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.

It’s not ideal, but it makes sense, says Amir AghaKouchak, a civil and environmental engineer at the University of California, Irvine.

“Under drought conditions, the priority is to use water for people and cities, and managers might prefer to burn gas for energy,” he says. “Because you have alternatives for getting energy from different sources—but you don't have alternatives for water.”

Big impacts

States like California, Washington, and Oregon that rely heavily on hydroelectric power during water-rich years were the hardest hit. And the climate costs were large. In California, for instance, the extra carbon dioxide emitted because of the drought added up to more than seven percent of its total carbon emissions. In Oregon and Washington, the extra emissions made up about 10 percent of their total.

That’s far from insignificant. Many of the western states have set out plans for how to aggressively reduce their emissions over the next few decades. California, for instance, is trying to get its emissions down to 80 percent below 1990 levels by 2050. But drought bumps all the western states strongly in the wrong direction, making it harder for them to meet their emissions reduction goals.

Future megadrought?

Over the past decade, the western U.S. experienced some of the worst periods of drought it’s seen in centuries. Now, some scientists think that the American Southwest is settling in to a “megadrought”—a drought that lingers on for at least 20 years. And forecasts for the future predict that dry places are likely to get even drier, stressing out the hydroelectric system even further.

“Droughts are going to get worse, and that could mean more natural gas and coal being burned,” says Peter Gleick, a water expert at the Pacific Institute, a research organization in Oakland. “And that's a positive feedback—which in climate change is a bad thing.”

It’s easy to get caught in this positive feedback loop, he says—but the more aware of it managers are, the better able they’ll be to plan their way out of it.

AghaKouchak points out that the carbon cost of this particular stretch of drought, in this particular place, is large—but at any given point, many different regions of the world are in their own water-stressed states. “And if you add up the cumulative impact of all these extreme events on CO₂ emissions,” he says, before pausing. “Well. It's way beyond what you may think.”

But Diffenbaugh points out that with this study and a slew of others from the past few years, we’ve learned more and more about when and why carbon-free energy sources struggle. Armed with this information, he says, and with ever improving forecasting of coming stresses on the system like drought, energy managers can figure out how to fill the gaps in energy demand with more renewables.