Scientists Say Deserts Are Expanding As Jet Streams Move Toward the Poles
By ANDREW BRIDGES
Associated Press Writer
(AP) 03:34:41 PM (ET), Thursday, May 25, 2006 (WASHINGTON)
Deserts in the American Southwest and around the globe are creeping toward heavily populated areas as the jet streams shift, researchers reported Thursday.
The result: Areas already stressed by drought may get even drier.
Satellite measurements made from 1979 to 2005 show that the atmosphere in the subtropical regions both north and south of the equator is heating up. As the atmosphere warms, it bulges out at the altitudes where the northern and southern jet streams slip past like swift and massive rivers of air. That bulging has pushed both jet streams about 70 miles closer to the Earth’s poles.
Since the jet streams mark the edge of the tropics, in essence framing the hot zone that hugs the equator, their outward movement has allowed the tropics to grow wider by about 140 miles. That means the relatively drier subtropics move as well, pushing closer to places like Salt Lake City, where Thomas Reichler, co-author of the new study, teaches meteorology.
“One of the immediate consequences one can think of is those deserts and dry areas are moving poleward,” said Reichler, of the University of Utah. Details appear in Thursday’s Science Express, the online edition of the journal Science.
The movement has allowed the subtropics to edge toward populated areas, including the American Southwest, southern Australia and the Mediterranean basin. In those places, the lack of precipitation already is a worry.
Additional creep could move Africa’s Sahara Desert farther north, worsening drought conditions that are already a serious problem on that continent and bringing drier weather to the countries that ring the Mediterranean Sea.
“The Mediterranean is one region that models consistently show drying in the future. That could be very much related to this pattern that we are seeing in the atmosphere,” said Isaac Held, a senior research scientist with the National Oceanic and Atmospheric Administration. He was not connected with the research.
A shift in where subtropical dry zones lie could make climate change locally noticeable for more people, said Karen Rosenlof, a NOAA research meteorologist also unconnected to the study.
“It is a plausible thing that could be happening, and the people who are going to see its effects earliest are the ones who live closer to the tropics, like southern Australia,” said Rosenlof. Her own work suggests the tropics have actually compressed since 2000, after growing wider over the previous 20 years.
Reichler suspects global warming is the root cause of the shift, but said he can’t be certain. Other possibilities include variability and destruction of the ozone layer. However, he and his colleagues have noted similar behavior in climate models that suggest global warming plays a role.
Moving the jet streams farther from the equator could disrupt storm patterns, as well as intensify individual storms on the poleward side of the jet streams, said lead author Qiang Fu, a University of Washington atmospheric scientist.
In Europe, for example, that shift could mean less snow falling on the Alps in winter. That would be bad news for skiers, as well as for farmers and others who rely on rivers fed by snowmelt.
“This definitely favors or enhances the frequency of droughts,” Fu said of such a shift.
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“Until he extends the circle of his compassion to all living things, man will not himself find peace.”
“Short-term climate fluctuations give rise to an El Niño event every few years. But an overall weakening of the Walker system could cause an increase in the severity or frequency of these events, and some experts even fear that the Pacific could be plunged into a permanent El Niño.”
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Published online: 3 May 2006; | doi:10.1038/news060501-5
Global warming weakens Pacific winds
Dwindling circulation could worsen El Niño effect.
Climate change is weakening a vast system of circulating winds that traverses the Pacific Ocean from coast to coast, say climate experts. Global warming has caused the system, which is crucial for monsoon rains in Southeast Asia and fisheries in South America, to decline since the advent of industrial times.
The system, known as the Walker circulation, has weakened by more than 3% since the mid-nineteenth century, report climate modellers led by Gabriel Vecchi of the US National Oceanic and Atmospheric Administration in Princeton, New Jersey. The cause, they say, is greenhouse gases. And with emissions still climbing, Pacific winds could potentially decline by more than 10% by the end of the century, they predict.
The observations, reported in Nature (1), back up climate model predictions that these winds should weaken, says Vecchi. “This is one of the most robust predictions of climate research,” he says.
Variations in the Walker circulation are one of the factors that lead to El Niño climate events. These periodic events, which feature a weakening of the easterly winds that blow from the Americas to Southeast Asia, have a range of effects, from droughts in Indonesia to poor fish harvests in Chile.
Short-term climate fluctuations give rise to an El Niño event every few years. But an overall weakening of the Walker system could cause an increase in the severity or frequency of these events, and some experts even fear that the Pacific could be plunged into a permanent El Niño.
Wet and wild
Global warming can disrupt winds because the rate of evaporation from the ocean increases more rapidly with increasing temperatures than does the rate of precipitation. Usually, in the Walker circulation, water evaporates from the warm waters of the eastern Pacific and travels across on the trade winds to Southeast Asia, where it rises and feeds rain. The dry air then heads east again at a higher altitude, completing the cycle (see graphic).
But as sea temperatures rise, the increase in rainfall cannot keep pace with the increase in evaporation. This means that moist air gets stalled, and the winds in both directions weaken.
Vecchi and his colleagues studied records of sea-level atmospheric pressure from 1861 to 1992. Because winds are driven by differences in atmospheric pressure, the difference between pressures on the two sides of the Pacific indicates the strength of the winds. The data show a decline in winds of around 3%, they found, with the trend most marked over roughly the last 50 years of the study.
They also used computer climate models to simulate the past and future performance of the Walker circulation, helping to work out how much of the effect is due to man-made greenhouse emissions. The answer, says Vecchi, is pretty much all of it. “At least 80% of this is attributable to human activities,” he says.
When the researchers ran the model without including human factors such as industrial greenhouse emissions, the change in wind strength all but disappeared. “Over the past 140 years, there should have been next to no trend,” Vecchi says.
Winds of change
It’s unclear exactly how this change to the Walker circulation will affect El Niño patterns, because weakening of winds is both a cause and an effect of this weather event, says Mick Kelly, a researcher at the Climate Research Unit at the University of East Anglia in Norwich, UK. “You can’t separate the chicken and egg,” he says.
Models run in Kelly’s lab have predicted slightly more subtle effects on the Walker circulation, such as more local eddies, which could shift the areas that experience the most drought and flooding. All the flood-vulnerable coastal villages and communities that depend on Pacific fisheries should be prepared for change, Kelly says. “At the moment we can’t have enough confidence in any individual model’s projections. But we can’t afford to wait until the science is absolutely certain; it will be too late.”
1. Vecchi G. A., et al. Nature, 441. 73 – 76 (2006). | Article |
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