A provocative image: “beachfront roads littered with coconuts, avocados and tree limbs”

It’s hard to imagine the intellectual and emotional pressure facing the person at the National Hurricane Center responsible for making the forecasts. Vast sums of money and lives literally hang on their words. So it’s interesting to note that if you look at the bottom of each advisory, the forecasters always sign their name.

Since this has come up repeatedly in the recent discussions of hurricanes and global climate change, I think it’s worth revisiting the issue of extreme events and the extent to which it’s reasonable to attribute them to global climate change.

David Appell articulates this well over at Technology Review:

The right way to describe these kind of events seems to be that they “are consistent with” the consequences of global climate change, because no storm outside of The Day After Tomorrow can definitely be ascribed to a warmer world. It?s a peculiar phrase, but the best scientists are likely to ever be able to do.

As has been discussed here and elsewhere, hurricanes are not a terribly good example – the connection between greenhouse climate change and more or stronger hurricanes is tenuous at best.

But there are situations in which it’s reasonable to make strong assertions about the connection between a single event and the predictions made by the greenhouse warming hypothesis. David’s right on the epistemological point – the best science can do is assert consistency with the hypothesis. But there are situations in which the consistency, even in the case of a single event or cluster of events – can get mighty strong.

Europe’s 2003 heat wave is a good case in point. I’ve written about this before (and also here), but it’s worth revisiting.

Most weather and climate phenomena tend to be normally distributed, along a typical bell-shaped curve with most cases falling near the middle and fewer cases out along the more extreme tails.

When you have an extreme event, it’s useful to not only look at how much it differs from the norm (say, for example, how many degrees warmer than average it is), but where it fits along that curve. If it’s so far outside the curve that it doesn’t make sense, then you’re in all likelihood onto something interesting in statistical terms. (Well, to be careful, the most likely thing is that you’ve got bad data, but once you’ve ruled that out then things get interesting.)

That’s what Christoph Sch?r and colleagues did with the European summer of 2003, publishing their data in January in Nature (Nature \ 427, 332 – 336 (22 January 2004); doi:10.1038/nature02300). They concluded that the only way to make sense of this single extreme event – the hot summer of 2003 – was in the context of greenhouse climate models that predict not only increased warming, but also a substanial widening of the bell curve.

We find that an event like that of summer 2003 is statistically extremely unlikely, even when the observed warming is taken into account. We propose that a regime with an increased variability of temperatures (in addition to increases in mean temperature) may be able to account for summer 2003. To test this proposal, we simulate possible future European climate with a regional climate model in a scenario with increased atmospheric greenhouse-gas concentrations, and find that temperature variability increases by up to 100%, with maximum changes in central and eastern Europe.

In other words, it not only could be expected to get warmer, but also more events at either extreme – a wider standard distribution – should be expected.

That makes the summer of 2003 a great example of a single climatological event that’s, to paraphrase David’s careful wording, “frighteningly consistent with the consequences of global climate change.”

Think of Charley and Frances as blocking backs. Watch Ivan.

Seth Borenstein at Knight-Ridder has, it seems to me, done this just about right. He’s got a recap of hurricane data since 1995, which captures intense hurricane years subsequent to the IPCC data I cited earlier. He also notes with some care the plausible “natural variability” argument:

Hurricanes go through multidecade cycles of many storms and few storms, Gray said in his most recent forecast. In the 1950s and 1960s, there were a lot of storms, then few from 1970 to 1994. Gray theorized that it’s based on changes of temperature and salinity in the ocean on a massive scale that then changes atmospheric conditions.

This is not some fringe climate contrarian he’s quoting here, but William Gray, dean of the hurricane forecasters. The money line:

Why it’s been so busy lately is widely debated. Some experts point to global warming; others say it’s just part of natural cycles.

That seems like about the right way to pitch this one.

Chris Mooney has sparked a bit of discussion about the media’s coverage of hurricane Frances, and whether reporters ought to be pointing out to the public the linkage between hurricanes and greenhouse-induced climate change. Mooney, using Ross Gelbspan’s words as something of a surrogate, seems to be suggesting that reporters, if they are being honest with the science, ought to be sketching out the connection. Implicit in this is the assumption, common currency in the initial post and the comments that follow, that there’s a global warming->more and stronger hurricanes link.

Well, maybe. But maybe not. And I think this is an important point that should be handled with some care, lest Mooney, Gelbspan and their ilk leave themselves open to the same sort of accusations of manipulation of the science that they so frequently accuse the climate change contrarians of committing.

Let’s look first at Gelbspan’s words, as quoted by Mooney:

Given the dramatic increase in extreme weather events, one might think that journalists, in covering these stories, would include the line: “Scientists associate this pattern of violent weather with global warming.” They don’t.

It would be lovely if Gelbspan cited a source for this claim that there has been a “dramatic increase in extreme weather events.” I’m not sure he does. But let’s check out the Intergovernmental Panel on Climate Change’s dreaded summary for policymakers. On this crucial question of hurricanes, which is the whole raison d’etre for Mooney’s post, the IPCC concludes that there is no evidence for increased hurricane intensity over the last half of the twentieth century. They go on to say that it’s “likely” (a modestly strong assertion in the IPCC’s lingo) that hurricanes will get stronger in the future under greenhouse climate change. On the question of hurricane frequency (here you have to burrow back into the detailed text of the full report) the IPCC concludes that there is no evidence of an increase in hurricane frequency in the past (one wonders where Gelbspan’s “dramatic increase in extreme weather events” fits here) and , “there is some evidence that regional frequencies of tropical cyclones may change.” (emphasis added)

This is not exactly the strong and vibrant consensus of the scientific community that Mooney seems to want the news media to be sharing as Frances does the lock-and-load thing off the Florida coast.

Don’t get me wrong here. There is an important consensus that an array of climatological extremes is a clear likely consequence of greenhouse climate change, especially in the area of droughts, heat waves and more intense precipitation events. And that’s important. The most significant climate-linked human problems come from swings – weather that’s wetter than you expect, or drier. But Mooney seems to be wanting the news media to be doing something over the next few days that seems at best weakly supported by the science.