Since I think a lot of this discussion of global and national GDP obscures the most worrying cost of global warming -- namely, the vast impact climate change will have on developing nations -- I thought I would dig up some charts on global warming and global agriculture production. The most rigorous study on this subject that I know of is William Cline's Global Warming and Agriculture. (You can actually download the chapters from that link.)
The basic points of Cline's book are that, by the end of the 21st century, (1) climate change will lead to a slight decline in global agricultural productivity; and (2) climate change will lead to a giant decline in agricultural productivity in Africa, South America and India.
Here are two charts that sum this up. The first is the change in agricultural productivity (by 2080) taking into account the potential benefits of "carbon fertilization" (the increase in yield that occurs in a carbon rich environment*):
Not hard to see why Russia and China might find a climate bill unappetizing. (Likewise the good people of Minnesota.) Here is the change in agricultural productivity without assuming benefits from carbon fertilization:
As a sidenote, I think it's important to recognize that deep brick color falling over most of Africa, South Asia and Latin America -- all places where agricultural productivity will fall by more than 25% -- actually hides big differences. For example, Cline reports that the southern regions of India would experience potential output declines of 30-35%, while northern regions would experience declines of 60%.
*Quick note about this: The effects of carbon fertilization are very uncertain, and depend crucially on the availability of other resources -- water for irrigation, say -- that will also be affected by global warming. I'm not in a position to parse the various bits of evidence. But it's worth noting that, even if carbon fertilization yields large benefits, Cline estimates a decline in global agricultural productivity.