A three-person panel at Pennsylvania State University has cleared faculty member Michael E. Mann of charges that he destroyed or misused data, or engaged in a conspiracy to distort climate science. Mann, a central figure in the so-called climategate scandal, is the person behind the controversial "hockeystick" graph (above) that shows a sharp rise in global temperatures in the last century. The graph, reproduced prominently in the 2001 IPCC climate assessment report, has been a lightning rod for global warming skeptics. Some scientists such as John Christy, see the 20th century temperature record as critically important, but others place more emphasis on the long-term temperature record or on modeling results. In effect there are within climate science several scientific subcultures: some put the most faith in theory, others in simulations, and yet others in empirical results--and even among the empiricists, opinions differ as to what kind of evidence is most compelling.
Image: This chart is Figure 1(b) from the Intergovernmental Panel on Climate Change Third Assessment Report, (c) 2001 The Intergovernmental Panel on Climate Change. The source of this image is a PDF file that can be downloaded here: http://www.grida.no/climate/ipcc_tar/wg1/pdf/WG1_TAR-FRONT.PDF