The Heat Is Online

Aerosol Cooling Would Cut Rainfall, Increase Drought

"Volcano Cure" for Warming? Not So Fast, Study Says


National Geographic News, Aug. 17, 2007


A controversial theory proposes mimicking volcanoes to fight global warming. But throwing sulfur particles into the sky may do more harm than good, a new study says.

The temporary solution would pump particles of sulfur high into the atmospheresimulating the effect of a massive volcano by blocking out some of the sun's rays. This intervention, advocates argue, would buy a little time to reduce greenhouse gas emissions.

But as well as cooling the planet, the sulfur particles would reduce rainfall and cause serious global drought, a new study says.

"It is a Band-Aid fix that does not work," said study co-author Kevin Trenberth of the National Center for Atmospheric Research (NCAR) in Boulder, Colorado.

It's just one of several drastic measures proposed to combat global warming, now that most scientists are in agreement that carbon dioxide, primarily from burning fossil fuels, is changing Earth's climate.

Drying Effects

Trenberth and NCAR colleague Aiguo Dai studied worldwide rainfall and streamflow records for the world's largest rivers between 1950 and 2004.

During this period three major volcanic eruptions occurred: Mount Agung in
Indonesia in 1963, El Chichón in Mexico in 1982, and Mount Pinatubo in the Philippines in 1991.

It's well known that particles thrown into the atmosphere by volcanic eruptions cause a global cooling effect by reflecting back sunlight.

In the case of Mount Pinatubo, global temperatures dropped by an average of 0.9 degrees Fahrenheit (0.5 degrees Celsius) the following year. But until now, no one had been able to pin down the effect that these volcanoes might have had on rainfall.

By carrying out statistical analysis on rainfall and streamflow records, the researchers were able to detect a significant drying effect after Mount Pinatubo's eruption.

There was less rainfall over land, and a record decrease in runoff and ocean discharge into the ocean from October 1991 to September 1992, the scientists report this week in the journal Geophysical Research Letters.

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 Niñoan abnormal warming of surface ocean waters in the eastern tropical Pacific.

Reduced Rainfall

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 oceansand 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.

Solar Shield?

The findings should ring alarm bells for those considering pumping sulfur into the skiesand creating a solar shieldas a solution to global warming, the researchers warned.

"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.