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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/>
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==
As pointed out above, future man-made change of climate  is accepted as likely by the national science academies of the eleven largest countries;<ref name="academies" /> they  follow in this the conclusions of the  [[IPCC]] (Intergovernmental Panel on Climate Change).  On the other hand, there is a vocal minority of skeptics who believe that the present climate change, if it exists at all, is not man-made and hence is unavoidable. Many of these skeptics  deny the immediate danger and do not see the need for a large reduction in the use of fossil fuels and change of consumption patterns. The dispute will continue for some time in the future, because of the difficulty of the underlying science and the uncertainty in the climate data. The latter is fully recognized by the IPCC and is accounted for by them as much as possible.


There is a growing tendency to measure the credibility of IPCC's conclusions by the number of scientists that question IPCC's conclusions;  consequently the alleged number  of dissenting scientists has become a sensitive issue too.  For instance, when an FPS (Forum on Physics & Society) Editor of the APS (American Physical Society) wrote the following comment: "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 this 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>. Clearly, the suggestion that there is a "considerable presence" of scientists disagreeing with the IPCC is already politically laden. Yet, the US Senate recently was able to quote more than 400 scientists&mdash;with areas of expertise in climate matters&mdash; who dispute man-made global warming<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>.
Earlier in this article the point of view<ref name=grida7 />  of the IPCC  was outlined and some scattered attention was paid to the arguments of IPCC's dissidents.
Some of the arguments against the IPCC conclusions, carried forward by skeptical scientists,  will now be reviewed.
===Difficulties in climate change science===
All participants in the dispute agree on one fact, namely that ''climate change'' science is a field full of difficult problems. Some skeptics state that many of its problems are too difficult to be solvable at all.  Climate science itself is already a very hard area of study.  In the first place it is an observational science (like astronomy), meaning that one cannot perform experiments to falsify or verify certain hypotheses.
In the second place its object of study, the Earth, is an extremely complex system, much more complex than natural scientists usually dare to tackle. Most of the physical sciences is based on a [[reductionism|reductionistic approach]], that is, systems of study are reduced to smaller ones that are easier to understand, but still possess their essential characteristics. In climate science,  such an approach is impossible, the atmosphere, the oceans, and the landmasses are tightly coupled subsystems and consequently the  energy and mass exchanges between the three major subsystems of the Earth must be included simultaneously. Further, it goes without saying that the [[electromagnetic radiation|radiation]] balance, i.e., insolation (solar irradiation), energy absorption and back radiation  by the Earth, plays a crucial role and cannot be omitted, which means that even the Earth itself is not a closed system.
Third, there exists no encompassing theory, like [[Newton]]'s equations in [[classical mechanics]], that predicts the characteristics of the climate and of which the truth is accepted by all climatologists. Theories are ad-hoc and taken from many different&mdash;and not the simplest&mdash;areas of applied physics: turbulent and dissipative systems, convective and radiative transport phenomena, non-linear (chaotic) systems and their inherent sensitivity to initial conditions (discovered by by [[Edward Norton Lorenz]]), and so on. Further, there is a paucity of reliable data to gauge the existing models. Hence, by necessity even models that try to explain the ''present'' world climate are based on choices that are subjective and open to criticism. The problems are compounded for predictions of  world-wide ''changes'' in climate. So, it is no surprise that many workers in the field do not have much faith in climate models' ability to predict the future.<ref>Interviews among 558 scientists working in the field of climate change&mdash;mainly climatologist and meteorologists&mdash;by Dennis Bray and Hans von Storch[http://coast.gkss.de/staff/storch/pdf/070511.bray.GKSS.pdf ]</ref>
It is also no wonder that global warming, and its possible anthropogenic origin, has been disputed&mdash;although, again, to what extent it is disputed is itself a matter of dispute.
Some scientists are skeptical about the interpretation of [[proxy]] data&mdash;indirect data giving information about past temperatures around the world (such as year rings of trees and isotopic composition of arctic and antarctic ice). Proxy data are  used to construct historical mean temperature profiles, yielding, for instance,  the widely discussed [[hockey stick model|hockey stick shaped]] graph.<ref>A famous plot of mean temperature  over the last 1&thinsp;000 years. The plot is flat  on average from the years 1000 to 1900.  The flat part forms the hockey stick's "shaft." After 1900, and especiallly after 1980, temperatures appear to shoot up, forming the hockey stick's "blade." [http://www.caenvirothon.com/Resources/Mann,%20et%20al.%20Global%20scale%20temp%20patterns.pdf M. E. Mann et al.] Nature, vol. '''392''', pp. 779-787 (1998) </ref> The same people questioning the reliability of the past temperature profiles usually have  doubts about the uniqueness of the present global warming; they argue that the world has seen warm periods before, even without human intervention. They often refer to the discovery of Greenland  by the Vikings around the year 1000, when Greenland was green, and the time before the dinosaurs became extinct.
Others question the validity of the computer models predicting the climate a few decades ahead, referring to the unreliability of computer models in general.  One of their arguments is the inability of current computer models to predict  the weather for more than 10 days in advance.  Also the failure of computer models to forecast the formation of tropical cyclones is  pointed to.
From reading the dissenting literature, one gathers the impression that the great majority of skeptical scientists admit  a definite increase in CO<sub>2</sub> in the atmosphere, due to the  growing use of fossil fuels, but question that this increase in CO<sub>2</sub> concentration will lead to a world-wide catastrophe. The dissidents reject the warnings by the former US vice-president and Nobel Peace Prize winner [[Al Gore]] as unscientific and needlessly alarmist.
===Open letter to Ban Ki-Moon from skeptical scientists===
During the [[United Nations]] Climate Conference on the Indonesian island [[Bali]] in December 2007,
more than a 100 skeptical scientists (climatologists, physicists, biologists, meteorologists, statisticians, and others) took the initiative to write 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>  In this letter they express their opinion that "the 2007 UN climate conference [is] taking the World in entirely the wrong direction".
They recognize that a climate change is going on but they state that it is a natural phenomenon that is impossible to stop and they express their doubts that "it is possible to significantly alter global climate through cuts in human greenhouse gas emissions."
People expressing fears of catastrophic climate change invariably refer to the IPCC  Assessment Reports of 2001 and 2007. In the open letter to the UN Secretary-General the skeptical scientists cast doubt on the the procedures leading to these reports. They write that "the reports are prepared  by a relatively small core writing team with the final drafts approved line-by-line by ­government ­representatives". Further they write:  "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".
===Climate sensitivity===
In a contribution to the APS Forum on Physics & Society of July 2008,<ref>[http://www.aps.org/units/fps/newsletters/200807/monckton.cfm Christopher Monckton of Brenchley, Forum on Physics & Society of the American Physical Society, July 2008]</ref>  [[Christopher Monckton, 3rd Viscount Monckton of Brenchley |Christopher Monckton]], a known critic of anthropogenic causes of global warming, takes issue with the 2007 IPCC report.  Moncktons' contribution  will now be given some attention, because it follows closely the arguments of the IPCC report and yet comes to different conclusions.  A review of Monckton's criticism, which is mainly directed at climate sensitivity, gives us a chance to delve deeper into IPCC's scientific reasonings and to  give a discussion that is more quantitative than presented earlier in this article. The main purpose  of this section is to illustrate that, even when the very same physical effects are accounted for, different estimates of the same parameters lead to different conclusions. Of course, the IPCC is very much aware of this and discusses likelihoods of parameter values wherever possible, but still it is of interest to see how different parameter choices work out in practice. We present Monckton's and IPCC's values vis-à-vis, but we  do not phrase opinions about which of the opposing parties is likely to be correct.
Monckton begins with presenting  data  plots showing that the globally averaged land and sea surface absolute temperature (''T''<sub>S</sub>) has not risen since 1998<ref> Opponents of Monckton point out that 1998 is not a good reference year because it was unusually warm with the strongest El Nino on record.</ref>  and may have fallen since late 2001, in contrast to the prediction of further rapid warming by the IPCC. 
After having made this introductory point, he  directs his criticism at the value of [[climate sensitivity]] found by the IPCC. The concept of climate sensitivity arose when the IPCC members asked themselves the question how much the temperature on Earth would change by an increase of CO<sub>2</sub> in the atmosphere. To give a quantitative answer, it is necessary to define a reference concentration and a quantitative amount of increase of CO<sub>2</sub>. The IPCC accepted the following formal 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.'' That is, as the reference increase in CO<sub>2</sub> they take a doubling with respect to the concentration at the  start of the [[Industrial Revolution]] (1750).
The IPCC gives an estimate of  3.26 °C for climate sensitivity,  the Earth's temperature response to a possible CO<sub>2</sub> concentration doubling since 1750. In contrast, Monckton gives detailed arguments that 0.58 °C is the more reliable value.
The concept of [[radiative forcing]] plays an important role in the discussion of climate sensitivity. Basically, this is the gross amount of solar radiation absorbed by  CO<sub>2</sub> in the atmosphere. As for the climate sensitivity, it is more expedient to give a relative value, &Delta;F, i.e., an increase or decrease in absorption, rather than the absolute value ''F'' of the absorption  itself. The IPCC assumes a logarithmic dependence on concentration and relates the concentration ''C'' of CO<sub>2</sub> in the atmosphere  to radiative forcing, &Delta;F, by several formulas, the following being the simplest,
:<math> \scriptstyle
\Delta F_\mathrm{CO_2} = 5.35 \, \ln(C/C_0), \qquad\qquad\qquad\qquad(1)
</math>
where  C<sub>0</sub> is the CO<sub>2</sub> concentration before the Industrial Revolution.
The factor 5.35 has dimension W/m<sup>2</sup> (solar energy  absorbed per second per square meter by carbondioxide) and plays a pivotal role in the discussion. Note that &Delta;''F''<sub>CO<sub>2</sub></sub>  represents  a ''gross'' absorption: back radiation of the Earth into space and feedback effects (evaporation of water, etc., see earlier in this article) are not yet included.
By means of equation (1)  changes in CO<sub>2</sub> concentration can be expressed in the unit of &Delta;''F''<sub>CO<sub>2</sub></sub> (W/m<sup>2</sup>).  At this point Monckton remarks that at 1990 the total &Delta;F<sub>CO<sub>2</sub></sub> was ~27 W/m<sup>2</sup> and that from 1995-2005, the CO<sub>2</sub> concentration rose  from 360 to 378 W/m<sup>2</sup>, with a consequent increase in radiative forcing of  0.26 W/m<sup>2</sup>, which is less than  1% of the 1990 value of ~27 W/m<sup>2</sup>. The 2007 IPCC report states:  "The CO<sub>2</sub> radiative forcing increased by 20% in the last 10 years (1995-2005)".  Monckton, noticing that the true value is one-twentieth of the value given by the IPCC, states:
:The absence of any definition of radiative forcing in the 2007 IPCC Summary, led many to believe that the effect of CO<sub>2</sub> on ''T''<sub>S</sub>  had increased by 20% in 10 years. The IPCC – despite requests for correction – retained this confusing statement in its report.
By equation (1) a doubling of CO<sub>2</sub> concentration gives  a radiative forcing of  <math>\scriptstyle \Delta F_{2x}</math> = 5.35 ln2 = 3.71  W/m<sup>2</sup>.  When this value is corrected for  aerosols etc., it becomes slightly smaller  <math>\scriptstyle \Delta F_{2x}</math> = 3.405 W/m<sup>2</sup> [see Table 1 in Monckton (2008)].
In his explanation of the origin of the factor 5.35 in equation (1), Monckton shows  plots of different models  considered by the IPCC for relative warming rates of the atmosphere as a function of altitude and latitude. All these plots show  a strong dependence of warming rate on altitude and in particular they exhibit a tropical mid-troposphere "hot-spot".  Monckton argues that observations from satellites and by  radiosondes do not show this hot-spot: they show that not only absolute temperatures but also warming rates decline with altitude. Therefore he concludes: "Since the great majority of the incoming solar radiation incident upon the Earth strikes the tropics, any reduction in tropical radiative forcing has a disproportionate effect on mean global forcings. On the basis of Lindzen (2007),<ref>R.S. Lindzen, ''Taking greenhouse warming seriously.'' Energy & Environment vol. '''18''' pp. 937-950 (2007)</ref> the anthropogenic radiative forcing as established is divided by 3 to take account of the observed failure of the tropical mid-troposphere to warm as projected by the models". Accepting the corrected value of 3.405 he arrives at a readjusted value of radiative forcing:  <math>\scriptstyle \Delta F_{2x}</math> = 1.135 W/m<sup>2</sup>. Hence, the allegedly observed absence of hot-spots in the troposphere gives a diminishing of IPCC's climate sensitivity (predicted temperature change) by a factor 3.
Obviously a certain amount of radiation is transmitted back: the Earth is a source of [[blackbody radiation]]. Deviating in a non-essential way from the IPCC report, Monckton introduces a parameter &kappa; that gives the fraction of the absorbed solar radiation  that is re-emitted by the Earth in the form of blackbody (infra-red) radiation. The parameter &kappa; is referred to as ''climate sensitivity parameter'' and is introduced as a multiplying factor. If, e.g., &kappa; = 0.3, it means that effectively 70% of the insolation is radiated back into space. Some basic laws of physics state that the energy content of black-body radiation depends on absolute temperature,  meaning that &kappa;  is a function of absolute  temperature [see equation (21) of Monckton].
To give an estimate of the value of &kappa; one may note that at the Earth’s surface, ''T''<sub>S</sub> &asymp; 288 K (kelvin) &asymp; 15 °C, implying the surface value  &kappa;<sub>S</sub> = 0.185 K·m<sup>2</sup>/W. At the characteristic altitude at which incoming and outgoing radiative fluxes balance, the temperature ''T''<sub>C</sub> &asymp; 254 K &asymp; &minus;19 °C, giving &kappa;<sub>C</sub> = 0.269 K·m<sup>2</sup>/W. Monckton then writes:  "The IPCC’s value for &kappa; is dependent upon temperature at the surface and radiant-energy flux at the tropopause, so that its implicit value &kappa; &asymp; 0.313 K·m<sup>2</sup>/W is considerably higher than either &kappa;<sub>S</sub> or &kappa;<sub>C</sub>". Then some papers  quoted by the IPCC are mentioned by Monckton and he continues:  "None of these papers provides any theoretical or empirical justification for a value as high as the &kappa; &asymp; 0.313 K·m<sup>2</sup>/W chosen by the IPCC".
Monckton then proceeds to give an alternative estimate of &kappa;, but since he needs the feedback factor ''f'' for this, we digress and consider feedback first.
The effect of solar absorption is strengthened by feedback mechanisms, such as water evaporation, etc., see earlier in this article. To account for these effects the radiative forcing is multiplied by a dimensionless feedback factor ''f'' > 1.  Typical values of ''f'' are 2.095 or 3.077 (roughly doubling or tripling the effect of the solar absorption, as estimated by Monckton and the IPCC, respectively).
The following equation for ''f'' is presented  in the IPCC report:
:<math>
f = \frac{1}{(1 - b\kappa)}, \qquad\qquad\qquad\qquad\qquad(2)
</math>
where ''b'' is the sum of all climate-relevant temperature feedbacks.  Equation (2) is taken from linear feedbacks  for electronic circuits.<ref>{{cite book|author=Hendrick W. Bode|title=Network analysis and feedback amplifier design|edition=1st Edition|publisher=D. Van Nostrand Inc.|year=1945 |id=OCLC No. 1692825}}</ref> Obviously, the value of ''f'' is as important as the values of <math>\scriptstyle \Delta F_{2x}</math> and &kappa;.  Monckton observes that equation (2) is of "questionable utility because it was not designed to model feedbacks in non-linear objects such as the climate".
Equation (2) gives a singularity (infinite value) for ''f'' and  a corresponding blow up  for the climate sensitivity  <math>\scriptstyle \Delta T_{\lambda}</math>, when ''b'' = 1/&kappa; = 1/0.313 = 3.19 W/(m<sup>2</sup>·K). (Note that  0.313 is IPCC's implicit value  for &kappa;.)  The IPCC estimates an upper limit ''b''<sub>max</sub> = 3.38 W/(m<sup>2</sup>·K), which is close to the singularity. Monckton argues that it is very unlikely that ''b'' will exceed the value 3.19 W/(m<sup>2</sup>·K), because of its runaway temperature effect that even not has occurred in the Cambrian atmosphere. During that period the CO<sub>2</sub> concentration approached 20 times today’s,  with an inferred mean global surface temperature no more than 7 °C higher than today’s. He adds that "a runaway greenhouse effect would occur even in today’s climate when ''b'' &ge;  3.2 W/(m<sup>2</sup>·K),  but has not occurred"  and:  "The IPCC’s high-end estimates of the magnitude of individual temperature feedbacks are very likely to be excessive, implying that its central estimates are also likely to be excessive".  After a few more critical comments Monckton concludes that it is "prudent and conservative" to restore ''f'' to the 50 % lower value ''f'' &asymp; 2.08 that is implicit in the 2001 IPCC report. He  adjusts its value a little to maintain consistency with his earlier equations and proposes the value ''f'' = 2.095.
After this digression to the feedback, we return to the climate sensitivity parameter.
The value of &kappa; cannot be directly observed. In order to obtain an empirical estimate, Monckton rewrites &kappa; as
:<math>
\kappa = \frac{\Delta T_\lambda}{\Delta F_{2x} + b\,\Delta T_\lambda} \qquad\qquad\qquad\qquad(3)
</math>
by means of equation (2).
Now, &kappa; can be computed by equation (3), provided the three parameters, appearing in its right hand side, over a given period are known. Monckton compares the years 1980 and 2005 giving a spread of a quarter of a century.  After discussing  his choices for these parameters, Monckton arrives at &kappa; = 0.242 K·m<sup>2</sup>/W. This value is bracketed by the values deduced from atmospheric temperatures: &kappa;<sub>S</sub> = 0.185 and &kappa;<sub>C</sub> = 0.269 K·m<sup>2</sup>/W, which adds to its credibility.
====Conclusion about climate sensitivity====
Following Monckton and slightly deviating from the IPCC, we wrote implicitly the climate sensitivity as a product
:<math>  \scriptstyle
\Delta T_\lambda = \Delta F_{2x} \cdot \kappa\cdot f .
</math>
The climate sensitivity, from the 2007 IPCC parameters that were discussed above,  is:
:<math>  \scriptstyle
\Delta T_\lambda = 3.405 \times 0.313 \times 3.077 = 3.28\,\, ^0\mathrm{C}
</math>.
The IPCC (2007) reports<ref>[http://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr.pdf Climate Change 2007: Synthesis Report] (p. 38; pdf-page 16)</ref> a likely range of <math>\scriptstyle \Delta T_{\lambda}</math> of 2.0 to 4.5 ºC, with a  best estimate of about 3 °C, demonstrating that Monckton has faithfully replicated IPCC’s method with IPCC's parameters. The IPCC report adds that a value of less than 1.5 ºC is very unlikely (probability less than 10%).
Using his own revised values, Monckton gives the following final estimate for the climate sensitivity
:<math> \scriptstyle
\Delta T_\lambda = 1.135 \times 0.242 \times 2.095 = 0.58\,\, ^0\mathrm{C},
</math>
which is in the range considered to be very unlikely by the IPCC.
Monckton concludes:
:If this equation is correct, the IPCC’s estimates of climate sensitivity must have been very much exaggerated. There may, therefore, be a good reason why, contrary to the projections of the models on which the IPCC relies, temperatures have not risen for a decade and have been falling since the phase-transition in global temperature trends that occurred in late 2001. Perhaps real-world climate sensitivity is very much below the IPCC’s estimates. Perhaps, therefore, there is no "climate crisis" at all. At present, then, in policy terms there is no case for doing anything. The correct policy approach to a non-problem is to have the courage to do nothing.
Since Monckton includes faithfully the very same physical effects as the IPCC, but differs in well-argued choices of parameters, this discussion of climate sensitivity illustrates that serious workers can have different interpretations of the same observations on climate.


==References==
==References==
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{{reflist|2}}

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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.

Global average air temperature near the Earth's surface rose 0.74 ± 0.18 °C (1.33 ± 0.32 °F) from 1906 to 2005. The prevailing scientific view, as represented by the science academies of the major industrialized nations[1] and the Intergovernmental Panel on Climate Change (IPCC),[2] is that most of the temperature increase since the mid-20th century has been very likely caused by increases in atmospheric greenhouse gas concentrations produced by human activity.

Climate models project that avarage 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.[2] 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.[2]

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].”[3] We cannot rule out other effects of global warming that may initiate self-acceleration.

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. 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.

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. Template:TOC-right

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.[4] The terms "anthropogenic global warming" and "anthropogenic climate change" are sometimes used when focusing on human-induced changes.

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[5] 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,[6] or that warming is caused by changes in cloud cover due to variations in galactic cosmic rays.[7]

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.[8]

Greenhouse gases in the atmosphere

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 °C (59 °F). Without this natural greenhouse effect, the average temperature of Earth would be about -18 °C (0 °F) making the planet uninhabitable.[9] 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%.[10]

The present atmospheric concentration of CO2 is about 383 parts per million (ppm) by volume.[11] From geological evidence it is believed that CO2 values this high were last attained 20 million years ago.[12] About three-fourths 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.[13] 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 volcanoes or the oceans.[14]

Future CO2 concentrations will 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.[15] Fossil fuel reserves are sufficient to reach these levels and continue emissions past 2100, if coal, tar sands, or methane clathrates are extensively used.[16] 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[17] not included in climate models cited by the IPCC.[2]

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 CO2 alone.[18]

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.[18]

Another important process is ice-albedo feedback.[19] 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.

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.[20]

Solar variation

It has been hypothesized that variations in solar output, possibly amplified by cloud feedbacks, may have been a secondary contributor to recent warming.[21] 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.[22] 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.[23] 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.[24] 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) relative to the period 1860–1900. This measured temperature increase is not significantly affected by the urban heat island effect.[25][26][27] Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade).[28] 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, widespread instrumental measurements became available in the late 1800s, exceeding the previous record set in 1998 by a few hundredths of a degree.[29] Estimates prepared by the World Meteorological Organization and the Climatic Research Unit concluded that 2005 was the second warmest year, behind 1998.[30][31] Global temperatures in 1998 were exceptionally warm because the strongest El Niño in the instrumental record occurred in that year.[32]

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 twentieth century,[33] though the cooling may also be due in part to natural variability.

Climate models

Scientists have studied global warming with 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 CO2 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 °C (2.0 to 11.5 °F) hotter than the average temperatures from 1980-1999.[2]

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.[34] 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.

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.[35][36][37]

The representation of clouds is one of the main sources of uncertainty in present-generation models, though progress is being made on this problem.[38] 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.[39] 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.

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.[40] 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.[39] 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.[2]

Additional anticipated effects include sea level rise of 110 to 770 millimeters (0.36 to 2.5 ft) between 1990 and 2100,[41] 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 future climate projections.[42] 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.[43]

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 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.[44] 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.[45]

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.

Related climatic issues

A variety of issues are often raised in relation to global warming. One is ocean acidification. Increased atmospheric CO2 increases the amount of CO2 dissolved in the oceans.[46] 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,[47] 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.[2][48] Since 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.[49]

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.[2]


References

  1. ↑ http://nationalacademies.org/onpi/06072005.pdf
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Summary for Policymakers (PDF). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Intergovernmental Panel on Climate Change (2007-02-05). Retrieved on 2007-02-02.
  3. ↑ Walker G. (2007) Climate Change 2007: A world melting from the top down. Nature 446, 718-221 (12 April 2007)
  4. ↑ United Nations Framework Convention on Climate Change, Article I. United Nations Framework Convention on Climate Change. Retrieved on 2007-01-15.
  5. ↑ Joint science academies' statement: The science of climate change (ASP). Royal Society (2001-05-17). Retrieved on 2007-04-01. “The work of the Intergovernmental Panel on Climate Change (IPCC) represents the consensus of the international scientific community on climate change science”
  6. ↑ Bard, Edouard; Frank, Martin (2006-06-09). "Climate change and solar variability: What's new under the sun?". Earth and Planetary Science Letters 248 (1-2): 1-14. Retrieved on 2007-09-17.
  7. ↑ Svensmark, Henrik (July 2000). "Cosmic Rays and Earth's Climate" (PDF). Space Science Reviews 93 (1-2): 175-185. Retrieved on 2007-09-17.
  8. ↑ Meehl, Gerald A.; et al. (2005-03-18). "How Much More Global Warming and Sea Level Rise". Science 307 (5716): 1769–1772. DOI:10.1126/science.1106663. Retrieved on 2007-02-11. Research Blogging.
  9. ↑ (December 2002). Living with Climate Change – An Overview of Potential Climate Change Impacts on Australia. Summary and Outlook (PDF). Australian Greenhouse Office. Retrieved on 2007-04-18.
  10. ↑ Kiehl, J. T.; Kevin E. Trenberth (February 1997). "Earth’s Annual Global Mean Energy Budget" (PDF). Bulletin of the American Meteorological Society 78 (2): 197-208. Retrieved on 2006-05-01.
  11. ↑ Tans, Pieter. Trends in Atmospheric Carbon Dioxide – Mauna Loa. National Oceanic and Atmospheric Administration. Retrieved on 2007-04-28.
  12. ↑ Pearson, Paul N.; Palmer, Martin R. (2000-08-17). "Atmospheric carbon dioxide concentrations over the past 60 million years". Nature 406 (6797): 695–699. DOI:10.1038/35021000. Research Blogging.
  13. ↑ 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. Intergovernmental Panel on Climate Change (2001-01-20). Retrieved on 2007-01-18.
  14. ↑ [1]
  15. ↑ Prentice, I. Colin; et al. (2001-01-20). 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. Intergovernmental Panel on Climate Change. Retrieved on 2007-04-28.
  16. ↑ 4.4.6. Resource Availability. IPCC Special Report on Emissions Scenarios. Intergovernmental Panel on Climate Change. Retrieved on 2007-04-28.
  17. ↑ Sample, Ian. Warming Hits 'Tipping Point', The Guardian, 2005-08-11. Retrieved on 2007-01-18.
  18. ↑ 18.0 18.1 Soden, Brian J.; Held, Isacc M. (2005-11-01). "An Assessment of Climate Feedbacks in Coupled Ocean–Atmosphere Models" (PDF). Journal of Climate 19 (14). Retrieved on 2007-04-21. “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”
  19. ↑ Stocker, Thomas F.; et al. (2001-01-20). 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. Intergovernmental Panel on Climate Change. Retrieved on 2007-02-11.
  20. ↑ 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 316: 567-570.
  21. ↑ Marsh, Nigel; Henrik, Svensmark (November 2000). "Cosmic Rays, Clouds, and Climate" (PDF). Space Science Reviews 94: 215–230. DOI:10.1023/A:1026723423896. Retrieved on 2007-04-17. Research Blogging.
  22. ↑ Hegerl, Gabriele C.; et al. (2007-05-07). Understanding and Attributing Climate Change (PDF). 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. Intergovernmental Panel on Climate Change. Retrieved on 2007-05-20.
  23. ↑ Scafetta, Nicola; West, Bruce J. (2006-03-09). "Phenomenological solar contribution to the 1900–2000 global surface warming" (PDF). Geophysical Research Letters 33 (5). DOI:10.1029/2005GL025539. L05708. Retrieved on 2007-05-08. Research Blogging.
  24. ↑ Stott, Peter A.; et al. (2003-12-03). "Do Models Underestimate the Solar Contribution to Recent Climate Change?". Journal of Climate 16 (24): 4079–4093. DOI:10.1175/1520-0442(2003)016%3C4079:DMUTSC%3E2.0.CO;2. Retrieved on 2007-04-16. Research Blogging.
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  27. ↑ Thomas C. Peterson (2003). "Assessment of urban versus rural in situ surface temperatures in the contiguous United States: no difference found". Journal of Climate 16: 2941–2959. (PDF)
  28. ↑ Smith, Thomas M.; Reynolds, Richard W. (2005-05-15). "A Global Merged Land–Air–Sea Surface Temperature Reconstruction Based on Historical Observations (1880–1997)" (PDF). Journal of Climate 18 (12): 2021–2036. Retrieved on 2007-03-14.
  29. ↑ Hansen, James E.; et al. (2006-01-12). Goddard Institute for Space Studies, GISS Surface Temperature Analysis. NASA Goddard Institute for Space Studies. Retrieved on 2007-01-17.
  30. ↑ Global Temperature for 2005: second warmest year on record (PDF). Climatic Research Unit, School of Environmental Sciences, University of East Anglia (2005-12-15). Retrieved on 2007-04-13.
  31. ↑ WMO STATEMENT ON THE STATUS OF THE GLOBAL CLIMATE IN 2005 (PDF). World Meteorological Organization (2005-12-15). Retrieved on 2007-04-13.
  32. ↑ National Oceanic and Atmospheric Administration: Global Warming Frequently Asked Questions
  33. ↑ Mitchell, J. F. B.; et al. (2001-01-20). 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. Intergovernmental Panel on Climate Change. Retrieved on 2007-01-04.
  34. ↑ 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. Intergovernmental Panel on Climate Change (2001-01-20). Retrieved on 2007-04-28.
  35. ↑ Torn, Margaret; Harte, John (2006-05-26). "Missing feedbacks, asymmetric uncertainties, and the underestimation of future warming". Geophysical Research Letters 33 (10). L10703. Retrieved on 2007-03-04.
  36. ↑ Harte, John; et al. (2006-10-30). "Shifts in plant dominance control carbon-cycle responses to experimental warming and widespread drought". Environmental Research Letters 1 (1). 014001. Retrieved on 2007-05-02.
  37. ↑ Scheffer, Marten; et al. (2006-05-26). "Positive feedback between global warming and atmospheric CO2 concentration inferred from past climate change.". Geophysical Research Letters 33. DOI:10.1029/2005gl025044. Retrieved on 2007-05-04. Research Blogging.
  38. ↑ Stocker, Thomas F.; et al. (2001-01-20). 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. Intergovernmental Panel on Climate Change. Retrieved on 2007-03-04.
  39. ↑ 39.0 39.1 Climate Change 2001: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Intergovernmental Panel on Climate Change (2001-02-16). Retrieved on 2007-03-14.
  40. ↑ Summary for Policymakers (PDF). Climate Change 2007: Impacts, Adaptation and Vulnerability. Working Group II Contribution to the Intergovernmental Panel on Climate Change Fourth Assessment Report. Intergovernmental Panel on Climate Change (2007-04-13). Retrieved on 2007-04-28.
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  42. ↑ Thomas, Chris D.; et al. (2004-01-08). "Extinction risk from climate change" (PDF). Nature 427 (6970): 145-138. DOI:10.1038/nature02121. Retrieved on 2007-03-18. Research Blogging.
  43. ↑ McLaughlin, John F.; et al. (2002-04-30). "Climate change hastens population extinctions" (PDF). PNAS 99 (9): 6070–6074. DOI:10.1073/pnas.052131199. Retrieved on 2007-03-29. Research Blogging.
  44. ↑ Kyoto Protocol Status of Ratification (PDF). United Nations Framework Convention on Climate Change (2006-07-10). Retrieved on 2007-04-27.
  45. ↑ Climate talks face international hurdles, by Arthur Max, Associated press, 5/14/07.
  46. ↑ The Ocean and the Carbon Cycle. NASA (2005-06-21). Retrieved on 2007-03-04.
  47. ↑ Jacobson, Mark Z. (2005-04-02). "Studying ocean acidification with conservative, stable numerical schemes for nonequilibrium air-ocean exchange and ocean equilibrium chemistry" (PDF). Journal of Geophysical Research 110 (D7). DOI:10.1029/2004JD005220. D07302. Retrieved on 2007-04-28. Research Blogging.
  48. ↑ Caldeira, Ken; Wickett, Michael E. (2005-09-21). "Ocean model predictions of chemistry changes from carbon dioxide emissions to the atmosphere and ocean". Journal of Geophysical Research 110 (C09S04): 1–12. Retrieved on 2006-02-14.
  49. ↑ Raven, John A.; et al. (2005-06-30). Ocean acidification due to increasing atmospheric carbon dioxide (ASP). Royal Society. Retrieved on 2007-05-04.
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