Yahoo News 3 Dec 07;
The tropics appear to have already expanded by at least the same margin as models predict for this century
The greenhouse effect is causing Earth's zone of tropical climate to creep towards the poles, according to a study whose release on Sunday coincided with the eve of a major UN conference on climate change.
The poleward expansion of the tropics will have far-reaching impacts, notably in intensifying water scarcity in the Mediterranean and the US "Sun belt" as well as southern Africa and southern Australia, it warns.
The paper, appearing in a new journal, Nature Geoscience, is an overview of the latest published research into atmospheric systems at the tropics.
For cartographers, the tropical belt is defined quite simply by the tropics of Cancer and Capricorn, at latitudes 23.5 degrees north and south of the equator.
For climatologists, though, where the tropics end is fuzzier because of the complexity of a powerful high-altitude wind pattern, known as the Hadley circulation.
These powerful jet streams are what determine precipitation patterns of the tropics, which is characterised by lots of rain in the central part of the belt near the Equator and by dryness at its fringes.
Some years ago, the first credible computer simulations predicted that, as the Earth warmed, the Hadley jet streams and their associated wind and rainfall patterns would move poleward.
Under the most extreme scenario, the tropics were on average predicted to expand by about two degrees latitude, equivalent to around 200 kilometres (120 miles) over the 21st century.
The new paper looks at a batch of recent studies based on five different types of measurement from 1979-2005.
It concludes that this change in the tropical jet streams has already happened -- and the worst-case scenario has already been surpassed.
"Remarkably, the tropics appear to have already expanded -- during only the last few decades of the 20th century -- by at least the same margin as models predict for this century," it says.
"The observed widening appears to have occurred faster than climate models predict in their projections of anthropogenic [man-made] climate change."
The five datasets variously find expansion ranging from two degrees to 4.8 degrees latitude over the 25 years, or 200-480 kms (120-300 miles).
Many worrisome questions arise from these findings, says the paper.
First and foremost is about the accuracy of climate models that drive scientific conclusions about the pace of global warming and, in turn, inform policymakers about how to address the problem.
Also unclear are the mechanisms that have caused the widening of the tropics, says the paper. Possible factors include the ozone hole, warming of the sea's surface and an increase in the tropopause, a boundary layer between the troposphere and the stratosphere.
In any case, a widening of the tropics carries "worldwide implications," according to the study.
"The edges of the tropical belt are the outer boundaries of the sub-tropical dry zones, and their poleward shift could lead to fundamental shifts in ecosystems and in human settlements.
"Shifts in precipitation would have obvious implications for agriculture and water resources and could present serious hardships in marginal areas."
It voices particular concern for semi-arid regions that are at the fringes of the sub-tropical dry zone, including the Mediterranean, the southwestern United States and 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 increase moisture to others," it warns.
The paper is lead-authored by Dian Seidel of the US National Center for Atmospheric Research (NOAA) Air Resources Laboratory at Silver Spring, Maryland.
Publication on Sunday came on the eve of a UN conference in Bali, Indonesia aimed at revamping efforts to tackle global warming.
Around 10,000 delegates are expected to attend the 11-day gathering, which is tasked with agreeing on a blueprint for two years of negotiations for a pact that will slash greenhouse-gas emissions from 2012 and beef up support for poor countries that will bear the brunt of climate change.
Expanding tropics could spur storms: study
Deborah Zabarenko, Reuters 3 Dec 07;
WASHINGTON (Reuters) - Earth's tropical belt is expanding much faster than expected, and that could bring more storms to the temperate zone and drier weather to parts of the world that are already dry, climate scientists reported on Sunday.
"Remarkably, the tropics appear to have already expanded -- during only the last few decades of the 20th century -- by at least the same margin as models predict for this century," the scientists said in the current edition of Nature Geoscience.
Scientists forecast the tropic belt would spread by about 2 degrees of latitude north and south of the Equator by the end of the 21st century. That amount of tropical expansion has already occurred, and was confirmed by five independent ways of measuring it, the report found.
For mapmakers and astronomers, there is no question about where the tropic zone ends: it is at 23.5 degrees north and south of the Equator at the Tropics of Capricorn and Cancer, boundaries determined by Earth's tilt on its axis. These tropical borders are the furthest point from the Equator where the sun shines directly overhead at the summer solstice.
But climate scientists define the tropic band by what happens on the land, in the water and in the air, and that is what is changing, the study said.
'SERIOUS HARDSHIPS'
Tropical temperatures are warm, and it rains a lot, with little seasonal or day-to-day change. The subtropics, by contrast, are generally dry. If the warm, wet tropical climate is spreading poleward, the dry subtropic climate may head for the poles too.
Those dry subtropical bands could include some of the most heavily populated places on Earth, the scientists said: the Mediterranean, the U.S. Southwest, northern Mexico, southern Australia, southern Africa and parts of South America.
"Shifts in precipitation patterns would have obvious implications for agriculture and water resources and could present serious hardships in marginal areas," the authors wrote.
For those who live in the middle latitudes, like most U.S., European and Asian residents, the change could affect the storm tracks that largely determines weather in these areas, said co-author Dian Seidel of the U.S. National Oceanic and Atmospheric Administration.
"Those storm tracks are linked with the position of the jet stream, which is one way we use to delineate the width of the tropics," Seidel said by telephone from NOAA's Air Resources Laboratory outside Washington. "The jet streams are moving poleward, and so, presumably, would the storm tracks."
This poleward migration of storm tracks is in line with predictions by the Intergovernmental Panel on Climate Change. This worldwide panel of scientists and policymakers is convening in Bali, Indonesia this week to determine how to deal with climate change.
(Editing by Vicki Allen)
Earth's tropics belt expands
Seth Borenstein, Associated Press, Yahoo News 2 Dec 07;
Earth's tropical belt seems to have expanded a couple hundred miles over the past quarter century, which could mean more arid weather for some already dry subtropical regions, new climate research shows.
Geographically, the tropical region is a wide swath around Earth's middle stretching from the Tropic of Cancer, just south of Miami, to the Tropic of Capricorn, which cuts Australia almost in half. It's about one-quarter of the globe and generally thought of as hot, steamy and damp, but it also has areas of brutal desert.
To meteorologists, however, the tropics region is defined by long-term climate and what's happening in the atmosphere. Recent studies show changes that indicate an expansion of the tropical atmosphere.
The newest study, published Sunday in the new scientific journal Nature Geoscience, shows that by using the weather definition, the tropics are expanding toward Earth's poles more than predicted. And that means more dry weather is moving to the edges of the tropics in places like the U.S. Southwest.
Independent teams using four different meteorological measurements found that the tropical atmospheric belt has grown by anywhere between 2 and 4.8 degrees latitude since 1979. That translates to a total north and south expansion of 140 to 330 miles.
One key determination of the tropical belt is called the Hadley circulation, which is essentially prevailing rivers of wind that move vertically as well as horizontally, carrying lots of moisture to rainy areas while drying out arid regions on the edges of the tropics. That wind is circulating over a larger area than a couple decades ago.
But that's not the only type of change meteorologists have found that shows an expansion of the tropics. They've seen more tropical conditions by measuring the amount of ozone in the atmosphere, measuring the depth of the lower atmosphere, and the level of dryness in the atmosphere at the edges of the tropics.
Climate scientists have long predicted a growing tropical belt toward the end of the 21st century because of man-made global warming. But what has happened in the past quarter century is larger and more puzzling than initially predicted, said Dian Seidel, a research meteorologist with the National Oceanic and Atmospheric Administration lab in Silver Spring, Md. She is the author of the newest study.
"They are big changes," she said. "It's a little puzzling."
She said this expansion may only be temporary, but there's no way of knowing yet.
Seidel said she has not determined the cause of this tropical belt widening. While a leading suspect is global warming, other suspects include depletion in the ozone layer and changes in El Nino, the periodic weather phenomenon in the Pacific Ocean.
Other climate scientists are split on the meaning of the research because it shows such a dramatic change — beyond climate model predictions. Some scientists, such as Richard Seager at Columbia University's Lamont-Doherty Earth Observatory, say changes in El Nino since the 1970s probably are a big factor and could make it hard to conclude there's a dramatic expansion of the tropical belt.
But climate scientists Andrew Weaver of the University of Victoria and Richard Somerville of the Scripps Institution of Oceanography said Seidel's work makes sense and that computer models have consistently been underestimating the ill effects of global warming.
"Every time you look at what the world is doing it's always far more dramatic than what climate models predict," Weaver said.
Both Weaver and Seidel said the big concern is that dry areas on the edge of the tropics — such as the U.S. Southwest, parts of the Mediterranean and southern Australia — could get drier because of this.
"You're not expanding the tropical jungles, what you're expanding is the area of desertification," Weaver said.
Nature Geoscience http://www.nature.com/ngeo
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