In the past few years, scientific societies and scientists have released statements and studies showing the growing consensus on climate change science. A common objection to taking action to reduce our heat-trapping emissions has been uncertainty within the scientific community on whether or not global warming is happening and whether it is caused by humans. However, there is now an overwhelming scientific consensus that global warming is indeed happening and humans are contributing to it. Below are links to documents and statements attesting to this consensus. 

Intergovernmental Panel on Climate Change

• Climate Change 2007: The Physical Science Basis, IPCC, 2007. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
  
  “Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level”
  
  “Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.”
  
  IPCC defines "very likely" as greater than 90% probability of occurrence.

 Scientific Societies

• American Meteorological Society: Climate Change: An Information Statement of the American Meteorological Society
  
  "Indeed, strong observational evidence and results from modeling studies indicate that, at least over the last 50 years, human activities are a major contributor to climate change." (February 2007)
  

• American Geophysical Union: Human Impacts on Climate
  
  "The Earth's climate is now clearly out of balance and is warming. Many components of the climate system—including the temperatures of the atmosphere, land and ocean, the extent of sea ice and mountain glaciers, the sea level, the distribution of precipitation, and the length of seasons—are now changing at rates and in patterns that are not natural and are best explained by the increased atmospheric abundances of greenhouse gases and aerosols generated by human activity during the 20th century." (Adopted December 2003, Revised and Reaffirmed December 2007)
  

• American Association for the Advancement of Science: AAAS Board Statement on Climate Change
  
  "The scientific evidence is clear: global climate change caused by human activities is occurring now, and it is a growing threat to society." (December 2006)
  

• Geological Society of America: Global Climate Change
  
  "The Geological Society of America (GSA) supports the scientific conclusions that Earth’s climate is changing; the climate changes are due in part to human activities; and the probable consequences of the climate changes will be significant and blind to geopolitical boundaries."  (October 2006)

• American Chemical Society: Statement on Global Climate Change
  
  "There is now general agreement among scientific experts that the recent warming trend is real (and particularly strong within the past 20 years), that most of the observed warming is likely due to increased atmospheric greenhouse gas concentrations, and that climate change could have serious adverse effects by the end of this century." (July 2004)
  

National Science Academies

• U.S. National Academy of Sciences: Understanding and Responding to Climate Change (pdf)
  
  "The scientific understanding of climate change is now sufficiently clear to justify taking steps to reduce the amount of greenhouse gases in the atmosphere." (2005)

• International academies: Joint science academies’ statement: Global response to climate change (pdf)
  
  "Climate change is real. There will always be uncertainty in understanding a system as complex as the world’s climate. However there is now strong evidence that significant global warming is occurring." (2005, 11 national academies of science)

• International academies: The Science of Climate Change 
  
  "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." (2001, 16 national academies of science)

Research

• Beyond the Ivory Tower: The Scientific Consensus on Climate Change, Naomi Oreskes
  
  Oreskes analyzed 928 abstracts published in refereed scientific journals between 1993 and 2003 and listed in the ISI database with the keywords "climate change." Of all the papers, 75 percent either explicitly or implicitly accepted the consensus view that global warming is happening and humans are contributing to it; 25 percent dealt with methods or ancient climates, taking no position on current anthropogenic (human-caused) climate change. Remarkably, none of the papers disagreed with the consensus position.

UCS Sign-on Statements

• State of Climate Science Sign-on Statement
  
  "Anthropogenic climate change, driven by emissions of greenhouse gases, is already under way and likely responsible for most of the observed warming over the last 50 years." (2003)
  
  
• Increase Your Leadership on Global Warming: A Letter from California Scientists
  
  "If emissions continue unabated, the serious consequences of a changing climate for California are likely to include a striking increase in extreme heat and heat-related mortality, significant reductions in Sierra snowpack with severe impacts on water supply, mounting challenges to agricultural production, and sea-level rise leading to more widespread erosion of California’s beaches and coastline." (2005)