Wednesday, January 12, 2005

Global Warming major factor in Drought increasing globally

Image Label: 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 Society.)

According to a new analysis by scientists at the National Center for Atmospheric Research (NCAR) in the USA the percentage of Earth's land area stricken by serious drought more than doubled from the 1970s to the early 2000s. Iguo Dai, lead author of the study, said that rising global temperatures appear to be a major factor. Widespread drying occurred over much of Europe and Asia, Canada, western and southern Africa, and eastern Australia.

Climate Change major factor in Drought increasing

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 already begun."

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 NiƱo, a more frequent visitor since the 1970s, tends to inhibit precipitation.

Future Heat Waves: More Severe, More Frequent, Longer Lasting

In August a new modeling study by two scientists at the National Center for Atmospheric Research (NCAR) in the United States predicted Heat Waves will become more intense, more frequent, and longer lasting in the 21st century.

Model results show that an increase in heat-absorbing greenhouse gases intensifies an unusual atmospheric circulation pattern already observed during heat waves in Europe and North America. As the pattern becomes more pronounced, severe heat waves occur in the Mediterranean region and the southern and western United States. Other parts of France, Germany, and the Balkans also become more susceptible to severe heat waves.

"It's the extreme weather and climate events that will have some of the most severe impacts on human society as the climate changes," says Meehl.

Heat waves can kill more people in a shorter time than almost any other climate event. By a measure of excess death reports, 739 people died as a result of Chicago's heat wave July 14-20, 1995. Fifteen thousand are estimated to have died from heat in France in August 2003, along with thousands of farm animals.

For the study, Meehl and Tebaldi compared present (1961-1990) and future (2080-2099) decades to determine how greenhouse gases and sulfate aerosols might affect future climate in Europe and the United States, focusing on Paris and Chicago. They assumed little in the way of policy intervention to slow the buildup of greenhouse gases.

During the Paris and Chicago heat waves, changes in atmospheric pressure produced clear skies and prolonged hot conditions at the surface. In the model, similar atmospheric pressure changes are enhanced during heat waves in both regions as carbon dioxide accumulates in the atmosphere.

See: Meehl, G.A. and C. Tebaldi, 2004: More Intense, More Frequent, and Longer Lasting Heat Waves in the 21st Century (PDF) Science, 305, 994-997.