From National Public Radio (Lauren Sommer):
Almost half the country’s population is facing dry conditions. Soils are parched. Mountain snowpacks produce less water. Wildfire risk is already extreme. The nation’s largest reservoir, Lake Mead, is headed to its lowest level since it was first filled in the 1930s.
The past year has been the driest or second driest in most Southwestern states since record-keeping began in 1895. Farms and cities have begun imposing water restrictions, but Western states are facing a threat that goes deeper than a single bad year. The hotter climate is shrinking water supplies, no matter what the weather brings.
Warming temperatures make it less likely for a raindrop or snowflake to reach a reservoir due to increased evaporation. As a result, the people who manage the West’s complex water systems are realizing that with climate change, they can no longer rely on the past to predict the future.
That’s creating a fundamental threat to the way Western water systems operate, because they were built around the idea that the climate would remain constant. Historical climate data such as river flows and rainfall totals told engineers how big to build reservoirs and canals. The data also told them how much water was available to divide up among cities and farms.
Climate change is putting that system under increasing stress, shrinking water supplies for tens of millions of people and for the farmland that produces most of the country’s fruits and vegetables. Water cutbacks are reverberating through California’s $50 billion agricultural industry, which employs tens of thousands of people in many small towns.
Southwestern states recently negotiated a temporary agreement to use less water as reservoirs keep falling. But tough conversations remain about how the West and its complex system of water rights will adapt to a future for which it wasn’t designed.
“We can really no longer look at the past and say: The amount of water we’ve had in the last 100 years is what we can expect in the future,” says Eric Kuhn, an author who worked on water policy for decades at the Colorado River Water Conservation District. “That is no longer true because of climate change.”
Climate amplifying bad luck
Like a run-of-the-mill streak of bad luck, droughts are normal in the West. Now, climate change is exacerbating their effects.
“Over the last 22 years or so, there’s been quite a bit of bad luck because precipitation totals have on average been low,” says Park Williams, associate professor of hydroclimatology at UCLA. “But the effect of that bad luck has been really amplified because of warmer temperatures.”
A hotter atmosphere is thirstier, drawing water out of plants and soils and into the air. Snowpacks melt earlier, which in turn boosts that evaporation, because without the reflective surface that snow creates, soils heat up faster. And when soils are dry, they act like a sponge. They need to soak up more moisture before they’re saturated enough for the water to run off into rivers and streams.
Studies show that since 2000, about half the reduction in the Colorado River’s flow has been due to warmer temperatures. For every degree Celsius of warming, the river’s flow is expected to shrink by 9%, according to another study.