For instance, the Diablo Canyon reactor that California plans to close produces 14 times as much power as the Topaz solar farm, which requires 500 times as much land, according to Environmental Progress. The Wind Catcher farm in Oklahoma occupies 2,400 times as much land as Diablo Canyon but produces half as much energy.

The most worrisome aspect about the all-out push for a future powered by renewables has to do with cost: The price of turbines and solar panels may be falling, but the cost of integrating these intermittent sources of energy — on when the wind blows and the sun shines; off when they don’t — is not. This alone will sharply curtail the climate benefits of renewable power.

Integrating renewable sources requires vast investments in electricity transmission — to move power from intermittently windy and sunny places to places where power is consumed. It requires maintaining a backstop of idle plants that burn fossil fuel, for the times when there is no wind or sun to be had. It requires investing in power-storage systems at a large scale.

These costs will ultimately be reflected in power prices. One concern is that by raising the retail cost of electricity they will discourage electrification, encouraging consumers to rely on alternative energy sources like gas — and pushing CO 2 emissions up.

Another concern is that they will drive wholesale energy prices down too far. Because they produce the most energy when the sun is up and the wind is blowing, renewable generators can flood the grid at critical times of the day, slashing the price of power. This not only threatens the solvency of nuclear reactors, which cannot shut down on a dime and must therefore pay for the grid to accept their power, but also reduces the return on additional investment in renewables.

A study by Lion Hirth of the Hertie School of Governance in Berlin found that the value of wind power falls from 110 percent of the average power price to 50 to 80 percent as the penetration of wind rises from zero to 30 percent of total consumption. “Competitive large-scale renewables deployment will be more difficult to accomplish than many anticipate,” he concluded.

The diplomats in Bonn may be tempted to wave away these concerns. Thomas Bruckner of the University of Leipzig argues that in the case of Germany, expanding renewables to supply 80 percent of power by 2050 is “not a significant burden.” Heavy German investment in renewable energy technologies over the last decade succeeded in bringing prices down, he contends; it will be much cheaper to go the rest of the way.