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Lake Powell.

Photo Courtesy of NASA/GSFC

Soil moisture 30 cm below ground projected through 2100 for high emissions scenario RCP 8.5. The soil moisture data are standardized to the Palmer Drought Severity Index and are deviations from the 20th century average.

A report released last week predicts the worst drought in history in on the way for the central and southwest United States. And some of the data for the report came from an unlikely source.

A NASA report comparing a thousand years of historical data to several climate projections of temperature increase over the next century is paying close attention to moisture in the ground.

“We focus on soil moisture because, at the end of the day, it’s really soil moisture that plants care about,” said climate scientist Benjamin Cook. He says western trees are well suited to tell the tale of historical drought, being highly drought sensitive and tend to live long lives. The data is based on the width of tree trunk rings. The wider the ring, the more soil moisture that year.

“And what we found with these projections is a pretty robust response," Cook said. "A tendency of trends towards a much drier conditions in places like the southwest and the central plains. But more importantly, this drying is very likely to be the most extreme that we’ve seen in the last 1,000 years.”

If the multiple climate models predicting higher temperatures hold true, soil moisture evaporation rates will increase. Cook said data suggests an 80 percent chance that natural drought tendencies on top of climate change shifts will bring unprecedented drought conditions.