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How volcanoes can hurt the world's largest rivers

Scientists have long known the enormous climatic effects of giant volcanoes, but are only now discovering implications for the world's largest rivers.

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Guillermo Granja/Reuters
The Cotopaxi volcano, one of the world's highest active volcanoes, spews smoke as seen from Quito, Ecuador, September 21, 2015. Ecuadorian authorities are monitoring activity at Cotopaxi volcano, which prompted Ecuador's President Rafael Correa to declare a state of emergency and authorities to maintain a yellow alert for eruptions as bursts of ash keep spewing from the snow-encircled crater of the volcano and falling in gusts on residential communities.

Volcanic eruptions can reduce flow in some of the world’s largest rivers, which millions depend upon for clean drinking water, say researchers from the University of Edinburgh.

How it works: during a volcanic eruption, erupted ash is flung into the atmosphere, where it can linger for years and block sunlight, which cools the atmosphere and often leads to reduced rainfall, explains the study’s press release. And less rain means smaller rivers.

"It was known that volcanic eruptions affect global rainfall," explained lead researcher of the study Dr. Carley Iles to ABC Science, "but it was previously unclear to what extent this translated into changes in river flow."

A new study, published Monday in Nature Geoscience, analyzed annual water flow in 50 rivers around the world and used computer models to look for changes to the data from volcanic eruptions such as the 1963 eruption of Indonesia’s Agung, the 1982 eruption of Mexico’s El Chichón, and the 1991 eruption of the Philippines’ Pinatubo.

Impacts vary according to region, they found. In tropical regions, eruptions were followed by one to two years of reduced river flow. The estimated that that the Amazon and the Nile would drop in volume by about 10 percent for two years after a volcanic eruption. 

On the other hand, river flow increased in some drier, sub-tropical regions, including the Southwestern US and parts of South America, because of disrupted atmospheric circulation patterns. 

Rivers provide drinking water for millions of people. Some 20 million depend on the Amazon for their potable water, and the Nile could have an even higher level of human dependence, say the researchers.

"Future volcanic eruptions could substantially affect global water availability," they conclude, an issue of increasing importance in the 21st century. 

They therefore argue against looking to geo-engineering, or artificially modifying Earth's climate systems, as a solution to global warming.

Proponents of one geo-engineering technique, Solar Radiation Management, suggest mimicking volcanoes' ability to "inject particles – sulfate aerosols – high up into the atmosphere," explained Dr. Iles, in hopes that they will "spread out and reflect sunlight back out into space."

In other words, erupted ash particles can limit the amount of sunlight reaching Earth’s surface and therefore lower average global temperatures – and geo-engineers want to replicate this by seeding the sky with sulfur dioxide.

That poses the same risks to water availability as an eruption, says Iles.

"These kinds of geo-engineering schemes are likely to have side effects on river flow, so caution is advised," she warns.

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