The reason, say the researchers, is that you have to take into account the sort of energy that's likely to be displaced by the new solar panels and wind turbines.
Out in Texas or Oklahoma, wind turbines would crowd out relatively clean natural gas-fired plants. That would cut down on carbon-dioxide emissions and other pollutants. But it's not quite as big a deal as, say, building a wind farm in Ohio — where the wind would displace coal power and lead to a 20 percent bigger reduction in pollutants that are heating the planet or causing respiratory problems.
The regions with the biggest wind or solar resources aren't always the most beneficial places to build renewable energy, at least in the very near term. Here's a map of the most wind-rich areas in the United States (the redder the breezier):
And next we have a map of where wind power would deliver the most benefits, once you factor in the reductions from pollutants like sulfur or particulates, which have been linked to health problems from asthma to heart attacks:
By the same metric, here are the most beneficial places to stick solar panels:
Note that California sees somewhat fewer benefits — in part because the state has already spent so many years cleaning up its energy supply compared with other states.
Now, to be clear, the researchers aren't saying that California's clean-energy spree in recent years has been pointless. Switching from natural gas to renewable energy has still led to a drop in carbon-dioxide emissions, which helps slow the pace of climate change at the margins and curbs other pollutants. The authors note that "the social benefits from existing wind farms are roughly 60% higher than the cost of the Production Tax Credit, an important federal subsidy for wind energy."
But the authors do suggest that Congress could take these regional variations into account when structuring tax benefits for clean energy. "[T]hat same investment," they note, "could achieve greater health, environmental, and climate benefits if it were differentiated by region."
One final point: This study largely takes into account the existing power grid in the United States and focuses on near-term benefits. But in theory, it would also be possible to build gigantic wind farms in the Great Plains and then transmit that electricity all over the United States, to places like Ohio and West Virginia, through new high-voltage transmission lines. That would be a way of developing the most promising resources and delivering high benefits.
But there's a big hitch. As Matthew Wald explained in the New York Times this weekend, these grand schemes always run afoul of the balkanized nature of the power grid. "The technology, the engineering skill and even the money are all available, experts say, but the ability to reach agreement on such a grid is not," Wald wrote. "For now, there is simply no momentum for a transmission system that would connect the best sites for renewable energy with the biggest areas of demand."