[xoxoxoBruce]

The overall correlation between our CO2 andCH4 records and the Antarctic isotopic temperature 5,9,16 is remarkable (r2 ¼ 0:71 and 0.73 for CO2 and CH4, respectively). This high correlation indicates that CO2 and CH4 may have contributed to the glacial–interglacial changes over this entire period by amplifying the orbital forcing along with albedo, and possibly other changes15,16. We have calculatedthe direct radiative forcing corresponding to the CO2, CH4 and N2O changes16. The largest CO2 change, which occurs between
stages 10 and 9, implies a direct radiative warming of DTrad ¼ 0:75 8C. Adding the effects of CH4 and N2O at this termination increases the forcing to 0.95 8C (here we assume that N2O varies with climate as during termination I37). This initial forcing is amplified by positive feedbacks associated with water vapour, sea ice, and possibly clouds (although in a different way for a ‘doubled CO2’ situation than for a glacial climate38). The total glacial–interglacial forcing is important (,3Wm2), representing 80% of that corresponding to the difference between a ‘doubled CO2’ world and modern CO2 climate. Results from various climate simulations 39,40 make it reasonable to ssume that greenhouse gases have, at a global scale, contributed significantly (possibly about half, that is, 2–3 8C) to the globally averaged glacial–interglacial temperature change.