This idea was already in the air in 2003 when Stott traveled though the worst heat wave in recorded European history on a wedding anniversary trip to Italy and Switzerland. One of the striking consequences he noticed was that the Swiss mountains were missing their usual melodious tinkling of cowbells. "There was no water in the mountains, and the farmers had to take all their cows down in the valley," he says. He decided to see if he could pin part of the blame on climate change after he returned to his office in Exeter, England. "I didn't expect to get a positive result," he says
But he did. In fact, the signal of a warming climate was quite clear in Europe, even using data up to only 2000. In a landmark paper in Nature Stott and colleagues concluded that the chances of a heat wave like the 2003 event have more than doubled because of climate change. (Scientific American is part of Nature Publishing Group.) Data collected since then show that the odds are at least four times higher compared with pre-industrial days. "We are very aware of the risks of misattribution," Stott says. "We don't want to point to specific events and say that they are part of climate change when they really are due to natural variability. But for some events, like the 2003 heat wave, we have the robust evidence to back it up."
Case in point: Hurricane Katrina
Another event with a clear global warming component, says Kevin Trenberth, head of climate analysis at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., was Hurricane Katrina. Trenberth calculated that the combination of overall planetary warming, elevated moisture in the atmosphere, and higher sea-surface temperatures meant that "4 to 6 percent of the precipitation—an extra inch [2.5 centimeters] of rain—in Katrina was due to global warming," he says. "That may not sound like much, but it could be the straw that breaks the camel's back or causes a levee to fail." It was also a very conservative estimate. "The extra heat produced as moisture condenses can invigorate a storm, and at a certain point, the storm just takes off," he says. "That would certainly apply to Nashville." So climate change's contribution to Katrina could have been twice as high as his calculations show, he says. Add in higher winds to the extra energy, and it is easy to see how storms can become more damaging.
This science of attribution is not without controversies. Another case in point: the 2010 Russian heat wave, which wiped out one quarter of the nation's wheat crop and darkened the skies of Moscow with smoke from fires. The actual meteorological cause is not in doubt. "There was a blocking of the atmospheric circulation," explains Martin Hoerling, a research meteorologist at the NOAA's Earth System Research Laboratory, also in Boulder. "The jet stream shifted north, bringing a longer period of high pressure and stagnant weather conditions." But what caused the blocking? Hoerling looked for an underlying long-term temperature trend in western Russia that might have increased the odds of a heat wave, as Stott had done for the 2003 European event. He found nothing. "The best explanation is a rogue black swan—something that came out of the blue," he says.
Wrong, retorts NCAR's Trenberth. He sees a clear expansion of the hot, dry Mediterranean climate into western Russia that is consistent with climate change predictions—and that also intensified the Pakistan monsoon. "I completely repudiate Marty—and it doesn't help to have him saying you can't attribute the heat wave to climate change," he says. "What we can say is that, as with Katrina, this would not have happened the same way without global warming."
Yet even this dispute is smaller than it first appears. What is not in doubt is that the Russian heat wave is a portent—a glimpse of the future predicted by climate models. Even Hoerling sees it as a preview of coming natural disasters. By 2080, such events are expected to happen, on average, once every five years, he says: "It's a good wake-up call. This type of phenomenon will become radically more common."