A Climate Whodunit

Science nails the blame game.

To those who are convinced that the science of global warming is sound, as well as to those on the fence, the refusal of climate scientists to attribute any single episode of extreme weather to greenhouse-induced climate change has been either exasperating … or suspicious.

You mean you guys can’t definitely say human-caused climate change is why 135 daily rainfall records were broken along the East Coast during September’s deluges (Wilmington, N.C.: 19.7 inches over three days)? You can’t say climate change is why 2010 is eclipsing 1998 as the hottest year on record, or why in August an ice island four times the size of Manhattan broke off from a Greenland glacier? How about why 2000–09 was the warmest decade on record, that 153 of the 1,218 U.S. weather stations recorded their hottest summer since 1895, why Moscow suffered a once-in-centuries heat wave this summer, or why one fifth of Pakistan flooded?

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In short, no. No matter how bizarre the weather, the mantra of climatologists has been that one cannot attribute any single event to changing climate. All science can do is conclude that extreme events are getting more likely as humankind pumps more greenhouse gases into the atmosphere.

Finally, climate scientists see a way to stop being so wishy-washy and start assigning blame, through a technique called “fractional risk attribution.” This technique uses mathematical models of how the atmosphere would work if we had not goosed carbon dioxide to 389 ppm (from 278 before the Industrial Revolution), plus data about ancient (“paleo”) climates and historical (more recent) weather. The idea is to calculate how many times an extreme event should have occurred absent human interference, explains climate scientist Ben Santer of Lawrence Livermore National Lab, and the probability of the same extreme event in today’s greenhouse-forced atmosphere. Result: putting numbers on extreme weather.

In their biggest success, climate scientists led by Peter Stott of the British Met Office analyzed the 2003 European heat wave, when the mercury rose higher than at any time since the introduction of weather instruments (1851), and probably since at least 1500. After plugging in historical and paleo data, and working out climate patterns in a hypothetical world without a human-caused greenhouse effect, they conclude that our meddling was 75 percent to blame for the heat wave. Put another way, we more than doubled the chance that it would happen, and it’s twice as likely to be human-caused than natural. That’s one beat shy of “Yes, we did it,” but better than “There’s no way to tell.”

Scientists are now applying the technique to other extreme weather, especially deluges and droughts. They have reason to be optimistic. One of the signal successes of climate science has been identifying the “fingerprints” of the culprits behind rising temperatures, fierce storms, and other signs that a 10,000-year-old climate regime has been knocked for a loop. Fingerprinting has shown that the rise in global temps follows the pattern you’d expect from the greenhouse effect and not an increase in the sun’s output, for instance. A hotter sun would heat the upper atmosphere more than the lower, but in fact the upper layers have cooled while the lower have warmed, Santer explains. Fingerprinting has also nailed the greenhouse effect for warming the oceans. Natural forces such as El Niño warm some seas and cool others, but every major ocean is hotter than in the 1950s. Similar analyses have been done for today’s extreme rainfall patterns (drought followed by deluge, not precipitation spread out evenly) and the retreat of arctic sea ice. “Natural causes alone can’t explain any of these,” Santer says. “You need a large human contribution.”

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The word “interesting” covers a lot of sins, which is why it’s the perfect word for the world’s current response to climate change. That response is no response, as shown by the low expectations for the international climate meeting this week in Cancún, by China’s voracious appetite for coal, and by the Senate’s failure to pass a climate bill. It’s interesting that people refuse to make changes today to stave off disasters years hence. It’s interesting that memories—of killer storms and heat waves—are so short, with people apparently viewing them as one-offs rather than harbingers of what we’ll suffer regularly in a greenhouse world. It’s interesting that we saw Muscovites and Pakistanis dying, and blithely thought, too bad, but hey, it isn’t me. All of which means that the climate we are creating will be … interesting.