Page last updated at 02:40 GMT, Tuesday, 4 December 2007

'Tropics expand' as world warms

Drought in India. Image: AP
Expansion of the tropics could mean an increase in droughts

Climate change is causing the tropics to widen, with possible impacts on the global food supply, research suggests.

Scientists examined five different measures of the width of the tropical belt, and found it expanded by between 2 and 4.8 degrees latitude since 1979.

Other researchers meanwhile said climatic change could increase the number of thunderstorms in the US.

The findings emerged as delegates met in Bali for UN climate talks focusing on reducing greenhouse gas emissions.

The capacity of poorer countries - many of them in the tropics - to respond and adapt to impacts of climate change will be another major theme of the talks.

Widening belts

The new analysis of tropical expansion comes from a team of US scientists who reviewed five separate strands of evidence, all gathered from satellite data.

While geographers define "The Tropics" rigidly as the region between 23.5 degrees North and 23.5 degrees South, to atmospheric scientists it is a more variable zone marked by features such as the jet stream and the circulation known as Hadley cells.

Food bowl. Image: Irri

On these measures, the tropics have expanded since the era of reliable satellite observation began in 1979.

"The edges of the tropical belt are the outer boundaries of the subtropical dry zones, and their poleward shift could lead to fundamental shifts in ecosystems and in human settlements," the researchers write in the journal Nature Geoscience.

"Shifts in precipitation patterns would have obvious implications for agriculture and water resources, and could present serious hardships in marginal areas."

The Intergovernmental Panel on Climate Change (IPCC) warned in its series of reports this year that serious impacts on food and water supplies lie ahead, including:

  • 75-250 million people across Africa could face water shortages by 2020
  • Crop yields could increase by 20% in East and South East Asia, but decrease by up to 30% in Central and South Asia
  • Agriculture fed by rainfall could drop by 50% in some African countries by 2020

The scientists behind the new study note that the tropical zone appears to be expanding much faster than predicted by computer models.

Thunder rolls

While impacts on agriculture could prove important for developing countries, a bigger concern for richer nations such as the US may be the damage wrought by extreme weather.

The IPCC forecasts stronger hurricanes in the future, but possibly fewer of them. Now another US team is suggesting an increase in thunderstorms over the country as well.

In the journal Proceedings of the National Academy of Sciences (PNAS), researchers report a computer modelling study that projects a doubling of the frequency of weather conditions right for the formation of severe thunderstorms.

Already, they write, extreme weather events are costing the US economy more than $2bn (970m) each year.




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