When It Rains...

Like every discussion about weather and climate, this one has to be preceded by the standard disclaimer "that no particular weather event can be attributed to the global climate change that's underway as a result of the greenhouse effect. But to tell you the truth, that reluctance to say, "see this? This is global warming," is getting a little stale.

The simple fact is that climate, the long-term average of individual weather events, has shifted. The climate is warmer, and marked by more extreme weather events. The no'easter that pummeled the eastern seaboard April 15, causing floods and forcing evacuations of low-lying communities, brought a deluge for the record books "New York's Central Park got 7.8 inches of rain in 24 hours. But in coming years that won't seem so anomalous: for the last decade, the number of extreme precipitation events in the U.S. has soared well above the historical average. Climatologists consider that yet another sign of climate change, which means only one thing: get used to it.

The scientist who has pioneered the study of extreme precipitation "when it rains it pours "is Thomas Karl, director of the National Climatic Data Center, part of the National Oceanic and Atmospheric Administration. A decade ago he and colleagues introduced the U.S. Climate Extremes Index to chronicle unusual temperature, precipitation and drought. Just as there have been more heat waves beginning in the 1990s, so have there been more deluges and droughts. In particular, as Karl reported, there is "a rather remarkable trend [in] the proportion of precipitation in the United States that is falling from precipitation events of two inches or more on any given day. That percentage has increased from the order of 8 or 9 percent in the early part of the century to somewhere in the neighborhood of about 11 percent." That was in the 1990s, and that trend has only gotten stronger, particularly in North America, as this graph shows.

Karl told a Senate committee in 2001 that the frequency of intense precipitation events had already increased by about 20 percent between the beginning and end of the 20^th century. This is exactly what models of greenhouse-induced climate change predict for a warming world. For one thing, warmer air holds more moisture than cool. Also, some of the excess heat in the atmosphere goes into evaporating more water from the planet's surface. (The seeming paradox of an increase in droughts as well as deluges reflects the fact that evaporated moisture is sometimes transported away from where atmospheric conditions are not favorable for precipitation; the water vapor is carried to, and falls over, other regions.) Climate models show at least a doubling, and in some cases a four-four increase, in the frequency of extreme precipitation in the U.S., which is defined as more than two inches in a 24-hour period. Most of the U.S. is not in for more frequent rainfalls, because precipitation requires particular circulation patterns that are not expected to shift much even as the world warms, but storms that do form will carry more moisture. This pattern, Karl finds, began to take hold in the 1970s.

For the eastern United States, global warming has made the Atlantic Ocean a greater-than-normal source of moisture, due to evaporation. That portends more intense snowstorms, too, as Newsweek explained after the blizzard of January 1996. In a warmer world, warm moist air that feeds blizzards is more likely to hover over the Atlantic; if a mass of cold air swoops over from Canada, the collision produces the ingredients of a massive snowstorm. So while it is indeed true that no single weather event can be blamed on overall climate change, once again we are experiencing exactly what the models forecast.