leads to an overestimate of future greenhouse gas emissions. The IPCC future climate change projections would have been overestimated. However, the difference in economic growth is offset by a difference in energy intensity. Some say these two opposite effects fully cancel, some say this is only partial. Overall, the effect of a switch from MER to PPP is likely to have a minimal effect on carbon dioxide concentrations in the atmosphere. Castles and Henderson later accepted this and acknowledged that they were mistaken that future greenhouse gas emissions had been significantly overestimated. But even though global climate change is not affected, it has been argued 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. Availability of fossil fuels As part of the SRES, IPCC authors assessed the potential future availability of fossil fuels for energy use. SRES assumptions about availability of fossil fuels is largely based on a 1997 study done by Rogner, who goes to great lengths to claim that there are enough fossil resources, i.e. hydrocarbon molecules in the crust, to theoretically sustain production for an extended period of time. The issue of whether or not the future availability of fossil fuels would limit future carbon emissions was considered in the Third Assessment Report; it concluded that limits on fossil fuel resources would not limit carbon emissions in the 21st century. Their estimate for conventional coal reserves was around 1,000 gigatonnes of carbon (GtC), with an upper estimate of between 3,500 and 4,000 GtC. This compares with cumulative carbon emissions up to the year 2100 of about 1,000 GtC for the SRES B1 scenario, and about 2,000 GtC for the SRES A1FI scenario. The carbon in proven conventional oil and gas reserves was estimated to be much less than the cumulative carbon emissions associated with atmospheric stabilization of CO2 concentrations at levels of 450 ppmv or higher. The Third Assessment Report suggested that the future makeup of the world's energy mix would determine whether or not greenhouse gas concentrations were stabilized in the 21st century. The future energy mix might be based more on the exploitation