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The tropics are expanding towards both poles and this could increase the threat of land degradation in arid regions and cause deserts to spread, say researchers.

The findings, published on Friday (26 May) in Science, are based on satellite data from 1979 to 2005.

According to the study, the northern and southern hemispheres' jet streams — fast wind currents high in the atmosphere — have each moved about one degree of latitude, or some 113 kilometres, nearer the poles.

The researchers say the jet streams are being pushed polewards because the atmosphere is warming in the subtropics faster than anywhere else.

"The jet streams mark the edge of the tropics, so if they are moving poleward that means the tropics are getting wider," said co-author John Wallace of the University of Washington in Seattle, United States.

"If they move another two to three degrees poleward in the century, very dry areas such as the Sahara Desert could nudge farther towards the poles, perhaps by a few hundred miles," he added.

Wallace said that other areas including the northern Middle East and South Africa would face increased drought and desertification if the tropics continue to expand.

The researchers say more studies are needed to assess whether the patterns are linked to natural climatic variation or are a response to human-induced phenomena such as climate change or the depletion of the atmosphere's ozone layer.

Link to full paper in Science

Reference: Science 312, 1179 (2006)

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