How did California’s drought get so bad?

The short answer is heat

No matter how you slice it, the drought in California is extremely, exceptionally bad.

The past 12 months were the driest in a century. Lake Mead, one of our most crucial water sources, has dropped to its lowest level ever. And Californians aren’t conserving anywhere close to the 15% cutback in water use that officials have called for.

With these grim fortunes in mind, Gov. Gavin Newsom this week declared a drought emergency for the entire state and called on Californians to “redouble our efforts to save water in every way possible.”

You might be wondering why this is such a big deal, given that periods of low rainfall have long been par for the course in California.

The short answer? Heat.

Because of human-induced climate change, California is experiencing warmer temperatures, and this summer was our hottest on record. That unusually balmy weather exacerbates drought conditions — and transforms what could be a normal fluctuation in precipitation into a full-blown crisis.

Here’s how this happens: California relies on snowpacks to store water, but during warm winters, that snow increasingly falls as rain instead. And what little snow does collect melts earlier in the year.

“By the time you reach late summer — August, September, October — and its scorching temperatures, there’s not a cloud in the sky and California must rely on water that’s fallen on the snowpack the previous winter,” Julien Emile-Geay, a climate scientist at the University of Southern California, told me. “Increasingly, that water is long gone by that point.”

This year, the Sierra Nevada snowpack had already dwindled to next to nothing by June.

There’s also another, slightly more complicated, piece of this “heat plus drought” equation, sometimes described as the atmosphere’s thirst.

When the air is warmer, it can hold more moisture — about 7% more with every 1 degree Celsius of warming. The air absorbs water from nearby lakes, plants and soil, further drying out the already parched land and depleting water supplies.

(In typically wetter climates, warmer air still holds more water, but it also releases it, which is why storms on the East Coast are becoming more intense.)

That “thirst” in California has created drought conditions similar to what we would normally experience only after four or five years of a dry spell. We’re in our second year of drought, yet Lake Oroville, a major reservoir, has less water than ever before.

And, despite rain blanketing much of the state this week, officials are predicting that this winter will bring La Niña, a weather phenomenon that typically leaves California exceptionally dry.

This article originally appeared in The New York Times.

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