As NASA climate modeler Gavin Schmidt explained in a post at RealClimate.org, “This is a complex issue, and one not well-suited to soundbite quotes and headlines The two new papers deal with the attribution of a single flood event, and the attribution of increased intensity of rainfall across the Northern Hemisphere. While these issues are linked, they are quite distinct, and the two approaches are very different too.” With that in mind, Schmidt presented “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 extremes in 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.
New York Times reporter-turned-blogger Andrew Revkin felt that the two studies published in Nature were not as “definitive” as they seemed and that their authors should have done more to highlight caveats in the research. “Did the authors stress the uncertainties in discussions with journalists?” he asked in a post at his blog, Dot Earth. “It sure doesn’t look that way. Should the journalists have pushed harder when confronted with definitive language? To my mind, yes.”
To some degree, Revkin has a point. Referring to the study that looked at the flood in England and Wales in 2000, an Associated Press article reported, “Researchers found that global warming more than doubled the likelihood of that flood occurring.” This seems to be what the paper’s authors told other reporters as well, but it doesn’t accurately reflect what they found. Ninety percent of the time, models showed that manmade warming increased the risk of floods by 20 percent; only two-thirds of the time did they show that warming increased the risk by more than 90 percent. In other words, a doubling of the odds was not the most likely scenario, even though that was the impression delivered by multiple articles. (Though, to be fair, the authors suspect that their models may underestimate the effects of manmade warming on the intensification of rainfall.)
On the other hand, the news coverage did, on balance, convey many of the caveats related to the Nature studies. The AP article cited above included a comment from climate scientist Jerry North, who expressed reservations about some of the data the researchers used (conversely, it was the only article to dig deeper into the “fingerprinting” technique used in the climate models, relying on NOAA’s Climate Indicators website to point out that it has been used to link greenhouse gases to more than a dozen other ecological changes). Richard Black reported for the BBC that “Both research groups were at pains to emphasize that these two papers are not the end of the road.”
Indeed, articles from The News York Times, The Washington Post, and NPR included sentences, respectively, stressing that models “cannot fully capture the complexity of the real world,” that “there is uncertainty in this work,” and that “not all extreme weather events can be blamed on climate change.” (Ironically, Nature News was the only one to employ the verb “causing” in relation to climate change and weather.)
Many articles (such as a good one from Scientific American) also did excellent work explaining why these types of modeling experiments are so difficult, and why results pertaining to particular phenomena cannot be used to extrapolate conclusions about other weather events, or weather in general. According to the Times, for instance:
That it took a decade to come to that conclusion [about the flood in the U.K. 2000] illustrates one of the major problems of climate science at the moment. Researchers are barraged with questions about weather extremes like the recent winters in Europe and the United States and the heat waves and droughts of last summer.
Yet, even when adequate weather statistics are available for an affected region, the scientists need years to run computer analyses of any specific event and calculate whether it was made more or less likely by global warming.