I'd for one would be very interested to see some greater explanation of how Tol derives his numbers. In brief encounters with him in the blogosphere he has demonstrated an underwhelming understanding of the functioning of ecosystems, and of the value of such functioning and of the biodiversity contained therein. And when I say "value" I mean value not only to current and near-future generations of Westerners, but to non-Western societies, to generations a century and more hence, and to the millions of other species with whom we share the planet.
Any economic analysis is effectively meaningless if it does not seriously and comprehensively account for the obvious as well as the subtle global ecological impacts of warming over the various increments of predicted temperature increase. Many of the impacts that will manifest with warming will be emergent - predicted by biologists in some instances and missed in others - and some of the most serious consequences are likely to occur in ways that were either not forseen, or were effectively ignored by everyone other than the ecologists and other scientific professionals who warn of them in the first place.
This matter is a weak link in the IPCC's reporting chain, and Tol is painting over the rust.