May 25, 2006

Subtropic warming could mean bigger deserts- study

By Deborah Zabarenko

WASHINGTON (Reuters) - Earth's atmosphere is warming faster over the subtropics than anywhere else, which could mean bigger deserts and more drought from Africa to Australia to the Middle East, researchers said on Thursday.

The fast-heating area girdles the globe at about 30 degrees north and south latitude, crossing the southern United States, southern China and north Africa in the Northern Hemisphere, and southern Australia, South Africa and southern South America in the Southern Hemisphere.

Based on 25 years of satellite data, researchers at the University of Washington also determined that the jet streams -- a pattern of westerly winds that help drive weather in both hemispheres -- have shifted about 70 miles toward their respective poles.

This is important because the jet streams mark the northern and southern boundaries of the tropic climate zones, said John Wallace, an atmospheric scientist and co-author of a research paper in this week's Science journal. The jet streams' shift toward the poles means the zones are expanding.

The research is not predictive, but does show a long-term trend, Wallace said by telephone.

"If (this jet-stream shift) is going to stop and it just ends up being 70 miles, that's not a big deal," he said. "But if it were to continue at the same rate over the next century, then that would amount to a couple of hundred miles (kilometres) and that would start to have significant effects."


The dry subtropical climate regions, which contain some of the world's major deserts, could encroach into temperate regions, Wallace said. Areas such as the Mediterranean, southern Europe and the northern part of the Middle East could have a tendency toward more drought, Wallace said.

The same might happen in southern Australia and South Africa, he said.

The study does not address whether this warming is due to the greenhouse effect or some other factor. It is different from previous models, which saw the fastest warming in the tropics, rather than the subtropics.

The greenhouse effect is seen as a major cause for global warming, in which so-called greenhouse gases, especially carbon dioxide, swaddle the Earth like a blanket, keeping the sun's warmth.

Some greenhouse warming is natural, but many scientists believe that accelerated warming over the last century was caused by human activities including coal-burning power plants and the use of other fossil fuels.

Faster subtropical warming in the lower atmosphere, which moves the jet streams, could push storm tracks toward the poles, possibly reducing winter precipitation in places like southern Europe, including the Alps, and southern Australia, the scientists said in a statement.