The National Academy of Sciences is the "Supreme Court" for science. Honest Abe created it 150 years ago to summarize "state of the art" scientific knowledge and advise Congress and the White House. It's been doing this every year since, under every administration.
Notes: President Lincoln created the National Academy in 1963 because state-of-the art military technology was crucial to the Union war effort. Since science is a highly contentious process of argument and counter-argument, the President needed an objective body to assess all the evidence and summarize the consensus among top experts. Every President since has asked the NAS to perform similar scientific assessments on questions of national importance.
President Bush asked the NAS, more than once, to provide assessments on the "state of the science" regarding climate.
Here's what the latest (2010) National Academy assessment concluded about climate change:
"A strong, credible body of scientific evidence shows that climate change is occurring, is caused largely by human activities, and poses significant risks for a broad range of human and natural systems...
Some scientific conclusions or theories have been so thoroughly examined and tested, and supported by so many independent observations and results, that their likelihood of subsequently being found to be wrong is vanishingly small. Such conclusions and theories are then regarded as settled facts. This is the case for the conclusions that the Earth system is warming and that much of this warming is very likely due to human activities." (NAS, Advancing the Science of Climate Change, 2010, p 21.)
Bite Source: Tom Smerling, inspired by Richard Ades
Image Source: here