The climate crisis has caused the ailing Colorado River basin, a system relied upon by 40 million people in the US west, to lose more than 10tn gallons of water in the last two decades, new research has found.
The volume of water lost due to rising global temperatures has been so enormous that it is equal to the entire storage capacity of Lake Mead, the US’s largest reservoir that was formed by the Hoover Dam, or enough water to fill around 15bn Olympic-sized swimming pools.
The Colorado River provides vital water supplies to people across the US west, as well as nourishes ecosystems and millions of acres of farmland, but has dwindled since 2000 due to a “megadrought” that has been significantly worsened by climate change.
Without the influence of human-caused global heating, researchers for the new study found, reservoir levels wouldn’t have slumped to such low levels that the first ever federally declared water shortage was declared, requiring a desperate, temporary deal to be struck between states in May to cut water use.
Benjamin Bass, a hydrological modeler at the University of California-Los Angeles and lead author of the study, said the researchers were “surprised” at how sensitive the Colorado River basin is to warming temperatures.
“The fact that warming removed as much water from the basin as the size of Lake Mead itself during the recent megadrought is a wakeup call to the climate change impacts we are living today,” Bass said.
The Colorado River basin covers the vast sweep of the US west, across seven states, that is drained by the Colorado River and its tributaries. This area, spanning around 250,000 sq miles, is critical as a water resource for cities, cropland and wildlife but has withered under the region’s worst drought in at least 1,200 years.
A new hydrological model developed for the new study shows that warming temperatures – the basin is 1.5C (2.7F) hotter on average than it was in 1880 – has led to a 10.3% reduction in water runoff. Parts of the basin that are usually snow-covered in winter, which builds up vital snowpack that then melts and nourishes the river in springtime, are losing water at twice the rate of snowless regions, suggesting the Rocky Mountain west is shifting to a more arid state rather than experiencing a mere prolonged drought, Bass said.
Rising global heat is responsible, in total, for 10tn gallons (or about 40tn liters) of water to be lost from the Colorado River system since 2000, the study concluded. This is enough to fill the entire Lake Mead, which has hit record-low levels due to the drought, or 15bn Olympic swimming pools.
The deal struck between California, Nevada and Arizona in May brought a temporary reprieve to the growing crisis around the Colorado River, as did the torrential bursts of rainfall and snowfall over winter that helped replenish many reservoirs.
But experts have pointed out that longer-term, structural changes need to be made to sustain the river, which has always been over-allocated in terms of its water use and is now seeing its supply shrink in the face of the climate crisis.
“Going into the future, we may get some natural variability, wet or dry swings, but this study highlights that there’s been a decreasing trend in runoff,” said Bass. “In the long run, that’s likely to continue if greenhouse gas emissions are not reduced.”
