This year began with huge bushfires in southeastern Australia that drove one community after another into temporary exile, killed an estimated billion animals, and turned Canberra’s air into the dirtiest on the planet. The temperatures across the continent broke records—one day, the average high was above 107 degrees, and the humidity so low that forests simply exploded into flames. The photos of the disaster were like something out of Hieronymus Bosch, with crowds gathered on beaches under blood-red skies, wading into the water as their only refuge from the flames licking nearby. But such scenes are only a chaotic reminder of what is now happening every hour of every day. This year wouldn’t have begun in such a conflagration if 2019 hadn’t been an extremely hot year on our planet—the second-hottest on record, and the hottest without a big El Niño event to help boost temperatures. And we can expect those numbers to be eclipsed as the decade goes on. Indeed, in mid-February the temperature at the Argentine research station on the Antarctic Peninsula hit 65 degrees Fahrenheit, crushing the old record for the entire continent.

It is far too late to stop global warming, but these next ten years seem as if they may be our last chance to limit the chaos. If there’s good news, it’s that 2019 was also a hot year politically, with the largest mass demonstrations about climate change taking place around the world.

We learned a great deal about the current state of the climate system in December, thanks to the annual confluence of the two most important events in the climate calendar: the UN Conference of the Parties to the Framework Convention on Climate Change, which met for the twenty-fifth time, this year in Madrid (it ended in a dispiriting semi-collapse), and the American Geophysical Union conference, which convened in San Francisco to listen to the newest data from researchers around the world. That latest news should help ground us as we enter this next, critical phase of the crisis.

The first piece of information emerged from a backward look at the accuracy of the models that scientists have been using to predict the warming of the earth. I wrote the Review’s first article about climate change in 1988, some months after NASA scientist James Hansen testified before Congress that what we then called the “greenhouse effect” was both real and underway. Even then, the basic mechanics of the problem were indisputable: burn coal and oil and gas and you emit carbon dioxide, whose molecular structure traps heat in the atmosphere.

Human activity was also spewing other gases with the same effect (methane, most importantly); it seemed clear the temperature would go up. But how much and how fast this…