On an afternoon in late June, the San Luis Reservoir — a 9-mile lake about an hour southeast of San Jose, California — shimmered in 102-degree heat. A dusty, winding trail led down into flatlands newly created by the shrinking waterline. Seven deer, including a pair of fawns, grazed on tall grasses that, in wetter times, would have been at least partially underwater.
That day, the reservoir, California’s sixth-largest and a source of water for millions of people, was just 40 percent full. An interactive graphic in the visitor’s center reported that this year’s snowpack was zero percent of the yearly average. Throughout the West, anxiety about drought is as palpable as the dryness of the air; talk of water fills newspapers and conversations alike. “Aridification kills civilizations. Is California next?” read one Los Angeles Times headline in June.
In February, scientists confirmed that the current, decades-long “megadrought” is the worst in 1,200 years. They also confirmed that rising temperatures — driven by human consumption of fossil fuels — were partly to blame. But the connection between climate change and drought is not as straightforward as it seems. One problem with linking drought and climate change is that there is little agreement on what drought actually is.
The clearest link between drought and climate change right now is not a lack of rainfall — it’s rising temperatures. The atmosphere is like a sponge: It sucks up water from soils, plants, rivers, oceans, and lakes. Any time rain falls, some of it will evaporate, returning back into the sky before it can be piped into homes, fields, or aqueducts. Scientists have a measure for how “thirsty” the atmosphere is, or how much water the sky absorbs: evaporative demand. As temperatures go up, evaporative demand increases.
“A warmer atmosphere can hold more water,” explained Christine Albano, an ecohydrologist at the Desert Research Institute in Reno, Nevada. And, she added, the changes are nonlinear — a small change in temperature could lead to a much larger change in how thirsty the sky is. As temperatures warm, the situation will get even worse. “For every raindrop, we’re going to get less of that going into our streams and rivers,” Albano said.