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The global warming controversy is a dispute regarding the nature, causes, and consequences of global warming. The disputed issues include the causes of increased global average air temperature, especially since the mid-20th century, whether this warming trend is unprecedented or within normal climatic variations, and whether the increase is wholly or partially an artifact of poor measurements. Additional disputes concern estimates of climate sensitivity, predictions of additional warming, what the consequences are, and what action should be taken (if any). The debate is vigorous in the popular media and on a policy level, with individuals, corporations, and political organizations all being involved.

History of public opinion

In the European Union, global warming has been a prominent and sustained issue. All European Union member states ratified the 1997 Kyoto Protocol, and many European countries had already been taking action to reduce greenhouse gas emissions prior to 1990 (for example, Margaret Thatcher advocated action against man-made climate change in 1988¹ and Germany started to take action after the Green Party took seats in Parliament in 1983). Both "global warming" and the more politically neutral "climate change" were listed by the Global Language Monitor as political buzzwords or catch phrases in 2005.² In Europe, the notion of human influence on climate gained wide acceptance more rapidly than in many other parts of the world, most notably the United States.³⁴

There has been a debate among public commentators about how much weight and media coverage should be given to each side of the controversy. Andrew Neil of the BBC stated that "There's a great danger that on some issues we're becoming a one-party state in which we're meant to have only one kind of view. You don't have to be a climate-change denier to recognise that there's a great range of opinion on the subject."⁵

The table below shows how public perceptions about the existence and importance of global warming have changed in the U.S. and elsewhere.⁶⁷⁸⁹

A June 2007 Ipsos Mori poll conducted in the UK found 56 percent of respondents believed scientists were still questioning climate change. The survey suggested that terrorism, graffiti and crime were all of more concern than climate change. Ipsos Mori's head of environmental research, Phil Downing, said people had been influenced by counter-arguments.¹⁰

The Canadian science broadcaster and environmental activist, David Suzuki, reports that focus groups organized by the David Suzuki Foundation showed the public has a poor understanding of the science behind global warming.¹¹ This is despite recent publicity through different means, including the films An Inconvenient Truth and The 11th Hour.

An example of the poor understanding is public confusion between global warming and ozone depletion or other environmental problems.¹²¹³

A 15-nation poll conducted in 2006 by Pew Global found that there "is a substantial gap in concern over global warming – roughly two-thirds of Japanese (66%) and Indians (65%) say they personally worry a great deal about global warming. Roughly half of the populations of Spain (51%) and France (46%) also express great concern over global warming, based on those who have heard about the issue. But there is no evidence of alarm over global warming in either the United States or China – the two largest producers of greenhouse gases. Just 19% of Americans and 20% of the Chinese who have heard of the issue say they worry a lot about global warming – the lowest percentages in the 15 countries surveyed. Moreover, nearly half of Americans (47%) and somewhat fewer Chinese (37%) express little or no concern about the problem."¹⁴ A 47-nation poll conducted in 2007 found that "Substantial majorities 25 of 37 countries say global warming is a 'very serious' problem".¹⁵

Controversy concerning the science

Existence of a scientific consensus

Main article: Scientific opinion on climate change

Environmental groups, many governmental reports, and the non-U.S. media often state that there is virtually unanimous agreement in the scientific community in support of human-caused global warming, although there is less agreement on the specific consequences of this warming. Opponents either maintain that most scientists consider global warming "unproved," dismiss it altogether, or highlight the dangers of focusing on only one viewpoint in the context of unsettled science.¹⁶¹⁷¹⁸ Others maintain that either proponents or opponents have been stifled or driven underground.¹⁹

The majority of climate scientists agree that global warming is primarily caused by human activities such as fossil fuel burning and deforestation.^20212223 The conclusion that global warming is mainly caused by human activity and will continue if greenhouse gas emissions are not reduced has been endorsed by more than 50 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries. The U.S. National Academy of Sciences,²⁴ the American Association for the Advancement of Science,²⁵ and the Joint Science Academies of the major industrialized and developing nations²⁶ explicitly use the word "consensus" when referring to this conclusion.

A 2004 essay by Naomi Oreskes in the journal Science reported a survey of 928 abstracts of peer-reviewed papers related to global climate change in the ISI database.²⁷ Oreskes claimed that "Remarkably, none of the papers disagreed with the consensus position. ... This analysis shows that scientists publishing in the peer-reviewed literature agree with IPCC, the National Academy of Sciences, and the public statements of their professional societies." Benny Peiser claimed to have found flaws in Oreskes' work,²⁸ but his attempted refutation is disputed²⁹³⁰³¹ and has not been published in a peer-reviewed journal. Peiser later withdrew parts of his criticism,³² also commenting that "the overwhelming majority of climatologists is agreed that the current warming period is mostly due to human impact. However, this majority consensus is far from unanimous."³⁰³³

A 2006 op-ed by Richard Lindzen in The Wall Street Journal challenged the claim that scientific consensus had been reached, and listed the Science journal study as well as other sources, including the IPCC and NAS reports, as part of "an intense effort to suggest that the theoretically expected contribution from additional carbon dioxide has actually been detected."³⁴ Lindzen wrote in The Wall Street Journal on April 12, 2006,³⁵

Similarly, Timothy Ball asserts that skeptics have gone underground for "job security and fear of reprisals. Even in University, where free speech and challenge to prevailing wisdoms are supposedly encouraged, academics remain silent."³⁶

At least one survey of the scientific community has found the opposite problem -- New Scientist notes that in surveys a much larger fraction of U.S. scientists consistently state that they are pressured by their employers or by U.S. government bodies to deny that global warming results from human activities¹⁹ or risk losing funding.

In response to claims of a consensus on global warming, some skeptics have compared the theory to a religion,³⁷³⁸³⁹ to scientific support for the eugenics movement,⁴⁰⁴¹ and to discredited scientific theories such as phlogiston⁴² and miasma.⁴³

In 2008, Fergus Brown, Roger A. Pielke and James Annan submitted a paper titled "Is there agreement amongst climate scientists on the IPCC AR4 WG1?"⁴⁴ It was rejected for publication by the AGU publication EOS and Nature Precedings. Pielke writes: “From this experience, it is clear that the AGU EOS and Nature Precedings Editors are using their positions to suppress evidence that there is more diversity of views on climate, and the human role in altering climate, than is represented in the narrowly focused 2007 IPCC report.”⁴⁵

Heartland Institute's list

On April 29, 2008, environmental journalist Richard Littlemore revealed that a list of "500 Scientists with Documented Doubts of Man-Made Global Warming Scares"⁴⁶ propagated by the Heartland Institute included at least 45 scientists who neither knew of their inclusion as "coauthors" of the article, nor agreed with its contents.⁴⁷ Many of the scientists asked the Heartland Institute to remove their names from the list; for instance, Gregory Cutter from the Old Dominion University was reported by Littlemore as saying,

However, the Heartland Institute refused to remove any names from the list. In a statement on May 5, 2008, Institute CEO Joseph Bast said that the title of the September 14, 2007 news release announcing the list had been changed to "500 Scientists Whose Research Contradicts Man-Made Global Warming Scares."⁴⁸ In the same statement, Bast also charged that the outraged scientists:

Bast also stated that:

Petitions

In 1997, the “World Scientists Call For Action” petition was presented to world leaders meeting to negotiate the Kyoto Protocol. The declaration asserted, “A broad consensus among the world's climatologists is that there is now ‘a discernible human influence on global climate.’" It urged governments to make “legally binding commitments to reduce industrial nations' emissions of heat-trapping gases”, and called global warming “one of the most serious threats to the planet and to future generations.”⁴⁹ The petition was conceived by the Union of Concerned Scientists as a follow up to their 1992 World Scientists' Warning to Humanity, and was signed by “more than 1,500 of the world's most distinguished senior scientists, including the majority of Nobel laureates in science”⁵⁰⁵¹

To support his claim of a lack of consensus, the website of prominent skeptic Fred Singer's Science and Environmental Policy Project (SEPP) lists four petitions. According to SEPP, these petitions show that "the number of scientists refuting global warming is growing."⁵² The petitions are:

• The 1992 "Statement by Atmospheric Scientists on Greenhouse Warming," signed by 47 scientists, claims "such policy initiatives [those concerning the Earth Summit scheduled to convene in Brazil in June 1992] derive from highly uncertain scientific theories. They are based on the unsupported assumption that catastrophic global warming follows from the burning of fossil fuels and requires immediate action. We do not agree."⁵³
• The "Heidelberg Appeal" (also from 1992), signed by over 4000 scientists including 72 Nobel Prize winners.⁵⁴ This appeal makes no mention of climate change or any other specific environmental issue, but is essentially a plea for policy based on "scientific criteria and not on irrational preconceptions".
• Singer's "Leipzig Declaration on Global Climate Change" (1995 and 1997). Critics point out that most of the signatories lack credentials in the specific field of climate research or even physical science in general.⁵⁵ Followup interviews found at least twelve signers who denied having signed the Declaration or had never heard of it.⁵⁶
• The "Oregon Petition", self-signed and unverified by third party, was started in 1998 by physicist Frederick Seitz, past president of the United States National Academy of Sciences. The identical petition card was circulated again in late 2007 and Arthur B. Robinson presented the petition of 31,000 claimed signatories in Washington DC on May 19, 2008.⁵⁷ Critics point out that many of the signatories of the petition lack a background in climate-related sciences⁵⁸ and that the petition itself mentions only "catastrophic heating" and not the broader issue of global warming. The petition's website claims that all of the 31,000 signatories are qualified scientists with "technical training suitable for the evaluation of the relevant research data."⁵⁹ However, anyone with a degree was entitled to sign the list and this would therefore include many who are not qualified to evaluate the complex data and modelling involved.⁶⁰

In April 2006, a group describing itself as "sixty scientists" signed an open letter⁶¹ to the Canadian Prime Minister Stephen Harper to ask that he revisit the science of global warming and "Open Kyoto to debate." As with the earlier statements, critics pointed out that many of the signatories were non-scientists or lacked relevant scientific backgrounds.⁶² For example, the group included David Wojick, a journalist, and Benny Peiser, a social anthropologist. More than half the signatories cited past or emeritus positions as their main appointments. Only two (Richard Lindzen and Roy Spencer) indicated current appointments in a university department or a recognized research institute in climate science.⁶³ One of the signatories has since publicly recanted, stating that his signature was obtained by deception regarding the content of the letter.⁶⁴ In response shortly afterward another open letter to Prime Minister Harper endorsing the IPCC report and calling for action on climate change was prepared by Gordon McBean and signed by 90 Canadian climate scientists initially, plus 30 more who endorsed it after its release.⁶⁵⁶⁶

The IPCC

Main article: Intergovernmental Panel on Climate Change

Statements Agreeing with the IPCC Positions

A joint statement issued by the Australian Academy of Sciences, Royal Flemish Academy of Belgium for Sciences and the Arts, Brazilian Academy of Sciences, Royal Society of Canada, Caribbean Academy of Sciences, Chinese Academy of Sciences, French Academy of Sciences, German Academy of Natural Scientists Leopoldina, Indian National Science Academy, Indonesian Academy of Sciences, Royal Irish Academy, Accademia Nazionale dei Lincei (Italy), Academy of Sciences Malaysia, Academy Council of the Royal Society of New Zealand, Royal Swedish Academy of Sciences, and Royal Society (UK) said:

The work of the Intergovernmental Panel on Climate Change (IPCC) represents the consensus of the international scientific community on climate change science. We recognise IPCC as the world’s most reliable source of information on climate change and its causes, and we endorse its method of achieving this consensus. Despite increasing consensus on the science underpinning predictions of global climate change, doubts have been expressed recently about the need to mitigate the risks posed by global climate change. We do not consider such doubts justified.⁶⁷

Many other science academies and scientific organizations support the conclusions of the IPCC.

In Naomi Oreskes's talk The American Denial of Global Warming,⁶⁸ Oreskes recounted the following incident:

Statements Disagreeing with the IPCC Positions

The work of the IPCC has attracted controversy and criticism, including some from experts invited by the IPCC to submit reports or serve on its panels.⁶⁹

In blog posts, Roger A. Pielke contends that the IPCC distorted the evidence by not including scientific results that questioned anthropogenic global warming.⁷⁰ These criticisms have been described as "failed" by William Connolley.⁷¹⁷² Pielke also perceived a conflict of interest in the IPCC assessment process, since the "same individuals who are doing primary research in the role of humans on the climate system are then permitted to lead the assessment! ... Assessment Committees should not be an opportunity for members to highlight their own research."⁷⁰ There is no obvious solution to this problem, since scientists with sufficient knowledge of the field to serve on the IPCC and scientists who have written noteworthy papers in the field are essentially the same group.⁷²

Stephen McIntyre said in his blog that portions of the report were based on in-press data. When he attempted to obtain this data from the authors, the IPCC told him he could not use his reviewer status to obtain in-press data outside the normal journal review process⁷³

Christopher Landsea, a hurricane researcher, said of "the part of the IPCC to which my expertise is relevant" that "I personally cannot in good faith continue to contribute to a process that I view as both being motivated by pre-conceived agendas and being scientifically unsound",⁷⁴ because of comments made at a press conference by Kevin Trenberth of which Landsea disapproved. Trenberth said that "Landsea's comments were not correct";⁷⁵ the IPCC replied that "individual scientists can do what they wish in their own rights, as long as they are not saying anything on behalf of the IPCC" and offered to include Landsea in the review phase of the AR4.⁷⁶ Roger Pielke, Jr. commented that "Both Landsea and Trenberth can and should feel vindicated... the IPCC accurately reported the state of scientific understandings of tropical cyclones and climate change in its recent summary for policy makers".⁷⁵

In 2005, the House of Lords Economics Committee wrote that "We have some concerns about the objectivity of the IPCC process, with some of its emissions scenarios and summary documentation apparently influenced by political considerations." It doubted the high emission scenarios and its "played-down" positive aspects of global warming.⁷⁷ The main claims of the House of Lords Economics Committee were rejected in the response made by the United Kingdom government⁷⁸ and by the Stern Review.

John Christy, an IPCC lead author and global warming skeptic, wrote that contributing authors and reviewers have little influence, so that "to say that 800 contributing authors or 2,000 reviewers reached consensus on anything describes a situation that is not reality."⁷⁹

While some critics have argued that the IPCC overstates likely global warming, others have made the opposite criticism. David Biello, writing in the Scientific American, argues that, because of the need to secure consensus among governmental representatives, the IPCC reports give conservative estimates of the likely extent and effects of global warming.⁸⁰ Climate scientist James Hansen argues that the IPCC's conservativeness seriously underestimates the risk of sea-level rise on the order of meters—enough to inundate many low-lying areas, such as the southern third of Florida.⁸¹. Roger A. Pielke Sr. has also stated that "Humans are significantly altering the global climate, but in a variety of diverse ways beyond the radiative effect of carbon dioxide. The IPCC assessments have been too conservative in recognizing the importance of these human climate forcings as they alter regional and global climate.".[7]

Causes

Attribution to greenhouse gases

Attribution of recent climate change discusses how global warming is attributed to anthropogenic GHGs. Correlation of CO₂ and temperature is not part of this evidence. Nonetheless, one argument against anthropogenic global warming claims that rising levels of carbon dioxide (CO₂) and other greenhouse gases (GHGs) do not correlate with global warming.⁸²

Reproduction of the temperature record using historical forcings

• General circulation models and basic physical considerations predict that in the tropics the temperature of the troposphere should increase more rapidly than the temperature of the surface. Models and observations agree on this amplification for monthly and interannual time scales but not for decadal time scales in most observed data sets. It is uncertain whether the discrepancy is attributable to deficiencies in model formulation, biases in the observations, or both. The present view is that because of large uncertainties in observed tropospheric temperature trends along with other evidence for tropospheric warming (such as the increasing height of the tropopause), the more likely explanation is observational bias.⁸³ Furthermore, if greenhouse gases were causing the climate warming then scientists would expect the troposphere to be warming faster than the surface, but observations do not bear this out.⁸⁴ Satellite temperature measurements show that tropospheric temperatures are increasing with "rates similar to those of the surface temperature," leading the IPCC to conclude that this discrepancy is reconciled.⁸⁵
• Studies of ice cores show that carbon dioxide levels rise and fall with or after (as much as 1000 years) temperature variations.⁸⁶ This argument assumes that current climate change can be expected to be similar to past climate change. While it is generally agreed that variations before the industrial age are mostly timed by astronomical forcing,⁸⁷ the current variations, of whatever size, are claimed to be timed by anthropogenic releases of CO₂ (thus returning the argument to the importance of human CO₂ emissions). Analysis of carbon isotopes in atmospheric CO₂ shows that the recent observed CO₂ increase cannot have come from the oceans, volcanoes, or the biosphere, and thus is not a response to rising temperatures as would be required if the same processes creating past lags were active now.⁸⁸
• Between 1940 and 1970, global temperatures went down slightly, even though carbon dioxide levels went up. This has been attributed to the cooling effect of sulphate aerosols.⁸⁹⁹⁰
• Carbon dioxide accounts for about 383 parts per million by volume (ppm) of the Earth's atmosphere, increasing from 278 ppm in the 1880s to over 380 ppm in 2005. Carbon dioxide causes between 9 and 26% of the natural greenhouse effect.
• Seth Young, a doctoral student in earth sciences at Ohio State, presented his analysis that the Earth had a much higher level of CO₂ within the atmosphere during an ice age. In the Ordovician period of the Paleozoic era, he states the Earth had an atmospheric CO₂ concentration estimated at 4400ppm⁹¹ (or 0.44% of the atmosphere). However, a recent study suggests the Ordovician period began with a reduction in CO₂.⁹²

As noted above, climate models are only able to simulate the temperature record of the past century when GHG forcing is included, being consistent with the findings of the IPCC which has stated that: "Greenhouse gas forcing, largely the result of human activities, has very likely caused most of the observed global warming over the last 50 years"⁹³ (See also: attribution of recent climate change.)

Alternate hypotheses

See also: Solar variation theory

400 year history of sunspot numbers.

Last 30 years of solar variability.

Scientists opposing the mainstream scientific assessment of global warming express varied opinions concerning the cause of global warming. Some say only that it has not yet been ascertained whether humans are the primary cause of global warming (e.g., Balling, Lindzen, and Spencer). Others attribute global warming to natural variation (e.g., Soon and Baliunas), ocean currents (e.g., Gray), increased solar activity (e.g., Shaviv and Veizer), cosmic rays (e.g., Svensmark), or unknown natural causes (e.g., Leroux).

A few studies claim that the present level of solar activity is historically high as determined by sunspot activity and other factors. Solar activity could affect climate either by variation in the Sun's output or, more speculatively, by an indirect effect on the amount of cloud formation. Solanki and co-workers suggest that solar activity for the last 60 to 70 years may be at its highest level in 8,000 years; Muscheler et al. disagree, suggesting that other comparably high levels of activity have occurred several times in the last few thousand years.⁹⁴ Both Muscheler et al. and Solanki et al. conclude that "solar activity reconstructions tell us that only a minor fraction of the recent global warming can be explained by the variable Sun."⁹⁵⁹⁶

Another point of controversy is the correlation of temperature with solar variation.⁹⁷

Solar physicists Mike Lockwood and Claus Fröhlich reject the claim that the warming observed in the global mean surface temperature record since about 1850 is the result of solar variations.⁹⁸ Lockwood and Fröhlich conclude that:

There are many interesting palaeoclimate studies that suggest that solar variability had an influence on pre-industrial climate. There are also some detection–attribution studies using global climate models that suggest there was a detectable influence of solar variability in the first half of the twentieth century and that the solar radiative forcing variations were amplified by some mechanism that is, as yet, unknown. However, these findings are not relevant to any debates about modern climate change. Our results show that the observed rapid rise in global mean temperatures seen after 1985 cannot be ascribed to solar variability, whichever of the mechanisms is invoked and no matter how much the solar variation is amplified.

Svensmark and Friis-Christensen dispute this in a recent reply⁹⁹ arguing that tropospheric air temperature records, as opposed to the surface air temperature data used by Lockwood and Fröhlich, do show a significant negative correlation between cosmic-ray flux and air temperatures up to 2006. A linear warming trend of about 0.14 K/decade is however left unaccounted for. As of October 2007, this reply has not yet been published in a peer-reviewed journal.

The consensus position (as represented for example by the IPCC Fourth Assessment Report) says that solar radiation may have increased by 0.12 W/m² since 1750, compared to 1.6 W/m² for the net anthropogenic forcing.¹⁰⁰ The TAR said, "The combined change in radiative forcing of the two major natural factors (solar variation and volcanic aerosols) is estimated to be negative for the past two, and possibly the past four, decades."¹⁰¹ The AR4 makes no direct assertions on the recent role of solar forcing, but the previous statement is consistent with the AR4's figure 4.

Modification by lower aerosols

The "pause" in warming from the 1940s to 1960's is generally attributed to aerosol forcing, which acts to cool the climate. More recently, this forcing has (relatively) declined, which may have enhanced warming, though the effect is regionally varying. See global dimming. Another example of this is Ruckstuhl et al.(2008) who found a 60% reduction in aerosol concentrations over Europe causing solar brightening.¹⁰²

. . .the direct aerosol effect had an approximately five times larger impact on climate forcing than the indirect aerosol and other cloud effects. The overall aerosol and cloud induced surface climate forcing is ~+1 W m−2 dec−1 and has most probably strongly contributed to the recent rapid warming in Europe.

Instrumental temperature record

Main articles: Instrumental temperature record and Urban heat island

Skeptics have questioned the accuracy of the instrumental temperature record on the basis of the urban heat island effect, the quality of the surface station network and what they view as unwarranted adjustments to the temperature record.

Urban heat island

Skeptics contend that stations located in more populated areas could show warming due to increased heat generated by cities, rather than a global temperature rise^{citation needed}. The IPCC Third Assessment Report acknowledges that the urban heat island is an important local effect, but cites analyses of historical data indicating that the effect of the urban heat island on the global temperature trend is no more than 0.05 °C (0.09 °F) degrees through 1990.¹⁰³ More recently, Peterson (2003) found no difference between the warming observed in urban and rural areas.¹⁰⁴ Stephen McIntyre analyzed Peterson's raw data. He claimed to find "actual cities have a very substantial trend of over 2 °C per century relative to the rural network—and this assumes that there are no problems with rural network—something that is obviously not true since there are undoubtedly microsite and other problems."¹⁰⁵ McIntyre has not published his results in a peer-reviewed journal.

Parker (2006) found that there was no difference in warming between calm and windy nights. Since the urban heat island effect is strongest for calm nights and is weak or absent on windy nights, this was taken as evidence that global temperature trends are not significantly contaminated by urban effects.¹⁰⁶ Pielke and Matsui published a paper disagreeing with Parker's conclusions.¹⁰⁷

Surface station siting and adjustments

More recently, Roger A. Pielke and Stephen McIntyre have criticized the US instrumental temperature record and adjustments to it, and Pielke and others have criticized the poor quality siting of a number of weather stations in the United States.¹⁰⁸¹⁰⁹ In response, Anthony Watts began a volunteer effort to photographically document the siting quality of these stations.¹¹⁰ Based on the work of Watts, Stephen McIntyre has completed a reconstruction of U.S. temp history using only those weather stations identified so far as meeting the requirements to be CRN level 1 (excellent) or level 2 (good) stations. The higher quality stations indicate the warmest years in the U.S. were 1934 and 1921, followed by 1998 and 2006.¹¹¹ McIntyre has made all of his methods, data and code available for others to reproduce his findings. McIntyre's analysis has not been published in the peer-reviewed literature.

Estimates of climate sensitivity

Equilibrium climate sensitivity refers to the equilibrium change in global mean surface temperature following a doubling of the atmospheric (equivalent) CO2 concentration. This value is estimated by the IPCC Fourth Assessment Report as “likely to be in the range 2 to 4.5 °C with a best estimate of about 3 °C.”

Using a combination of surface temperature history and ocean heat content, Stephen E. Schwartz has proposed an estimate of climate sensitivity of 1.9 ± 1.0 K for doubled CO2. ¹¹², revised upwards from 1.1 ± 0.5 K ¹¹³. Grant Foster, James Annan, Gavin Schmidt, and Michael E. Mann¹¹⁴¹¹⁵ argue that there are errors in both versions of Schwartz's analysis. Astronomer Nir Shaviv also has proposed a low value for climate sensitivity.¹¹⁶¹¹⁷ Petr Chylek and co-authors have also proposed low climate sensitivity to doubled CO2, estimated to be 1.6 K ± 0.4 K.¹¹⁸

Stabilizing "Infrared Iris" Effect

Richard Lindzen proposed an Infrared Iris hypothesis of compensating meteorological processes that tend to stabilize climate change.¹¹⁹ Roy Spencer et al. discovered "a net reduction in radiative input into the ocean-atmosphere system" in tropical intraseasonal oscillations that "may potentially support" the idea of an "Iris" effect, although they point out that their work is concerned with much shorter time scales.¹²⁰ If confirmed, this effect might reduce the positive "amplifying" feedback assumed in climate models.

Internal Radiative Forcing

Roy Spencer hypothesizes there is an "Internal Radiative Forcing" affecting climate variability,¹²¹¹²²

" . . .mixing up of cause and effect when observing natural climate variability can lead to the mistaken conclusion that the climate system is more sensitive to greenhouse gas emissions than it really is. . . . it provides a quantitative mechanism for the (minority) view that global warming is mostly a manifestation of natural internal climate variability."

". . .low frequency, internal radiative forcing amounting to little more than 1 W m-2, assumed to be proportional to a weighted average of the Southern Oscillation and Pacific Decadal Oscillation indices since 1900, produces ocean temperature behavior similar to that observed: warming from 1900 to 1940, then slight cooling through the 1970s, then resumed warming up to the present, as well as 70% of the observed centennial temperature trend."

Predictions of greenhouse gas rises

The "standard" set of scenarios for future atmospheric greenhouse gases are the IPCC SRES scenarios. The purpose of the range of scenarios is not to predict what exact course the future of emissions will take, but what it may take under a range of possible population, economic and societal trends.¹²³ Climate models can be run using any of the scenarios as inputs to illustrate the different outcomes for climate change. No one scenario is officially preferred, but in practice the "A1b" scenario roughly corresponding to 1%/year growth in atmospheric CO₂ is often used for modelling studies.

There is debate about the various scenarios for fossil fuel consumption. Global warming skeptic Fred Singer stated that "some good experts believe" that atmospheric CO₂ concentration will not double since economies are becoming less reliant on carbon.¹²⁴

However, The Stern report,¹²⁵ like many other reports, notes the past correlation between CO₂ emissions and economic growth and then extrapolates using a "business as usual" scenario to predict GDP growth and hence CO₂ levels, concluding that:

According to Lawrence Livermore National Laboratory, "the earth would warm by 8 degrees Celsius (14.4 degrees Fahrenheit) if humans use the entire planet’s available fossil fuels by the year 2300."¹²⁶

Predictions of temperature rises

Conventional predictions of future temperature rises depend on estimates of future GHG emissions (see SRES) and the climate sensitivity. Models referenced by the Intergovernmental Panel on Climate Change (IPCC) predict that global temperatures are likely to increase by 1.1 to 6.4 °C (2.0 to 11.5 °F) between 1990 and 2100. Others have proposed that temperature increases may be higher than IPCC estimates. One theory is that the climate may reach a "tipping point" where positive feedback effects lead to runaway global warming; such feedbacks include decreased reflection of solar radiation as sea ice melts, exposing darker seawater, and the potential release of large volumes of methane from thawing permafrost.¹²⁷

Some scientists, such as David Orrell or Henk Tennekes, say that climate change cannot be accurately predicted. Orrell says that the range of future increase in temperature suggested by the IPCC rather represents a social consensus in the climate community, but adds that "we are having a dangerous effect on the climate".¹²⁸

A 2007 study by David Douglass and coworkers concluded that the 22 most commonly used global climate models used by the IPCC were unable to accurately predict accelerated warming in the troposphere when tuned to match actual surface warming, concluding that "projections of future climate based on these models should be viewed with much caution." This result contrasts a similar study of 19 models which found that discrepancies between model predictions and actual temperature were likely due to measurement errors.¹²⁹

Confidence in GCM forecasts

The IPCC states it has increased confidence in forecasts coming from General Circulation Models or GCMs. Chapter 8 of AR4 reads:

There is considerable confidence that climate models provide credible quantitative estimates of future climate change, particularly at continental scales and above. This confidence comes from the foundation of the models in accepted physical principles and from their ability to reproduce observed features of current climate and past climate changes. Confidence in model estimates is higher for some climate variables (e.g., temperature) than for others (e.g., precipitation). Over several decades of development, models have consistently provided a robust and unambiguous picture of signifi cant climate warming in response to increasing greenhouse gases.¹³⁰

Certain scientists, skeptics and otherwise, believe this confidence in the models’ ability to predict future climate is not earned.¹³¹¹³² A recent peer-reviewed article has assessed the performance of global climate models by comparing their outputs to historical time series on the local level and concludes "At the annual and the climatic (30-year) scales, GCM interpolated series are irrelevant to reality."¹³³

Computer models versus Evidence-based forecasting

Climate models are systems of differential equations based on the basic laws of physics, fluid motion, and chemistry. To “run” a model, scientists divide the planet into a 3-dimensional grid, apply the basic equations, and evaluate the results. The atmospheric component of the model calculates winds, heat transfer, radiation, relative humidity, surface hydrology, and surface fluxes of heat and moisture within each grid and evaluates interactions with neighboring points. The ocean component calculates currents, heat content and salinity. The atmospheric and oceanic components interact, for example with evaporation from the oceans into the atmosphere and with atmospheric winds affecting ocean currents. Different models vary in such basics as grid size and therefore do not give the same results.

Kesten Green and J. Scott Armstrong have criticized the validity of model projections of future climate, arguing "Advocates of complex climate models claim that they are based on well-established laws of physics. But there is clearly much more to the models than physical laws, otherwise the models would all produce the same output, which they do not, and there would be no need for confidence estimates for model forecasts, which there certainly is. Climate models are, in effect, mathematical ways for experts to express their opinions."¹³⁴ Green and Armstrong contend that the possibility of accurate long-term climate forecasts has never been proven, and argue that simple methods always outperform more complex forecasting methods.¹³⁴ The work of Green and Armstrong has been criticized for showing insufficient domain knowledge to evaluate their own criteria and for failing to distinguish between forecasts based on past experience and projections based on physical models.¹³⁵¹³⁶

The poles

Arctic sea ice

Main article: Arctic shrinkage

Arctic Sea ice as of 2007 compared to 2005 and also compared to 1979-2000 average

Northern Hemisphere ice trends

One unsettled question related to temperature rises is if or when the Arctic sea may become ice-free in the summer (winter sea ice remains in all scenarios). Arctic specialist Mark Serreze said, following the record low in 2007,¹³⁷ "If you asked me a couple of years ago when the Arctic could lose all of its ice then I would have said 2100, or 2070 maybe. But now I think that 2030 is a reasonable estimate."¹³⁸

However, a 2003 paper in Nature claims that computer models predictions poorly represent observed changes in Arctic sea ice:¹³⁹

Roger A. Pielke claims melting Arctic sea ice is a result of regional warming and not global warming.¹⁴⁰

Antarctic cooling

Main article: Antarctica cooling controversy

Antarctic Skin Temperature Trends between 1981 and 2007, based on thermal infrared observations made by a series of NOAA satellite sensors. Skin temperature trends do not necessarily reflect air temperature trends.

The Antarctica cooling controversy relates to the question of whether or not current temperature trends in Antarctica contradict or cast doubt on the theory of global warming. Observations unambiguously show the Peninsula to be warming. The trends elsewhere show both warming and cooling but are smaller and dependent on season and the timespan over which the trend is computed.¹⁴³¹⁴⁴ Climate models predict that future trends in Antarctica are much smaller than in the Arctic.¹⁴⁵

To the extent that a controversy exists it is confined to the popular press and blogs; there is no evidence of a related controversy within the scientific community. Various skeptics, most notably Michael Crichton,¹⁴⁶ have asserted the findings of Doran et al.¹⁴⁷ contradict global warming. Peter Doran, the lead author of the paper, stated that "... our results have been misused as "evidence" against global warming by Crichton in his novel 'State of Fear'..."¹⁴⁸ Others, for example RealClimate, agree there is no contradiction.¹⁴⁹

Dispute over data archiving and sharing

Scientific journals and funding agencies generally require authors of peer-reviewed research to archive all of the data necessary to reproduce their research. If another scientist attempts to reproduce the research and needs additional data, authors are expected (with few exceptions) to provide the data, metadata, methods and source code that may be necessary.

Dr David Legates has claimed that Mann, Bradley and Hughes 1998, famous for its hockey stick shaped historic temperature reconstruction, serves as an example of climate scientists not abiding by these policies and suggested that legislators might ultimately take action to enforce them.¹⁵⁰

Political, economic, and social aspects of the controversy

See also: Politics of global warming and Economics of global warming

Washington Monument illuminated with a message criticizing American environmental policy

In the U.S. global warming is often a partisan political issue. Republicans tend to oppose action against a threat that they regard as unproved, while Democrats tend to support actions that they believe will reduce global warming and its effects through the control of greenhouse gas emissions.^{151dead link} Recently, bipartisan measures have been introduced.¹⁵²

Kevin E. Trenberth stated:

As more evidence has become available over the existence of global warming debate has moved to further controversial issues, including:

1. The social and environmental impacts
2. The appropriate response to climate change
3. Whether decisions require less uncertainty

The single largest issue is the importance of a few degrees rise in temperature:

The other point that leads to major controversy—because it could have significant economic impacts—is whether action (usually, restrictions on the use of fossil fuels to reduce carbon-dioxide emissions) should be taken now, or in the near future; and whether those restrictions would have any meaningful effect on global temperature.^{citation needed}

Due to the economic ramifications of such restrictions, there are those, including the Cato Institute, a libertarian think tank, who feel strongly that the negative economic effects of emission controls outweigh the environmental benefits.¹⁵⁵ They claim that even if global warming is caused solely by the burning of fossil fuels, restricting their use would have more damaging effects on the world economy than the increases in global temperature.¹⁵⁶

Conversely, others feel strongly that early action to reduce emissions would help avoid much greater economic costs later, and would reduce the risk of catastrophic, irreversible change.¹⁵⁷ In his December 2006 book, Hell and High Water, energy technology expert Joseph J. Romm

Ultimately, however, a strictly economic argument for or against action on climate change is limited at best, failing to take into consideration other potential impacts of any change.

Kyoto Protocol

Main article: Kyoto Protocol

The Kyoto protocol is the most prominent international agreement on climate change, and is also highly controversial. Some argue that it goes too far¹⁵⁹ or not nearly far enough¹⁶⁰ in restricting emissions of greenhouse gases. Another area of controversy is the fact that China and India, the world's two most populous countries, both ratified the protocol but are not required to reduce or even limit the growth of carbon emissions under the present agreement even though when listed by greenhouse gas emissions per capita, they have rankings of 121st largest per capita emitter at 3.9 Tonnes of CO₂e and 162nd largest per capita emitter at 1.8 Tonnes of CO₂e respectively, compared with for example the USA at position of the 14th largest per capita CO₂e emitter at 22.9 Tonnes of CO₂e. Nevertheless, China is the world's second largest producer of greenhouse gas emissions, and India 4th (see: countries by greenhouse emissions). Various predictions see China overtaking the US in total greenhouse emissions between late 2007 and 2010,^161162163 and according to many other estimates, this already occurred in 2006.^164165166

Additionally, high costs of decreasing emissions may cause significant production to move to countries that are not covered under the treaty, such as India and China, claims Fred Singer.¹⁶⁷ As these countries are less energy efficient, this scenario is claimed to cause additional carbon emissions.

The only major developed nation which has signed but not ratified the Kyoto protocol is the USA (see signatories). The countries with no official position on Kyoto are mainly African countries with underdeveloped scientific infrastructure or are oil producers^{citation needed}.

Funding for partisans

Both sides of the controversy have alleged that access to funding has played a role in the willingness of credentialed experts to speak out.

Funding for scientists who do not acknowledge anthropogenic global warming

Several skeptical scientists—Fred Singer, Fred Seitz and Patrick Michaels—have been linked to organizations funded by ExxonMobil and Philip Morris for the purpose of promoting global warming skepticism (see section: Risks of passive smoking). Similarly, groups employing global warming skeptics, such as the George C. Marshall Institute, have been criticized for their ties to fossil fuel companies.¹⁶⁸

On February 2, 2007, The Guardian stated¹⁶⁹¹⁷⁰ that Kenneth Green, a Visiting Scholar with AEI, had sent letters¹⁷¹ to scientists in the UK and the U.S., offering US$10,000 plus travel expenses and other incidental payments in return for essays with the purpose of "highlight[ing] the strengths and weaknesses of the IPCC process," specifically regarding the IPCC Fourth Assessment Report.

A furor was raised when it was revealed that the Intermountain Rural Electric Association (an energy cooperative that draws a significant portion of its electricity from coal-burning plants) donated $100,000 to Patrick Michaels and his group, New Hope Environmental Services, and solicited additional private donations from its members.^172173174

The Union of Concerned Scientists have produced a report titled 'Smoke, Mirrors & Hot Air',¹⁷⁵ that criticizes ExxonMobil for "underwriting the most sophisticated and most successful disinformation campaign since the tobacco industry" and for "funnelling about $16 million between 1998 and 2005 to a network of ideological and advocacy organizations that manufacture uncertainty on the issue." In 2006 Exxon claimed that it was no longer going to fund these groups¹⁷⁶ though that claim has been challenged by Greenpeace.¹⁷⁷

The Center for the Study of Carbon Dioxide and Global Change, a skeptic group, when confronted about the funding of a video they put together ($250,000 for "The Greening of Planet Earth" from an oil company) stated, "We applaud Western Fuels for their willingness to publicize a side of the story that we believe to be far more correct than what at one time was 'generally accepted.' But does this mean that they fund The Center? Maybe it means that we fund them!"¹⁷⁸

Donald Kennedy, editor-in-chief of Science, has said that skeptics such as Michaels are lobbyists more than researchers, and that "I don't think it's unethical any more than most lobbying is unethical," he said. He said donations to skeptics amounts to "trying to get a political message across."¹⁷⁹

Funding for scientists who acknowledge anthropogenic global warming

A number of global warming skeptics, such as the following, assert that grant money is given preferentially to supporters of global warming theory. Atmospheric scientist Reid Bryson said in June 2007 that "There is a lot of money to be made in this... If you want to be an eminent scientist you have to have a lot of grad students and a lot of grants. You can't get grants unless you say, 'Oh global warming, yes, yes, carbon dioxide.'"¹⁸⁰ Similar claims have been advanced by climatologist Marcel Leroux,¹⁸¹ NASA's Roy Spencer, climatologist and IPCC contributor John Christy, University of London biogeographer Philip Stott,¹⁸² and Accuracy in Media.¹⁸³

Richard Lindzen, the Alfred P. Sloan Professor of Meteorology at MIT, makes the specific claim that "[in] the winter of 1989 Reginald Newell, a professor of meteorology [at MIT], lost National Science Foundation funding for data analyses that were failing to show net warming over the past century." Lindzen also suggests four other scientists "apparently" lost their funding or positions after questioning the scientific underpinnings of global warming.¹⁸⁴ Lindzen himself, however, has been the recipient of money from energy interests such as OPEC and the Western Fuels Association, including "$2,500 a day for his consulting services",¹⁸⁵ as well as funding from federal sources including the National Science Foundation, the Department of Energy, and NASA.¹⁸⁶

Changing position of some skeptics

In recent years some skeptics have changed their positions regarding anthropogenic global warming. Ronald Bailey, author of Global Warming and Other Eco-Myths (published by the Competitive Enterprise Institute in 2002), stated in 2005, "Anyone still holding onto the idea that there is no global warming ought to hang it up".¹⁸⁷ By 2007, he wrote "Details like sea level rise will continue to be debated by researchers, but if the debate over whether or not humanity is contributing to global warming wasn't over before, it is now.... as the new IPCC Summary makes clear, climate change Pollyannaism is no longer looking very tenable".¹⁸⁸ Others have shifted from claims that global warming is unproven to advocating