It also occurs to me to wonder... what would be so BAD about another "Medieval Warm Period", making *practical* arable and habitable places like the Greenland coast, central Canada, and parts of Siberia? Yeah, you might sacrifice a relatively smaller area elsewhere as desert, but wouldn't it be a net gain for human habitability?
It might be a net gain for human habitability, if only there was a way of moving humans without them slaughtering each other. Unfortunately, we're stuck with silly things like national borders, and if bajillions of Chinese and Indian coastal dwellers decide they want to move to newly habitable Siberia due to rising sea levels and Russia objects, shit will hit the fan and nukes start falling.
As to species preservation, all well and good, but species come and go all the time; that's the nature of a non-static biosphere.
So they do, but never before they have gone this fast, and it's not as if replacements just pop into being overnight even if punctuated equilibrium is given. And we still don't understand the biosphere fully, we may not know until it's too late which species were of vital importance to us.
Seems to my our job is to adapt as needed like any other viable species, not to attempt to freezeframe nature at some theoretically optimal point, lest the nonviable perish. What happens when your freezeframe inevitably collapses and you're stuck with a biosphere that's not *had* to adapt, and is now a large Fail?
There's no practical difference between "freezeframe collapsing" and a rate of change too fast for biosphere to adapt to, which is already happening.