A Commonsense Climate Index

Wondering whether that wet winter was a fluke? Climatologists have come up with a new index that may help people find out if climate change is happening right in their own backyards. According to this "commonsense" index, described in tomorrow's Proceedings of the National Academy of Sciences, longtime residents of Alaska and parts of Asia should now be able to notice the imprints of global warming. Elsewhere, such effects are drowned out by annual variations, the scientists say--but that may change if warming intensifies.

Most researchers believe that greenhouse gases like carbon dioxide from fossil fuels and other sources can alter Earth's climate. But the careful language of climatologists, who speak of trends measured in hundredths of a degree per year, isn't so accessible to laypeople. Some predicted effects of global warming are more tangible, such as blazing hot summer days or increasingly intense rains. If those changes routinely surpass a region's normal weather fluctuations, longtime residents might be able to grasp that they may be dealing with a new climate.

A team led by climatologist James Hansen of NASA's Goddard Institute for Space Studies in New York City unified the most obvious of these effects in a single index. The data include total rainfall, numbers of heavy storms, average seasonal temperature, and "degree days"--a measure of the heating or cooling required to maintain an inside temperature of 18 degrees Celsius (65 degrees Fahrenheit). When these combined values exceed +1 in Hansen's index for many years, his team argues, the trend should become perceptible. Since 1951, only Alaska and Siberia have achieved +1 persistently, but many other parts of North America, Europe, and Asia are verging on that mark. "We think this pattern will spread in the next decade or so," says Hansen's colleague, physicist Makiko Sato.

The index may help raise public awareness of climate change, says climatologist Stephen Schneider of Stanford University. For that to occur, Schneider notes, media must publish the index regularly--a "Dow Jones of the climate" with simple numbers that people could track, just like their favorite stocks.