Human influences on climate
Anthropogenic factors are acts by humans that change the environment and influence climate. The biggest factor of present concern is the increase in CO2 levels due to emissions from fossil fuel combustion, followed by aerosols (particulate matter in the atmosphere) which exerts a cooling effect. Other factors, including land use, ozone depletion, animal agriculture  and deforestation also impact climate.
 Fossil fuels
Carbon dioxide variations over the last 400,000 years, showing a rise since the industrial revolution.
Carbon dioxide variations over the last 400,000 years, showing a rise since the industrial revolution.
Beginning with the industrial revolution in the 1850s and accelerating ever since, the human consumption of fossil fuels has elevated CO2 levels from a concentration of ~280 ppm to more than 370 ppm today. These increases are projected to reach more than 560 ppm before the end of the 21st century. Along with rising methane levels, these changes are anticipated to cause an increase of 1.4–5.6 °C between 1990 and 2100 (see global warming).
Anthropogenic aerosols, particularly sulphate aerosols from fossil fuel combustion, are believed to exert a cooling influence; see graph. This, together with natural variability, is believed to account for the relative "plateau" in the graph of 20th century temperatures in the middle of the century.
 Land use
Prior to widespread fossil fuel use, humanity's largest impact on local climate is likely to have resulted from land use. Irrigation, deforestation, and agriculture fundamentally change the environment. For example, they change the amount of water going into and out of a given locale. They also may change the local albedo by influencing the ground cover and altering the amount of sunlight that is absorbed. For example, there is evidence to suggest that the climate of Greece and other Mediterranean countries was permanently changed by widespread deforestation between 700 BC and 0 BC (the wood being used for ship-building, construction and fuel), with the result that the modern climate in the region is significantly hotter and drier, and the species of trees that were used for ship-building in the ancient world can no longer be found in the area.
A controversial hypothesis by William Ruddiman called the early anthropocene hypothesis  suggests that the rise of agriculture and the accompanying deforestation led to the increases in carbon dioxide and methane during the period 5000–8000 years ago. These increases, which reversed previous declines, may have been responsible for delaying the onset of the next glacial period, according to Ruddimann's overdue-glaciation hypothesis.
 Animal agriculture
According to a 2006 United Nations report, animal agriculture is responsible for 18% of the world’s greenhouse gas emissions as measured in CO2 equivalents. By comparison, all transportation emits 13.5% of the CO2. In addition to increased CO2 emissions, animal agriculture produces 65% percent of human-related nitrous oxide (which has 296 times the global warming potential of CO2) and 37% of all human-induced methane (which is 23 times as warming as CO2).http://en.wikipedia.org/wiki/Climate_change
Statements by organizations
Various prominent bodies have commented on global warming, most notably the Intergovernmental Panel on Climate Change (IPCC). National and international scientific groups have issued statements both detailing and summarizing the current state of scientific knowledge on the earth's climate.
 Intergovernmental Panel on Climate Change (IPCC)
Main article: Intergovernmental Panel on Climate Change
In 2007, as part of its Fourth Assessment Report, the IPCC concluded that human actions are "very likely" the cause of global warming, meaning a 90% or greater probability.
"The world's leading climate scientists said global warming has begun, is very likely caused by man, and will be unstoppable for centuries, ... . The phrase very likely translates to a more than 90 percent certainty that global warming is caused by man's burning of fossil fuels. That was the strongest conclusion to date, making it nearly impossible to say natural forces are to blame."
"The report said that an increase in hurricane and tropical cyclone strength since 1970 more likely than not can be attributed to man-made global warming. The scientists said global warming's connection varies with storms in different parts of the world, but that the storms that strike the Americas are global warming-influenced."
"On sea levels, the report projects rises of 7-23 inches by the end of the century. That could be augmented by an additional 4-8 inches if recent surprising polar ice sheet melt continues."
 Joint science academies’ statement
In 2005 the national science academies of the G8 nations, plus Brazil, China and India, three of the largest emitters of greenhouse gases in the developing world, signed a statement on the global response to climate change. The statement stresses that the scientific understanding of climate change is now sufficiently clear to justify nations taking prompt action , and explicitly endorsed the IPCC consensus.
 US National Research Council, 2001
In 2001 the Committee on the Science of Climate Change of the National Research Council published Climate Change Science: An Analysis of Some Key Questions . This report explicitly endorses the IPCC view of attribution of recent climate change as representing the view of the science community:
The changes observed over the last several decades are likely mostly due to human activities, but we cannot rule out that some significant part of these changes is also a reflection of natural variability. Human-induced warming and associated sea level rises are expected to continue through the 21st century... The IPCC's conclusion that most of the observed warming of the last 50 years is likely to have been due to the increase in greenhouse gas concentrations accurately reflects the current thinking of the scientific community on this issue. 
 American Meteorological Society
The American Meteorological Society (AMS) statement adopted by their council in 2003 said:
There is now clear evidence that the mean annual temperature at the Earth's surface, averaged over the entire globe, has been increasing in the past 200 years. There is also clear evidence that the abundance of greenhouse gases in the atmosphere has increased over the same period. In the past decade, significant progress has been made toward a better understanding of the climate system and toward improved projections of long-term climate change... Human activities have become a major source of environmental change. Of great urgency are the climate consequences of the increasing atmospheric abundance of greenhouse gases... Because greenhouse gases continue to increase, we are, in effect, conducting a global climate experiment, neither planned nor controlled, the results of which may present unprecedented challenges to our wisdom and foresight as well as have significant impacts on our natural and societal systems. 
 Federal Climate Change Science Program, 2006
On May 2, 2006, the Federal Climate Change Science Program commissioned by the Bush administration in 2002 released the first of 21 assessments that concluded that there is clear evidence of human influences on the climate system (due to changes in greenhouse gases, aerosols, and stratospheric ozone) . The study said that observed patterns of change over the past 50 years cannot be explained by natural processes alone, though it did not state what percentage of climate change might be anthropogenic in nature.
 Other organizations
Other scientific organizations have made position statements on climate change.
* American Geophysical Union position statement on greenhouse gases and climate change (also endorsed by the American Institute of Physics)
* Climate Change Science: An Analysis of Some Key Questions, National Academy of Sciences, Commission on Geosciences, Environment and Resources, (Washington, DC: National Academy Press, 2001).
* Joint statement on the Science of Climate Change, 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).
* A position paper of the Stratigraphy Commission of the Geological Society of London.
* Position Statement on Global Climate Change adopted by the Geological Society of America
* Policy Statement on Climate Variability and Change by the American Association of State Climatologists (AASC)
* Australian Medical Association statement on climate change
* American Chemical Society statement on Global Climate Change
The only major scientific organization that rejects the finding of human influence on recent climate is the American Association of Petroleum Geologists.
 Recent Surveys of scientists
Various surveys have been conducted to determine a scientific consensus on global warming. Only one has been conducted within the last ten years.
 Oreskes, 2004
In December 2004, an article by geologist and historian of science Naomi Oreskes summarized a study of the scientific literature on climate change. The essay concluded that there is a scientific consensus on the reality of anthropogenic climate change. The author analyzed 928 abstracts of papers from refereed scientific journals between 1993 and 2003, listed with the keywords "global climate change". The abstracts were divided into six categories: explicit endorsement of the consensus position, evaluation of impacts, mitigation proposals, methods, paleoclimate analysis, and rejection of the consensus position. 75% of the abstracts were placed in the first three categories, thus either explicitly or implicitly accepting the consensus view; 25% dealt with methods or paleoclimate, thus taking no position on current anthropogenic climate change; none of the abstracts disagreed with the consensus position, which the author found to be "remarkable". It was also pointed out, "authors evaluating impacts, developing methods, or studying paleoclimatic change might believe that current climate change is natural. However, none of these papers argued that point."
 Older surveys
 Survey of US state climatologists 1997
The neutrality of this section is disputed.
Please see the discussion on the talk page.
In 1997, the conservative advocacy group Citizens for a Sound Economy surveyed America's 48 official state climatologists on questions related to climate change . Of the 36 respondents, 44% considered global warming to be a largely natural phenomenon, compared to 17% who considered warming to be largely manmade. The survey further found that 58% of the climatologists disagreed or somewhat disagreed with then-President Clinton's assertion that "the overwhelming balance of evidence and scientific opinion is that it is no longer a theory, but now fact, that global warming is for real". Eighty-nine percent of the climatologists agreed that "current science is unable to isolate and measure variations in global temperatures caused ONLY by man-made factors," and 61% said that historical data do not indicate "that fluctuations in global temperatures are attributable to human influences such as burning fossil fuels."
60% of the respondents said that reducing man-made CO2 emissions by 15% below 1990 levels would not prevent global temperatures from rising, and 86% said that reducing emissions to 1990 levels would not prevent rising temperatures. By a 39% to 33% margin, more climatologists agreed that "evidence exists to suggest that the earth is headed for another glacial period"  though the time scale for the next glacial period was not specified.
 Bray and von Storch, 1996
In 1996, Dennis Bray and Hans von Storch undertook a survery of climate scientists on attitudes towards global warming and related matters. The results were subsequently published in Bulletin of the American Meteorological Society Vol. 80, No. 3, March 1999 439-455.  The paper addressed the views of climate scientists, with a response rate of 40% from a mail survey questionnaire to 1000 scientists in Germany, the USA and Canada. Most of the scientists believed that global warming was occurring and appropriate policy action should be taken, but there was wide disagreement about the likely effects on society and almost all agreed that the predictive ability of currently existing models was limited.
The abstract says:
The international consensus was, however, apparent regarding the utility of the knowledge to date: climate science has provided enough knowledge so that the initiation of abatement measures is warranted. However, consensus also existed regarding the current inability to explicitly specify detrimental effects that might result from climate change. This incompatibility between the state of knowledge and the calls for action suggests that, to some degree at least, scientific advice is a product of both scientific knowledge and normative judgment, suggesting a socioscientific construction of the climate change issue.
The survey was extensive, and asked numerous questions on many aspects of climate science, model formulation, and utility, and science/public/policy interactions. To pick out some of the more vital topics, from the body of the paper:
The resulting questionnaire, consisting of 74 questions, was pre-tested in a German institution and after revisions, distributed to a total of 1,000 scientists in North America and Germany... The number of completed returns was as follows: USA 149, Canada 35, and Germany 228, a response rate of approximately 40%...
...With a value of 1 indicating the highest level of belief that predictions are possible and a value of 7 expressing the least faith in the predictive capabilities of the current state of climate science knowledge, the mean of the entire sample of 4.6 for the ability to make reasonable predictions of inter-annual variability tends to indicate that scientists feel that reasonable prediction is not yet a possibility... mean of 4.8 for reasonable predictions of 10 years... mean of 5.2 for periods of 100 years...
...a response of a value of 1 indicates a strong level of agreement with the statement of certainty that global warming is already underway or will occur without modification to human behavior... the mean response for the entire sample was 3.3 indicating a slight tendency towards the position that global warming has indeed been detected and is underway.... Regarding global warming as being a possible future event, there is a higher expression of confidence as indicated by the mean of 2.6.
 Other older surveys of scientists
It should be noted that these surveys are over 15 years old and the state of climate science has changed radically since their time; current beliefs of the scientific community are different as shown in the reviews cited above.
* Global Environmental Change Report, 1990: GECR climate survey shows strong agreement on action, less so on warming. Global Environmental Change Report 2, No. 9, pp. 1-3
* Stewart, T.R., Mumpower, J.L., and Reagan-Cirincione, P. (1992). Scientists' opinions about global climate change: Summary of the results of a survey. NAEP (National Association of Environmental Professionals) Newsletter, 17(2), 6-7.
* A 1991 Gallup poll of 400 members of the American Geophysical Union and the American Meteorological Society
o Fairness and Accuracy in Reporting states that the report said that 66 % of the scientists said that human-induced global warming was occurring, with 10 % disagreeing and the rest undecided. In a correction Gallup stated: "Most scientists involved in research in this area believe that human-induced global warming is occurring now."
o George Will reported "53 percent do not believe warming has occurred, and another 30 percent are uncertain." (Washington Post, September 3, 1992)
o A 1993 publication by the politically conservative Heartland Institute states: "A Gallup poll conducted on February 13, 1992 of members of the American Geophysical Union and the American Meteorological Society - the two professional societies whose members are most likely to be involved in climate research - found that 18 percent thought some global warming had occurred, 33 percent said insufficient information existed to tell, and 49 percent believed no warming had taken place."
 Scientists opposing consensus opinions
Main article: List of scientists opposing global warming consensus
 Alleged US governmental interference in reporting
According to an Associated Press release of 30 January 2007 :
"Climate scientists at seven government agencies say they have been subjected to political pressure aimed at downplaying the threat of global warming.
"The groups presented a survey that shows two in five of the 279 climate scientists who responded to a questionnaire complained that some of their scientific papers had been edited in a way that changed their meaning. Nearly half of the 279 said in response to another question that at some point they had been told to delete references to "global warming" or "climate change" from a report."
 See also
* Attribution of recent climate change
* Global warming controversy
1. ^ "Warming 'very likely' human-made", BBC News, BBC, 2007-02-01. Retrieved on 2007-02-01.
2. ^ Naomi Oreskes (December 3, 2004). "Beyond the Ivory Tower: The Scientific Consensus on Climate Change". Science 306 (5702): 1686. DOI:10.1126/science.1103618. (see also for an exchange of letters to Science)
 External links
* Sherwood Rowland (Nobel Laureate for work on ozone depletion) gives his opinion on climate change 2006 Freeview video provided by the Vega Science Trust.
* Newer reports on EPA website
* Climate Change Science: An Analysis of Some Key Questions, National Academy of Sciences
* Joint Science Academies' Statement: Global Response to Climate Change, National Academy of Sciences
* Climate change special: State of denial New Scientist 4 November 2006
* The Denial Machine CBC Television 15 November 2006http://en.wikipedia.org/wiki/Scientific_opinion_on_climate_change