Spare a little pity for Jerry Brown. The California governor has been standing up admirably to Donald Trump on many issues, but especially on climate change—even threatening to launch scientific satellites to replace the ones that Washington wants to ground. This week, he’s in Bonn, Germany, at the global climate talks, spearheading the drive to show that America’s states and cities have not forsaken the promises made last year in Paris. On Saturday, barely a minute into his big prime-time talk, Brown was rewarded for his pains with booing. He was visibly startled when demonstrators interrupted his speech and began chanting, “Keep it in the ground!”

Pity him, then, but not too much. For one thing, Brown responded to the challenge Trumpishly—“let’s put you in the ground,” he told the protesters, who were led by indigenous and climate-justice activists. And, for another, they were absolutely right; their slogans illustrated the contradiction at the heart of the planet’s climate policy, one that Brown, if he wanted to, could play a key role in solving.

There are two halves to the climate dilemma: demand and supply. We use too much coal and gas and oil, and we’ve begun to address that through the rapid adoption of renewable energy, the spread of conservation measures, and ideas such as a price on carbon. Brown’s California has been a leader in much of this work. But we also produce too much fossil fuel, and that endless production makes it harder to drive down demand. In fact, it will make it impossible to meet even the modest goals of the Paris accords. A remarkable study, published last year by Oil Change International, found that the world’s developed oil and gas fields—the ones we’re already pumping—contain enough carbon to carry us past the 1.5-degree-Celsius temperature increase agreed to in Paris. (Add coal to the mix and we go way past two degrees, without ever discovering another seam or field.) That’s why campaigners from around the world, meeting in Lofoten, Norway, this summer, signed a declaration calling on governments to begin the “managed decline” of the world’s fossil-fuel-production zones.

Five hundred N.G.O.s—including 350.org, which I helped found—have signed that declaration, but not many political leaders. In fact, heads of governments tend to fall into one of two camps. The first, populated largely by Trump and his followers, sees climate change as nonsense and aims to increase both supply and demand. The other, which includes everyone from Barack Obama to Canada’s Justin Trudeau to Brown, offers inspiring rhetoric on fighting global warming but refuses to rein in fossil-fuel exploration and development. Trudeau, for instance, said at an oil-and-gas conference in Texas this year that “no country would find a hundred and seventy-three billion barrels of oil in the ground and leave them there,” a reference to Alberta’s tar sands. Give Trudeau high marks for honesty—he’s gone all out to build the pipelines necessary to drain that oil—but low marks for math. There’s no way to burn those hundred and seventy-three billion barrels without overwhelming the atmosphere; they would take us thirty per cent of the way to 1.5 degrees, and that from a nation with less than one per cent of the planet’s population.

California is a big oil-and-gas producer, too—the third largest in the United States—and Brown has so far declined to curtail even fracking and urban drilling, the dirtiest and most dangerous kinds. In his Bonn speech, he offered the most tired of explanations: “If I could turn off the oil today, thirty-two million vehicles would stop, and ten million jobs would be destroyed overnight.” But, of course, no one is talking about turning off the flow of oil overnight. That’s the point of “managed decline”—an orderly retreat from the fossil-fuel precipice. And, in truth, no one is better situated than Brown to lead it. California doesn’t depend on oil and gas the way that, say, Russia or Saudi Arabia or Oklahoma does; the state is full of the world’s most vigorous entrepreneurs, many of them making fortunes on the energy transition.

And then there’s the fact that Brown is on the way out, term-limited and hence insulated from the political power of the fossil-fuel industry. He could do his successor—and the rest of the world—a huge favor by, for instance, announcing that California will no longer grant new permits for exploration or major infrastructure development. Such a commitment would shut down nothing except the petroleum industry’s scientifically and economically flawed assumption that it can maintain its business model indefinitely.

The pressure is not just on Brown. Over the weekend, climate activists occupied a German coalfield, and there are increasing calls on Chancellor Angela Merkel to announce a phase-out of coal mining before the Bonn summit wraps up, at the end of this week. Merkel has been pretty steadfast on climate policy, so she might do it. But Brown is something different: he aspires to lead Earth’s fight against climate change, having called a huge conference next autumn, in San Francisco, of governors, mayors, and other “subnational actors” from around the world. That could really be a turning point in the battle, and a way to bypass Trump—but only if Brown and others are willing to get serious about supply as well as demand.