Questioning the cooling effects of volcanoes.

Questioning the cooling effects of volcanoes

Sleet, snow and bleak temperatures besieged New England in the summer of 1816. August frosts gripped farms, devastating crops. Some scientists have blamed this "year without a summer" on the 1815 eruption of the Indonesian volcano Tambora, noting that volcanic debris can significantly dim sunlight over large portions of the Earth. Still, researchers disagree over how much volcanoes actually change the climate.

Two atmospheric scientists now conclude that, while major volcanoes can lower temperatures slightly, the crazy cold snap of 1816 and other dramatic temperature dips stemmed not from volcanoes but from other, unidentified freaks of nature.

Clifford F. Mass and David A. Portman of the University of Washington in Seattle undertook an extensive study of climate and temperature records from the years surrounding nine volcanic eruptions, including Tambora. Mass says the most important aspect of the study is that he and Portman separated out temperature changes caused by the El Nino-Southern Oscillation (ENSO), the unpredictable ocean phenomenon that wreaks havoc on weather patterns every few years. They also avoided some of the "noise" from random weather fluctuations, he says, by averaging the temperature trends associated with several volcanoes. "We are struggling to pull a weak signal out of a lot of random flux," says Mass.

After subtracting ENSO's influence, Mass and Portman saw clear temperature drops following five major eruptions, including Indonesia's Krakatoa in 1883 and Mexico's El Chichon in 1982. But Mass says these dips were too small in relation to the effects of ENSO and other year-to-year variations to set off a cold spell as drastic as the one in 1816.

The researchers estimate in the June JOURNAL OF CLIMATE that the most prolific eruptions lowered hemispheric temperatures by only about 0.3[deg.]C -- a subtle change, Mass says, considering previous observations that major volcanic debris can block 10 to 30 percent of incoming sunlight. He adds that the debris blocking incoming sunlight may also hold heat at the Earth's surface, mitigating the cooling through a greenhouse-type warming.

Other scientists dispute Mass and Portman's conclusions. Alan Robock of the University of Maryland in College Park thinks volcanoes have a definite influence on climate, asserting that even a fraction of a degree of hemispheric cooling can bring profound effects. A small average cooling over a hemisphere could involve extreme chilling of a few sensitive areas, and New England may have been such an area during the shivery summer of 1816, he says.