Based mostly on empirical observations of species density in existing habitat, Chris Thomas and others have offered
(through three different methods) model results which suggest 18-35% of extant species may be extinct (or irreversibly commited toward extinction) in the next fifty years (see also this perspective
on the research appearing in the same issue of Nature). These are big numbers and further butress my suggestion
, in response to Michael Crichton, that there is a compelling social interest in mitigating climate change.
As might be expected the Thomas paper has been the subject of a lot of controversey. Some of the relevant issues have already been raised in this excellent post
by Carl Zimmer. I'll flesh out a few more of the relevant issues in subsequent posts.