During the darkest days of the drought（干旱）that hit the western US since the early 2000s，rivers went dry from north to south. Consequently, low river flows severely reduced the amount of carbon-free electricity that could be produced by the thousands of hydroelectric power plants（水力发电厂）along rivers across the West.
Now, a group of researchers have figured out that an extra 100 million tons of carbon ended up in the atmosphere because people had to use carbon-emitting（碳排放）power sources instead of hydroelectric power during drought. That's equal to adding about 1.4 million cars to road for every one of those years. "That's a big piece of the pic, "says Noth Diffenbaugh a climate scientist at Stanford and one of the authors of the study.
In normal years, over 20% of the electicity comes from hydroelectric power plants. But that number fluctuates with the ebb（退潮）and flow of water. And when water is not enough. the amount of hydroelectric power decreases.
States like Califomia. Washirgton, and Orgon that rely on hydroelectic power during water-rich years were the hardest hit. In Califormia, for example, the extra carbon dioxide emitted because of the drought added up to over 7% of its total carbon emissions.
That's not a minor problem. Many of the western states have made plans for how to reduce their emissions over the next few decades. Califormia, for example, is trying to get its emissions down to 80% below 1990 levels by 2050. But drought makes it harder to meet the emissions reduction goals.
While the carbon cost of drought is large, Diffenbaugh points out, with this study and a lot of others from the past few years, we've learned more and more about when and why carbon-free energy sources face challenge. Armed with this information, he says, energy managers can figure out how to fill the gaps in energy demand with more renewable resources.
