A climate map of the Western U.S. 21,000 years ago may help predict what the region could be in for in the coming decades -- and that prediction includes a possible "megadrought," researchers say.
Based on paleoclimate trends, scientists are putting the odds of the U.S. Southwest sliding into a decades-long megadrought at around 50/50.
In contrast, the U.S. Pacific Northwest -- although likely to be slightly drier during summers -- should see much wetter winters, they say.
Researchers at Vanderbilt University and Stanford University say their comprehensive map of the region's climate 21,000 years ago is helping them test and improve climate models that can predict how rainfall patterns might change in the future.
In a time period 21,000 years ago called the Last Glacial Maximum, at the height of the last Ice Age, the Southwest was wetter than is seen now -- a lot wetter -- while the Northwest was much drier, the researchers report.
The nature of the climate back then offers insight into the history of climate change taking place in the region, and yields clues to future trends, the researchers say.
"Most of the previous research of the past climate in this region is based on detailed studies of specific sites," says lead study author Jessica Oster, a Vanderbilt professor of environmental and earth sciences. "We combined these records to create a detailed map of past climate change in the American West. We then compared this map to computer climate models to understand what caused these changes."
Some previous site-specific studies in the Northwest had yielded evidence of a drier climate during the Last Glacial Maximum, while similar studies in the Southwest found evidence for a wetter climate.
The difference is the result of a transition zone running through the region, the researchers explain, with strong competing forces including changes in atmospheric circulation, changes in ocean surface temperature during La Niña and El Niño periods, and the moving dynamics of storm tracks all interacting to affect rainfall in the Western United States.
"As a result, understanding the exact nature of how these different effects express themselves to form the north/south transition zone will be extremely important for freshwater resource management in major population centers across the Western U.S.," says study co-author Mathew Winnick of Stanford University.
If global warming continues there is little chance of the ancient weather pattern -- a drier Northwest and a wetter Southwest -- returning in the foreseeable future, which suggests warnings of a megadrought need to be taken seriously, the researchers say.
