To predict the future climate of the southwestern United States and the Central Plains, the researchers used 17 climate models and ran simulations based on a “business as usual” scenario, in which the growth in greenhouse gas emissions followed current trends, or based on a “moderate reduction” scenario, in which growth in greenhouse gas emissions was more modest.
The study used three measures of drought, all of them indications of levels of soil moisture available to plants. These were soil moisture content from surface to 30-cm depth; soil moisture content from surface to 2-m depth; and the Palmer Drought Severity Index (PDSI), a measure of the difference between soil moisture supply (from precipitation) and soil moisture demand (from evaporation and uptake by plants). In the PDSI, negative values mean drier conditions—drought—and positive values mean wetter conditions. When the researchers combined data from the past, present, and future, the results were stunning—and troubling.
The climate models predicted unprecedented levels of drought in both regions through 2100, with high levels of agreement between the 17 climate models and the three measures of soil moisture. The study concluded that the drying of soils was not being driven by drastic reductions in precipitation but rather by increased evaporative pressure, that is, warmer temperatures leading to higher rates of evaporation and elevated rates of soil water uptake by plants