NOT that long ago, the world wondered whether clean energy could survive without lavish government support. Now the question is how far it can spread. The number of electric vehicles, which breached 1m in 2015, last year reached 2m; countries like France and firms like Volvo are looking ahead to the demise of the internal combustion engine. In electricity generation, too, momentum is with the greens. In June the Chinese province of Qinghai ran for seven consecutive days on renewable energy alone; in the first half of this year wind, solar and hydro generated a record 35% of Germany’s power.

Greater success is breeding greater ambition. California is proposing to reach 60% renewable energy by 2030; 176 countries have clean-energy goals. Hawaii, America’s most oil-dependent state, has pledged to be 100% renewable by the middle of the century. So have 48 poor countries vulnerable to climate change. This week the number of multinationals making a commitment to running their operations on 100% renewable energy rose to 100.

Even if such targets are never met, they galvanise effort. They also provide reassurance about long-term climate policy, despite reversals such as America’s withdrawal from the Paris agreement. The resulting economies of scale help bring down the costs of wind and solar projects.

But not every target is helpful. To see why, consider that goal of 100% renewable energy. It makes solving climate change seem deceptively easy. In fact, though wind and solar can generate all a country’s electricity on some days, renewables still account for less than 8% of the world’s total power output. Moreover, cleaning up electricity is only part of the battle. Even though gas-fired heating and cooking can be at least as big a source of greenhouse-gas emissions, renewable heating gets minuscule attention. Transport policy is erratic, too. Carmakers may hit their goal of annual sales of 10m electric vehicles in a decade, but battery-powered road haulage, shipping and aviation are dreams. A much-quoted claim that America could rely on wind, solar and hydro alone for its electricity has recently been witheringly criticised by a group of respected academics (see article).

Most important, a 100% renewables target confuses means with ends. The priority for the planet is to stop net emissions of greenhouse gases, especially carbon dioxide. Putting too much emphasis on wind, solar and other renewables may block off better carbon-reduction paths. After decades of investment, it is wrong to leave nuclear power off the table. Carbon emissions in Germany actually rose because it chose to phase out nuclear power and so burned more coal. New technologies, such as “direct air capture” systems designed to separate carbon dioxide from the air, may in time prove vital.

Likewise, greater energy efficiency could reduce emissions by even more than deploying renewables would. Indians last year consumed twice as much energy from newly installed air conditioners as they produced from new solar farms. More accurate metering of energy consumption could encourage companies and households to rein in power demand.

Putting the wind up

It would be better, as the Paris agreement urges, for countries to focus on reductions in emissions rather than to set goals for renewable energy. Global emissions have stabilised in the past three years, which is encouraging. But to stand a chance of mitigating global warming, they must start falling sharply and keep doing so for decades. The world will move in that direction with the help of wind and solar. It will not get there without big advances on every other front as well.