Food securityLoss of summer rains lead to long droughts in southwest U.S.
Long-term droughts in southwestern North America often mean failure of both winter and summer rains, according to new research. The finding contradicts a commonly held belief regarding the region — that a dry winter rainy season is generally followed by a wet summer season, and vice versa. In fact, when severe, decades-long droughts have struck the area in centuries past, both winter and summer rains generally were sparse year after year.
Long-term droughts in southwestern North America often mean failure of both winter and summer rains, according to new tree-ring research. The finding contradicts a commonly held belief regarding the region — that a dry winter rainy season is generally followed by a wet summer season, and vice versa. In fact, when severe, decades-long droughts have struck the area in centuries past, both winter and summer rains generally were sparse year after year, the new study shows.
“One of the big questions in drought studies is what prompts droughts to go on and on,” said lead author Daniel Griffin, a doctoral candidate in the School of Geography and Development of the University of Arizona in Tucson. “This gives us some indication that the monsoon and its failure is involved in drought persistence in the Southwest.”
An American Geophysical Union release reports that a monsoon is a season of heavy rains caused by air rising over warm land, which draws in cooler, more humid air from the ocean. In most of Arizona, western New Mexico, and parts of northern Mexico — where the monsoon lasts from late spring to early fall—moisture-laden winds blow in from the Gulf of California and the eastern tropical Pacific Ocean.
The new study’s results surprised Griffin because rain gauge records for the Southwest from 1950 to 2000 show that dry seasons alternated with wet ones. The team’s new 470-year-long record, going from 2008 all the way back to 1539, shows, however, that the wet/dry pattern of the latter part of the 20th century is not the norm — either prior to the twentieth century or now, he said.
The research report by Griffin and his colleagues was published 11 March in Geophysical Research Letters, a journal of the American Geophysical Union.
“This is the first time researchers have used tree rings to take a closer look at the monsoon in a large and important area of the American Southwest,” Griffin said. “Monsoon droughts of the past were more severe and persistent than any of the last 100 years,” he added. “These major monsoon droughts coincided with decadal winter droughts.”
Those droughts had major environmental and social effects, Griffin said, pointing out that the late-sixteenth-century megadrought caused landscape-scale vegetation changes, a seventeenth-century drought has been implicated in the Pueblo Revolt of 1680 and the 1882-1905 drought killed more than 50 percent of Arizona’s cattle.
“The thing that’s interesting about these droughts
