Kate Prengaman, writing for Ars Technica:
Current US renewable energy policy encourages development where the best resources, wind or sunshine, exist to create the most power. But the authors of a new study argue that analyzing the benefits of renewable energy projects should include the environmental cost of the electricity being supplanted.
Basically, if you install a solar array in the sunny Southern California desert, that power mostly replaces electricity made with relatively clean natural gas. In contrast, if you installed the same solar panels in considerably less sunny Ohio, you’d be primarily replacing capacity at a coal-burning power plant, reducing far more air pollutants than in California.