Volcanoes, El Ninos: climatic ties?

Volcanoes, El Ninos: Climatic ties?

Scientists have long recognized the link between volcanic eruptions and climatic changes. Benjamin Franklin may have been the first to suggest such a link in 1784 after the Laki volcano in Iceland erupted, sending a blue haze over Europe. Franklin suspected that the haze shielded the land from the sun's rays and resulted in an unusually severe winter. But researchers have not always found the predictd tropospheric cooling after major eruptions. A few months following El Chichon's large eruption in 1982, for example, global air temperatures actually rose.

"The evidence for cooling is rather mixed, to put it mildly," says James K. Angell, a meteorologists at the National Oceanic and Atmospheric Administration (NOAA) in Silver Spring, Md.

Now Angell proposes a reason: The cooling effect of volvanic aerosals has been overwhelmed by the occurrence of an El Nino, an unusual warming of Pacific waters that also warms the air (see p. 184. "Looking back at past eruptions, there's at least a hint that this is plausible," he says. "Eruptions followed by El Ninos are the ones for which evidence of cooling is pretty weak." By subtracting the temperatur increases due to the 1982-83 El Nino, Angell was able to show in a simple calculation that El Chichon by itself would have caused cooling.

Paul Handler, a physicist at the University of Illinois at Urbana-Champaign, thinks the interfering effects of El Ninos are no coincidence. He has argued that eruptions of volcanic aerosols into the stratosphere actually trigger El Ninos (SN: 5/5/84, p. 287). Now he says there is an even stronger statistical link between eruptions and the strength of Indian monsoons, another weather change thought to be related to El Ninos.

According to Handler, who also presented a paper at the symposium, the monsoons can be thought of as a giant sea breeze in which moist air is carried from the ocean onto the land, producing clouds and rain. When volcanic aerosols shade the land, the land temperature drops and so does the temperature difference between the land and sea which drives the monson winds. A large volcanic eruption at low latitudes would cause a weaker monsoon. The injection of aerosols from high latitudes would strengthen the monsoon, says Handler. These effects, he notes, don't become evident unless aerosols are in teh stratosphere for three to six months and solar radiation at the ground has decreased by at least a few tenths of a percent. "From 1942 through 1984, the Indian monsoon can be explained [in this way] without a contrary case," he says.

Handler's ideas about volcanoes, El Ninos and now perhaps monsoons have been somewhat controversial. Handler himself thinks the reason other scientists haven't accepted his theory is that it is so simple. He also argues that one reason climatologists have been unable to see clear trends in the climatic effects of volcanoes is that their studies lump all eruptions together, regardless of latitude. He notes that, as he predicted, sea surface temperatures off the coast of Peru have now begun to rise, three months after an eruption in Colombia.

Other scientists, such as J. Murray Mitchell at NOAA, question the statistical validity of Handler's results since there have been so few major volcanic eruptions in th last century. "I think it's too soon to know if his conclusions will stand the test of time," says Mitchell.

Mitchell and others say they are most concerned about the climatic effects of volcanoes because these effects can obscure the potentially more serious climatic changes resulting from carbon dioxide and other "greenhouse" gases (SN: 9/14/85, p. 170). In order to show policymakers that "we're facing an important greenhouse warming in the future, we have to show that this warming is already taking place," says Mitchell. "Volcanic eruptions can confound that picture and delay the day we can pin down these effects to carbon dioxide.