The chronic drought that hit the West from 2000-2004 left dying forests and depleted river basins in its wake and was the strongest such dry period in 800 years, a team of scientists led by an Oregon State University professor report.
They warn that those conditions will become the new normal for most of the coming century.
Such climactic extremes have increased as a result of global warming, a group of 10 researchers report in the current issue of "Nature Geoscience."
As bad as conditions were in the 2000-2004 drought, they may eventually be seen as the good old days, the researchers conclude.
Climate models and precipitation projections indicate this period will actually be closer to the wet end of a drier hydroclimate during the last half of the 21st Century.
Aside from its impact on forests, crops, rivers and water tables the drought also cut carbon sequestration by an average of 51% in a massive region of the West, the report states. As vegetation withered, this released more carbon dioxide into the atmosphere, with the effect of amplifying global warming.
"Climate 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," says Beverly Law, co-author of the study who is a professor of global change biology and terrestrial systems science in the OSU College of Forestry.
The reduction of carbon sequestration during the drought has significant implications, Law adds, commenting that "If global carbon emissions don't come down, the future will be even worse,"
The research was supported by the National Science Foundation, the National Aeronautics and Space Administration, the U.S. Department of Energy and other agencies.
The lead author is Christopher Schwalm of Northern Arizona University, with other collaborators from the University of Colorado and other institutions.
It is not clear whether or not the current drought, now being called one of the worst since the Dust Bowl, is related to these same forces, Law says. The study did not address that, and there are some climate mechanisms in the West that affect that region more than any other part of the country.
But in the West, the 2000-2004 drought was unlike anything seen in many centuries, based on tree ring data. In the last two periods with drought event of similar severity were in the Middle Ages, 977-981, and 1146-1151.
Ordinarily, points out Law, the land sink in North America is able to sequester the equivalent of about 30% of the carbon emitted into the atmosphere by the use of fossil fuels in the same region. However, based on projected changes in precipitation and drought severity, the scientists say this carbon sink, at least in the West, could disappear by the end of this century.
"Areas that are already dry in the West are expected to get drier," says Law. "We expect more extremes. It's these extreme periods that can really cause ecosystem damage."
Towards the end of the 21st Century the precipitation regime associated with the turn of the century drought will represent an outlier of extreme wetness, the report concludes.
These long-term trends are consistent with a "megadrought," they add.