last 30 years, the earth doubles it's drought
The percentage of Earth's land area stricken by serious
drought more than doubled from the 1970s to the early 2000s,
according to a new analysis by scientists at the National
Center for Atmospheric Research (NCAR). Widespread drying
occurred over much of Europe and Asia, Canada, western and
southern Africa, and eastern Australia. Rising global
temperatures appear to be a major factor, says NCAR's Aiguo
Dai, lead author of the study.
January 13, 2005
This depiction of linear trends in the Palmer Drought Severity Index
from 1948 to 2002 shows drying (reds and pinks) across much of Canada,
Europe, Asia, and Africa and moistening (green) across parts of the
United States, Argentina, Scandinavia, and western Australia.
(Illustration courtesy Aiguo Dai and the American Meteorological
Dai will present the new findings on January 12 at the American
Meteorological Society's annual meeting in San Diego. The work also
appears in the December issue of the Journal of Hydrometeorology in a
paper also authored by NCAR's Kevin Trenberth and Taotao Qian. The study
was supported by the National Science Foundation, NCAR's primary
Dai and colleagues found that the fraction of global land experiencing
very dry conditions (defined as -3 or less on the Palmer Drought
Severity Index) rose from about 10-15% in the early 1970s to about 30%
by 2002. Almost half of that change is due to rising temperatures rather
than decreases in rainfall or snowfall, according to Dai.
"Global climate models predict increased drying over most land areas
during their warm season, as carbon dioxide and other greenhouse gases
increase," says Dai. "Our analyses suggest that this drying may have
Even as drought has expanded across Earth's land areas, the amount of
water vapor in the air has increased over the past few decades. The
average global precipitation has also risen slightly. However, as Dai
notes, "surface air temperatures over global land areas have increased
sharply since the 1970s." The large warming increases the tendency for
moisture to evaporate from land areas. Together, the overall area
experiencing either very dry or very wet conditions could occupy a
greater fraction of Earth's land areas in a warmer world, Dai says.
Though most of the Northern Hemisphere has shown a drying in recent
decades, the United States has bucked that trend, becoming wetter
overall during the last 50 years, says Dai. The moistening is especially
notable between the Rocky Mountains and Mississippi River. Other parts
of the world showing a moistening trend include Argentina and parts of
western Australia. These trends are related more to increased
precipitation than to temperature, says Dai.
"Droughts and floods are extreme climate events that are likely to
change more rapidly than the average climate," says Dai. "Because they
are among the world's costliest natural disasters and affect a very
large number of people each year, it is important to monitor them and
perhaps predict their variability."
To see how soil moisture has evolved over the last few decades, Dai and
colleagues produced a unique global-scale analysis using the Palmer
index, which for decades has been the most widely used yardstick of U.S.
drought. The index is a measure of near-surface moisture conditions and
is correlated with soil moisture content.
Since the Palmer index is not routinely calculated in most of the world,
Dai and colleagues used long-term records of temperature and
precipitation from a variety of sources to derive the index for the
period 1870-2002. The results were consistent with those from a
historical simulation of global land surface conditions, produced by a
comprehensive computer model developed by scientists at NCAR, NASA,
Georgia University of Technology, the University of Texas at Austin, and
the University of Arizona.
By factoring out rainfall and snowfall, Dai and colleagues estimated how
much of the global trend in soil moisture was due solely to rising
temperatures through the extra evaporation they produce.
"The warming-induced drying has occurred over most land areas since the
1970s," says Dai, "with the largest effects in northern mid and high
latitudes." In contrast, rainfall deficits alone were the main factor
behind expansion of dry soils in Africa's Sahel and East Asia. These are
regions where El Niño, a more frequent visitor since the 1970s, tends to
Source: National Center for Atmospheric Research
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