To get through those lengthy periods, renewables-dominated systems have to be supersized, installing three to eight times more power capacity than peak demand so they can fill the void when solar and wind output is diminished. That means they also produce way too much electricity when favorable conditions return. That excess is either wasted or stored (at a cost) for later use. And, presently, storage technologies are not available to do the job cost effectively at the scale we are talking about. If such a system were to be built, it would feature a lot of poorly utilized, capital-intensive wind, solar and storage assets. Unless the cost of those technologies falls to extremely low levels, we’ll be forced to choose between clean energy and affordability. The recent riots in Paris show how that conflict might play out.

Clean power systems with a mix of fuels can more easily match generation to demand and productively use all assets more often, making these systems more economical to operate. There’s no waste of large surpluses of energy or the need to develop seasonal energy storage. These advantages make the economics of a balanced low-carbon power system more attractive than those that rely solely on weather-dependent renewables. Maintaining fuel diversity also minimizes the zero-sum politics of an all-renewables grid. We don’t need to shutter the nuclear and fossil fuel industries to achieve environmental goals.

To be clear, firm energy technologies face daunting challenges of their own. No one believes it is easy to build nuclear or carbon-capture power plants, which would capture carbon dioxide emissions and utilize or permanently store them. Enhanced geothermal energy (which uses techniques similar to fracking to unlock renewable energy) is still an emerging technology. But the value of these resources to a decarbonized power system would be very high, justifying efforts to accelerate their development and deployment.

We are not alone in our conclusions. In his State of the State address Tuesday, Gov. Andrew Cuomo of New York deliberately outlined a transition to a 100-percent clean, carbon-free energy system by 2040 — not an all-renewables system . California recently did the same. When two of the most environmentally progressive states both see the wisdom of a broad path to decarbonization, activists would be wise to heed their pragmatic conclusions.

Expanding and improving the set of firm, carbon-free energy resources would make it much more affordable and feasible to eliminate greenhouse gas emissions from electric power plants. That’s something Green New Dealers and Americans of all political stripes could get behind.

Jesse Jenkins is a postdoctoral environmental fellow at Harvard. Samuel Thernstrom is the founder and executive director of the Energy Innovation Reform Project, and a senior fellow at the Center for the National Interest.

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