科學家預言：美國西北將面臨21世紀「超級大旱」

The climate's "new normal" for most of the coming century will parallel the long-term drought that hit western North America from 2000 to 2004 - the most severedrought in 800 years - scientists report in a study published Sunday.

"The severity and incidence of climatic extremes, including drought, have increasedas a result of climate warming," the researchers said, adding that these long-termtrends are consistent with a 21st century "megadrought."

Crops and forests died and river basins dried, but as bad as conditions were duringthe 2000-04 drought, in the future they may be seen as the good old days, a group of10 researchers warned Sunday in the journal "Nature Geoscience."

Pinyon pine forests near Los Alamos, New Mexico, had begun to turn brown fromd rought stress in 2002, left. Another photo taken in 2004 from the same vantagepoint, right, show them grey and dead. (Photo by Craig Allen, U.S. Geological Survey)

Climate models and precipitation projections indicate this period will be closer tothe "wet end" of a drier hydroclimate during the last half of the 21st century, thescientists said.

"Climatic extremes such as this will cause more large-scale droughts and forest mortality, and the ability of vegetation to sequester carbon is going to decline,"said Beverly Law, a co-author of the study, professor of global change biology and terrestrial systems science at Oregon State University, and former science director of AmeriFlux, an ecosystem observation network.

The 2000-04 drought had the effect of amplifying climate change as vegetation withered and could no longer take up the greenhouse gas carbon dioxide from the atmosphere.

This drought cut carbon sequestration by an average of 51 percent in the western United States, Canada and Mexico, the scientists calculate, although some areas were hit much harder than others. As the plants died, they released more carbon dioxide into the atmosphere, with the effect of amplifying global warming.

"During this drought, carbon sequestration from this region was reduced by half," Law said. "That's a huge drop. And if global carbon emissions don't come down, the future will be even worse."

The effects are driven by human-caused increases in temperature, with associated lower soil moisture and decreased runoff in all major water basins of the western United States, researchers said in the study.

Drought has affected Colorado farm lands near Strasburg, Colorado, July 21, 2012.(Photo by Lance Cheung, USDA)

It is not clear whether or not the current drought in the West and Midwest, now being called one of the worst since the Dust Bowl, is related to these same forces, Law said. This study did not address that, and there are some climate mechanisms in western North America that affect that region more than other parts of the country.

But in the West, this multi-year drought was unlike anythin seen in many centuries, based on tree ring data. The last two periods with drought events of similar severity were in the Middle Ages, from 977-981 and 1146-1151. The 2000-04 drought affected precipitation, soil moisture, river levels, crops, forests and grasslands.

Ordinarily, Law said, the land sink in North America is able to sequester the equivalent of about 30 percent of the carbon emitted into the atmosphere by the use of fossil fuels in the same region.

But based on projected changes in precipitation and drought severity, scientists said that this carbon sink, at least in western North America, could disappear by the end of the century.

"Areas that are already dry in the West are expected to get drier," Law said. "We expect more extremes. And it's these extreme periods that can really cause ecosystem damage, lead to climate-induced mortality of forests, and may cause some areas to convert from forest into shrublands or grassland."

During the 2000-04 drought, runoff in the upper Colorado River basin was cut in half.
Crop productivity in much of the West fell five percent. The productivity of forests  and grasslands declined, along with snowpacks.

Evapotranspiration decreased the most in evergreen needleleaf forests, about 33 percent.

Although regional precipitations patterns are difficult to forecast, the researchers said in this report that climate models are underestimating the extent and severity of drought, compared to actual observations.

They say the situation will continue to worsen, and that 80 of the 95 years from 2006 to 2100 will have precipitation levels as low as, or lower than, this "turn of the century" drought from 2000-04.

This research was supported by the National Science Foundation, NASA, U.S. Department of Energy, and other government agencies. The lead author was Christopher Schwalm at Northern Arizona University. Other collaborators were from the University of Colorado, University of California at Berkeley, University of British Columbia and San Diego State University.