The south-western United States is almost certain to experience a “megadrought” lasting decades if global warming continues unchecked, researchers have warned.
The once-mighty Colorado river, which has regularly failed to reach the ocean since the 1960s, is already in the grip of the worst 15-year drought on record with the flow of water in the 21st century nearly a fifth lower than the 20th-century average, a new study found.
And scientists warned the river could be reduced by anything from 35 to 55 per cent by the end of this century if nothing is done to reduce greenhouse gas emissions.
Rising temperatures cause increased evaporation from the river, but also prompt plants to use more water.
A paper about the study in the journal Water Resources Research said: “With continued anthropogenic [human-caused] warming, the risk of multi-decadal megadrought in the Southwest increases to over 90 per cent over this century if there is no increase in mean precipitation.
“Even if modest precipitation increases do occur, the risk will still exceed 70 per cent.”
In the event of “huge and unlikely” increases in rainfall, there would still be a megadrought risk of just under 50 per cent.
Some 40 million people in seven US states and more in Mexico rely on the Colorado river for water.
But the researchers found that the river’s flow between 2000 and 2014 was 19 per cent lower than the average from 1906 to 1999 – equivalent to the amount of water used by two million people over a year.
One of the paper’s lead authors, Bradley Udall, of Colorado State University, said: “The future of Colorado river is far less rosy than other recent assessments have portrayed.
“Current planning understates the challenge that climate change poses to the water supplies in the American Southwest.
“A clear message to water managers is that they need to plan for significantly lower river flows.”
However he added that a prolonged drought – such as has occurred in the past – could make things substantially worse.
“A megadrought in this century will throw all our operating rules out the window,” Mr Udall said.
The paper said the current rate of warming was faster than seen during previous megadroughts.
“During the 12th century, flow reductions of approximately 16 per cent occurred during one 25-year period,” it said.
“Evidence indicates that hemispheric and Southwest temperature anomalies were significantly smaller during past megadroughts than the rapid ongoing current warming that could easily exceed 4C to 5C by the end of century under business-as-usual emissions.”
The researchers said there was one way to stop the rapid reduction in the size of the river.
“Temperature-driven threats to the flows of the Colorado are large and real,” the paper said. “The only way to curb substantial risk of long-term mean declines in Colorado river flow is thus to work towards aggressive reductions in the emissions of greenhouse gases into the atmosphere.
“The record warm nature of the ongoing Colorado River drought indicates that this drought is not just a natural drought, and our work demonstrates that flows are unlikely to return to the 20th-century averages if we only wait.
“Unusually wet periods like the 1920s and 1990s will still continue to occur, but they will co-occur with higher temperatures that will increase water demand from plants, soil, snow and humans.”
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