Unfortunately, it is also where we’re going. Last year, the hottest ever recorded, was about 1.55 degrees warmer than the world before the Industrial Revolution. Long-term averages are lower—perhaps 1.36 degrees, depending on how one measures—but they’re rising rapidly. (Scientists usually measure temperatures at the Earth’s surface and average it out over a decade—so by the time they confirm that the line has been crossed, it may be behind us.) Global emissions haven’t even started to decline from all-time highs, and President Trump is taking months off the clock by jackhammering pillars of environmentalism, such as the Inflation Reduction Act and the Environmental Protection Agency. “Current policies mean we would have 3°C of warming by the end of the century,” Piers Forster, a physicist who co-authored several landmark I.P.C.C. reports, told me in an e-mail. “What we should do is acknowledge that our inaction—or insufficient action—has generated death and destruction,” Marina Romanello, the executive director of the Lancet Countdown, a research initiative focussed on health and climate change, said. “That is a cross that we will have to carry.” The moment the line is crossed, she added, should serve as a grim occasion to renew global ambitions. “It’s not to give us more space, or time, or wiggle room,” she told me. “It’s about keeping temperatures as low as physically possible.” The knowledge that we can’t afford to crash past 1.5 degrees, and that we’re on track to do so nonetheless, has sparked a debate about whether the goalpost should move. “It would be a huge mistake to deviate from 1.5,” Johan Rockström, the co-director of the Potsdam Institute for Climate Impact Research, told me. Beyond that point, he said, there’s an escalating risk of climate tipping points—the collapse of ice sheets, the disruption of ocean currents, the sudden thawing of permafrost. He preferred the metaphor of a landing zone: the more we overshoot, the rougher the landing, and the more we’ll struggle to remove carbon from the atmosphere in the future. “If we readjusted the target every few years, any sense of urgency would be lost,” Jean-Pascal van Ypersele, a former vice-chair of the I.P.C.C., said. (He has a necktie that says “I ♡ 1.5°C”—although on the day we met, at an awards ceremony for environmental scientists called the Frontiers Planet Prize awards ceremony, it was too hot for him to wear it.) Even if it’s difficult to make highways safer, he pointed out, no leader would ever set a target of hundreds of thousands of traffic deaths per decade. Why aim for lethal levels of warming? One problem with countdowns, and with points of no return, is that they don’t tell you much about what comes afterward. “I think we need to be honest about where we’re most likely headed,” Daniel Swain, a climate scientist who studies extreme weather at the California Institute for Water Resources, told me in an e-mail. “Best-case scenarios from a decade ago are, unfortunately, probably off the table.” He urged not only emissions cuts but also adaptation to a hotter and more dangerous planet. The climate crisis has been described as a ticking time bomb, but this gives the false impression that the bomb has not gone off yet; in truth, we are not so much working to defuse the explosives as we are trying to contain the blast. Still, Swain argued that a second number in the Paris Agreement—“well below 2°C”—could still be reachable, with a fight. The good news is that we know how to get there: by phasing out the coal, oil, and gas that caused the crisis; by protecting the ecosystems we depend on; by electrifying buildings and vehicles; and, as Bill McKibben wrote on Wednesday, by scaling green-energy sources such as solar. Ayana Elizabeth Johnson, a marine biologist and climate advocate, does not identify as an optimist. Even so, she is the author of a book called “What If We Get It Right?: Visions of Climate Futures.” When I asked her about the 1.5-degree target, she told me, “Some people feel like, if you exceed it, it’s all over, and you can just give up.” But the difference between a narrow miss and a big one, she went on, could be hundreds of millions of lives. It could mean whether or not the places you love continue to exist. At well below two degrees, coral reefs struggle to survive; at two degrees, they may simply go extinct. “All I can really come up with is, like, Don’t be a quitter! Why are we giving up on the future of life on Earth so fucking easily?” Johnson said. “Where is our tenacity? Where is our fortitude? We can do hard things.” ♦