Figure — Reasons for concern about climate change impacts (source: IPCC WG 2 TAR, figure SPM-2). The left part of the figure displays the observed temperature increase up to 1990 and the range of projected increases after 1990 as estimated by IPCC, WG I (IPCC, 2001a) for scenarios from the Special Report on Emission Scenarios (SRES — see the Emission scenarios). The right panel displays conceptualizations of five reasons for concern regarding climate change risks evolving through 2100. White indicates neutral or small negative or positive impacts or risks, yellow indicates negative impacts for some systems, and red means negative impacts or risks that are more widespread or greater in magnitude. This figure shows that the most potentially dangerous impacts (the red colors on the figure) typically occur after a few degrees warming — thus, my later use of 3.5 oC as a tentative “threshold” for serious climate damages is very conservative. The risks of adverse impacts from climate change increase disproportionately with the magnitude of climate change.