An unexpected side effect of drought: Higher carbon emissions.

During the darkest days of the drought that has gripped the western U.S. since the early 2000s, fires raged and crops withered. Dust storms rolled across plains and valleys. And rivers shriveled from north to south.

But the drought had less obvious effects on climate and the environment, too: Low river flows drastically hampered the amount of carbon-free electricity that could be produced by the thousands of hydroelectric power plants dotted along rivers and reservoirs across the West.

(Learn more about how the West is drying out, slowly but surely.)

Now, a group of researchers has done the carbon math to see how big that effect was. They figured out that an extra 100 megatons of carbon ended up in the atmosphere because utilities had to use carbon-emitting power sources instead of hydroelectric power during drought, added up over the 15 years they studied. That’s the equivalent of adding about 1.4 million cars to the road for every one of those years.

“That’s a sizable slice of the pie,” says Noah Diffenbaugh, a climate scientist at Stanford and one of the authors of the study, which was published Friday in Environmental Research Letters.

In a normal year, a little more than 20 percent of the electricity produced across the western U.S. comes from hydroelectric plants. But that number fluctuates with the ebb and flow of water. And when water is scarce, the amount of energy produced by those plants plummets.

But people need lights and heat and air conditioning in a drought as much (and sometimes more) than they do in times of water plenty. If energy utilities can't get the power they need from hydroelectric sources, they have to fill that gap with something else. Most of the time, the researchers found, the utilities fell back on carbon-emitting sources like natural gas and coal to fill their power needs.

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