How Volcanoes Affect Climate -- and Rainfall

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As continuing eruptions of Mount Merapi in central Java threaten thousands of Indonesians and impose on the diplomatic mission of President Obama, their immediate impact on humans seems pretty clear. Less obvious are the impacts of volcanoes in the long-term on other features of our lives such as weather and climate.

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Since Benjamin Franklin’s day, earth scientists have been thinking about the ability of erupting volcanoes to cast ghastly, sun-blocking veils into the atmosphere that cool the climate, at least for a year or so.

Most famously, the 1815 eruption of Mount Tambora on the Indonesian island of Sumbawa provoked the infamous “Year Without a Summer” over much of the globe in 1816, although it would be a century before physicist William Humphreys at the U.S. Weather Bureau was able to put the two events together scientifically.

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WATCH VIDEO: Underwater volcanic eruptions and magma flows on the sea floor have been seen for the first time ever thanks to video captured by NOAA.

Major, explosive volcanic eruptions send tons of gaseous sulfur compounds high into the atmosphere. These compounds chemically form minute sulfate aerosol particles that together act like a parasol, defecting incoming solar radiation and temporarily cooling the planet.

This natural effect has inspired the “geoengineering” idea that humans could arrest the progress of global warming by pumping a bunch of sulfur dioxide into the atmosphere. Researchers, however, warn that the impact of such a scheme may not be so simple, or predicable.

WATCH: Could humans make a volcano erupt? Find out here.

The most recent research on the climate impact of volcanoes has produced some surprising results. Conventional thinking has it that cooler temperatures mean less evaporation, and in simple models, less evaporation translates into less rainfall. However, a more complicated picture emerges from a study by tree-ring researchers at Columbia University’s Lamont-Doherty Earth Observatory and elsewhere of data from 330 sites across Asia, spanning 800 years and 54 volcanic eruptions.

Rather than simply diminishing rainfall, they found that volcanic eruptions rearrange the rainfall patterns over the Asian monsoon region, where storms irrigate crops for nearly half the world’s population. Big eruptions cause drought in much of central Asia, the study finds. But contrary to model predictions, the eruptions bring more rain to southeast Asia — Vietnam, Laos, Cambodia, Thailand and Myanmar.

WATCH: Volcano photographer Richard Roscoe describes what it was like sitting a few feet away from an erupting volcano.

The study, published in the journal Geophysical Research Letters, also points to a complicated relationship between the effect of volcanic eruptions and cycles of El Nino, which changes temperatures over the Pacific and Indian oceans. In some instances El Nino seems to counteract the volcanic effects; in others, they seem to reinforce one another.

Rosanne D’Arrigo of Lamont-Doherty, a co-author of the study, notes that the data to test the models became available only recently. “Now,” she said, “it’s obvious there’s a lot of work to be done to understand how all these different forces interact.”

So how would artificially injecting tons of sulfur compounds into the atmosphere affect the rainfall that waters crops for half the world’s population? The answer, at the moment, seems to be anybody’s guess.

Image credit: NASA

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