'A Threat To Millions'
tropical belt that girdles the Earth is expanding north and south, which
could have dire consequences for large regions of the world where the
climate is likely to become more arid or more stormy, scientists have
warned in a seminal study published today.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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,"
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.
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.
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.
in precipitation patterns would have obvious implications for agriculture
and water resources and could present serious hardships in marginal
areas," she said.
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.
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.
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.
case for rapid action on greenhouse gas emissions becomes that much
more compelling," he said.
feature of our climate system
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.
Independent News and Media Limited
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