U. ARIZONA (US) — Tree rings in southwestern North America show that long-term droughts have been caused by a lack of both summer and winter rains.
The findings, published in the journal Geophysical Research Letters, contradict the commonly held belief that a dry winter rainy season is generally followed by a wet monsoon season, and vice versa.
The new research shows that for the severe, multi-decadal droughts that occurred from 1539 to 2008, generally both winter and summer rains were sparse year after year.
“One of the big questions in drought studies is what prompts droughts to go on and on,” says lead author Daniel Griffin, a doctoral candidate in the School of Geography and Development at the University of Arizona. “This gives us some indication that the monsoon and its failure is involved in drought persistence in the Southwest.”
The new 470-year-long history of summer precipitation in the Southwest covers most of Arizona, western New Mexico, and parts of northern Mexico.
“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,” says Griffin.
“Monsoon droughts of the past were more severe and persistent than any of the last 100 years,” he says. “These major monsoon droughts coincided with decadal winter droughts.”
Those droughts had major environmental and social effects, Griffin says, pointing out that the late-16th-century megadrought caused landscape-scale vegetation changes, a 17th-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 is that we’ve reconstructed the winter precipitation, but we’ve never known what the summers were like,” says co-author Connie A. Woodhouse, associate head and associate professor of geography and development.
Because winter precipitation has the strongest influence on annual tree growth, previous large-scale, long-term tree-ring reconstructions of the region’s precipitation history had focused only on the winter rainy season.
“Now we see—wow—the summers were dry, too,” she says. “That has a big impact.”
“In the Southwest, the winter precipitation is really important for water supply. This is the water that replenishes reservoirs and soil moisture,” Woodhouse says. “But the monsoon mediates the demand for water in the summer.”
Until recently, most tree-ring researchers, known as dendrochronologists, have looked at the total width of trees’ annual rings to reconstruct past climate. Few teased out the seasonal climate signal recorded in the narrow part of the growth ring laid down in late summer known as latewood.
To figure out the region’s past history of monsoon precipitation, the scientists needed to measure latewood from tree-ring samples stored in the archives of the Laboratory of Tree-Ring Research and go into the field to take additional samples of tree rings.
The team looked at annual growth rings from two different species, Douglas-fir (Pseudotsuga menziesii) and ponderosa pine (Pinus ponderosa) throughout the weather forecast region called North American Monsoon Region 2, or NAM2, which covers most of Arizona, western New Mexico and northern parts of the Mexican states of Sonora and Chihuahua.
In all, the researchers used samples from 50 to 100 trees at each of 53 different sites throughout southwestern North America. The team measured almost 1 million tree rings in four years.
The results were a surprise because rain gauge records for the Southwest from 1950-2000 show dry seasons alternated with wet ones.
But the team’s new multi-century record going back to 1539 shows that the wet/dry pattern of the latter part of the 20th century is not the norm—either prior to the 20th century or now, Woodhouse says.
One possible next step is to expand the current project to other areas of the Southwest and into Mexico, where the monsoon has a bigger influence on annual precipitation.
Another would be using tree-ring reconstructions of the Southwest’s fire histories to see how wildfires are related to summer precipitation.
“Before I moved to the Southwest, I didn’t realize how critically important the summer rains are to the ecosystems here. The summer monsoon rains have allowed humans to survive in the Southwest for at least 4,000 years.”
Researchers from the University of Arkansas in Fayetteville contributed to the study, which was supported by the National Science Foundation, the National Oceanic and Atmospheric Administration, and the US Environmental Protection Agency.
Source: University of Arizona