We've all heard about the dire straits polar bears are facing if they lose their icy habitat to global warming. But just how many species may global warming drive extinct? One way to find out is to look over the mass extinctions of the past--and the picture there's not pretty, as I explain in my new article, "Biodiversity in the Balance." It appears today in the new publication Yale Environment 360, an online environment magazine from the Yale School of Forestry & Environmental Studies. They've already got some pieces from some big names, like Bill McKibben and Carl Safina. So check the whole place out.