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Sunday, February 20, 2011

Gavin Schmidt: Going to extremes

Going to extremes

by Gavin Schmidt, RealClimate, February 17, 2011

There are two new papers in Nature this week that go right to the heart of the conversation about extreme events and their potential relationship to climate change. This is a complex issue, and one not well-suited to soundbite quotes and headlines, and so we’ll try and give a flavour of what the issues are and what new directions these new papers are pointing towards.

Let’s start with some very basic, but oft-confused points:
  • Not all extremes are the same. Discussions of ‘changes in extremes’ in general without specifying exactly what is being discussed are meaningless. A tornado is an extreme event, but one whose causes, sensitivity to change and impacts have nothing to do with those related to an ice storm, or a heat wave or cold air outbreak or a drought.
  • There is no theory or result that indicates that climate change increases extremesin general. This is a corollary of the previous statement – each kind of extreme needs to be looked at specifically – and often regionally as well.
  • Some extremes will become more common in future (and some less so). We will discuss the specifics below.
  • Attribution of extremes is hard. There are limited observational data to start with, insufficient testing of climate model simulations of extremes, and (so far) limited assessment of model projections.
The two new papers deal with the attribution of a single flood event (Pall et al.), and the attribution of increased intensity of rainfall across the Northern Hemisphere (Min et al.). While these issues are linked, they are quite distinct, and the two approaches are very different too.

The aim of the Pall et al. paper was to examine a specific event – floods in the UK in Oct-Nov 2000. Normally, with a single event there isn’t enough information to do any attribution, but Pall et al set up a very large ensemble of runs starting from roughly the same initial conditions to see how often the flooding event occurred. Note that flooding was defined as more than just intense rainfall – the authors tracked runoff and streamflow as part of their modelled setup. Then they repeated the same experiments with pre-industrial conditions (less CO2 and cooler temperatures). If the amount of times a flooding event would occur increased in the present-day setup, you can estimate how much more likely the event would have been because of climate change. The results gave varying numbers but in 9 out of 10 cases the chance increased by more than 20%, and in 2 out of 3 cases by more than 90%. This kind of fractional attribution (if an event is 50% more likely with anthropogenic effects, that implies it is 33% attributable) has been applied also to the 2003 European heatwave, and will undoubtedly be applied more often in future. One neat and interesting feature of these experiments was that they used the set up to harness the power of the public’s idle screensaver time.

The second paper is a more standard detection and attribution study. By looking at the signatures of climate change in precipitation intensity and comparing that to the internal variability and the observation, the researchers conclude that the probability of intense precipitation on any given day has increased by 7% over the last 50 years – well outside the bounds of natural variability. This is a result that has been suggested before (i.e., in the IPCC report (Groisman et al. 2005), but this was the first proper attribution study (as far as I know). The signal seen in the data though, while coherent and similar to that seen in the models, was consistently larger, perhaps indicating the models are not sensitive enough, though the El Niño of 1997/8 may have had an outsize effect.

Both papers were submitted in March last year, prior to the 2010 floods in Pakistan, Australia, Brazil or the Philippines, and so did not deal with any of the data or issues associated with those floods. However, while questions of attribution come up whenever something weird happens to the weather, these papers demonstrate clearly that the instant pop-attributions we are always being asked for are just not very sensible. It takes an enormous amount of work to do these kinds of tests, and they just can’t be done instantly. As they are done more often though, we will develop a better sense for the kinds of events that we can say something about, and those we can’t.

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