The researchers found that Mount Pinatubo's eruption still kept much of the world dry, even after taking into consideration the drying effects of El Niño—an abnormal warming of surface ocean waters in the eastern tropical Pacific.
It's not clear why the sulfur particles reduce rainfall, but the team has a theory.
"First the particles block out the sun and cool off the land, making the rain move over the oceans," Trenberth said.
"Then they cool off the oceans—and that reduces evaporation and thus global precipitation."
In the case of the Agung and El Chichón eruptions, there was no detectable reduction in global precipitation. That's probably because these volcanoes didn't have quite as much oomph as Mount Pinatubo, experts say.
(Related: "Volcanic Activity Triggered Deadly Prehistoric Warming" [April 26, 2007].)
The findings should ring alarm bells for those considering pumping sulfur into the skies—and creating a solar shield—as a solution to global warming, the researchers warned. (Get the basics on global warming.)
"Our results suggest that major adverse effects, including drought, could arise from such 'geoengineering' solutions," Trenberth said.
Not everyone agrees that these findings rule out a solar shield.
"While looking at volcanoes as an analog for geoengineering is a good approach, we should be careful not to overinterpret the analogy," said Ken Caldeira, a geoscientist with the Carnegie Institution of Washington.
Caldeira has modeled the effect that a sulfur solar shield would have.
Volcanic eruptions throw tons of particles into the atmosphere in just a few days. But planned schemes would inject particles slowly over decades.
Caldeira said such differences make it difficult to compare planned schemes with volcanoes.
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