Gradual Trends and Extreme Events

http://krugman.blogs.nytimes.com/2011/02/08/gradual-trends-and-extreme-events/

February 8, 2011, 7:51 am

I’ve spent a lot of the last several days reading about climate change, extreme weather events, food prices, and so on. And one thing that became clear to me is that there’s widespread misunderstanding of the relationship between the gradual trend of rising temperatures and the extreme weather events that have become so much more common. What I’m about to say may seem obvious, because it is obvious, at least if you approach it the right way; but I still think it needs saying.

So, let’s start with an observation: weather varies. (Duh.) Heat waves and other stuff happens. Think of it in terms of a probability distribution for temperatures, with the area under the curve over some range representing the probability of temperatures in that range in a given place over a given period. And define an extreme event as a case in which the temperature exceeds some threshold. The the picture looks like this:

----- (see article for graph)

What happens is that the right tail gets fatter: the probability, and hence the frequency, of extreme events goes up.

Two immediate implications. First, there will still be cold stretches: global warming shifts the distribution, it doesn’t eliminate the left side of the distribution. So there will still be cold spells; that proves nothing.

Second, no individual weather event can properly be said to have been “caused” by global warming. Heat waves happened 30 years ago; there’s no way to prove that any individual heat wave now might not have happened even if we hadn’t emitted all that CO2.

But the pattern should have changed: we should be getting lots of record highs, and not as many record lows — which is exactly what we do see. And we should be seeing 100-year heat waves and similar events much more often than history would have suggested likely; again, that’s what we actually do see.

The point is that the usual casual denier arguments — it’s cold outside; you can’t prove that climate change did it — miss the point. What you’re looking for is a pattern. And that pattern is obvious.


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