[Sharlin]

Earth has not been considerably warmer prehistorically (a term that, technically speaking, refers to the period between the appearance of tool-using hominins and the invention of written history). If you mean paleontological periods before the appearance of genus Homo, yes, but that's not very relevant given how the GP talked about the human civilization (which, incidentally, did not exist in the prehistoric era either!)

As far as we know, the loss of arable land caused by the climate change (and other anthropogenic environmental changes) far outweighs possible gains elsewhere, and even if it didn't, agricultural land area is not exactly fungible.

[ppf]

Ok, I used the wrong word. I was also referring to OP's point that "nature was just fine" - the earth has been up to 12degC warmer than present, before humans could make any impact. My point is that, taken in isolation, a warmer earth is not fundamentally bad, nor new.

As for his point about humans preferring warmer temperatures - hopefully that is self-evident.

>As far as we know, the loss of arable land caused by the climate change (and other anthropogenic environmental changes) far outweighs possible gains elsewhere

Would like to see an analysis of arable land gained vs. lost. This is all purely hypothetical, of course - such a change would be drastic, and as you point out, arable land isn't exactly fungible.

[NeedMoreTea]

A warmer earth with humans on it is fundamentally new, as we've not been around that long. Even forerunner species like Homo habilis only go back three million years. An eye blink in the geologic record, and long, long after the five mass extinctions.

12C warmer would put vast amounts of the land area of the planet outside habitable conditions for humanity. We'd probably be restricted to former arctic regions, and not much else. So no, I don't think it's self evident at all. It won't take many degrees rise to rule us out of equatorial regions, then tropical...

[ppf]

True, it would be new to humans, but (at least initially) it's not fundamentally a problem (please note, I'm arguing in the most theoretical sense here). Large amounts of the equatorial regions are already uninhabitable - it would be interesting to see an analysis of the total inhabitable land loss vs gain for each degree in temperature rise.

[NeedMoreTea]

Given a slow enough rate of change, species would adapt and migrate. Presuming there aren't farms, cities, roads, railways, dams and fences preventing smooth migration to the newly appropriate regions.

My concern is we're changing the climate at geologically unprecedented rates, likely far too quickly for species to evolve and migrate, even if we hadn't locked up 50% of the world's landmass for our own use. That will play interesting havoc with food chains no matter what former permafrost and arctic is freed up for use (with its own emissions load on melting).

[ppf]

Fair enough - so how are humans sustainable?

[cmendel]

GP's point is that they aren't. We are looking down the barrel of a total ecological collapse, and the trigger has likely already been pulled.

[Sharlin]

And the "hothouse" Earths were drastically different from Earth today. Given time, life adapts, but the point is that there's no time. There have been sudden, drastic global climate changes in Earth's history before. It's just that they have been accompanied by massive extinction events. Whatever we do, we probably won't destroy the whole ecosystem. But that's a rather ridiculously low bar to clear!

[ppf]

Sure, the "hothouse" earths are at the drastic end of the scale, but earth has spent significant periods of time at temperatures a few degrees above current. "There is no time" is a complex assertion that needs at least some research, and if there really isn't, what do you propose we do?