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