When the fate of the planet is at stake, a single precedent starts to seem like a blueprint.

Most Americans, as far as pollsters can tell, want the United States to honor its commitment under the Paris Agreement on climate change. According to that pact, the United States must, by 2025, cut its carbon emissions 26 percent below their all-time peak. That will be hard. To make the Paris goal, the U.S. would have to cut carbon by 2.6 percent every year for the next seven years. And it has simply never cut its emissions that fast in such a sustained way before.