David Henderson calls this point from David Friedman "great", but I'm not quite sure that's the word for it:
"Humans were "designed" for earth's climate over a period for most of which its climate was quite different from the present--we are currently in an interglacial, a relatively short period of warmth in the current long ice age. Hence the evolutionary argument provides no support for your claim [a commenter's claim that Friedman is wrong to think climate change is as likely to be beneficial as harmful]."
The concern isn't whether our bodies have evolved to deal with a warmer climate. Of course physically we'll be fine. The simple fact that humans live and are able to travel through radically different climates will tell you that. Hell, the fact that we can withstand the temperature swings of a seasonal change will tell you that we can physically withstand any warming that will come.
That's not the concern - the concern is that in the last two hundred years (i.e. - not over evolutionary time spans), we've managed to erect a civilization that can support 7 billion people through a complex web of advanced production and exchange that hasn't been tested to its limits yet. How will that system adapt to (by all indications) permanent shocks to the climate? Will our ability to produce energy keep up with demand from population growth and even more importantly, demand from income growth?
None of this is clear. We have good reason to believe that modern, industrial, market democracies are quite robust. But we are projecting out of sample here, and the shocks we're considering are not shocks that this system has evolved to deal with.
It's not a "great" thought from Friedman in my opinion. It's a practically irrelevant thought. Humans made it through big climatic changes with a very different way of living. I'm not an anthropologist, but I'm guessing they managed this feat with huge population swings along the way. Yes, we've seen lots of temperature fluctuations in our time - but in one ice age the human population is thought to have potentially been reduced from as much as 100,000 to as few as 10,000. This is what evolution is, guys. And that was at a time when humans were not reliant on a dispersed network for their food and shelter.
I'm not saying that's what will happen as a result of climate change, but this is what having "evolved to live in X climate" means - lots and lots of death. If our losses due to climate change are even a fraction of those losses, it will be catastrophe the likes of which none of us could even imagine.
Not to mention the fact that we now have some really nasty weapons that people will be inclined to use if resources get scarce.
I'm not a climate change alarmist. We do have good institutions and they will help. Those of us in the West will probably do OK, although even we might not enjoy the experience. But this is a big deal. Of course we don't know exactly what will happen. Of course a scientific break through could make all this worrying moot. And of course, the climate science could just end up being wrong. But this attitude that sharp, discontinuous change like this isn't serious business isn't wise, in my opinion.