The National Science Foundation, the National Oceanic and Atmospheric Administration and the U.S. Environmental Protection Agency supported the research.

"In the Southwest, the winter precipitation is really important for water supply. This is the water that replenishes reservoirs and soil moisture," Woodhouse said. "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 UA 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.

In all, the researchers used samples from 50 to 100 trees at each of 53 different sites throughout southwestern North America. The team’s climate analyses focused on NAM2, which covers most of Arizona, western New Mexico and northern parts of the Mexican states of Sonora and Chihuahua.

Griffin said, "It was a massive undertaking – we employed about 15 undergraduates over a four-year period to measure almost 1 million tree rings."

The results surprised him because rain gauge records for the Southwest from 1950-2000 show dry seasons alternated with wet ones.

However, 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, he said.

One possible next step, Woodhouse said, 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.

Griffin said, "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."

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