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==Skepticism about global climate change and its anthropogenic origin==
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Although the national science academies of many countries accept the IPCC's conclusion that future man-made climate change is likely,<ref name="academies"/> a minority of skeptical scientists and others believe that the present climate change, if it exists at all, is not man-made and is unavoidable. The skeptics dispute any immediate danger and any need for large reductions of man-made CO<sub>2</sub>  emissions. The skepticism is based on the complex problems associated with the underlying science and the uncertainty of the available climate data. The latter has been acknowledged and accounted for by the IPCC as much as possible.
 
There is a tendency to downplay the number of skeptical scientists or to ignore them. For example, 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> Clearly, any suggestion of a "considerable presence" of scientists disagreeing with the IPCC is politically undesirable.
None-the-less, over 400 prominent scientists from more than two dozen countries, testifying before a [[U.S. Senate]] committee in 2007, voiced objections to major aspects of the so-called "consensus" on man-made global warming. Many 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>.
 
===The complexities and problems of climate change science===
 
All participants in the dispute, between those who agree with the IPPC's conclusions and those who do not agree, recognize that ''climate change'' science involves difficult problems. Some of the skeptical scientists believe that many of those problems are too difficult to be solvable. In the first place, it is an observational science (like astronomy), meaning that experiments cannot be performed to verify or disprove certain hypotheses.
 
In the second place, the Earth's climate is an extremely complex system, much more complex than physical scientists usually tackle. Most of the physical sciences is based on a [[reductionism|reductionistic approach]], in which systems are reduced to smaller ones that are easier to understand but still possess their essential characteristics. In climate science,  such an approach is impossible because the atmosphere, the oceans, and the land masses are tightly coupled subsystems and consequently the energy and mass exchanges between those three major subsystems of the Earth must be studied simultaneously and cannot be reduced to separate smaller systems. Further, the [[electromagnetic radiation]] balance (i.e., insolation of solar irradiation) between energy absorption and back radiation by the Earth plays a crucial role and cannot be omitted, which means that the Earth's climate system  itself is not a closed system.
 
Third, there exists no encompassing theory that predicts the characteristics of the climate and which is accepted by all climatologists. Theories are ad-hoc and 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. Hence, even models trying to explain the ''present'' world climate are based on subjective choices open to criticism. The problems are compounded for predictions of  world-wide ''climate changes''. Thus, many climate scientists do not have much 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> The validity of computer models predicting the climate a few decades ahead is questioned by those who refer to the inability of current computer models to predict the weather for more than 10 days in advance.
 
Another problem is the use of ''proxy data'' (i.e., 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 M. E. Mann et al. (1998),  Nature, vol. '''392''', pp. 779-787.] A famous 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.
 
The majority of skeptical scientists admit a definite increase of CO<sub>2</sub> in the atmosphere, due to the  growing use of fossil fuels, but doubt that the increase in CO<sub>2</sub> concentration will lead to a world-wide catastrophe.
 
===Open letter to the UN Secretary-General from skeptical scientists===
During the [[United Nations]] Climate Conference on the Indonesian island of [[Bali]] in December 2007, more than 100 scientists (climatologists, physicists, biologists, meteorologists, statisticians, and others) 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> The letter expressed 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."
 
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 further 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".
 
===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]]. It is currently (2008) about 385 ppm. The concept of climate sensitivity arose when the IPCC members asked themselves how much the temperature on Earth would change by an increase of CO<sub>2</sub> in the atmosphere. To answer that question, 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.''
 
More simply put, the IPCC defined climate sensitivity as the temperature change, in the Earth's surface temperature, <math>\scriptstyle \Delta T_\lambda</math>, that would be caused by doubling the pre-1750  atmospheric CO<sub>2</sub> concentration of 278 ppm to 556 ppm which is currently expected to incur later in this century.
 
The IPCC estimated the climate sensitivity to be 3.26 °C. In other words, when the atmospheric concentration of CO<sub>2</sub> 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 150 years ago (1750).
 
In a contribution to the American Physical Society (APS) Forum on Physics & Society of July 2008, entitled "Climate sensitivity reconsidered",<ref name=Monckton>[http://www.aps.org/units/fps/newsletters/200807/monckton.cfm ''Climate Sensitivity Reconsidered''], 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. Monckton is a British journalist and he acknowledges that he was  assisted in various ways during the preparation of his APS Forum contribution by physicists, meteorologists and others at Harvard University and Massachusetts Institute of Technology (both in Cambridge, Massachusetts), Imperial College (London, England), St. Andrews University (Scotland) and other universities.
 
The major point made in Monckton's contribution in the APS Forum on Physics & Society is that the IPCC's estimated climate sensitivity of 3.26 °C is much too high. Using the same methodology and the same physical effect parameters as did the IPCC, but estimating his own values of those parameters, Monckton obtains a climate sensitivity of 0.58 °C. In other words, Monckton predicts that when the atmospheric CO<sub>2</sub> concentration reaches 556 ppm (expected later this century), the Earth's surface temperature will be only 0.58 °C higher than it was in 1750.
 
Monckton documents and references his calculations in much detail. Although Monckton is a journalist, his work should not be discounted simply because it has not been published in a peer-reviewed scientific journal.

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