Rev Bob: one of the main concerns is a rapid pace of warming such that ecosystems will be unable to adapt to the change. When rate-of-change is the concern, the actual temperature compared to any other time is not as important. Extinction rates go way, way up whenever climate change is rapid. (And there are sound reasons for that.)
Here's a quote from page 295 of Prothero:
"But recent paleoclimatic studies have shown that the last glacial-interglacial transition was amazingly abrupt, taking place in a few decades or less, and was characterized by extreme swings in climate before the interglacial warming took hold."
It also appears, from several studies, that warming cycles happen much faster than cooling cycles.
My point is not that rapid climate change won't have some deleterious (as well as beneficial) effects, only that climate change is something that has happened again and again throughout Earth's history, and that, if forced to chose between the two, warming (at least at this point in Earth's history) is far better than cooling. Moreover, I think we often underestimate the ability of plants and animals to adapt to changing conditions. !8,000 years ago Chicago was under ice. The Tundra, now limited to the Arctic, stretched across Iowa and Southern Illinois. That's a lot of migration in such a short time. The Boreal Spruce Forest, shifted similarly, from the Central U.S. right up into Canada.
I'm no longer as much of a sceptic as I once was -- though I'm not sold on the idea that this is an ENTIRELY man-made phenomenon -- I just get tired of the "we're all going to die" hysteria that so permeates news coverage of these matters.
All best. :-)