Climate change is having a dramatic impact on the tropics by pushing their boundaries towards the poles at an unprecedented rate not foreseen by computer models, which had predicted this sort of poleward movement only by the end of the century.
The report comes as representatives from 191 countries around the world assemble on the island of Bali in Indonesia, to negotiate a new international treaty to cut down on greenhouse gas emissions. Scientists have found that, during the past 25 years the equatorial region classified as climatologically tropical has expanded polewards by about 172 miles which has meant that a further 8.5 million sq miles of the Earth are now experiencing a tropical climate, compared to 1980.
The scientists warned there are grave implications for the many millions of people living in dry, subtropical regions bordering the tropics, which are at risk of becoming even more arid because of changes to rainfall patterns and wind directions.
"Several lines of evidence show that, during the past few decades, the tropical belt has expanded. This expansion has potentially important implications for subtropical societies and may lead to profound changes to the global climate system," the scientists say in their study published online in the journal Nature Geoscience.
"Most importantly, poleward movement of large-scale atmospheric circulation systems, such as jet streams and storm tracks, could result in shifts in precipitation patterns affecting natural ecosystems, agriculture and water resources," they say.
They are particularly concerned about the poleward movement of subtropical dry belts that could affect water supplies and agriculture over vast areas of the Mediterranean, the south-western United States, northern Mexico, southern Australia, southern Africa and parts of South America.
"A poleward expansion of the tropics is likely to bring even drier conditions to these heavily populated regions but may bring increased moisture to other areas," the scientists warn.
"An increase in the width of the tropics could bring an increase in the area affected by tropical storms, or could change climatologically tropical cyclone development regions and tracks," they say.
They also point out that the expansion of the tropical band could exacerbate global warming by increasing the rate at which water vapour an important greenhouse gas is being pumped naturally into the upper atmosphere. They warn that could lead to irreversible climate change.
The study was carried out by Dian Seidel of the US National Oceanic and Atmospheric Administration in Washington, her colleagues from the National Centre for Atmospheric Research in Boulder, Colorado, and the universities of Washington in Seattle and Utah in Salt Lake City.
They found that, during the past quarter-century, the area defined as tropical, based on a list of five recognised climatological criteria, has moved further north and south by about 2.5 degrees of latitude, or about 172 miles in total in both directions. That is greater than the predicted shift of 2 degrees by 2100 predicted under the "extreme scenario" envisaged by the Intergovernmental Panel on Climate Change.
"We looked at how certain aspects of the structure and circulation of the atmosphere have been altered over the past few decades and how models predict they may change as the climate changes in the future," Dr Seidel said. "We are seeing indications that a warming climate is associated with expansion of the tropical region towards the poles, and the rate of expansion that has occurred in recent decades is greater than projected by climate models to occur in the 21st century," she said.
Climatologists have long suspected that a warmer world will lead to an expansion of the tropics, which are defined by patterns of wind circulation, ozone concentrations and the height of the troposphere, but few had predicted that the dramatic shift observed by Dr Seidel and her colleagues would have occurred already.
Computer models of the global climate, for instance, had suggested a polewards shift of the tropics by as much as 2 degrees of latitude by the end of the 21st century. "Remarkably, the tropics appear to have already expanded during only the last few decades of the 20th century by at least the same margins as models predict for this century," Dr Seidel said.
"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.
"Shifts in precipitation patterns would have obvious implications for agriculture and water resources and could present serious hardships in marginal areas," she said.
Australia is one of the countries likely to be worst affected by the shifting tropics because westerly winds bringing much-needed rain to the continent's arid south coast are likely to be pushed further south, dumping their water over open ocean rather than on land, scientists said.
"An expansion of tropical pathogens and their insect vectors is almost certainly sure to follow the expansion of tropical zones," said Professor Barry Brook of the University of Adelaide.
"The global implication is the unexpectedly rapid expansion of the tropical belt constitutes yet another signal that climate change is occurring sooner than expected," Professor Brook said.
"The case for rapid action on greenhouse gas emissions becomes that much more compelling," he said.
A defining feature of our climate system
The tropics are one of the defining features of the Earth's climate system. Their existence is due to the fact that the region receives the greatest amount of the Sun's energy per unit of surface area. Map makers define the boundaries as the Tropic of Cancer, about 23.5 degrees north of the equator, and the Tropic of Capricorn in the south. These are the points where the Sun is directly overhead during the summer and winter solstices. However, climatologists define the tropical boundaries in a more complicated manner, based on five different sets of criteria, which are mostly connected to the way the air and oceans circulate around the hot equatorial region. Directly over the equator, the hot air rises, bringing with it moisture that accounts for tropical storms. Further away from the equator, the air descends, which tends to make these subtropical regions drier. Scientists have found that the boundaries of the tropics are shifting polewards.
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