Special Report on Emissions Scenarios
[6] The most prominently publicized criticism of SRES focused on the fact that all but one of the participating models compared gross domestic product (GDP) across regions using market exchange rates (MER), instead of the more correct purchasing-power parity (PPP) approach.
Under the six illustrative SRES scenarios, the IPCC Third Assessment Report (2001)[18] projects the atmospheric concentration of carbon dioxide (CO2) in the year 2100 as between 540 and 970 parts per million (ppm).
The United States Environmental Protection Agency has also produced projections of future atmospheric GHG concentrations using the SRES scenarios.
[17] These projections are shown opposite, and are subject to the uncertainty described earlier regarding the future role of carbon sinks and changes to the Earth's biosphere.
Between the 1990s and 2000s, the growth rate in CO2 emissions from fossil fuel burning and industrial processes increased (McMullen and Jabbour, 2009, p. 8).
Using MER, the SRES scenarios overstate income differences in past and present, and overestimate future economic growth in developing countries.
[31] Castles and Henderson later accepted this and acknowledged that they were mistaken that future greenhouse gas emissions had been significantly overestimated.
[32] But even though global climate change is not affected, it has been argued[33] that the regional distribution of emissions and incomes is very different between an MER and a PPP scenario.
This would influence the political debate: in a PPP scenario, China and India have a much smaller share of global emissions.
It would also affect vulnerability to climate change: in a PPP scenario, poor countries grow more slowly and would face greater impacts.
As part of the SRES, IPCC authors assessed the potential future availability of fossil fuels for energy use.
[37] The Third Assessment Report suggested[37] that the future makeup of the world's energy mix would determine whether or not greenhouse gas concentrations were stabilized in the 21st century.
[40]This persistent problem has been criticized for a long time as many assumptions used for fossil fuel availability and future production have been optimistic at best and implausible at worst.
The SRES and RCP scenarios have been criticized for being biased towards "exaggerated resource availability" and making "unrealistic expectations on future production outputs from fossil fuels.
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.
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.
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.
[54] IPCC (2007)[54] noted that post-SRES scenarios had used lower values for some drivers for emissions, notably population projections.
In IPCC Fifth Assessment Report released in 2014, SRES projections were superseded by Representative Concentration Pathways (RCPs) models.
