I'm actually a tad skeptical about the high claimed extinction rates due to global warming, as in the Nature study. Yes, you can portray the climatic range within which a species currently occurs but that does not necessarily prove that that species can only exist within that temperature range. Temperature may not be, and frequently isn't, the factor determining a species' distribution. There will be extinctions through it, and for some species this is thought to be happening or have happened, but I doubt it will be anything like the scale suggested.

It would be a very interesting exercise to run the same simulations but imagine global cooling instead. Then see how many species modelled as extinct by that model had survived actually existing conditions in the past.

I'm not sceptical about the greenhouse effect or about it being largely human-caused, though there are some ultra-alarmist projections around that I don't take very seriously. It's also worth noting that in the Australian case, the "six degree warming" projection is a worst case scenario based on events feeding off each other; the expected outcome is nothing like that bad.