climate models predict that average global surface temperatures will increase by a further 1.1 to 6.4 °C (2.0 to 11.5 °F) by the end of the century, relative to 1980–1999.<ref name=grida7/> The range of values reflects differing assumptions of future [[greenhouse gas]] emissions and results of models that differ in their sensitivity to increases in greenhouse gases.<ref name=grida7/>  


Scientists have not yet quantitatively assessed the potential self-accelerating effects of global-warming itself, either on threshold or rate.  Melting of permafrost, for example, causes increased production and atmospheric release of such newly produced as well as anciently stored methane gas, which reportedly “….packs a far greater warming punch than its oxidized sibling [carbon dioxide],”<ref name=walker2007>Walker G. (2007) [http://dx.doi.org/10.1038/446718a Climate Change 2007: A world melting from the top down.] Nature 446, 718-221 (12 April 2007)</ref> possibly as much as 25 times that of carbon dioxide per unit mass.<ref name=simpson2009>Simpson (2009) [http://www.ScientificAmerican.com/Earth3 "The Peril Below the Ice"]. ''Scientific American Earth 3.0'', June 2009, pp. 30-37.</ref> We cannot rule out other effects of global warming that may initiate self-acceleration.
Scientists have not yet quantitatively assessed the potential self-accelerating effects of global-warming itself, either on threshold or rate.  Melting of [[permafrost]], for example, causes increased production and atmospheric release of such newly produced as well as anciently stored methane gas, which “….packs a far greater warming punch than [carbon dioxide] (CO<sub>2</sub>),”<ref name=walker2007>Walker G (2007) [http://dx.doi.org/10.1038/446718a Climate Change 2007: A world melting from the top down] ''Nature'' 446:718-21</ref> possibly as much as 25 times that of CO<sub>2</sub> per unit mass.<ref name=simpson2009>Simpson (2009) [http://www.ScientificAmerican.com/Earth3 "The Peril Below the Ice"] ''Scientific American Earth 3.0'' pp 30-7</ref>  


An increase in global temperatures will in turn cause sea level rise, [[glacier]] retreat, melting of sea ice, and changes in the amount and pattern of [[precipitation (meteorology)|precipitation]]. There may also be changes in the frequency and intensity of extreme weather events, though it is difficult to connect specific events to global warming. These changes to the climate will produce a range of practical effects, such as changes in agricultural yields and impacts on human health.<ref>[http://www.ipcc.ch/publications_and_data/ar4/wg2/en/ch19s19-3-6.html Schneider et al. (2007)]. [http://www.ipcc.ch/publications_and_data/ar4/wg2/en/ch19.html Assessing key vulnerabilities and the risk from climate change]. In M.L. Parry et al. (eds) ''[http://www.ipcc.ch/publications_and_data/ar4/wg2/en/contents.html Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change]''. Cambridge: Cambridge University Press. pp.779-810.</ref>
An increase in global temperatures will cause the sea level to rise, [[glacier]]s to retreat, sea ice to melt, and changes in the amount, geographical distribution and seasonal pattern of [[precipitation (meteorology)|precipitation]]. There may also be changes in the frequency and intensity of extreme weather events. These will have many practical consequences, including changes in agricultural yields and impacts on human health.<ref>[http://www.ipcc.ch/publications_and_data/ar4/wg2/en/ch19s19-3-6.html Schneider ''et al.'' (2007)]. [http://www.ipcc.ch/publications_and_data/ar4/wg2/en/ch19.html Assessing key vulnerabilities and the risk from climate change]. In Parry ML ''et al.'' (eds) ''[http://www.ipcc.ch/publications_and_data/ar4/wg2/en/contents.html Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change]'' Cambridge University Press pp 779-810  
 
*"There is new and stronger evidence of observed impacts of climate change on unique and vulnerable systems (such as polar and high-mountain communities and ecosystems), with increasing levels of adverse impacts as temperatures increase (very high confidence).  
Remaining scientific uncertainties include the exact degree of climate change expected in the future, and how changes will vary from region to region around the globe. There is ongoing political and public debate regarding what, if any, action should be taken to reduce future warming or to adapt to its consequences. The [[Kyoto Protocol]], an international agreement aimed at reducing greenhouse gas emissions, was adopted by 169 nations.
*There is new evidence that observed climate change is likely to have already increased the risk of certain extreme events such as heatwaves, and it is more likely than not that warming has contributed to the intensification of some tropical cyclones, with increasing levels of adverse impacts as temperatures increase (very high confidence).
*The distribution of impacts and vulnerabilities is still considered to be uneven, and low-latitude, less-developed areas are generally at greatest risk due to both higher sensitivity and lower adaptive capacity; but there is new evidence that vulnerability to climate change is also highly variable within countries, including developed countries." </ref> Scientific uncertainties include the extent of climate change expected in the future, and how changes will vary around the globe. There is political and public debate about what action should be taken to reduce future warming or to adapt to its consequences. The [[Kyoto Protocol]], an international agreement aimed at reducing greenhouse gas emissions, was adopted by 169 nations.
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==Terminology==
==Terminology==
The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes.<ref>{{cite web | title = United Nations Framework Convention on Climate Change, Article I | publisher = [[United Nations Framework Convention on Climate Change]] | url = http://unfccc.int/essential_background/convention/background/items/2536.php | accessdate = 2007-01-15 }}</ref>  The terms "anthropogenic global warming" and "anthropogenic climate change" are sometimes used when focusing on human-induced changes.
The ''weather'' is the day-by-day temperature, humidity, wind and rainfall in a given region; ''climate'' encompasses long- term weather patterns. The Earth's atmosphere is heated by radiation from the sun, and how much of that heat is retained rather than reflected depends critically on the composition of the atmosphere. In particular, the burning of fossil fuels releases "greenhouse gases" into the atmosphere that are causing "climate change."
 
The ''United Nations Framework Convention on Climate Change'' (UNFCCC) uses this term, "climate change", for human-caused change, and "climate variability" for other changes.<ref>{{cite web | title = United Nations Framework Convention on Climate Change, Article I | url = http://unfccc.int/essential_background/convention/background/items/2536.php | accessdate = 2007-01-15 }}</ref>  The terms "anthropogenic global warming" and "anthropogenic climate change" are sometimes also used for human-induced changes.
 
The ''[[Intergovernmental Panel on Climate Change]]'' (IPCC)<ref>[http://www.ipcc.ch/ Intergovernmental Panel on Climate Change]</ref>  is a scientific body that was established by the ''United Nations Environment Programme'' <ref>[http://www.unep.org/ United Nations Environment Programme] Mission: "To provide leadership and encourage partnership in caring for the environment by inspiring, informing, and enabling nations and peoples to improve their quality of life without compromising that of future generations."</ref> and the ''World Meteorological Organization'' (WMO), a specialized agency of the UN.<ref>[http://www.wmo.int/pages/about/index_en.html ''World Meteorological Organization'']</ref> It was established to provide a clear scientific view on the current state of knowledge in climate change, and its potential environmental and socio-economic impacts. The IPCC works by reviewing and assessing current scientific, technical and socio-economic information, and thousands of scientists from all over the world contribute to this, encompassing a range of views and expertise.  Governments participate in the review process and in the plenary sessions, where decisions about the IPCC work programme are taken and where reports are presented, revised and approved.  The IPCC is endorsed by the UN General Assembly, and currently has 194 member countries. In 2007, the IPCC was awarded the [[Nobel Peace Prize]] jointly with [[Al Gore]]  "for their efforts to build up and disseminate greater knowledge about man-made climate change, and to lay the foundations for the measures that are needed to counteract such change"<ref>[http://nobelprize.org/nobel_prizes/peace/laureates/2007/ The Nobel Peace Prize 2007} Intergovernmental Panel on Climate Change, Al Gore</ref>
 
The ''[[Kyoto Protocol]]'' is an agreement that set binding targets for 37 industrialized countries, including the European community, for reducing greenhouse gas emissions. Acknowledging that developed countries are mainly responsible for the current high levels of greenhouse gas emissions, the Protocol placed a greater burden on them under the principle of "common but differentiated responsibilities."<ref>[http://unfccc.int/resource/docs/convkp/kpeng.pdf "Kyoto Protocol to the United Nations Framework Convention on Climate Change"] Full text</ref>. Under the [[George W. Bush|Bush]] Administration, the USA refused to ratify the protocol, and the [[Barack Obama|Obama]] administration followed suit<ref>http://www.american.com/archive/2009/november/the-quiet-yet-historic-death-of-the-kyoto-protoco The Quiet Death of the Kyoto Protocol] ''The American'' November 5, 2009</ref> With the Kyoto Protocol set to expire in 2012, in December 2009, governments met at an international Climate Conference in Copenhagen to negotiate a continuation of international efforts to minimise global warming. That conference concluded with the ''Copenhagen accord'' which asserted a "strong political will to urgently combat climate change in accordance with the principle of common but differentiated responsibilities and respective capabilities." Signatories agree that deep cuts in global emissions are required, recognize the crucial role of reducing emission from deforestation, and agree that developed countries shall provide financial resources, technology and capacity-building to support adaptation in developing countries. The USA has signed, along with most other countries of the world.<ref>[http://unfccc.int/home/items/5262.php Copenhagen Accord]</ref><ref>[http://www.usclimatenetwork.org/policy/copenhagen-accord-commitments Who's On Board With The Copenhagen Accord?] USCAN climate action network</ref>


==Causes==
==Causes==
The climate system varies both through internal processes and in response to external forcing.  External forcing includes solar activity, volcanic emissions, variations in Earth's orbit , and atmospheric composition. The scientific consensus<ref>{{cite web |title=Joint science academies' statement: The science of climate change | url=http://www.royalsoc.ac.uk/displaypagedoc.asp?id=13619 | format=[[Active Server Pages|ASP]] | quote=The work of the Intergovernmental Panel on Climate Change (IPCC) represents the consensus of the international scientific community on climate change science| publisher = [[Royal Society]] | date =[[2001-05-17]] |accessdate=2007-04-01}}</ref> is that most of the warming observed since the mid-twentieth century is very likely due to increased atmospheric concentrations of greenhouse gases produced by human activity. Some other hypotheses have been offered to explain most of the observed increase in global temperatures but these are not broadly supported in the scientific community. Among these are that the warming is caused by natural fluctuations in the climate, that warming is mainly a result of variations in solar radiation,<ref>{{cite journal |last=Bard |first=Edouard |coauthors=Frank, Martin |date=[[2006-06-09]] |title=Climate change and solar variability: What's new under the sun? |journal=Earth and Planetary Science Letters |volume=248 |issue=1-2 |pages=1-14 |url=http://www.ifm-geomar.de/fileadmin/personal/fb1/p-oz/mfrank/Bard_and_Frank_2006.pdf |accessdate= 2007-09-17}}</ref> or that warming is caused by changes in cloud cover due to variations in galactic cosmic rays.<ref>{{cite journal |last=Svensmark |first=Henrik |authorlink=Henrik Svensmark |year=2000 | month=July |format=[[Portable Document Format|PDF]] |title=Cosmic Rays and Earth's Climate |journal=Space Science Reviews |volume=93 |issue=1-2 |pages=175-185 |url=http://winnetou.lcd.lu/physique/OSCIE2003/global_warming/Cosmic_rays_and_Earth_Climate_new_sven0606.pdf |accessdate= 2007-09-17}}</ref>
{|align="right" style="width:50%;"
|
<font color="darkblue"><b>
:''"Human-induced warming of the climate system is widespread. Anthropogenic warming of the climate system can be detected in temperature observations taken at the surface, in the troposphere and in the oceans. Multi-signal detection and attribution analyses, which quantify the contributions of different natural and anthropogenic forcings to observed changes, show that greenhouse gas forcing alone during the past half century would likely have resulted in greater than the observed warming if there had not been an offsetting cooling effect from aerosol and other forcings.
:''"It is extremely unlikely (<5%) that the global pattern of warming during the past half century can be explained without external forcing, and very unlikely that it is due to known natural external causes alone. The warming occurred in both the ocean and the atmosphere and took place at a time when natural external forcing factors would likely have produced cooling.  
:''"Greenhouse gas forcing has very likely caused most of the observed global warming over the last 50 years. This conclusion takes into account observational and forcing uncertainty, and the possibility that the response to solar forcing could be underestimated by climate models. It is also robust to the use of different climate models, different methods for estimating the responses to external forcing and variations in the analysis technique."
:(IPCC 4th Assessment Report: Climate Change 2007<ref>IPCC 4th Assessment Report Climate Change 2007 Chapter 9: [http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch9.html Understanding and Attributing Climate Change]</ref>
{{col-end}}


The effects of forcing are not instantaneous. Due to the thermal inertia of the oceans and the slow responses of some feedback processes, Earth's climate is never in perfect equilibrium with the imposed forcing. Climate commitment studies indicate that even if greenhouse gases were stabilized at present day levels there would be a further warming of about 0.5 °C (0.9 °F) as the climate continued to adjust toward equilibrium.<ref>{{cite journal |last=Meehl |first=Gerald A. |coauthors=''et al.'' |date=[[2005-03-18]] |title=How Much More Global Warming and Sea Level Rise |journal=[[Science (journal)|Science]] |volume=307 |issue=5716 |pages=1769–1772 |doi=10.1126/science.1106663 |url=http://www.sciencemag.org/cgi/content/full/307/5716/1769 |accessdate=2007-02-11}}</ref>
There is no serious dispute that there has been a large and continuing increase in atmospheric CO<sub>2</sub> concentrations since the middle of the twentieth century; there is no dispute that there has been a parallel increase in fossil fuel use over this time, and there is no dispute that there has been an increase in mean global temperature over the same period. Although some skeptics still doubt that the rise in CO<sub>2</sub> is substantially a consequence of man's activities, all national science academies that have issued statements on the matter accept the IPCC's conclusion that they probably are, and that future man-made climate change is likely<ref name="academies"/>.


===Greenhouse gases in the atmosphere===
The climate system varies through internal processes and in response to ''external forcing''. External forcing includes fossil fuel emissions, but also solar activity, volcanic emissions, variations in the Earth's orbit, and variations in atmospheric composition. The scientific consensus<ref>{{cite web |title=Joint science academies' statement: The science of climate change | url=http://www.royalsoc.ac.uk/displaypagedoc.asp?id=13619 | quote=The work of the Intergovernmental Panel on Climate Change (IPCC) represents the consensus of the international scientific community on climate change science| publisher = Royal Society| date =2001}}</ref> is that most of the warming observed since the mid-twentieth century is  due to increased atmospheric concentrations of greenhouse gases produced by human activity. Alternative mechanisms seem able to account for only a small part of the observed increase in global temperatures. These other mechanisms include warming as a result of natural fluctuations in the climate, and warming as a result of variations in solar radiation,<ref>{{cite journal |last=Bard |first=E|coauthors=Frank M |date=2006|title=Climate change and solar variability: What's new under the sun? |journal=Earth and Planetary Science Lett |volume=248  |pages=1-14 |url=http://www.ifm-geomar.de/fileadmin/personal/fb1/p-oz/mfrank/Bard_and_Frank_2006.pdf }}</ref> or that warming is caused by changes in cloud cover due to variations in galactic cosmic rays.<ref>{{cite journal |last=Svensmark |first=H |year=2000  |title=Cosmic Rays and Earth's Climate |journal=Space Science Rev |volume=93  |pages=175-85 |url=http://winnetou.lcd.lu/physique/OSCIE2003/global_warming/Cosmic_rays_and_Earth_Climate_new_sven0606.pdf }}</ref>


Existence of the [[greenhouse effect]] itself is not disputed. It is the process by which emission of infrared radiation by atmospheric gases warms a planet's atmosphere and surface. Naturally occurring greenhouse gases warm the Earth by about 33&nbsp;°C (59&nbsp;°F). Without this natural greenhouse effect, the average temperature of Earth would be about -18&nbsp;°C (0&nbsp;°F) making the planet uninhabitable.<ref>{{cite paper|title=Living with Climate Change – An Overview of Potential Climate Change Impacts on Australia. Summary and Outlook |publisher=[[Australian Greenhouse Office]] |date=December 2002 |format=[[Portable Document Format|PDF]] |accessdate=2007-04-18 |url=http://www.greenhouse.gov.au/impacts/overview/pubs/overview4.pdf}}</ref> The major natural greenhouse gases are [[water vapor]], which causes about 36–70% of the greenhouse effect (not including clouds); [[carbon dioxide]] (CO<sub>2</sub>), which causes 9–26%; [[methane]] (CH<sub>4</sub>), which causes 4–9%; and [[ozone]], which causes 3–7%.<ref>{{cite journal| url=http://www.atmo.arizona.edu/students/courselinks/spring04/atmo451b/pdf/RadiationBudget.pdf| title=Earth’s Annual Global Mean Energy Budget| first=J. T.| last=Kiehl| coauthors= Kevin E. Trenberth| format=PDF | journal=Bulletin of the American Meteorological Society| pages=197-208| volume=78| issue=2| month=February| year=1997| accessdate=2006-05-01}}</ref>  
The effects of external forcing on the climate are not instantaneous, due to the thermal inertia of the oceans and the slow responses of some feedback processes. Climate models indicate that, even if greenhouse gases were stabilized at present day levels, there would be a further warming of about 0.5 °C (0.9 °F) as the climate continued to adjust toward equilibrium.<ref>{{cite journal |last=Meehl |first=GA |coauthors=''et al.''|date=2005 |title=How much more global warming and sea level rise |journal=Science|volume=307 |pages=1769–72 |url=http://www.sciencemag.org/cgi/content/full/307/5716/1769 }}
*"Two global coupled climate models show that even if the concentrations of greenhouse gases in the atmosphere had been stabilized in the year 2000, we are already committed to further global warming of about another half degree and an additional 320% sea level rise caused by thermal expansion by the end of the 21st century. ...At any given point in time, even if concentrations are stabilized, there is a commitment to future climate changes that will be greater than those we have already observed."</ref>


The present atmospheric concentration of CO<sub>2</sub> is about 383 parts per million ([[ppm]]) by volume.<ref>{{cite web | title = Trends in Atmospheric Carbon Dioxide Mauna Loa | last = Tans | first = Pieter | url = http://www.esrl.noaa.gov/gmd/ccgg/trends/ | publisher = [[National Oceanic and Atmospheric Administration]] | accessdate = 2007-04-28}}</ref> From geological evidence it is believed that CO<sub>2</sub> values this high were last attained 20 million years ago.<ref>{{cite journal| first=Paul N.| last=Pearson| coauthors=Palmer, Martin R.| journal=[[Nature (journal)|Nature]]| title= Atmospheric carbon dioxide concentrations over the past 60 million years| date=[[2000-08-17]]| volume=406| issue=6797| pages=695–699| url=http://www.nature.com/nature/journal/v406/n6797/abs/406695a0.html| doi=10.1038/35021000}}</ref> About three-fourths of man-made CO<sub>2</sub> emissions over the past 20 years have come from the burning of fossil fuels. Most of the rest is due to land-use change, mainly deforestation.<ref>{{cite web |url=http://www.grida.no/climate/ipcc_tar/wg1/006.htm |title=Summary for Policymakers |work=Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change |accessdate=2007-01-18 |date=[[2001-01-20]] |publisher=[[Intergovernmental Panel on Climate Change]]}}</ref> Measured trends in atmospheric composition and isotope ratios (namely the simultaneous depletion of <sup>13</sup>C, <sup>14</sup>C, and O<sub>2</sub>) confirm that the increased atmospheric CO<sub>2</sub> mainly comes from fossil fuels and not from other sources such as volcanoes or the oceans.<ref>[http://www.sciencemag.org/cgi/content/abstract/256/5053/74]</ref>  
===Greenhouse gases ===
The [[greenhouse effect]] is the process by which emission of infrared radiation by atmospheric gases warms a planet's atmosphere and surface. Naturally occurring greenhouse gases warm the Earth by about 33&nbsp;°C (59&nbsp;°F); without this, the average temperature of the Earth would be about -18&nbsp;°C (0&nbsp;°F) making the planet uninhabitable.<ref>{{cite paper|title=Living with Climate Change An Overview of Potential Climate Change Impacts on Australia. Summary and Outlook |publisher=Australian Greenhouse Office|date=2002|url=http://www.greenhouse.gov.au/impacts/overview/pubs/overview4.pdf}}</ref> The major natural greenhouse gases are [[water vapor]], which causes about 36–70% of the greenhouse effect (not including clouds); [[carbon dioxide]] (CO<sub>2</sub>), which causes 9–26%; [[methane]] (CH<sub>4</sub>), which causes 4–9%; and [[ozone]], which causes 3–7%.<ref>{{cite journal| url=http://www.atmo.arizona.edu/students/courselinks/spring04/atmo451b/pdf/RadiationBudget.pdf| title=Earth’s Annual Global Mean Energy Budget| first=JT| last=Kiehl| coauthors= KE Trenberth|journal=Bull Am Meteorol Soc| pages=197-208|volume=78|year=1997}}</ref>  


Future CO<sub>2</sub> concentrations will depend on uncertain economic, sociological, technological, and natural developments. The IPCC [[Special Report on Emissions Scenarios]] gives a wide range of future CO<sub>2</sub> scenarios, ranging from 541 to 970 ppm by the year 2100.<ref>{{cite web |url=http://www.grida.no/climate/ipcc_tar/wg1/123.htm |last = Prentice |first = I. Colin |coauthors = ''et al.'' |title = 3.7.3.3 SRES scenarios and their implications for future CO2 concentration |work = Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change |accessdate=2007-04-28 |date=[[2001-01-20]] |publisher=[[Intergovernmental Panel on Climate Change]]}}</ref> Fossil fuel reserves are sufficient to reach these levels and continue emissions past 2100, if coal, tar sands, or [[methane clathrate]]s are extensively used.<ref>{{cite web |url=http://www.grida.no/climate/ipcc/emission/104.htm |title=4.4.6. Resource Availability |work=IPCC Special Report on Emissions Scenarios |accessdate=2007-04-28 |publisher=[[Intergovernmental Panel on Climate Change]]}}</ref> Positive feedback effects such as the release of methane from the melting of [[permafrost]] [[peat]] [[bog]]s in [[Siberia]] (possibly up to 70,000 million [[tonne]]s) may lead to significant additional sources of greenhouse gas emissions<ref>{{cite news | first=Ian | last=Sample | title=Warming Hits 'Tipping Point' | date=[[2005-08-11]] | url=http://www.guardian.co.uk/climatechange/story/0,12374,1546824,00.html | publisher=[[The Guardian]] | accessdate=2007-01-18}}</ref> not included in climate models cited by the IPCC.<ref name=grida7/>
The present atmospheric concentration of CO<sub>2</sub> is about 383 parts per million (ppm) by volume.<ref>{{cite web | title = Trends in Atmospheric Carbon Dioxide – Mauna Loa | last = Tans | first = P| url = http://www.esrl.noaa.gov/gmd/ccgg/trends/ | publisher = [[National Oceanic and Atmospheric Administration]]}}</ref> From geological evidence, it is believed that CO<sub>2</sub> values this high were last attained 20 million years ago.<ref>{{cite journal| first=PN| last=Pearson| coauthors=Palmer MR| journal=Nature| title= Atmospheric carbon dioxide concentrations over the past 60 million years| date=2000| volume=406| pages=695–9| url=http://www.nature.com/nature/journal/v406/n6797/abs/406695a0.html}}</ref> About three-quarters of man-made CO<sub>2</sub> emissions over the past 20 years have come from the burning of fossil fuels; most of the rest is due to land-use change, mainly deforestation.<ref>{{cite web |url=http://www.grida.no/climate/ipcc_tar/wg1/006.htm |title=Summary for Policymakers |work=Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change|date=2001}}</ref> Measured trends in atmospheric composition and isotope ratios (namely the simultaneous depletion of <sup>13</sup>C, <sup>14</sup>C, and O<sub>2</sub>) confirm that the increased atmospheric CO<sub>2</sub> mainly comes from fossil fuels and not from other sources such as volcanos or the oceans.<ref>Quay PD ''et al.'' (1992) Oceanic uptake of fossil fuel CO2: carbon-13 evidence ''Science'' [http://www.sciencemag.org/cgi/content/abstract/256/5053/74 256:74-9]</ref>  


The warming due to atmospheric carbon dioxide from burning fossil fuels releases methane from the Arctic surface in at least three ways: (1) by forming lakes of melted ice whose waters melts the underlying permafrost, allowing methane-producing microbes to increase methane production by exposing thawing vegetative and animal matter for them to consume; (2) by opening channels in the attenuated permafrost cap for release into the atmosphere of old trapped methane hydrates; (3) by thawing offshore layers of permafrost capping methane hydrates.<ref name=simpson2009/>
Future CO<sub>2</sub> concentrations depend on uncertain economic, sociological, technological, and natural developments. The IPCC [[Special Report on Emissions Scenarios]] gives a wide range of future CO<sub>2</sub> scenarios, ranging from 541 to 970 ppm by the year 2100.<ref>{{cite web |url=http://www.grida.no/climate/ipcc_tar/wg1/123.htm |last = Prentice |first = IC|coauthors = ''et al.''|title = 3.7.3.3 SRES scenarios and their implications for future CO2 concentration |work = Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change|date=2001}}</ref> Fossil fuel reserves are sufficient to reach these levels and continue emissions past 2100, if coal, tar sands, or [[methane clathrate]]s are  used extensively.<ref>{{cite web |url=http://www.grida.no/climate/ipcc/emission/104.htm |title=4.4.6. Resource Availability |work=IPCC Special Report on Emissions Scenarios}}</ref> Positive feedback effects such as the release of methane from the melting of [[permafrost]] [[peat]] [[bog]]s in [[Siberia]] (possibly up to 70,000 million [[tonne]]s) may lead to significant additional sources of greenhouse gas emissions<ref>{{cite news| first=Ian | last=Sample |title=Warming Hits 'Tipping Point'|date=2005| url=http://www.guardian.co.uk/climatechange/story/0,12374,1546824,00.html | publisher=''The Guardian''|accessdate=2007-01-18}}</ref> not included in climate models cited by the IPCC.<ref name=grida7/>
 
The warming due to atmospheric CO<sub>2</sub> from burning fossil fuels releases methane from the Arctic surface in at least three ways: (1) by forming lakes of melted ice whose waters melts the underlying permafrost, allowing methane-producing microbes to increase methane production by exposing thawing vegetative and animal matter for them to consume; (2) by opening channels in the attenuated permafrost cap for release into the atmosphere of old, trapped methane hydrates; (3) by thawing offshore layers of permafrost capping methane hydrates.<ref name=simpson2009/>


===Feedbacks===
===Feedbacks===
The effects of forcing agents on the climate are modified by feedback processes. One of the most important feedbacks is caused by the evaporation of water. Increased greenhouse gases from human activity cause a warming of the [[Earth's atmosphere]] and surface. The increased warmth in turn increases the evaporation of water into the atmosphere. Since water vapor itself is a greenhouse gas, this causes still more warming; the warming causes more water vapor to be evaporated, and so on.  Eventually a new dynamic equilibrium concentration of water vapor is reached at a slight increase in humidity and with a much larger greenhouse effect than that due to CO<sub>2</sub> alone.<ref name=soden1>{{cite journal| first= Brian J. | last= Soden | coauthors= Held, Isacc M. | journal= [[Journal of Climate]] | title= An Assessment of Climate Feedbacks in Coupled Ocean–Atmosphere Models | date= [[2005-11-01]] | volume= 19 | issue= 14 | page= 3354–3360 | url= http://www.gfdl.noaa.gov/reference/bibliography/2006/bjs0601.pdf | format= [[Portable Document Format|PDF]] | accessdate= 2007-04-21 | quote=Interestingly, the true feedback is consistently weaker than the constant relative humidity value, implying a small but robust reduction in relative humidity in all models on average" "clouds appear to provide a positive feedback in all models}}</ref>  
The effects of forcing agents on the climate are modified by feedback processes, one of the most important of which is caused by the evaporation of water. Increased greenhouse gases from human activity cause a warming of the [[Earth's atmosphere]] and surface, which increases the evaporation of water into the atmosphere. As water vapor is itself a greenhouse gas, this causes further warming, causing yet more water vapor to be evaporated, and so on.  Eventually a new dynamic equilibrium concentration of water vapor is reached at a slight increase in humidity and with a much larger greenhouse effect than that due to CO<sub>2</sub> alone.<ref name=soden1>{{cite journal| first= BJ| last= Soden | coauthors= Held IM| journal= J Climate| title= An assessment of climate feedbacks in coupled ocean–atmosphere models | date= 2005 | volume= 19| page= 3354–60 | url= http://www.gfdl.noaa.gov/reference/bibliography/2006/bjs0601.pdf| quote=Interestingly, the true feedback is consistently weaker than the constant relative humidity value, implying a small but robust reduction in relative humidity in all models on average" "clouds appear to provide a positive feedback in all models}}</ref>  


The radiative effects of clouds are a major source of uncertainty in climate projections. Seen from below, clouds emit infrared radiation to the surface, and so have a warming effect. Seen from above, clouds reflect sunlight and emit infrared radiation to space, and so have a cooling effect. The cloud feedback effect is influenced not only by the amount of clouds but also by their distribution; for example, high clouds are at colder temperatures than low clouds, and thus radiate less energy to space. Increased global water vapor content may or may not cause an increase in global or regional cloud cover, since cloud cover is affected by [[relative humidity]] rather than the absolute concentration of water vapor. Cloud feedback is second only to water vapor feedback and has been found to have a net warming effect in all the models that contributed to the [[IPCC Fourth Assessment Report]].<ref name=soden1/>  
The radiative effects of clouds are a source of uncertainty in climate projections. Seen from below, clouds emit infrared radiation to the surface, and so have a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so have a cooling effect. The cloud feedback effect is influenced not only by the amount of clouds but also by their distribution; high clouds are colder than low clouds, and thus radiate less energy to space. Increased global water vapor content may or may not cause an increase in global or regional cloud cover, since cloud cover is affected by [[relative humidity]] rather than the absolute concentration of water vapor. Cloud feedback is second only to water vapor feedback and has a net warming effect in all the models that contributed to the [[IPCC Fourth Assessment Report]].<ref name=soden1/>  


Another important process is ice-albedo feedback.<ref>{{cite web |url=http://www.grida.no/climate/ipcc_tar/wg1/295.htm |last = Stocker |first = Thomas F. |coauthors = ''et al.'' |title = 7.5.2 Sea Ice |work = Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change |accessdate=2007-02-11 |date=[[2001-01-20]] |publisher=[[Intergovernmental Panel on Climate Change]]}}</ref> Warming of the Earth's surface leads to melting of ice near the poles. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and the cycle continues.
Global warming also leads to melting of ice near the poles, exposing more land surface. Land (and open water) are less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and the cycle continues (the ''"ice albedo feedback"''). Sea ice also has an important role in moderating heat exchange between the ocean and atmosphere at high latitudes, and by feedbacks involving ice growth and melt and the fresh water balance at the ocean surface<ref>{{cite web |url=http://www.grida.no/climate/ipcc_tar/wg1/295.htm |last = Stocker |first = TF |coauthors = ''et al.'' |title = 7.5.2 Sea Ice |work = Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change |date=2001}}</ref> The ocean's ability to sequester carbon is expected to decline as it warms, because the resulting low nutrient levels of the [[mesopelagic zone]] limits the growth of [[diatoms]] in favor of smaller [[phytoplankton]] that are poorer [[biological pump]]s of carbon.<ref>Buesseler KO ''et al.'' (2007) Revisiting carbon flux through the ocean's twilight zone ''Science'' 316:567-70</ref>
 
The ocean's ability to sequester carbon is expected to decline as it warms, because the resulting low nutrient levels of the [[mesopelagic zone]] limits the growth of [[diatoms]] in favor of smaller [[phytoplankton]] that are poorer [[biological pump]]s of carbon.<ref>Buesseler, K.O., C.H. Lamborg, P.W. Boyd, P.J. Lam, T.W. Trull, R.R. Bidigare, J.K.B. Bishop, K.L. Casciotti, F. Dehairs, M. Elskens, M. Honda, D.M. Karl, D.A. Siegel, M.W. Silver, D.K. Steinberg, J. Valdes, B. Van Mooy, S. Wilson. (2007) "Revisiting carbon flux through the ocean's twilight zone." ''[[Science (journal)|Science]]'' '''316''': 567-570.</ref>


===Solar variation===
===Solar variation===
It has been hypothesized that variations in [[solar variation|solar output]], possibly amplified by cloud feedbacks, may have been a secondary contributor to recent warming.<ref>{{cite journal | first=Nigel | last=Marsh | coauthors=Henrik, Svensmark | title=Cosmic Rays, Clouds, and Climate | journal=Space Science Reviews | volume=94 | number=1–2 | pages=215–230 | year=2000 | month=November | url=http://www.dsri.dk/~hsv/SSR_Paper.pdf | format=[[Portable Document Format|PDF]] | doi=10.1023/A:1026723423896 | accessdate=2007-04-17}}</ref> Natural phenomena, such as solar variation and [[volcano]]es, probably had a net warming effect from pre-industrial times to 1950 and a small cooling effect since 1950.<ref>{{cite web | url=http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Pub_Ch09.pdf | format=[[Portable Document Format|PDF]] | title=Understanding and Attributing Climate Change | work=Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change | accessdate=2007-05-20 | date=[[2007-05-07]] | publisher=[[Intergovernmental Panel on Climate Change]] | last=Hegerl | first=Gabriele C. | coauthors=''et al.'' | pages=690}}</ref> Some research indicate that the Sun's contribution may have been underestimated. These results suggest that the Sun may have contributed about 40–50% of the global surface warming between 1900 and 2000 and about 25–35% of the warming between 1980 and 2000.<ref>{{cite journal | first=Nicola | last=Scafetta | coauthors=West, Bruce J. | title=Phenomenological solar contribution to the 1900–2000 global surface warming | url = http://www.fel.duke.edu/~scafetta/pdf/2005GL025539.pdf | format = [[Portable Document Format|PDF]] | date=[[2006-03-09]] | journal=[[Geophysical Research Letters]] | volume=33 | issue=5 | id=L05708 | doi=10.1029/2005GL025539 | accessdate=2007-05-08}}</ref> Stott and coauthors suggest that climate models overestimate the relative effect of greenhouse gases compared to solar forcing; they also suggest that the cooling effects of volcanic dust and sulfate aerosols have been underestimated.<ref>{{Cite journal | first=Peter A. | last=Stott | coauthors=''et al.'' | title=Do Models Underestimate the Solar Contribution to Recent Climate Change? | date=[[2003-12-03]] | journal=[[Journal of Climate]] | volume=16 | issue=24 | pages=4079–4093 | doi=10.1175/1520-0442(2003)016%3C4079:DMUTSC%3E2.0.CO;2 | accessdate=2007-04-16 | url=http://climate.envsci.rutgers.edu/pdf/StottEtAl.pdf}}</ref> Nevertheless, they conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming during the latest decades is attributable to the increases in greenhouse gases.
Variations in [[solar variation|solar output]], possibly amplified by cloud feedbacks, may have been a secondary contributor to recent global warming. <ref>{{cite journal|first=N | last=Marsh| coauthors=Svensmark H| title=Cosmic rays, clouds, and climate|journal=Space Science Rev| volume=94|pages=215–30|year=2000| url=http://www.dsri.dk/~hsv/SSR_Paper.pdf}}
*"Overall, the role of solar activity in climate changes—such as the Quaternary glaciations or the present global warming—remains unproven and most probably represents a second-order effect. Although we still require even more and better data, the weight of evidence suggests that solar changes have contributed to small climate oscillations occurring on time scales of a few centuries, similar in type to the fluctuations classically described for the last millennium: The so-called Medieval Warm Period (900–1400 A.D.) followed on by the Little Ice Age (1500–1800 A.D.)."</ref> Natural phenomena, such as solar variation and [[volcano]]es, probably had a net warming effect from pre-industrial times to 1950 and a small cooling effect since 1950.<ref>{{cite web | url=http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch9.html |title=Understanding and Attributing Climate Change|work=Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change| date=2007|last=Hegerl|first=GC| coauthors=''et al.''| pages=690}}
*"Overall, modelling and detection and attribution studies confirm a role of volcanic, greenhouse gas and probably solar forcing in explaining the broad temperature evolution of the last millennium" </ref> Some research indicate that the Sun's contribution may have been underestimated, and that the Sun may have contributed about 40–50% of the global surface warming between 1900 and 2000 and about 25–35% of the warming between 1980 and 2000.<ref>{{cite journal| first=N| last=Scafetta | coauthors=West BJ| title=Phenomenological solar contribution to the 1900–2000 global surface warming |url = http://www.fel.duke.edu/~scafetta/pdf/2005GL025539.pdf| date=2006 | journal=Geophys Res Lett| volume=33|id=L05708}}</ref> Some authors suggest that climate models overestimate the relative effect of greenhouse gases compared to solar forcing, and that the cooling effects of volcanic dust and sulfate aerosols have been underestimated.<ref>{{Cite journal|first=PA|last=Stott| coauthors=''et al.''| title=Do Models Underestimate the Solar Contribution to Recent Climate Change?| date=2003| journal=J Climate|volume=16|pages=4079–93| url=http://climate.envsci.rutgers.edu/pdf/StottEtAl.pdf}}</ref> Nevertheless, they conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming during the latest decades is attributable to the increases in greenhouse gases.


==Climate change since the Industrial Revolution==
==Climate change since the Industrial Revolution==
According to the [[instrumental temperature record]], mean global temperatures (both land and sea) have increased by 0.75&nbsp;°C (1.35&nbsp;°F) relative to the period 1860&ndash;1900. This measured temperature increase is not significantly affected by the [[urban heat island]] effect.<ref>{{cite journal|author=David E. Parker|year=2004|title=Climate: Large-scale warming is not urban|journal=[[Nature (journal)|Nature]]|volume=432|pages=290}}[http://www.nature.com/nature/journal/v432/n7015/abs/432290a.html </ref><ref> {{cite journal|author=David E. Parker|year=2006|title=A demonstration that large-scale warming is not urban|journal=[[Journal of Climate]]|volume=19|pages=2882–2895}}[http://ams.allenpress.com/perlserv/?request=get-document&doi=10.1175%2FJCLI3730.1](online)]</ref><ref>{{cite journal|author=Thomas C. Peterson|year=2003|title=Assessment of urban versus rural in situ surface temperatures in the contiguous United States: no difference found|journal=Journal of Climate|volume=16|pages=2941–2959}} [http://www.ncdc.noaa.gov/oa/wmo/ccl/rural-urban.pdf (PDF)]</ref> Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25&nbsp;°C per decade against 0.13&nbsp;°C per decade).<ref>{{cite journal| last = Smith | first = Thomas M. | coauthors= Reynolds, Richard W. | title = A Global Merged Land–Air–Sea Surface Temperature Reconstruction Based on Historical Observations (1880–1997) | journal = [[Journal of Climate]] |volume = 18 |issue = 12 | pages = 2021–2036 | url = http://www.ncdc.noaa.gov/oa/climate/research/Smith-Reynolds-dataset-2005.pdf | format = [[Portable Document Format|PDF]] | date = [[2005-05-15]] | accessdate = 2007-03-14}}</ref> Temperatures in the lower [[troposphere]] have increased between 0.12 and 0.22&nbsp;°C (0.22 and 0.4&nbsp;°F) per decade since 1979, according to [[satellite temperature measurements]]. [[Temperature record of the past 1000 years|Temperature]] is believed to have been relatively stable over the one or two thousand years before 1850, with possibly regional fluctuations such as the [[Medieval Warm Period]] or the [[Little Ice Age]].
According to the [[instrumental temperature record]], mean global temperatures (both land and sea) have increased by 0.75&nbsp;°C (1.35&nbsp;°F) since the period 1860&ndash;1900. This increase is not significantly affected by the [[urban heat island]] effect.<ref>{{cite journal|author=DE Parker|year=2004|title=Climate: Large-scale warming is not urban|journal=Nature|volume=432|pages=290}}[http://www.nature.com/nature/journal/v432/n7015/abs/432290a.html ]</ref><ref> {{cite journal|author=DE Parker|year=2006|title=A demonstration that large-scale warming is not urban|journal=J Climate|volume=19|pages=2882–95}}[http://ams.allenpress.com/perlserv/?request=get-document&doi=10.1175%2FJCLI3730.1](online)]</ref><ref>{{cite journal|author=Peterson TC|year=2003|title=Assessment of urban versus rural in situ surface temperatures in the contiguous United States: no difference found|journal=J Climate|volume=16|pages=2941–59}} [http://www.ncdc.noaa.gov/oa/wmo/ccl/rural-urban.pdf (PDF)]</ref> Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25&nbsp;°C per decade against 0.13&nbsp;°C per decade).<ref>{{cite journal| last = Smith| first = TM | coauthors= Reynolds RW | title = A global merged land–air–sea surface temperature reconstruction based on historical observations (1880–1997)| journal = J Climate|volume = 18| pages = 2021–36| url = http://www.ncdc.noaa.gov/oa/climate/research/Smith-Reynolds-dataset-2005.pdf| date = 2005}}</ref> Temperatures in the lower [[troposphere]] have increased between 0.12 and 0.22&nbsp;°C (0.22 and 0.4&nbsp;°F) per decade since 1979, according to [[satellite temperature measurements]]. [[Temperature record of the past 1000 years|Temperature]] is believed to have been relatively stable over the one or two thousand years before 1850, with possibly regional fluctuations such as the [[Medieval Warm Period]] or the [[Little Ice Age]].


Based on estimates by [[NASA]]'s [[Goddard Institute for Space Studies]], 2005 was the warmest year since reliable, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree.<ref>{{cite web |url= http://data.giss.nasa.gov/gistemp/2005/ |last= Hansen | first = James E. |authorlink= James Hansen |coauthors= ''et al.'' |title= Goddard Institute for Space Studies, GISS Surface Temperature Analysis |accessdate=2007-01-17 |date= [[2006-01-12]] |publisher= NASA [[Goddard Institute for Space Studies]]}}</ref> Estimates prepared by the [[World Meteorological Organization]] and the [[Climatic Research Unit]] concluded that 2005 was the second warmest year, behind 1998.<ref>{{cite web |url= http://www.cru.uea.ac.uk/cru/press/2005-12-WMO.pdf |title= Global Temperature for 2005: second warmest year on record |accessdate=2007-04-13 |date= [[2005-12-15]] |publisher= [[Climatic Research Unit]], School of Environmental Sciences, University of East Anglia |format = [[Portable Document Format|PDF]]}}</ref><ref>{{cite web |url=http://grdc.bafg.de/servlet/is/4226/Pressemitteilung-WMO-23-Dez-05-743_E1.pdf |format=[[Portable Document Format|PDF]] |title=WMO STATEMENT ON THE STATUS OF THE GLOBAL CLIMATE IN 2005 |accessdate=2007-04-13 |date=[[2005-12-15]] |publisher=[[World Meteorological Organization]]}}</ref> Global temperatures in 1998 were exceptionally warm because the strongest [[El Niño]] in the instrumental record occurred in that year.<ref>[http://www.ncdc.noaa.gov/oa/climate/globalwarming.html National Oceanic and Atmospheric Administration: Global Warming Frequently Asked Questions]</ref>
Based on estimates by [[NASA]]'s [[Goddard Institute for Space Studies]], 2005 was the warmest year since reliable measurements became available in the late 1800s.<ref>{{cite web |url= http://data.giss.nasa.gov/gistemp/2005/ |last= Hansen | first = JE |authorlink= Hansen J|coauthors= ''et al.'' |title= Goddard Institute for Space Studies, GISS Surface Temperature Analysis |date= 2006 |publisher= NASA [[Goddard Institute for Space Studies]]}}</ref> Estimates prepared by the [[World Meteorological Organization]] and the [[Climatic Research Unit]] concluded that 2005 was the second warmest year, behind 1998.<ref>{{cite web |url= http://www.cru.uea.ac.uk/cru/press/2005-12-WMO.pdf |title= Global Temperature for 2005: second warmest year on record |date= 2005|publisher= [[Climatic Research Unit]], School of Environmental Sciences, University of East Anglia }}</ref> <ref>{{cite web |url=http://www.wmo.int/pages/prog/wcp/wcdmp/statement/documents/WMO998_E.pdf |title=World Meteorological Organization Statement on the status of the Global Climate in 2005 |date=2005}}
*"Since the beginning of the 20th century, the global average surface temperature has increased by about 0.6°C. However, this increase has not been continuous and has risen sharply since 1976. Areas of significant warmth were widespread with large areas of Africa, Australia, Brazil, the Russian Federation, Scandinavia, Canada, China and the south-west United States showing significantly above average temperatures. Much of the North Atlantic and south-west Pacific Oceans were also significantly warm, as was the Gulf of Alaska. Sea surface temperatures in the North Atlantic in 2005 were the warmest on record."</ref> Global temperatures in 1998 were exceptionally warm because the strongest [[El Niño]] in the instrumental record occurred in that year.<ref>[http://www.ncdc.noaa.gov/oa/climate/globalwarming.html National Oceanic and Atmospheric Administration: Global Warming Frequently Asked Questions]</ref>


Anthropogenic emissions of other [[pollutant]]s—notably sulfate [[Particulate|aerosols]]—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the twentieth century,<ref>{{cite web |url=http://www.grida.no/climate/ipcc_tar/wg1/462.htm |last = Mitchell |first = J. F. B. |coauthors = ''et al.'' |title = 12.4.3.3 Space-time studies |work = Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change |accessdate=2007-01-04 |date=[[2001-01-20]] |publisher=[[Intergovernmental Panel on Climate Change]]}}</ref> though the cooling may also be due in part to natural variability.
Anthropogenic emissions of other [[pollutant]]s—notably sulfate [[Particulate|aerosols]]—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the 20th century,<ref>{{cite web |url=http://www.grida.no/climate/ipcc_tar/wg1/462.htm |last = Mitchell |first = JFB|coauthors = ''et al.'' |title = 12.4.3.3 Space-time studies |work = Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change |date=2001}}</ref> though the cooling may also be due in part to natural variability.


==Climate models==
==Climate models==
Scientists have created computer models of the climate. These models are based on physical principles of fluid dynamics, [[radiative transfer]], and other processes, with some simplifications being necessary because of limitations in computer power. These models predict that the net effect of adding greenhouse gases is to produce a warmer climate. However, even when the same assumptions of fossil fuel consumption and CO<sub>2</sub> emission are used, the amount of projected warming varies between models and there is a considerable range of [[climate sensitivity]]. Including uncertainties in future greenhouse gas concentrations and climate modeling, the IPCC report projects global surface temperatures averaged over 2090-2099 are likely to be 1.1 to 6.4&nbsp;°C (2.0 to 11.5&nbsp;°F) hotter than the average temperatures from 1980-1999.<ref name=grida7/>
The Earth's climate is an extremely complex system: the atmosphere, oceans, and land masses are tightly coupled subsystems and consequently the energy and mass exchanges between them must be studied simultaneously. Further, the [[electromagnetic radiation]] balance between energy absorption and back radiation by the Earth plays a crucial role. Encompassing all these factors, scientists have created computer models of the climate, based on physical principles of fluid dynamics, [[radiative transfer]], and other processes. These models predict that the net effect of adding greenhouse gases is to produce a warmer climate. However, the amount of projected warming varies between models and there is a considerable range of [[climate sensitivity]]. Including uncertainties in future greenhouse gas concentrations and climate modeling, the IPCC report projects global surface temperatures averaged over 2090-2099 are likely to be 1.1 to 6.4&nbsp;°C (2.0 to 11.5&nbsp;°F) hotter than the average temperatures from 1980-1999.<ref name=grida7/>


Models have also been used to help investigate the [[Attribution of recent climate change|causes of recent climate change]] by comparing the observed changes to those that the models project from various natural and human derived causes.  Climate models can produce a good match to observations of global temperature changes over the last century, but cannot yet simulate all aspects of climate.<ref>{{cite web |url=http://www.grida.no/climate/ipcc_tar/wg1/007.htm |title=Summary for Policymakers |work=Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change |accessdate=2007-04-28 |date=[[2001-01-20]] |publisher=[[Intergovernmental Panel on Climate Change]]}}</ref> These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects; however, they suggest that the warming since 1975 is dominated by man-made [[greenhouse gas]] emissions.
Models have also been used to help investigate the [[Attribution of recent climate change|causes of recent climate change]] by comparing the observed changes to those that the models project from various natural and human derived causes.  Climate models can produce a good match to observations of global temperature changes over the last century, but cannot yet simulate all aspects of climate.<ref>{{cite web |url=http://www.grida.no/climate/ipcc_tar/wg1/007.htm |title=Summary for Policymakers |work=Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change|date=2001 }}</ref> These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects, but they suggest that the warming since 1975 is dominated by man-made [[greenhouse gas]] emissions.


Global climate model projections of future climate are forced by imposed greenhouse gas scenarios, generally one from the IPCC [[Special Report on Emissions Scenarios]] (SRES). Less commonly, models may also include a simulation of the [[carbon cycle]]; this generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200&nbsp;ppm of CO<sub>2</sub>). Some observational studies also show a positive feedback.<ref>{{cite journal |last=Torn |first=Margaret |coauthors=Harte, John |date=[[2006-05-26]] |title=Missing feedbacks, asymmetric uncertainties, and the underestimation of future warming |journal=[[Geophysical Research Letters]] |volume=33 |issue=10 |id=L10703 |url=http://www.agu.org/pubs/crossref/2006/2005GL025540.shtml |accessdate=2007-03-04}}</ref><ref>{{cite journal |last=Harte |first=John |coauthors=''et al.'' |date=[[2006-10-30]] |title=Shifts in plant dominance control carbon-cycle responses to experimental warming and widespread drought |journal=Environmental Research Letters |volume=1 |issue=1 |id=014001 |url=http://www.iop.org/EJ/article/1748-9326/1/1/014001/erl6_1_014001.html |accessdate=2007-05-02}}</ref><ref>{{cite journal |last = Scheffer |first = Marten |coauthors = ''et al.'' |title = Positive feedback between global warming and atmospheric CO2 concentration inferred from past climate change. |journal = [[Geophysical Research Letters]] |volume = 33 |url = http://www.pik-potsdam.de/~victor/recent/scheffer_etal_T_CO2_GRL_in_press.pdf |doi = 10.1029/2005gl025044 |date = [[2006-05-26]] |accessdate = 2007-05-04}}</ref>
Global climate model projections of future climate are forced by imposed greenhouse gas scenarios, generally one from the IPCC [[Special Report on Emissions Scenarios]] (SRES). Less commonly, models may also include a simulation of the [[carbon cycle]]; this generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200&nbsp;ppm of CO<sub>2</sub>). Some observational studies also show a positive feedback.<ref>{{cite journal |last=Torn |first=M |coauthors=Harte J |date=2006 |title=Missing feedbacks, asymmetric uncertainties, and the underestimation of future warming |journal=Geophys Res Lett |volume=33|id=L10703 |url=http://www.agu.org/pubs/crossref/2006/2005GL025540.shtml }}</ref><ref>{{cite journal |last=Harte |first=J |coauthors=''et al.'' |date=2006 |title=Shifts in plant dominance control carbon-cycle responses to experimental warming and widespread drought |journal=Environmental Res Lett |volume=1 |id=014001 |url=http://www.iop.org/EJ/article/1748-9326/1/1/014001/erl6_1_014001.html}}</ref><ref>{{cite journal |last = Scheffer |first = M |coauthors = ''et al.'' |title = Positive feedback between global warming and atmospheric CO2 concentration inferred from past climate change|journal = Geophys Res Lett |volume = 33 |url = http://www.pik-potsdam.de/~victor/recent/scheffer_etal_T_CO2_GRL_in_press.pdf |date = 2006 }}</ref>


The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.<ref>{{cite web |url=http://www.grida.no/climate/ipcc_tar/wg1/271.htm |last = Stocker |first = Thomas F. |coauthors = ''et al.'' |title = 7.2.2 Cloud Processes and Feedbacks |work = Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change |accessdate=2007-03-04 |date=[[2001-01-20]] |publisher=[[Intergovernmental Panel on Climate Change]]}}</ref> There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of [[solar variation|solar variability]].
The representation of clouds is one of the main sources of uncertainty in present models.<ref>{{cite web |url=http://www.grida.no/climate/ipcc_tar/wg1/271.htm |last = Stocker |first = TF |coauthors = ''et al.'' |title = 7.2.2 Cloud Processes and Feedbacks |work = Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change |date=2001}}</ref> There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of [[solar variation|solar variability]].


==Attributed and expected effects==
==Attributed and expected effects==
Some effects on both the [[natural environment]] and [[civilization|human life]] are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that [[Retreat of glaciers since 1850|glacier retreat]], [[Ice shelf#Ice shelf disruption|ice shelf disruption]] such as the [[Larsen Ice Shelf]], [[sea level rise]], changes in rainfall patterns, and increased intensity and frequency of [[Effects of global warming#More extreme weather|extreme weather events]], are being attributed in part to global warming.<ref name="tar_wg2">{{cite web |title = Climate Change 2001: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Third Assessment Report of the Intergovernmental Panel on Climate Change |url = http://www.grida.no/climate/ipcc_tar/wg2/index.htm |publisher = [[Intergovernmental Panel on Climate Change]] |date = [[2001-02-16]] |accessdate = 2007-03-14}}</ref> While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects as a result of warmer temperatures include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects.
Some effects on both the [[natural environment]] and [[civilization|human life]] are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as the [[Larsen Ice Shelf]], [[sea level rise]], changes in rainfall patterns, and increased intensity and frequency of extreme weather events, are being attributed in part to global warming.<ref name="tar_wg2">{{cite web |title = Climate Change 2001: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Third Assessment Report of the Intergovernmental Panel on Climate Change |url = http://www.grida.no/climate/ipcc_tar/wg2/index.htm |date = 2001 }}</ref> While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects.
{|align="center" style="width:90%;font-size:95%;"
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<font face="Gill Sans MT">And water expands as it warms: around a third of the continuing rise in sea levels is due to water expansion. That sea level rise is expected to accelerate, putting many of the world's greatest cities at risk.</font><ref>.[http://goo.gl/4kelye Is it time to stop worrying about global warming?]. This article appeared in print under the headline "Causes for climate concern". From issue 2946 of New Scientist magazine, page 5.</ref>
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A summary of probable effects and recent understanding can be found in the report made for the [[IPCC Third Assessment Report]] by Working Group II.<ref name="tar_wg2"/> The newer [[IPCC Fourth Assessment Report]] summary reports that there is observational evidence for an increase in intense [[tropical cyclone]] activity in the North [[Atlantic Ocean]] since about 1970, in correlation with the increase in sea surface temperature, but that the detection of long-term trends is complicated by the quality of records before routine satellite observations. The summary also states that there is no clear trend in the annual number of tropical cyclones.<ref name=grida7/>


Increasing deaths, displacements, and economic losses projected due to [[extreme weather]] attributed to global warming may be exacerbated by growing population densities in affected areas, although temperate regions are projected to experience some minor benefits, such as fewer deaths due to cold exposure.<ref name="WGII SPM AR4">{{cite web |title = Summary for Policymakers |work = Climate Change 2007: Impacts, Adaptation and Vulnerability. Working Group II Contribution to the Intergovernmental Panel on Climate Change Fourth Assessment Report |url = http://www.ipcc.ch/SPM13apr07.pdf |format = [[Portable Document Format|PDF]] |publisher = [[Intergovernmental Panel on Climate Change]] |date = [[2007-04-13]] |accessdate = 2007-04-28}}</ref> A summary of probable effects and recent understanding can be found in the report made for the [[IPCC Third Assessment Report]] by Working Group II.<ref name="tar_wg2"/> The newer [[IPCC Fourth Assessment Report]] summary reports that there is observational evidence for an increase in intense [[tropical cyclone]] activity in the North [[Atlantic Ocean]] since about 1970, in correlation with the increase in sea surface temperature, but that the detection of long-term trends is complicated by the quality of records prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones.<ref name=grida7/>
Other anticipated effects include a sea level rise of 110-770 mm (0.36 - 2.5 ft) between 1990 and 2100,<ref>{{cite web |url=http://www.grida.no/climate/ipcc_tar/wg1/409.htm |last = Church |first = JA |coauthors = ''et al.'' |title = Executive Summary of Chapter 11 |work = Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change|date=2001 }}</ref> [[Climate change and agriculture|repercussions to agriculture]], [[Shutdown of thermohaline circulation|possible slowing of the thermohaline circulation]], reductions in the [[ozone layer]], increased intensity and frequency of [[Effects of global warming#More extreme weather|hurricanes and extreme weather events]], [[Ocean acidification|lowering]] of ocean [[pH]], and the spread of diseases such as [[malaria]] and [[dengue fever]]. One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on climate projections.<ref>{{cite journal |last= Thomas |first= CD |coauthors= ''et al.'' |date=2004 |title= Extinction risk from climate change |journal= Nature |volume= 427 |pages= 145-38 |url= http://www.geog.umd.edu/resac/outgoing/GEOG442%20Fall%202005/Lecture%20materials/extinctions%20and%20climate%20change.pdf }}</ref> McLaughlin ''et al.'' have documented two populations of [[Bay checkerspot butterfly]] being threatened by precipitation change.<ref name=mclaughlin2002>>{{cite journal |last= McLaughlin |first= JF|coauthors= ''et al.'' |date= 2002 |title= Climate change hastens population extinctions |journal= PNAS |volume= 99 |pages= 6070–4 |url= http://www.pnas.org/content/99/9/6070.full }}</ref>
 
Additional anticipated effects include sea level rise of 110 to 770 millimeters (0.36 to 2.5 ft) between 1990 and 2100,<ref>{{cite web |url=http://www.grida.no/climate/ipcc_tar/wg1/409.htm |last = Church |first = John A. |coauthors = ''et al.'' |title = Executive Summary of Chapter 11 |work = Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change |accessdate=2005-12-19 |date=[[2001-01-20]] |publisher=[[Intergovernmental Panel on Climate Change]]}}</ref> [[Climate change and agriculture|repercussions to agriculture]], [[Shutdown of thermohaline circulation|possible slowing of the thermohaline circulation]], reductions in the [[ozone layer]], increased intensity and frequency of [[Effects of global warming#More extreme weather|hurricanes and extreme weather events]], [[Ocean acidification|lowering]] of ocean [[pH]], and the spread of diseases such as [[malaria]] and [[dengue fever]]. One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be [[extinction risk from climate change|extinct]] by 2050, based on future climate projections.<ref>{{cite journal |last= Thomas |first= Chris D. |coauthors= ''et al.'' |date= [[2004-01-08]] |title= Extinction risk from climate change |journal= [[Nature (journal)|''Nature'']] |volume= 427 |issue= 6970 |pages= 145-138 |doi= 10.1038/nature02121 |url= http://www.geog.umd.edu/resac/outgoing/GEOG442%20Fall%202005/Lecture%20materials/extinctions%20and%20climate%20change.pdf |format= [[Portable Document Format|PDF]] |accessdate= 2007-03-18}}</ref> McLaughlin ''et al.'' have documented two populations of [[Bay checkerspot butterfly]] being threatened by precipitation change, though they state few mechanistic studies have documented extinctions due to recent climate change.<ref name=mclaughlin2002>>{{cite journal |last= McLaughlin |first= John F. |coauthors= ''et al.'' |date= [[2002-04-30]] |title= Climate change hastens population extinctions |journal= [[Proceedings of the National Academy of Sciences|PNAS]] |volume= 99 |issue= 9 |pages= 6070–6074 |doi= 10.1073/pnas.052131199 |url= http://www.pnas.org/content/99/9/6070.full |accessdate= 2010-05-31}}</ref>
 
If global warming impacts sufficiently negatively on human health and fecundity, a decline in the rate of population growth, or even a decline in population, may provide an offsetting effect on the increase in global warming, by reducing anthropogenic carbon dioxide generation. If a sufficiently large offset occurs before Earth becomes uninhabitable for human life, humanity might escape the extinction now occurring in other species and projected for more.<ref name=mclaughlin2002/>


==Mitigation and adaptation==
==Mitigation and adaptation==
The broad agreement among climate scientists that global temperatures will continue to increase has led nations, states, corporations, and individuals to implement actions to try to curtail global warming or adjust to it. Many environmental groups encourage action against global warming, often by the consumer, but also by community and regional organizations. There has been business action on climate change, including efforts at increased energy efficiency and (still limited) moves to [[alternative fuels]]. One innovation has been the development of greenhouse gas [[emissions trading]] through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances.
The broad agreement among climate scientists that global temperatures will continue to increase has led nations, states, corporations, and individuals to implement actions to try to curtail global warming or adjust to it. Many environmental groups encourage action against global warming, often by the consumer, but also by community and regional organizations. There has been business action on climate change, including efforts at increased energy efficiency and moves to [[alternative fuels]]. One innovation has been the development of greenhouse gas [[emissions trading]] through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances.


The world's primary international agreement on combating global warming is the [[Kyoto Protocol]], an amendment to the UNFCCC, negotiated in 1997. The Protocol now covers more than 160 countries globally and over 55% of global greenhouse gas emissions.<ref>{{cite web | url=http://unfccc.int/files/essential_background/kyoto_protocol/application/pdf/kpstats.pdf | format=[[Portable Document Format|PDF]] |title=Kyoto Protocol Status of Ratification | publisher=[[United Nations Framework Convention on Climate Change]] | date=[[2006-07-10]] | accessdate=2007-04-27}}</ref> The [[United States]] and [[Kazakhstan]] have not ratified the treaty. [[China]] and [[India]], two other large emitters, have ratified the treaty but, as developing countries, are exempt from its provisions. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed the current one.<ref>[http://www.boston.com/news/science/articles/2007/05/14/climate_talks_face_international_hurdles/ Climate talks face international hurdles], by Arthur Max, Associated press, 5/14/07.</ref>
The main international agreement on combating global warming is the [[Kyoto Protocol]], negotiated in 1997. The Protocol covered more than 160 countries accounting for 55% of global greenhouse gas emissions.<ref>{{cite web | url=http://unfccc.int/files/essential_background/kyoto_protocol/application/pdf/kpstats.pdf |title=Kyoto Protocol Status of Ratification | publisher=[[United Nations Framework Convention on Climate Change]] | date=2006 }}</ref> However, the biggest emitter, the USA never ratified the treaty. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed it.


The world's primary body for crafting a response is the [[Intergovernmental Panel on Climate Change]] (IPCC), a UN-sponsored activity which holds periodic meetings between national delegations on the problems of global warming, and issues working papers and assessments on the current status of the science of climate change, impacts, and mitigation. It convenes four different working groups examining various specific issues.
[[Bjorn Lomborg]] is a Danish-based scientist and author of ''The Skeptical Environmentalist'' (2001); in that influential book, he accepted that global warming is occurring as a result of man's activities, but argued that the costs of drastic, short-term action are too high. He argued that the world's poorest - those likely to be hardest hit by global warming - would benefit more from rich countries honouring pledges on aid, opening up their markets and investing in providing universal access to clean water than they would from aggressive reductions of greenhouse gas emissions. <ref>Bjørn Lomborg (2001) ''The Skeptical Environmentalist''  Cambridge University Press; ISBN-10: 0521010683  see [http://www.scientificamerican.com/article.cfm?id=skepticism-toward-the-ske Skepticism toward The Skeptical Environmentalist] ''Scientific American'' for criticism</ref> By 2010, Lomborg had changed his mind, and declared that global warming is "undoubtedly one of the chief concerns facing the world today" and "a challenge humanity must confront",<ref>[http://www.guardian.co.uk/environment/2010/aug/30/bjorn-lomborg-climate-change-u-turn Bjørn Lomborg: $100bn a year needed to fight climate change] ''Guardian'', 30 August 2010</ref>


==Related climatic issues==
==Related climatic issues==
A variety of issues are often raised in relation to global warming. One is [[ocean acidification]]. Increased atmospheric CO<sub>2</sub> increases the amount of CO<sub>2</sub> dissolved in the oceans.<ref>{{cite web |url=http://science.hq.nasa.gov/oceans/system/carbon.html |title=The Ocean and the Carbon Cycle |accessdate=2007-03-04 |date=[[2005-06-21]] |work=[[NASA]]}}</ref> CO<sub>2</sub> dissolved in the ocean reacts with water to form [[carbonic acid]] resulting in acidification. Ocean surface [[pH]] is estimated to have decreased from approximately 8.25 to 8.14 since the beginning of the industrial era,<ref>{{cite journal |last= Jacobson |first= Mark Z. |date= [[2005-04-02]] |title= Studying ocean acidification with conservative, stable numerical schemes for nonequilibrium air-ocean exchange and ocean equilibrium chemistry |journal= [[Journal of Geophysical Research]] |volume= 110 |issue= D7 |id= D07302 |url= http://www.stanford.edu/group/efmh/jacobson/2004JD005220.pdf |format=[[Portable Document Format|PDF]] |doi = 10.1029/2004JD005220 |accessdate=2007-04-28}}</ref> and it is estimated that it will drop by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO<sub>2</sub>.<ref name=grida7/><ref>{{cite journal| last = Caldeira | first = Ken | coauthors= Wickett, Michael E. | title = Ocean model predictions of chemistry changes from carbon dioxide emissions to the atmosphere and ocean | journal = [[Journal of Geophysical Research]] |volume = 110 |issue = C09S04 | doi:10.1029/2004JC002671 | pages = 1–12 | url = http://www.agu.org/pubs/crossref/2005/2004JC002671.shtml | date = [[2005-09-21]] | accessdate = 2006-02-14}}</ref> Since organisms and ecosystems are adapted to a narrow range of pH, this raises [[extinction]] concerns, directly driven by increased atmospheric CO<sub>2</sub>, that could disrupt food webs and impact human societies that depend on marine ecosystem services.<ref>{{cite paper |author=Raven, John A.; ''et al.'' |title= Ocean acidification due to increasing atmospheric carbon dioxide |publisher= [[Royal Society]] |date= [[2005-06-30]] |url= http://www.royalsoc.ac.uk/displaypagedoc.asp?id=13314 |format= [[Active Server Pages|ASP]] |accessdate= 2007-05-04}}</ref>
Increased atmospheric CO<sub>2</sub> increases the amount of CO<sub>2</sub> dissolved in the oceans.<ref>{{cite web |url=http://science.hq.nasa.gov/oceans/system/carbon.html |title=The Ocean and the Carbon Cycle |date=2005|work=[[NASA]]}}</ref> CO<sub>2</sub> dissolved in the ocean reacts with water to form [[carbonic acid]] resulting in acidification. Ocean surface [[pH]] is estimated to have decreased from approximately 8.25 to 8.14 since the beginning of the industrial era,<ref>{{cite journal |last= Jacobson |first= MZ |date= 2005 |title= Studying ocean acidification with conservative, stable numerical schemes for nonequilibrium air-ocean exchange and ocean equilibrium chemistry |journal= J Geophys Res|volume= 110|id= D07302 |url= http://www.stanford.edu/group/efmh/jacobson/2004JD005220.pdf }}</ref> and it is estimated that it will drop by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO<sub>2</sub>.<ref name=grida7/><ref>{{cite journal|last = Caldeira |first = K | coauthors= Wickett ME| title = Ocean model predictions of chemistry changes from carbon dioxide emissions to the atmosphere and ocean |journal = J Geophys Res|volume = 110 |pages = 1–12 | url = http://www.agu.org/pubs/crossref/2005/2004JC002671.shtml| date = 2005}}</ref> As organisms and ecosystems are adapted to a narrow range of pH, this raises [[extinction]] concerns, directly driven by increased atmospheric CO<sub>2</sub>, that could disrupt food webs and impact human societies that depend on marine ecosystem services.<ref>{{cite paper |author=Raven JA ''et al.'' |title= Ocean acidification due to increasing atmospheric carbon dioxide |publisher= [[Royal Society]]|date= 2005 |url= http://www.royalsoc.ac.uk/displaypagedoc.asp?id=13314}}</ref>
 
Another related issue that may have partially mitigated global warming in the late twentieth century is [[global dimming]], the gradual reduction in the amount of global direct [[irradiance]] at the Earth's surface. From 1960 to 1990, human-caused aerosols likely precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents.<ref name=grida7/>
 
==Skepticism about global climate change and its anthropogenic origin==
Although all national science academies that have issued statements on the matter accept the IPCC's conclusion that future man-made climate change is likely,<ref name="academies"/> a few skeptical scientists and others believe that the present climate change, if it exists at all, is not man-made and is unavoidable. Most skeptical scientists accept that there has been a large increase of CO<sub>2</sub> in the atmosphere due to the  growing use of fossil fuels, but doubt that there is any immediate danger and dispute any need for large reductions of man-made CO<sub>2</sub> emissions. Their skepticism is based on the complex problems associated with the underlying science and the uncertainty of available climate data.


When an FPS ([[Forum on Physics & Society]]) editor of the [[American Physical Society]] wrote: "There is a considerable presence within the scientific community of people who do not agree with the IPCC conclusion that anthropogenic CO<sub>2</sub> emissions are very probably likely to be primarily responsible for the global warming that has occurred since the [[Industrial  Revolution]]",<ref>[http://www.aps.org/units/fps/newsletters/200807/editor.cfm Editor's Comment, Forum on Physics & Society of the American Physical Society, July 2008]</ref> the FPS Executive Committee hastened to declare that his statement does not represent their views.<ref>[http://www.aps.org/units/fps/newsletters/200807/ Forum on Physics & Society of the American Physical Society, July 2008].</ref><ref>The APS's 2007 statement on Climate Change, in which the APS urges an enhanced effort to understand the effects of human activity on the Earth’s climate, is given [http://www.aps.org/policy/statements/07_1.cfm here].</ref>
A factor that may have  mitigated global warming in the late twentieth century is [[global dimming]] - a gradual reduction in the amount of global direct [[irradiance]] at the Earth's surface. From 1960 to 1990, human-caused aerosols likely precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents.<ref name=grida7/>
Nonetheless, in 2007 a [[U.S. Senate]] committee identified more than 400 prominent scientists from more than two dozen countries as voicing objections to aspects of the "consensus" on man-made global warming. Some of those scientists were participants in the IPCC.<ref>
[http://epw.senate.gov/public/index.cfm?FuseAction=Files.View&FileStore_id=bba2ebce-6d03-48e4-b83c-44fe321a34fa U. S. Senate Report: Over 400 Prominent Scientists Disputed Man-Made Global Warming Claims in 2007]</ref>. Some individuals listed objected to their inclusion and declared that the statement did not reflect their views. In 2009, a  book defending the skeptical view was published by the [[Heartland Institute]],<ref>S. Fred Singer and Craig Idso, ''Climate Change Reconsidered''  Heartland Institute, June 2009,  ISBN-13 – 978-1-934791-28-8</ref> an institute that takes a partisan position against what they call "myths of global warming". <ref>[http://www.nipccreport.org/index.html Climate Change Reconsidered (8Mb pdf)]</ref>  


A 2010 assessment of the IPCC report by the Netherlands Environmental Assessment Agency found no significant errors, but its authors reported that they had been unable to find the provenance of some of its conclusions, that the report had not taken account of the positive effects of climate change and that there was a tendency to highlight the upper ends of uncertainty ranges.<ref>[http://www.pbl.nl/images/500216002_tcm61-48119.pdf ''Assessing an IPCC assessment. An analysis of statements on projected regional impacts in the 2007 report', Netherlands Environmental Assessment Agency, 2010]</ref>.
==Skepticism==
Although it is widely believed that there is widespread disagreement amongst scientists about the role of man in global warming, in fact there seems to be very little disagreement amongst Earth Scientists. Summarising the results of a survey of over 3,000 Earth Scientists, Doran (2009) wrote: "It seems that the debate on the authenticity of global warming and the role played by human activity is largely nonexistent among those who understand the nuances and scientific basis of long-term climate processes. The challenge, rather, appears to be how to effectively communicate this fact to policy makers and to a public that continues to mistakenly perceive debate among scientists."<ref name = Doran/>


===The complexities of climate change science===
Nevertheless, a well-funded campaign by skeptical scientists, free-market think tanks and industry, through advertisements, lobbying and media attention, has argued first that the world is not warming; then that warming is not caused by human activities, now that warming will be harmless.<ref>[http://www.newsweek.com/2007/08/13/the-truth-about-denial.html The Truth About Denial] ''Newsweek'' Education site</ref> <ref>[http://www.guardian.co.uk/environment/2006/sep/19/ethicalliving.g2 The denial industry] George Monbiot ''The Guardian'' 19 September 2006</ref>This campaign has had a major effect in the USA, on both the public and Congress; the [[Tea Party]] is at the forefront of climate change denial.  
All participants in the dispute recognize that ''climate change'' science involves difficult problems. The Earth's climate is an extremely complex system: the atmosphere, the oceans, and the land masses are tightly coupled subsystems and consequently the energy and mass exchanges between them must be studied simultaneously. Further, the [[electromagnetic radiation]] balance (i.e., insolation of solar irradiation) between energy absorption and back radiation by the Earth plays a crucial role, which means that the Earth's climate system is not a isolated system.


There is no encompassing theory that predicts the characteristics of the climate which is accepted by all climatologists. Theories are taken from many different areas of applied physics: turbulent and dissipative systems, convective and radiative transport phenomena, non-linear (chaotic) systems and their inherent sensitivity to initial conditions, and so on. Further, there is not enough reliable data to evaluate the existing climate models. Thus, some climate scientists have little faith in the ability of climate models to predict the future.<ref>[http://coast.gkss.de/staff/storch/pdf/070511.bray.GKSS.pdf Climate Scientists’ Perceptions of Climate Change Science] Interviews among 558 scientists working in the field of climate change (mainly climatologist and meteorologists) by Dennis Bray and Hans von Storch</ref><ref>[http://ruby.fgcu.edu/courses/twimberley/EnviroPol/IssuesInCurrentStateOfClimateScience.pdf  Issues in the Current State of Climate Science] The Center for Science and Public Policy, Washington, DC, March 2006 (from the website of the Florida Gulf Coast University)</ref> The validity of computer models predicting the climate a few decades ahead is questioned by those who refer to the inability of models to predict the weather for more than 10 days in advance, though this neglects the difference between development of specific events (weather) and evolution of changes in statistical distributions (climate).
A few skeptical scientists believe that the present climate change is not man-made and is unavoidable. <ref>During the [[United Nations]] Climate Conference on [[Bali]] in 2007, more than 100 scientists wrote an open letter to [[Ban Ki-Moon]], the Secretary-General of the United Nations expressing their opinion that "the 2007 UN climate conference [is] taking the World in entirely the wrong direction". ([http://www.nationalpost.com/news/story.html?id=164002 Letter to Ban Ki-Moon]) They recognized that climate change is occurring, but state that it is a natural phenomenon which is impossible to stop and express doubts that "it is possible to significantly alter global climate through cuts in human greenhouse gas emissions." The letter criticised the IPCC Assessment Reports of 2001 and 2007, claiming that they "are prepared  by a relatively small core writing team with the final drafts approved line-by-line by ­government representatives". </ref>


Another problem is the use of ''proxy data'' (indirect data such as tree rings and the isotopic content of arctic and antarctic ice) used to determine past temperatures of the Earth. Proxy data are used to construct historical temperature profiles, yielding, for instance, the [[hockey stick model|hockey stick shaped]] graph.<ref> [http://www.caenvirothon.com/Resources/Mann,%20et%20al.%20Global%20scale%20temp%20patterns.pdf ME Mann ''et al.'' (1998), ''Nature'':779-87] A plot of mean temperature over the last 1,000 years which is flat on average from the years 1000 to 1900. The flat part forms the hockey stick's ''shaft''. After 1900, and especially after 1980, temperatures appear to shoot up, forming the hockey stick's ''blade''.</ref> Some scientists question the reliability of temperature profiles based on proxy data and doubt that the present global warming is unique. They argue that the Earth, without human intervention, has had warm periods before and they refer to the discovery of Greenland by the Vikings around the year 1000 when Greenland was green, and the era of the dinosaurs when the Earth was green.
Most accept that there has been a large increase of CO<sub>2</sub> in the atmosphere due to the use of fossil fuels, but doubt that there is any immediate danger and dispute any need for large reductions of CO<sub>2</sub> emissions. Their skepticism is based on the complex problems associated with the underlying science and the uncertainty of available climate data. Put most simply, the skeptical view is that we don't understand climate well enough to make reliable predictions, and that there have been large variations of climate in the past, so perhaps the observed changes are just part of a natural cycle that we don't understand. <ref>[http://coast.gkss.de/staff/storch/pdf/070511.bray.GKSS.pdf Climate Scientists’ Perceptions of Climate Change Science] Interviews among 558 scientists working in the field of climate change (mainly climatologist and meteorologists) by Dennis Bray and Hans von Storch</ref><ref>[http://ruby.fgcu.edu/courses/twimberley/EnviroPol/IssuesInCurrentStateOfClimateScience.pdf  Issues in the Current State of Climate Science] The Center for Science and Public Policy, Washington, DC, March 2006 (from the website of the Florida Gulf Coast University)</ref> They argue that we don't have good enough data from the past to develop and test computer models of the climate, and object to the use of ''proxy data'' - indirect data on temperature, such as tree rings and the isotopic content of Arctic and Antarctic ice. Proxy data are used to construct historical temperature profiles, yielding, for instance, the [[hockey stick model|hockey stick shaped]] graph, but have some problems - tree ring thickness for example is affected by both temperature and precipitation.<ref> [http://www.caenvirothon.com/Resources/Mann,%20et%20al.%20Global%20scale%20temp%20patterns.pdf ME Mann ''et al.'' (1998)  ''Nature'':779-87] A plot of mean temperature over the last 1,000 years which is flat on average from the years 1000 to 1900. The flat part forms the hockey stick's ''shaft''. After 1900, and especially after 1980, temperatures appear to shoot up, forming the hockey stick's ''blade''.</ref>  


There is also concern about the lack of transparency of analyses purporting global warming and inaccessibility to data to allow independent analyses.<ref>Revkin AC (November 27, 2009) [http://dotearth.blogs.nytimes.com/2009/11/27/a-climate-scientist-on-climate-skeptics/ A Climate Scientist Who Engages Skeptics] The New York Times</ref><ref>Curry J. (November 27, 2009) [http://climateprogress.org/2009/11/27/%C2%AD-climategate-judith-curry-open-letter-to-graduate-students-young-scientists-climate-research-hacked-cru-emails/ An open letter to graduate students and young scientists in fields related to climate research from Dr Judith Curry regarding hacked CRU emails]. Climate Progress</ref><ref>McIntyre S [http://www.climateaudit.org/ Climate Audit]</ref> Specifically, skeptics have requested access to data from the University of East Anglia's [http://www.cru.uea.ac.uk/ Climatic Research Unit] (CRU) and Penn State [http://www.meteo.psu.edu/ Department of Meteorology]. The CRU reports that 95% of their data is available for the public at http://www.cru.uea.ac.uk/cru/data/.<ref>Anonymous. (November 24, 2009) [http://www.uea.ac.uk/mac/comm/media/press/2009/nov/homepagenews/CRUupdate CRU climate data already ‘over 95%’ available (28 November)]. University of East Anglia - Communications Office</ref> The [http://www.realclimate.org RealClimate blog] also maintains a list of data sources at http://www.realclimate.org/index.php/data-sources/. There are examples of a [[peer review]]ed publication in a [[scientific journal]] by researchers at Penn State supporting global warming<ref>{{Cite journal
There is also concern about the lack of transparency of analyses purporting global warming and inaccessibility to data to allow independent analyses. <ref>Revkin AC (November 27, 2009) [http://dotearth.blogs.nytimes.com/2009/11/27/a-climate-scientist-on-climate-skeptics/ A Climate Scientist Who Engages Skeptics] The New York Times</ref><ref>Curry J (November 27, 2009) [http://climateprogress.org/2009/11/27/%C2%AD-climategate-judith-curry-open-letter-to-graduate-students-young-scientists-climate-research-hacked-cru-emails/ An open letter to graduate students and young scientists in fields related to climate research from Dr Judith Curry regarding hacked CRU emails]. Climate Progress</ref><ref>McIntyre S [http://www.climateaudit.org/ Climate Audit]</ref> Specifically, skeptics have requested access to data from the University of East Anglia's [http://www.cru.uea.ac.uk/ Climatic Research Unit] (CRU) and Penn State [http://www.meteo.psu.edu/ Department of Meteorology]. The CRU reports that 95% of their data is available for the public<ref>[http://www.cru.uea.ac.uk/cru/data/ CRU data]; Anonymous. (November 24, 2009) [http://www.uea.ac.uk/mac/comm/media/press/2009/nov/homepagenews/CRUupdate CRU climate data already ‘over 95%’ available (28 November)]. University of East Anglia - Communications Office</ref><ref> The [http://www.realclimate.org RealClimate blog] also maintains a list of data sources at http://www.realclimate.org/index.php/data-sources/</ref>
| doi = 10.1126/science.1177303 | volume = 326 | pages = 1256-60 | last = Mann | first = ME | coauthors = ''et al.''| title = Global Signatures and Dynamical Origins of the Little Ice Age and Medieval Climate Anomaly | journal = Science | accessdate = 2009-11-29 | date = 2009-11-27 | url = http://www.sciencemag.org/cgi/content/abstract/326/5957/1256 }}</ref> and a conflicting interpretation posted at the [http://www.climateaudit.org Climate Audit] blog<ref>S. Jean. (November 27, 2009) [http://www.climateaudit.org/?p=7855 Yet another Upside Down Mann out] Climate Audit</ref> of the same data.


===Open letter to the UN Secretary-General from skeptical scientists===
When an FPS ([[Forum on Physics & Society]]) editor of the [[American Physical Society]] wrote: "There is a considerable presence within the scientific community of people who do not agree with the IPCC conclusion that anthropogenic CO<sub>2</sub> emissions are very probably likely to be primarily responsible for the global warming that has occurred since the [[Industrial  Revolution]]",<ref>[http://www.aps.org/units/fps/newsletters/200807/editor.cfm Editor's Comment, Forum on Physics & Society of the American Physical Society, July 2008]</ref> the FPS Executive Committee hastened to declare that his statement does not represent their views.<ref>[http://www.aps.org/units/fps/newsletters/200807/ Forum on Physics & Society of the American Physical Society, July 2008].</ref><ref>The APS's 2007 statement on Climate Change, which urges an enhanced effort to understand the effects of human activity on the climate, is [http://www.aps.org/policy/statements/07_1.cfm here].</ref>  
During the [[United Nations]] Climate Conference on the Indonesian island of [[Bali]] in December 2007, more than 100 scientists wrote an open letter to [[Ban Ki-Moon]], the Secretary-General of the United Nations<ref>[http://www.nationalpost.com/news/story.html?id=164002 Letter to Ban Ki-Moon]</ref> expressing their opinion that "the 2007 UN climate conference [is] taking the World in entirely the wrong direction". They recognized that a climate change is occurring but they state that it is a natural phenomenon which is impossible to stop and express their doubts that "it is possible to significantly alter global climate through cuts in human greenhouse gas emissions."
Nonetheless, in 2007 a [[U.S. Senate]] committee identified more than 400 prominent scientists from more than two dozen countries as objecting to aspects of the "consensus" on man-made global warming. Some of those scientists were participants in the IPCC.<ref>
[http://epw.senate.gov/public/index.cfm?FuseAction=Files.View&FileStore_id=bba2ebce-6d03-48e4-b83c-44fe321a34fa U. S. Senate Report: Over 400 Prominent Scientists Disputed Man-Made Global Warming Claims in 2007] Note that some individuals listed objected to their inclusion and declared that the statement did not reflect their views.</ref>.  


The open letter casts doubt on the procedure of writing the IPCC Assessment Reports of 2001 and 2007. The letter stated that "the reports are prepared  by a relatively small core writing team with the final drafts approved line-by-line by ­government representatives". It  states that "the great ­majority of IPCC contributors and ­reviewers, and the tens of thousands of other scientists who are qualified to comment on these matters, are not involved in the preparation of these documents. The summaries therefore cannot properly be represented as a consensus view among experts".
A 2010 assessment of the IPCC report by the Netherlands Environmental Assessment Agency found no significant errors in the report, but its authors reported that they had been unable to find the provenance of some of its conclusions, that the report had not taken account of the positive effects of climate change and that there was a tendency to highlight the upper ends of uncertainty ranges.<ref>[http://www.pbl.nl/images/500216002_tcm61-48119.pdf ''Assessing an IPCC assessment. An analysis of statements on projected regional impacts in the 2007 report', Netherlands Environmental Assessment Agency, 2010]</ref>.  


===Climate sensitivity===
===Climate sensitivity===
{{main|Climate sensitivity}}
{{main|Climate sensitivity}}
The concentration of CO<sub>2</sub> in the Earth's atmosphere at the beginning of the [[Industrial Age]] (1750) was about 278 [[Parts-per notation|ppm]]; in 2008, it was about 385 ppm. The concept of climate sensitivity arose when the IPCC members asked how much the temperature on Earth would change by an increase of CO<sub>2</sub> in the atmosphere. To answer that, the IPCC adopted this definition:
The concentration of CO<sub>2</sub> in the Earth's atmosphere at the beginning of the [[Industrial Age]] (1750) was about 278 ppm; in 2008, it was about 385 ppm. The concept of climate sensitivity arose when the IPCC members asked how much the temperature on Earth would change by an increase of CO<sub>2</sub> in the atmosphere. To answer that, the IPCC adopted this definition:


:''Climate sensitivity is the equilibrium temperature change, <math>\scriptstyle \Delta T_\lambda</math>, in the surface temperature, T<sub>''S''</sub>,  caused by the doubling of the pre-industrial CO<sub>2</sub> concentration.''  
:''Climate sensitivity is the equilibrium temperature change, <math>\scriptstyle \Delta T_\lambda</math>, in the surface temperature, T<sub>''S''</sub>,  caused by the doubling of the pre-industrial CO<sub>2</sub> concentration.''  
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==References==
==References==
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Annual average global warming by the year 2060 simulated and plotted as color differences using EdGCM

Global warming is the increase in the average temperature of the Earth's near-surface air and oceans in recent decades and its projected continuation. There is strong evidence that significant global warming is occurring; this evidence comes from direct measurements of rising surface air temperatures and subsurface ocean temperatures and from phenomena such as increases in average global sea levels, retreating glaciers, and changes to many physical and biological systems. It is likely that most of the warming in recent decades is attributable to human activity, particularly the burning of fossil fuels and deforestation.

Global average air temperature near the Earth's surface rose by 0.74 ± 0.18 °C (1.33 ± 0.32 °F) from 1906 to 2005. The prevailing scientific view,

[1] as represented by the science academies of the major industrialized nations[2] and the Intergovernmental Panel on Climate Change,[3] it is very likely that most of the temperature increase since the mid-20th century has been caused by increases in atmospheric greenhouse gas concentrations produced by human activity. Climate models predict that average global surface temperatures will increase by a further 1.1 to 6.4 °C (2.0 to 11.5 °F) by the end of the century, relative to 1980–1999.[3] The range of values reflects differing assumptions of future greenhouse gas emissions and results of models that differ in their sensitivity to increases in greenhouse gases.[3]

Scientists have not yet quantitatively assessed the potential self-accelerating effects of global-warming itself, either on threshold or rate. Melting of permafrost, for example, causes increased production and atmospheric release of such newly produced as well as anciently stored methane gas, which “….packs a far greater warming punch than [carbon dioxide] (CO2),”[4] possibly as much as 25 times that of CO2 per unit mass.[5]

An increase in global temperatures will cause the sea level to rise, glaciers to retreat, sea ice to melt, and changes in the amount, geographical distribution and seasonal pattern of precipitation. There may also be changes in the frequency and intensity of extreme weather events. These will have many practical consequences, including changes in agricultural yields and impacts on human health.[6] Scientific uncertainties include the extent of climate change expected in the future, and how changes will vary around the globe. There is political and public debate about what action should be taken to reduce future warming or to adapt to its consequences. The Kyoto Protocol, an international agreement aimed at reducing greenhouse gas emissions, was adopted by 169 nations.

Terminology

The weather is the day-by-day temperature, humidity, wind and rainfall in a given region; climate encompasses long- term weather patterns. The Earth's atmosphere is heated by radiation from the sun, and how much of that heat is retained rather than reflected depends critically on the composition of the atmosphere. In particular, the burning of fossil fuels releases "greenhouse gases" into the atmosphere that are causing "climate change."

The United Nations Framework Convention on Climate Change (UNFCCC) uses this term, "climate change", for human-caused change, and "climate variability" for other changes.[7] The terms "anthropogenic global warming" and "anthropogenic climate change" are sometimes also used for human-induced changes.

The Intergovernmental Panel on Climate Change (IPCC)[8] is a scientific body that was established by the United Nations Environment Programme [9] and the World Meteorological Organization (WMO), a specialized agency of the UN.[10] It was established to provide a clear scientific view on the current state of knowledge in climate change, and its potential environmental and socio-economic impacts. The IPCC works by reviewing and assessing current scientific, technical and socio-economic information, and thousands of scientists from all over the world contribute to this, encompassing a range of views and expertise. Governments participate in the review process and in the plenary sessions, where decisions about the IPCC work programme are taken and where reports are presented, revised and approved. The IPCC is endorsed by the UN General Assembly, and currently has 194 member countries. In 2007, the IPCC was awarded the Nobel Peace Prize jointly with Al Gore "for their efforts to build up and disseminate greater knowledge about man-made climate change, and to lay the foundations for the measures that are needed to counteract such change"[11]

The Kyoto Protocol is an agreement that set binding targets for 37 industrialized countries, including the European community, for reducing greenhouse gas emissions. Acknowledging that developed countries are mainly responsible for the current high levels of greenhouse gas emissions, the Protocol placed a greater burden on them under the principle of "common but differentiated responsibilities."[12]. Under the Bush Administration, the USA refused to ratify the protocol, and the Obama administration followed suit[13] With the Kyoto Protocol set to expire in 2012, in December 2009, governments met at an international Climate Conference in Copenhagen to negotiate a continuation of international efforts to minimise global warming. That conference concluded with the Copenhagen accord which asserted a "strong political will to urgently combat climate change in accordance with the principle of common but differentiated responsibilities and respective capabilities." Signatories agree that deep cuts in global emissions are required, recognize the crucial role of reducing emission from deforestation, and agree that developed countries shall provide financial resources, technology and capacity-building to support adaptation in developing countries. The USA has signed, along with most other countries of the world.[14][15]

Causes

"Human-induced warming of the climate system is widespread. Anthropogenic warming of the climate system can be detected in temperature observations taken at the surface, in the troposphere and in the oceans. Multi-signal detection and attribution analyses, which quantify the contributions of different natural and anthropogenic forcings to observed changes, show that greenhouse gas forcing alone during the past half century would likely have resulted in greater than the observed warming if there had not been an offsetting cooling effect from aerosol and other forcings.
"It is extremely unlikely (<5%) that the global pattern of warming during the past half century can be explained without external forcing, and very unlikely that it is due to known natural external causes alone. The warming occurred in both the ocean and the atmosphere and took place at a time when natural external forcing factors would likely have produced cooling.
"Greenhouse gas forcing has very likely caused most of the observed global warming over the last 50 years. This conclusion takes into account observational and forcing uncertainty, and the possibility that the response to solar forcing could be underestimated by climate models. It is also robust to the use of different climate models, different methods for estimating the responses to external forcing and variations in the analysis technique."
(IPCC 4th Assessment Report: Climate Change 2007[16]

There is no serious dispute that there has been a large and continuing increase in atmospheric CO2 concentrations since the middle of the twentieth century; there is no dispute that there has been a parallel increase in fossil fuel use over this time, and there is no dispute that there has been an increase in mean global temperature over the same period. Although some skeptics still doubt that the rise in CO2 is substantially a consequence of man's activities, all national science academies that have issued statements on the matter accept the IPCC's conclusion that they probably are, and that future man-made climate change is likely[2].

The climate system varies through internal processes and in response to external forcing. External forcing includes fossil fuel emissions, but also solar activity, volcanic emissions, variations in the Earth's orbit, and variations in atmospheric composition. The scientific consensus[17] is that most of the warming observed since the mid-twentieth century is due to increased atmospheric concentrations of greenhouse gases produced by human activity. Alternative mechanisms seem able to account for only a small part of the observed increase in global temperatures. These other mechanisms include warming as a result of natural fluctuations in the climate, and warming as a result of variations in solar radiation,[18] or that warming is caused by changes in cloud cover due to variations in galactic cosmic rays.[19]

The effects of external forcing on the climate are not instantaneous, due to the thermal inertia of the oceans and the slow responses of some feedback processes. Climate models indicate that, even if greenhouse gases were stabilized at present day levels, there would be a further warming of about 0.5 °C (0.9 °F) as the climate continued to adjust toward equilibrium.[20]

Greenhouse gases

The greenhouse effect is the process by which emission of infrared radiation by atmospheric gases warms a planet's atmosphere and surface. Naturally occurring greenhouse gases warm the Earth by about 33 °C (59 °F); without this, the average temperature of the Earth would be about -18 °C (0 °F) making the planet uninhabitable.[21] The major natural greenhouse gases are water vapor, which causes about 36–70% of the greenhouse effect (not including clouds); carbon dioxide (CO2), which causes 9–26%; methane (CH4), which causes 4–9%; and ozone, which causes 3–7%.[22]

The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume.[23] From geological evidence, it is believed that CO2 values this high were last attained 20 million years ago.[24] About three-quarters of man-made CO2 emissions over the past 20 years have come from the burning of fossil fuels; most of the rest is due to land-use change, mainly deforestation.[25] Measured trends in atmospheric composition and isotope ratios (namely the simultaneous depletion of 13C, 14C, and O2) confirm that the increased atmospheric CO2 mainly comes from fossil fuels and not from other sources such as volcanos or the oceans.[26]

Future CO2 concentrations depend on uncertain economic, sociological, technological, and natural developments. The IPCC Special Report on Emissions Scenarios gives a wide range of future CO2 scenarios, ranging from 541 to 970 ppm by the year 2100.[27] Fossil fuel reserves are sufficient to reach these levels and continue emissions past 2100, if coal, tar sands, or methane clathrates are used extensively.[28] Positive feedback effects such as the release of methane from the melting of permafrost peat bogs in Siberia (possibly up to 70,000 million tonnes) may lead to significant additional sources of greenhouse gas emissions[29] not included in climate models cited by the IPCC.[3]

The warming due to atmospheric CO2 from burning fossil fuels releases methane from the Arctic surface in at least three ways: (1) by forming lakes of melted ice whose waters melts the underlying permafrost, allowing methane-producing microbes to increase methane production by exposing thawing vegetative and animal matter for them to consume; (2) by opening channels in the attenuated permafrost cap for release into the atmosphere of old, trapped methane hydrates; (3) by thawing offshore layers of permafrost capping methane hydrates.[5]

Feedbacks

The effects of forcing agents on the climate are modified by feedback processes, one of the most important of which is caused by the evaporation of water. Increased greenhouse gases from human activity cause a warming of the Earth's atmosphere and surface, which increases the evaporation of water into the atmosphere. As water vapor is itself a greenhouse gas, this causes further warming, causing yet more water vapor to be evaporated, and so on. Eventually a new dynamic equilibrium concentration of water vapor is reached at a slight increase in humidity and with a much larger greenhouse effect than that due to CO2 alone.[30]

The radiative effects of clouds are a source of uncertainty in climate projections. Seen from below, clouds emit infrared radiation to the surface, and so have a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so have a cooling effect. The cloud feedback effect is influenced not only by the amount of clouds but also by their distribution; high clouds are colder than low clouds, and thus radiate less energy to space. Increased global water vapor content may or may not cause an increase in global or regional cloud cover, since cloud cover is affected by relative humidity rather than the absolute concentration of water vapor. Cloud feedback is second only to water vapor feedback and has a net warming effect in all the models that contributed to the IPCC Fourth Assessment Report.[30]

Global warming also leads to melting of ice near the poles, exposing more land surface. Land (and open water) are less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and the cycle continues (the "ice albedo feedback"). Sea ice also has an important role in moderating heat exchange between the ocean and atmosphere at high latitudes, and by feedbacks involving ice growth and melt and the fresh water balance at the ocean surface[31] The ocean's ability to sequester carbon is expected to decline as it warms, because the resulting low nutrient levels of the mesopelagic zone limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.[32]

Solar variation

Variations in solar output, possibly amplified by cloud feedbacks, may have been a secondary contributor to recent global warming. [33] Natural phenomena, such as solar variation and volcanoes, probably had a net warming effect from pre-industrial times to 1950 and a small cooling effect since 1950.[34] Some research indicate that the Sun's contribution may have been underestimated, and that the Sun may have contributed about 40–50% of the global surface warming between 1900 and 2000 and about 25–35% of the warming between 1980 and 2000.[35] Some authors suggest that climate models overestimate the relative effect of greenhouse gases compared to solar forcing, and that the cooling effects of volcanic dust and sulfate aerosols have been underestimated.[36] Nevertheless, they conclude that even with an enhanced climate sensitivity to solar forcing, most of the warming during the latest decades is attributable to the increases in greenhouse gases.

Climate change since the Industrial Revolution

According to the instrumental temperature record, mean global temperatures (both land and sea) have increased by 0.75 °C (1.35 °F) since the period 1860–1900. This increase is not significantly affected by the urban heat island effect.[37][38][39] Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[40] Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age.

Based on estimates by NASA's Goddard Institute for Space Studies, 2005 was the warmest year since reliable measurements became available in the late 1800s.[41] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.[42] [43] Global temperatures in 1998 were exceptionally warm because the strongest El Niño in the instrumental record occurred in that year.[44]

Anthropogenic emissions of other pollutants—notably sulfate aerosols—can exert a cooling effect by increasing the reflection of incoming sunlight. This partially accounts for the cooling seen in the temperature record in the middle of the 20th century,[45] though the cooling may also be due in part to natural variability.

Climate models

The Earth's climate is an extremely complex system: the atmosphere, oceans, and land masses are tightly coupled subsystems and consequently the energy and mass exchanges between them must be studied simultaneously. Further, the electromagnetic radiation balance between energy absorption and back radiation by the Earth plays a crucial role. Encompassing all these factors, scientists have created computer models of the climate, based on physical principles of fluid dynamics, radiative transfer, and other processes. These models predict that the net effect of adding greenhouse gases is to produce a warmer climate. However, the amount of projected warming varies between models and there is a considerable range of climate sensitivity. Including uncertainties in future greenhouse gas concentrations and climate modeling, the IPCC report projects global surface temperatures averaged over 2090-2099 are likely to be 1.1 to 6.4 °C (2.0 to 11.5 °F) hotter than the average temperatures from 1980-1999.[3]

Models have also been used to help investigate the causes of recent climate change by comparing the observed changes to those that the models project from various natural and human derived causes. Climate models can produce a good match to observations of global temperature changes over the last century, but cannot yet simulate all aspects of climate.[46] These models do not unambiguously attribute the warming that occurred from approximately 1910 to 1945 to either natural variation or human effects, but they suggest that the warming since 1975 is dominated by man-made greenhouse gas emissions.

Global climate model projections of future climate are forced by imposed greenhouse gas scenarios, generally one from the IPCC Special Report on Emissions Scenarios (SRES). Less commonly, models may also include a simulation of the carbon cycle; this generally shows a positive feedback, though this response is uncertain (under the A2 SRES scenario, responses vary between an extra 20 and 200 ppm of CO2). Some observational studies also show a positive feedback.[47][48][49]

The representation of clouds is one of the main sources of uncertainty in present models.[50] There is also an ongoing discussion as to whether climate models are neglecting important indirect and feedback effects of solar variability.

Attributed and expected effects

Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events, are being attributed in part to global warming.[51] While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects.

And water expands as it warms: around a third of the continuing rise in sea levels is due to water expansion. That sea level rise is expected to accelerate, putting many of the world's greatest cities at risk.[52]

A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II.[51] The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature, but that the detection of long-term trends is complicated by the quality of records before routine satellite observations. The summary also states that there is no clear trend in the annual number of tropical cyclones.[3]

Other anticipated effects include a sea level rise of 110-770 mm (0.36 - 2.5 ft) between 1990 and 2100,[53] repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity and frequency of hurricanes and extreme weather events, lowering of ocean pH, and the spread of diseases such as malaria and dengue fever. One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on climate projections.[54] McLaughlin et al. have documented two populations of Bay checkerspot butterfly being threatened by precipitation change.[55]

Mitigation and adaptation

The broad agreement among climate scientists that global temperatures will continue to increase has led nations, states, corporations, and individuals to implement actions to try to curtail global warming or adjust to it. Many environmental groups encourage action against global warming, often by the consumer, but also by community and regional organizations. There has been business action on climate change, including efforts at increased energy efficiency and moves to alternative fuels. One innovation has been the development of greenhouse gas emissions trading through which companies, in conjunction with government, agree to cap their emissions or to purchase credits from those below their allowances.

The main international agreement on combating global warming is the Kyoto Protocol, negotiated in 1997. The Protocol covered more than 160 countries accounting for 55% of global greenhouse gas emissions.[56] However, the biggest emitter, the USA never ratified the treaty. This treaty expires in 2012, and international talks began in May 2007 on a future treaty to succeed it.

Bjorn Lomborg is a Danish-based scientist and author of The Skeptical Environmentalist (2001); in that influential book, he accepted that global warming is occurring as a result of man's activities, but argued that the costs of drastic, short-term action are too high. He argued that the world's poorest - those likely to be hardest hit by global warming - would benefit more from rich countries honouring pledges on aid, opening up their markets and investing in providing universal access to clean water than they would from aggressive reductions of greenhouse gas emissions. [57] By 2010, Lomborg had changed his mind, and declared that global warming is "undoubtedly one of the chief concerns facing the world today" and "a challenge humanity must confront",[58]

Related climatic issues

Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans.[59] CO2 dissolved in the ocean reacts with water to form carbonic acid resulting in acidification. Ocean surface pH is estimated to have decreased from approximately 8.25 to 8.14 since the beginning of the industrial era,[60] and it is estimated that it will drop by a further 0.14 to 0.5 units by 2100 as the ocean absorbs more CO2.[3][61] As organisms and ecosystems are adapted to a narrow range of pH, this raises extinction concerns, directly driven by increased atmospheric CO2, that could disrupt food webs and impact human societies that depend on marine ecosystem services.[62]

A factor that may have mitigated global warming in the late twentieth century is global dimming - a gradual reduction in the amount of global direct irradiance at the Earth's surface. From 1960 to 1990, human-caused aerosols likely precipitated this effect. Scientists have stated with 66–90% confidence that the effects of human-caused aerosols, along with volcanic activity, have offset some of global warming, and that greenhouse gases would have resulted in more warming than observed if not for these dimming agents.[3]

Skepticism

Although it is widely believed that there is widespread disagreement amongst scientists about the role of man in global warming, in fact there seems to be very little disagreement amongst Earth Scientists. Summarising the results of a survey of over 3,000 Earth Scientists, Doran (2009) wrote: "It seems that the debate on the authenticity of global warming and the role played by human activity is largely nonexistent among those who understand the nuances and scientific basis of long-term climate processes. The challenge, rather, appears to be how to effectively communicate this fact to policy makers and to a public that continues to mistakenly perceive debate among scientists."[1]

Nevertheless, a well-funded campaign by skeptical scientists, free-market think tanks and industry, through advertisements, lobbying and media attention, has argued first that the world is not warming; then that warming is not caused by human activities, now that warming will be harmless.[63] [64]This campaign has had a major effect in the USA, on both the public and Congress; the Tea Party is at the forefront of climate change denial.

A few skeptical scientists believe that the present climate change is not man-made and is unavoidable. [65]

Most accept that there has been a large increase of CO2 in the atmosphere due to the use of fossil fuels, but doubt that there is any immediate danger and dispute any need for large reductions of CO2 emissions. Their skepticism is based on the complex problems associated with the underlying science and the uncertainty of available climate data. Put most simply, the skeptical view is that we don't understand climate well enough to make reliable predictions, and that there have been large variations of climate in the past, so perhaps the observed changes are just part of a natural cycle that we don't understand. [66][67] They argue that we don't have good enough data from the past to develop and test computer models of the climate, and object to the use of proxy data - indirect data on temperature, such as tree rings and the isotopic content of Arctic and Antarctic ice. Proxy data are used to construct historical temperature profiles, yielding, for instance, the hockey stick shaped graph, but have some problems - tree ring thickness for example is affected by both temperature and precipitation.[68]

There is also concern about the lack of transparency of analyses purporting global warming and inaccessibility to data to allow independent analyses. [69][70][71] Specifically, skeptics have requested access to data from the University of East Anglia's Climatic Research Unit (CRU) and Penn State Department of Meteorology. The CRU reports that 95% of their data is available for the public[72][73]

When an FPS (Forum on Physics & Society) editor of the American Physical Society wrote: "There is a considerable presence within the scientific community of people who do not agree with the IPCC conclusion that anthropogenic CO2 emissions are very probably likely to be primarily responsible for the global warming that has occurred since the Industrial Revolution",[74] the FPS Executive Committee hastened to declare that his statement does not represent their views.[75][76] Nonetheless, in 2007 a U.S. Senate committee identified more than 400 prominent scientists from more than two dozen countries as objecting to aspects of the "consensus" on man-made global warming. Some of those scientists were participants in the IPCC.[77].

A 2010 assessment of the IPCC report by the Netherlands Environmental Assessment Agency found no significant errors in the report, but its authors reported that they had been unable to find the provenance of some of its conclusions, that the report had not taken account of the positive effects of climate change and that there was a tendency to highlight the upper ends of uncertainty ranges.[78].

Climate sensitivity

For more information, see: Climate sensitivity.

The concentration of CO2 in the Earth's atmosphere at the beginning of the Industrial Age (1750) was about 278 ppm; in 2008, it was about 385 ppm. The concept of climate sensitivity arose when the IPCC members asked how much the temperature on Earth would change by an increase of CO2 in the atmosphere. To answer that, the IPCC adopted this definition:

Climate sensitivity is the equilibrium temperature change, , in the surface temperature, TS, caused by the doubling of the pre-industrial CO2 concentration.

Put more simply, the IPCC defined climate sensitivity as the temperature change, in the Earth's surface temperature, , that would be caused by doubling the pre-1750 atmospheric CO2 concentration of 278 ppm to 556 ppm which is currently expected to occur later in this century.

The IPCC estimated the climate sensitivity to be 3.26 °C. In other words, when the atmospheric concentration of CO2 reaches 556 ppm (expected later this century), the IPCC predicts that the Earth's surface temperature will be 3.26 °C higher than it was more than 250 years ago (1750).

References

  1. 1.0 1.1 See Doran (2009) 'Examining the Scientific Consensus on Climate Change' for information on a poll of research-active climate scientists, other researchers and the public regarding the scientific consensus on global warming Eos 90: 21-2
  2. 2.0 2.1 Joint science academies’ statement: Global response to climate change
    • "There will always be uncertainty in understanding a system as complex as the world’s climate. However there is now strong evidence that significant global warming is occurring. The evidence comes from direct measurements of rising surface air temperatures and subsurface ocean temperatures and from phenomena such as increases in average global sea levels, retreating glaciers, and changes to many physical and biological systems. It is likely that most of the warming in recent decades can be attributed to human activities (IPCC 2001). This warming has already led to changes in the Earth's climate."
  3. 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 Summary for Policymakers. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (2007).
    • "Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations...Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns"
  4. Walker G (2007) Climate Change 2007: A world melting from the top down Nature 446:718-21
  5. 5.0 5.1 Simpson (2009) "The Peril Below the Ice" Scientific American Earth 3.0 pp 30-7
  6. Schneider et al. (2007). Assessing key vulnerabilities and the risk from climate change. In Parry ML et al. (eds) Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change Cambridge University Press pp 779-810
    • "There is new and stronger evidence of observed impacts of climate change on unique and vulnerable systems (such as polar and high-mountain communities and ecosystems), with increasing levels of adverse impacts as temperatures increase (very high confidence).
    • There is new evidence that observed climate change is likely to have already increased the risk of certain extreme events such as heatwaves, and it is more likely than not that warming has contributed to the intensification of some tropical cyclones, with increasing levels of adverse impacts as temperatures increase (very high confidence).
    • The distribution of impacts and vulnerabilities is still considered to be uneven, and low-latitude, less-developed areas are generally at greatest risk due to both higher sensitivity and lower adaptive capacity; but there is new evidence that vulnerability to climate change is also highly variable within countries, including developed countries."
  7. United Nations Framework Convention on Climate Change, Article I. Retrieved on 2007-01-15.
  8. Intergovernmental Panel on Climate Change
  9. United Nations Environment Programme Mission: "To provide leadership and encourage partnership in caring for the environment by inspiring, informing, and enabling nations and peoples to improve their quality of life without compromising that of future generations."
  10. World Meteorological Organization
  11. [http://nobelprize.org/nobel_prizes/peace/laureates/2007/ The Nobel Peace Prize 2007} Intergovernmental Panel on Climate Change, Al Gore
  12. "Kyoto Protocol to the United Nations Framework Convention on Climate Change" Full text
  13. http://www.american.com/archive/2009/november/the-quiet-yet-historic-death-of-the-kyoto-protoco The Quiet Death of the Kyoto Protocol] The American November 5, 2009
  14. Copenhagen Accord
  15. Who's On Board With The Copenhagen Accord? USCAN climate action network
  16. IPCC 4th Assessment Report Climate Change 2007 Chapter 9: Understanding and Attributing Climate Change
  17. Joint science academies' statement: The science of climate change. Royal Society (2001). “The work of the Intergovernmental Panel on Climate Change (IPCC) represents the consensus of the international scientific community on climate change science”
  18. Bard, E; Frank M (2006). "Climate change and solar variability: What's new under the sun?". Earth and Planetary Science Lett 248: 1-14.
  19. Svensmark, H (2000). "Cosmic Rays and Earth's Climate". Space Science Rev 93: 175-85.
  20. Meehl, GA; et al. (2005). "How much more global warming and sea level rise". Science 307: 1769–72.
    • "Two global coupled climate models show that even if the concentrations of greenhouse gases in the atmosphere had been stabilized in the year 2000, we are already committed to further global warming of about another half degree and an additional 320% sea level rise caused by thermal expansion by the end of the 21st century. ...At any given point in time, even if concentrations are stabilized, there is a commitment to future climate changes that will be greater than those we have already observed."
  21. (2002). Living with Climate Change – An Overview of Potential Climate Change Impacts on Australia. Summary and Outlook. Australian Greenhouse Office.
  22. Kiehl, JT; KE Trenberth (1997). "Earth’s Annual Global Mean Energy Budget". Bull Am Meteorol Soc 78: 197-208.
  23. Tans, P. Trends in Atmospheric Carbon Dioxide – Mauna Loa. National Oceanic and Atmospheric Administration.
  24. Pearson, PN; Palmer MR (2000). "Atmospheric carbon dioxide concentrations over the past 60 million years". Nature 406: 695–9.
  25. Summary for Policymakers. Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change (2001).
  26. Quay PD et al. (1992) Oceanic uptake of fossil fuel CO2: carbon-13 evidence Science 256:74-9
  27. Prentice, IC; et al. (2001). 3.7.3.3 SRES scenarios and their implications for future CO2 concentration. Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change.
  28. 4.4.6. Resource Availability. IPCC Special Report on Emissions Scenarios.
  29. Sample, Ian. Warming Hits 'Tipping Point', 'The Guardian', 2005. Retrieved on 2007-01-18.
  30. 30.0 30.1 Soden, BJ; Held IM (2005). "An assessment of climate feedbacks in coupled ocean–atmosphere models". J Climate 19. “Interestingly, the true feedback is consistently weaker than the constant relative humidity value, implying a small but robust reduction in relative humidity in all models on average" "clouds appear to provide a positive feedback in all models”
  31. Stocker, TF; et al. (2001). 7.5.2 Sea Ice. Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change.
  32. Buesseler KO et al. (2007) Revisiting carbon flux through the ocean's twilight zone Science 316:567-70
  33. Marsh, N; Svensmark H (2000). "Cosmic rays, clouds, and climate". Space Science Rev 94: 215–30.
    • "Overall, the role of solar activity in climate changes—such as the Quaternary glaciations or the present global warming—remains unproven and most probably represents a second-order effect. Although we still require even more and better data, the weight of evidence suggests that solar changes have contributed to small climate oscillations occurring on time scales of a few centuries, similar in type to the fluctuations classically described for the last millennium: The so-called Medieval Warm Period (900–1400 A.D.) followed on by the Little Ice Age (1500–1800 A.D.)."
  34. Hegerl, GC; et al. (2007). Understanding and Attributing Climate Change. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change 690.
    • "Overall, modelling and detection and attribution studies confirm a role of volcanic, greenhouse gas and probably solar forcing in explaining the broad temperature evolution of the last millennium"
  35. Scafetta, N; West BJ (2006). "Phenomenological solar contribution to the 1900–2000 global surface warming". Geophys Res Lett 33. L05708.
  36. Stott, PA; et al. (2003). "Do Models Underestimate the Solar Contribution to Recent Climate Change?". J Climate 16: 4079–93.
  37. DE Parker (2004). "Climate: Large-scale warming is not urban". Nature 432: 290. [1]
  38. DE Parker (2006). "A demonstration that large-scale warming is not urban". J Climate 19: 2882–95. [2](online)]
  39. Peterson TC (2003). "Assessment of urban versus rural in situ surface temperatures in the contiguous United States: no difference found". J Climate 16: 2941–59. (PDF)
  40. Smith, TM; Reynolds RW (2005). "A global merged land–air–sea surface temperature reconstruction based on historical observations (1880–1997)". J Climate 18: 2021–36.
  41. Hansen, JE; et al. (2006). Goddard Institute for Space Studies, GISS Surface Temperature Analysis. NASA Goddard Institute for Space Studies.
  42. Global Temperature for 2005: second warmest year on record. Climatic Research Unit, School of Environmental Sciences, University of East Anglia (2005).
  43. World Meteorological Organization Statement on the status of the Global Climate in 2005 (2005).
    • "Since the beginning of the 20th century, the global average surface temperature has increased by about 0.6°C. However, this increase has not been continuous and has risen sharply since 1976. Areas of significant warmth were widespread with large areas of Africa, Australia, Brazil, the Russian Federation, Scandinavia, Canada, China and the south-west United States showing significantly above average temperatures. Much of the North Atlantic and south-west Pacific Oceans were also significantly warm, as was the Gulf of Alaska. Sea surface temperatures in the North Atlantic in 2005 were the warmest on record."
  44. National Oceanic and Atmospheric Administration: Global Warming Frequently Asked Questions
  45. Mitchell, JFB; et al. (2001). 12.4.3.3 Space-time studies. Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change.
  46. Summary for Policymakers. Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change (2001).
  47. Torn, M; Harte J (2006). "Missing feedbacks, asymmetric uncertainties, and the underestimation of future warming". Geophys Res Lett 33. L10703.
  48. Harte, J; et al. (2006). "Shifts in plant dominance control carbon-cycle responses to experimental warming and widespread drought". Environmental Res Lett 1. 014001.
  49. Scheffer, M; et al. (2006). "Positive feedback between global warming and atmospheric CO2 concentration inferred from past climate change". Geophys Res Lett 33.
  50. Stocker, TF; et al. (2001). 7.2.2 Cloud Processes and Feedbacks. Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change.
  51. 51.0 51.1 Climate Change 2001: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Third Assessment Report of the Intergovernmental Panel on Climate Change (2001).
  52. .Is it time to stop worrying about global warming?. This article appeared in print under the headline "Causes for climate concern". From issue 2946 of New Scientist magazine, page 5.
  53. Church, JA; et al. (2001). Executive Summary of Chapter 11. Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change.
  54. Thomas, CD; et al. (2004). "Extinction risk from climate change". Nature 427: 145-38.
  55. >McLaughlin, JF; et al. (2002). "Climate change hastens population extinctions". PNAS 99: 6070–4.
  56. Kyoto Protocol Status of Ratification. United Nations Framework Convention on Climate Change (2006).
  57. Bjørn Lomborg (2001) The Skeptical Environmentalist Cambridge University Press; ISBN-10: 0521010683 see Skepticism toward The Skeptical Environmentalist Scientific American for criticism
  58. Bjørn Lomborg: $100bn a year needed to fight climate change Guardian, 30 August 2010
  59. The Ocean and the Carbon Cycle. NASA (2005).
  60. Jacobson, MZ (2005). "Studying ocean acidification with conservative, stable numerical schemes for nonequilibrium air-ocean exchange and ocean equilibrium chemistry". J Geophys Res 110. D07302.
  61. Caldeira, K; Wickett ME (2005). "Ocean model predictions of chemistry changes from carbon dioxide emissions to the atmosphere and ocean". J Geophys Res 110: 1–12.
  62. Raven JA et al. (2005). Ocean acidification due to increasing atmospheric carbon dioxide. Royal Society.
  63. The Truth About Denial Newsweek Education site
  64. The denial industry George Monbiot The Guardian 19 September 2006
  65. During the United Nations Climate Conference on Bali in 2007, more than 100 scientists wrote an open letter to Ban Ki-Moon, the Secretary-General of the United Nations expressing their opinion that "the 2007 UN climate conference [is] taking the World in entirely the wrong direction". (Letter to Ban Ki-Moon) They recognized that climate change is occurring, but state that it is a natural phenomenon which is impossible to stop and express doubts that "it is possible to significantly alter global climate through cuts in human greenhouse gas emissions." The letter criticised the IPCC Assessment Reports of 2001 and 2007, claiming that they "are prepared by a relatively small core writing team with the final drafts approved line-by-line by ­government representatives".
  66. Climate Scientists’ Perceptions of Climate Change Science Interviews among 558 scientists working in the field of climate change (mainly climatologist and meteorologists) by Dennis Bray and Hans von Storch
  67. Issues in the Current State of Climate Science The Center for Science and Public Policy, Washington, DC, March 2006 (from the website of the Florida Gulf Coast University)
  68. ME Mann et al. (1998) Nature:779-87 A plot of mean temperature over the last 1,000 years which is flat on average from the years 1000 to 1900. The flat part forms the hockey stick's shaft. After 1900, and especially after 1980, temperatures appear to shoot up, forming the hockey stick's blade.
  69. Revkin AC (November 27, 2009) A Climate Scientist Who Engages Skeptics The New York Times
  70. Curry J (November 27, 2009) An open letter to graduate students and young scientists in fields related to climate research from Dr Judith Curry regarding hacked CRU emails. Climate Progress
  71. McIntyre S Climate Audit
  72. CRU data; Anonymous. (November 24, 2009) CRU climate data already ‘over 95%’ available (28 November). University of East Anglia - Communications Office
  73. The RealClimate blog also maintains a list of data sources at http://www.realclimate.org/index.php/data-sources/
  74. Editor's Comment, Forum on Physics & Society of the American Physical Society, July 2008
  75. Forum on Physics & Society of the American Physical Society, July 2008.
  76. The APS's 2007 statement on Climate Change, which urges an enhanced effort to understand the effects of human activity on the climate, is here.
  77. U. S. Senate Report: Over 400 Prominent Scientists Disputed Man-Made Global Warming Claims in 2007 Note that some individuals listed objected to their inclusion and declared that the statement did not reflect their views.
  78. Assessing an IPCC assessment. An analysis of statements on projected regional impacts in the 2007 report', Netherlands Environmental Assessment Agency, 2010