Climate change

Is climate change mainly caused by humans?

The finding that the climate has warmed in recent decades and that this warming is likely attributable to human influence has been endorsed by every national science academy that has issued a statement on climate change, including the science academies of all major industrialised countries. At present, no scientific body of national or international standing has issued a dissenting statement. A small minority of professional associations have issued noncommittal statements, and some individual scientists have been publicly sceptical.

Environmental groups, many government 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. In Europe, the notion of human influence on climate has gained greater acceptance than in many other parts of the world, particularly the US.
Some maintain that either proponents or opponents have been stifled or driven underground. Opponents either say that most scientists consider global warming "unproved," dismiss it altogether, or highlight the dangers of focusing on only one viewpoint in the context of what they say is unsettled science.

In 1997, the "World Scientists Call For Action" petition presented to world leaders meeting to negotiate the Kyoto Protocol 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." The petition was put together by the Union of Concerned Scientists and was signed by "more than 1,500 of the world's most distinguished senior scientists, including the majority of Nobel laureates in science".

In 2001, 16 of the world's national science academies made a joint-statement on climate change that read in part:
"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."

Nevertheless, 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.

While some critics have argued that the IPCC overstates likely global warming,  others have said the opposite, suggesting that the IPCC reports are too conservative because of the need to secure consensus.

Arguments vary as to whether recent climate change can be mostly attributed to human activity - that is, that it is anthropogenic. Studies of ice cores tracking variations over 1000 years, for example, are used to argue that current climate change can be expected to be similar to past climate change, but there are other explanations for variations occurring before the advent of the industrial age.

The argument then returns to the importance of human CO2 emissions. According to the IPCC, "Greenhouse gas forcing, largely the result of human activities, has very likely caused most of the observed global warming over the last 50 years." Carbon dioxide causes between 9 and 26% of the natural greenhouse effect.

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.

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.  

There is also debate about various scenarios following decline in fossil fuel consumption. Some optimists see CO2 levels falling as a result of a reduced use of fossil fuels. However, The Stern report concluded that "Increasing scarcity of fossil fuels alone will not stop emissions growth in time. The stocks of hydrocarbons that are profitable to extract are more than enough to take the world to levels of CO2 well beyond 750ppm, with very dangerous consequences for climate change impacts."

Some scientists say that it has not yet been ascertained whether humans are the primary cause of global warming. Others attribute it to natural variation, ocean currents, increased solar activity, cosmic rays, or unknown natural causes.

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.

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, although the effect is regionally varying.

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.

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

Some global warming sceptics have been linked to organisations funded by fuel and tobacco companies for the purpose of promoting global warming scepticism, and groups employing global warming sceptics have been criticized for their ties to fossil fuel companies. They are often described as being lobbyists rather than
researchers.

Many climate scientists state that they are put under enormous pressure to distort or hide any scientific results that suggest that human activity is to blame for global warming. Some have reported pressure to use the term climate change rather than global warming as it is perceived to be more politically neutral.

Some American climatologists who have expressed doubts regarding the certainty of human influence in climate change have also been criticised by politicians and governmental agencies.

Scientists who agree with the consensus view have sometimes expressed concerns over what they view as the sensationalising of global warming by interest groups and the press.

Further reading

USA National Climatic Data Centre, Global warming FAQ, http://www.ncdc.noaa.gov/oa/climate/globalwarming.html
New Scientist, 'Climate Change: A guide for the perplexed', May 2007, http://www.newscientist.com/article/dn11462-climate-change-a-guide-for-the-perplexed.html
Marion Koshland Science Museum of the National Academy of Sciences, Global Warming, Facts and our Future, http://www.koshland-science-museum.org/exhibitgcc/index.jsp
Research Channel, 'The American Public's View of Climate Change', October 2007 (video), http://www.researchchannel.org/prog/displayevent.aspx?rID=21211&fID=345
Issues in Science and Technology Librarianship, Global Warming and Climate change science resources, http://www.istl.org/01-fall/internet.html
ICECAP (International Climate and Environmental Change Assessment Project) http://icecap.us/index.php
Canada National Post, 'Climate Change : The Deniers', June 2007, http://www.canada.com/nationalpost/news/story.html?id=c6a32614-f906-4597-993d-f181196a6d71
Centre for Research on Globalization, 'Climate of Fear: Global-warming alarmists intimidate dissenting scientists into silence', Richard Lindzen, April 2007, http://globalresearch.ca/index.php?context=va&aid=5294
Sourcewatch profiles of global warming sceptics, http://www.sourcewatch.org/index.php?title=Climate_change_sceptics
Washington Post, 'The tempest', Joel Achenbach, April 2006, http://www.washingtonpost.com/wp-dyn/content/article/2006/05/23/AR2006052301305_pf.html
Science & Public Policy Institute, http://scienceandpublicpolicy.org/
Canada Free Press, 'Global Warming, humans, Carbon Dioxide-Global Warming: The Cold, Hard Facts?', February 2007, http://www.canadafreepress.com/2007/global-warming020507.htm
USA Senate Committee on Environment and Public Works, Minority Report, http://epw.senate.gov/public/index.cfm?FuseAction=Minority.SenateReport
http://en.wikipedia.org/wiki/Consensus_on_climate_change_controversy
Intergovernmental Panel on Climate Change, http://www.ipcc.ch/
Australian Government Department of Climate Change and Energy Efficiency, http://www.climatechange.gov.au 

References

http://en.wikipedia.org/wiki/Scientific_consensus_on_climate_change
http://en.wikipedia.org/wiki/Global_warming_controversy
http://en.wikipedia.org/wiki/Scientific_opinion_on_climate_change
http://en.wikipedia.org/wiki/Global_warming
http://en.wikipedia.org/wiki/List_of_scientists_opposing_the_mainstream_scientific_assessment_of_global_warming