Snip from introduction to a National Academy of Sciences report issued today:
There is sufficient evidence from tree rings, retreating glaciers, and other “proxies” to say with confidence that the last few decades of the 20th century were warmer than any comparable period in the last 400 years, according to a new National Research Council report. There is less confidence in reconstructions of surface temperatures from 1600 back to A.D. 900, and very little confidence in findings on average temperatures before then.
Link to news release, full text of report (“recent warmth is unprecedented for at least the last 400 years and potentially the last several millennia… human activities are responsible for much of the recent warming”), and briefing notes. Here’s a related AP report summarizing the findings:
Their 155-page report said average global surface temperatures in the Northern Hemisphere rose about 1 degree during the 20th century. This is shown in boreholes, retreating glaciers and other evidence found in nature, said Gerald North, a geosciences professor at Texas A&M University who chaired the academy’s panel. The report was requested in November by the chairman of the House Science Committee, Rep. Sherwood Boehlert, R-New York, to address naysayers who question whether global warming is a major threat.