Via Michael Roberts, a new paper (pdf) by James Hansen and associates that helps clear up a couple of points about climate change.

The first is the relationship between extreme weather events and climate change. The normal, cautious thing is to say that there’s no way to attribute any particular event, like a heat wave in the Ukraine, to global warming — and news media have basically been bullied by this argument into rarely mentioning climate change even when reporting on extreme weather. But Hansen et al make an important point: this argument is much weaker when we’re talking about really extreme events, like temperatures more than 3 standard deviations above historical norms. Such events would almost never happen if there weren’t a rising trend in global temperatures; so when they become quite common, as they have, it’s fair to call them evidence of warming.

The second point is how we know that climate change is a bad thing — a question I sometimes get asked. The questioners wonder why the fact that, say, more of Canada becomes agriculturally viable doesn’t offset the damage in places that get too hot.

My first-pass answer is that we have a global economy that is adapted to historically normal climate — not just in terms of what is grown where, but in terms of where we locate our cities. In the long run, after a couple of centuries’ worth of urban development and infrastructure has been drowned by rising sea levels and/or made useless because previously habitable regions need to be abandoned, we might be able to reconstruct an equally productive economy; but in the long run …

But Hansen et al make a stronger point: life as we know it evolved to fit the historical range of planetary temperatures. In the long run it might be able to adapt to a changed world — but now we’re talking millions of years.

In the long run, we are all extinct.