2 . NOT that long ago, the world wondered whether clean energy could survive without government support. Now the question is how far it can spread. The number of electric vehicles. which was about 1 million in 2015, last year reached 2 million. In electricity generation, too, trend is with the greens. 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 promised 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.

But not every target is helpful. To see why, consider that goal of 100% renewable energy. It makes solving climate change seem easy. In fact, though wind and solar can generate the whole country's electricity some day, 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 little attention. Transport policy is unpredictable, too. Carmakers may hit their goal of annual sales of 10 million electric vehicles in a decade, but battery-powered road transport, 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 bitterly criticized by a group of respected academics.

Most importantly, a 100% renewables target confuses means with ends. The priority for the planet is to stop net emissions（净排放量）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 gradually 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 using renewables would.