Deserts May Be Creeping Closer to Cities
Deserts in the American Southwest and around the globe are creeping
toward heavily populated areas as the jet streams shift, researchers
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
"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 Friday in 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
"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.