Dr Chris Hope commented on the SRES A2 scenario, which is one of the higher emissions scenarios of the SRES. Hope assessed and compared the marginal damages of climate change using two versions of the A2 scenario. In one version of the A2 scenario, emissions were as the IPCC projected. In the other version of A2, Hope reduced the IPCC's projected emissions by a half (i.e., 50% of the original A2 scenario). In his integrated assessment model, both of these versions of the A2 scenario lead to almost identical estimates of marginal climate damages (the present-day value of emitting one tonne of CO2 into the atmosphere). Based on this finding, Hope argued that present day climate policy was insensitive to whether or not you accepted the validity of the higher emission SRES scenarios. IPCC author Prof Richard Tol commented on the strengths and weaknesses of the SRES scenarios. In his view, the A2 SRES marker scenario was, by far, the most realistic. UK Government departments Defra and HM Treasury argued that case for action on climate change was not undermined by the Castles and Henderson critique of the SRES scenarios. They also commented that unless effective action was taken to curb emissions growth, other bodies, like the International Energy Agency, expected greenhouse gas emissions to continue to rise into the future. Comparison with a “no policy” scenario In a report published by the MIT Joint Program on the Science and Policy of Global Change, Webster et al. (2008) compared the SRES scenarios with their own “no policy” scenario. Their no-policy scenario assumes that in the future, the world does nothing to limit greenhouse gas emissions. They found that most of the SRES scenarios were outside of the 90% probability range of their no-policy scenario (Webster et al., 2008, p. 1). Most of the SRES scenarios were consistent with efforts to stabilize greenhouse gas concentrations in the atmosphere. Webster et al. (2008, p. 54) noted that the SRES scenarios were designed to cover most of the range of future emission levels in the published scientific literature. Many such scenarios in the literature presumably assumed that future efforts would be made to stabilize greenhouse gas concentrations. Post-SRES projections As part of the IPCC Fourth Assessment Report, the literature on emissions scenarios was assessed. Baseline emissions scenarios published since the SRES were found to be comparable in range to those in the SRES.