Western North America Faces 21st Century 'Mega-drought'
Photo Credit: Lance Cheung, USDA
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CORVALLIS, Oregon, July 30, 2012 (ENS) - 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 severe drought in 800 years - scientists report in a study published Sunday.
"The severity and incidence of climatic extremes, including drought, have increased as a result of climate warming," the researchers said, adding that these long-term trends are consistent with a 21st century "megadrought."
Crops and forests died and river basins dried, but as bad as conditions were during the 2000-04 drought, in the future they may be seen as the good old days, a group of 10 researchers warned Sunday in the journal "Nature Geoscience."
Climate models and precipitation projections indicate this period will be closer to the "wet end" of a drier hydroclimate during the last half of the 21st century, the scientists 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.
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 anything 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.