Using the latest high-resolution global climate simulations, the scientists show evaporation caused by warming temperatures is likely to leave less water for the rivers that flow out of the high country in northern New Mexico and Colorado that supplies much of the state’s water.
The finding is consistent with earlier research. But by using the latest models, which can more accurately account for weather across the region’s complex terrain, the new research gives a clearer picture of the situation, said Richard Seager, the climate scientist who led the team doing the work.
“We were able to make a more fine-grained picture,” Seager said. “The story becomes more robust because of that.”
The research, published in December in the peer-reviewed Nature Climate Change, is the latest in a series of research efforts going back to the early 1990s that have come to the same conclusion – as atmospheric levels of greenhouse gases increase, driving up temperatures, water supplies across the arid western United States will decline.
The use of the latest models is an important development, according to Brad Udall, who heads up a Colorado River water policy research group at the University of Colorado.
Scientists disagree about whether we are already seeing the phenomenon in current drought conditions. An analysis by Seager and a colleague in 2010 concluded that natural drought patterns could be sufficient to explain the drought conditions across the Colorado River Basin and much of the southwestern United States over the first decade of the 21st century.
Those natural patterns also can lead to long wet periods, Seager said in a telephone interview. “Climate change does not shut down natural variability,” he said.
But superimposed on those natural wet and dry cycles, which can last for decades, the long-term warming trend will inevitably sap the region’s water supplies, he said.
The mountain headwaters of the Colorado River Basin, which supplies a significant share of New Mexico’s water, illustrates the problem. The region is actually projected, on average, to see a slight increase in precipitation, Seager and his colleagues found. But increased evaporation as a result of warming temperatures wipes out any water supply benefit. For water management, farming and ecosystem health, it is precipitation minus evaporation that matters, the scientists wrote.
In Texas, Seager and his colleagues concluded that warming temperatures will lead to a 10 percent reduction in streamflow over the 2021-2040 time period. In California, the simulations suggest a 20 percent drop in the key spring runoff time period. For the Colorado River Basin, the average annual runoff drops 10 percent, with a 25 percent drop during the key spring runoff season.
Seager and his colleagues did not directly analyze runoff in the Rio Grande Basin, New Mexico’s other major water supply. But, sandwiched as it is between Texas and the Colorado River Basin, the outcome on the Rio Grande is likely to be similar, he said.
The study comes amid renewed discussion about the gap between water supply and demand in the western United States, with a new state-federal report released in mid-December showing increasing shortfalls as rising population increases water demand in New Mexico and the six other US states that share the Colorado River’s water, while climate change reduces supply.
— This article appeared on page C1 of the Albuquerque Journal