A Climate Whodunit

Article excerpt

Byline: Sharon Begley

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?

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. …