Mount PinatuboUSGS

Here are five big volcano-related questions that have been answered in the last decade by volcanologists, according to Alan Robock of the Department of Environmental Sciences at Rutgers University. He summarized them in this week's issue of Eos, a publication of the American Geophysical Union (AGU).

1) What was the largest volcanic eruption on Earth since the historic Mount Pinatubo eruption on 15 June 1991?

It depends on what you are measuring, some of the big ones were Soufrière Hills, Montserrat (May 2006), Kasatochi in Alaska’s Aleutian Islands (August 2008), and Sarychev, in Russia’s Kuril Islands (June 2009). But the one that launched the most climate-cooling sulfur (1.5 megatons) into the stratosphere since Pinatubo was the Nabro, Eritrea eruption in June of 2011.

Toba, SumatraNASA/JPL

2) Was the Toba supereruption 74,000 years ago, the largest in the past 100,000 years, responsible for a human genetic bottleneck or a 1000-year long glacial advance?

New research shows that the glaciers were advancing before Toba blew its top. So nix the idea that Toba sparked a 1,000-year ice age. However that doesn't mean it could not have caused a shorter period of cooling and hardship for humans. Models that use Pinatubo as an example, but multiply its sulfur ejection into the stratosphere hundreds of times, show that a decade of cooling could have followed Toba. But the human half of the question is still unresolved.

ANALYSIS: Mount Doom: Don't Say We Didn't Warn You

A Scene on the Iceby Hendrick Avercamp via Wikimedia Commons

3) Did small volcanic eruptions play a role in the reduced global warming of the past decade? Did an eruption cause the Little Ice Age?

Yes, and yes. A bunch of eruptions injected sulfur into the stratosphere over the last decade (2000-2010), where they have slightly reduced (by a mean of -0.1 watts per meter) the amount of sunlight reaching the surface, and thereby contributed to less global warming during that period than in the previous several decades. But they didn't do it alone.

ANALYSIS: 'Halted' Global-Warming Tied to Pacific Cooling

They had working with them more human-made air pollution, a stubborn little girl -- a.k.a. La Niña -- slightly lower output from the Sun and less water vapor in the atmosphere. Meanwhile the anthropogenic warming added +0.3 watts per square meter. Likewise, climate modeling research and evidence from various locations has now found that a large eruption in the second half of the 13th century cooled Earth so much that it enlarged the Arctic sea ice, which reinforced the cooling until the 19th century. Which volcano? Probably Indonesia's Rinjani in 1258.

ANALYSIS: Little Ice Age Wasn't So Little

Eyjafjallajökull, IcelandNASA

4) Was the April 2010 Eyjafjallajökull eruption in Iceland important for climate change?

Nope. Not even close. In fact if it had happened in Alaska you probably wouldn't have even heard about it. Eyjafjallajökull failed to get its emissions high enough to reach the stratosphere, where they can hang around long enough to cool down the planet. As a result the aerosols from that eruption were washed out of the atmosphere in just days.

PHOTOS: Eruption of Iceland's Grimsvotn Volcano

A map of global aerosols. Dust (red) is lifted from the surface, sea salt (blue) swirls inside cyclones, smoke (green) rises from fires, and sulfate particles (white) stream from volcanoes and fossil fuel emissions.William Putman, NASA/Goddard

5) Do volcanic eruptions teach us about new ideas on geoengineering and nuclear winter?

Yes. Absolutely. They teach us that geoengineering by injecting aerosols into the stratosphere (as volcanoes do) is not really a great idea, and that nuclear winter is one of the worst ideas ever devised. Volcanoes can cool the climate, but they are an imprecise tool. The cooling they create can also cause droughts and alter monsoons. As for nuclear winter, we still have the nuclear armaments to do it and it would be the most severe and abrupt climate changer humanity has ever faced. There are no winners in that scenario, except cockroaches and rats, of course.

Full disclosure: Besides being a correspondent for Discovery News, Larry O'Hanlon is the blogosphere manager for AGU. He had no involvement with the Eos article or its author.