It’s one of the lesser-known costs of California’s drought: the drying-up of the state’s normally abundant cheap hydroelectric power.

A hydro shortage has raised California’s electricity costs by a combined $2 billion the past four years, according to a report released Tuesday by the Pacific Institute, a water policy think tank based in Oakland. In addition, the institute said the drought has contributed to climate change: California’s fossil-fuel power plants have increased greenhouse gas emissions by 10 percent to make up for the hydro shortage.