'Sunshade' for global warming could cause drought

NewScientist.com news service
Catherine Brahic

Pumping sulphur particles into the atmosphere to mimic the cooling effect of a large volcanic eruption has been proposed as a last-ditch solution to combating climate change – but doing so would cause problems of its own, including potentially catastrophic drought, say researchers.

Sulphur "sunshades" are just one example of a "geo-engineering" solution to climate change. Such solutions involve artificially modifying our climate to counteract the effects of human greenhouse gas emission. Other examples include space mirrors and iron fertilisation of the ocean (see also Sunshade for the planet.

Recent research has suggested that sulphur sunshades could rapidly cool the climate back down to pre-industrial temperatures (see Solar shield could be quick fix for global warming).

However, a study, led by Ken Caldeira of the Carnegie Institution of Washington in the US, warned that failing to correctly deploy or maintain such a scheme would result in sudden warming – which would be worse than the long-term warming that had been avoided because of its swiftness.

Now, Kevin Trenberth and Aiguo Dai of the National Center for Atmospheric Research in Colorado, US, have shown that – even if correctly deployed – a sulphur sunshade could have deleterious effects on the environment by reducing rainfall.

Cooling cloud

Sulphur sunshades are inspired by the cooling effects of large volcanic eruptions, which blast sulphate particles into the stratosphere. The particles reflect part of the Sun's radiation back into space, reducing the amount of heat that reaches the Earth. In 1991, the eruption of Mount Pinatubo in the Philippines cooled Earth by a few tenths of a degree for several years.

To study the effects that sulphur sunshades might have on rainfall, Trenberth and Dai looked at trends in precipitation and continental run-off from 1950 to 2004 to try to detect the impact of the eruptions of Mount Agung in Indonesia 1963, El Chichón in Mexico in 1982, and Pinatubo in 1991.

The researchers had to account for the effects of El Niño, which tends to decrease rain over land, and increase it over the oceans. After this, a marked decrease in rainfall and run-off in the year after the Pinatubo eruption was clear (see graph, right).

However, the Agung and El Chichón eruptions did not produce a detectable signal in the precipitation records. Pinatubo is thought to have pumped significantly more particles into the atmosphere than Agung and El Chichón, releasing aerosols that increased the optical density of the atmosphere by about 10 times more than each of the other two. "We think those two were not strong enough to have an effect on precipitation," says Dai.

Dai and Trenberth say their results suggest that artificially putting large amounts of sulphate particles into the atmosphere in order to decrease solar radiation could have catastrophic effects on the planet's water cycle. "Creating a risk of widespread drought and reduced freshwater resources does not seem like an appropriate fix," they say.

They note that the negative effects experienced after Pinatubo erupted were harshest in the tropics.

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Journal reference: Geophysical Research Letters (DOI:10.1029/2007GL030524).