Illustration by João Fazenda

Three years ago, when world leaders met in Paris to negotiate a treaty on climate change, one of the sticking points was where to set what might be called the Doomsday Thermometer. For reasons that had to do mostly with politics, rather than with geophysics, industrialized nations wanted to define “dangerous” warming as an average global-temperature increase of two degrees Celsius (3.6 degrees Fahrenheit). But island states, such as the Maldives and Mauritius, along with developing countries like Ethiopia and Cambodia, were resistant. Well before the world warmed by two degrees, their countries would be devastated—some of them underwater. Why should they endorse what amounted to a death sentence?

“We will not sign off on any agreement that represents a certain extinction of our people,” a delegate to the talks from Barbados told Politico. Together with a group of nearly fifty “climate vulnerable” countries, the island nations pressed for a limit of 1.5 degrees Celsius (2.7 degrees Fahrenheit). The compromise reached—more Monty Hall than Solomon—was to endorse both figures. The Paris agreement calls for “holding” warming below two degrees, while “pursuing efforts” to limit it to 1.5 degrees.

Last week, the United Nations’ scientific advisory board delivered its assessment of those numbers. The findings of the group, the Intergovernmental Panel on Climate Change, were almost universally—and justifiably—described as “dire.” Even 1.5 degrees’ worth of warming, the I.P.C.C. warned, is likely to be disastrous, with consequences that include, but are not limited to, the loss of most of the world’s coral reefs, the displacement of millions of people by sea-level rise, and a decline in global crop yields. Meanwhile, at the current rate of emissions, the world will have run through the so-called carbon budget for 1.5 degrees within the next decade or so. “It’s like a deafening, piercing smoke alarm going off in the kitchen,” Erik Solheim, the executive director of the U.N. Environment Program, told the Washington Post.

But, if a smoke alarm rings in the kitchen and everyone’s watching “Fox & Friends” in the den, does it make a sound? Asked about the report last week, Donald Trump said, “I want to look at who drew it—you know, which group drew it.” The answer seemed to indicate that the President had never heard of the I.P.C.C., a level of cluelessness that, while hardly a surprise, was nevertheless dismaying. The next day, as a devastating hurricane hit Florida—one made that much more destructive by the warming that’s already occurred—the President flew to Pennsylvania to campaign for Lou Barletta, a climate-change-denying Republican congressman running for the Senate.

Though the Administration often seems incapable of systematic action, it has spent the past eighteen months systematically targeting rules aimed at curbing greenhouse-gas emissions. One of these rules, which required greater fuel efficiency for cars and trucks, would have reduced CO 2 emissions by an estimated six billion tons over the lifetime of the affected vehicles. In a recent filing intended to justify the rollback, the Administration predicted that, by the end of this century, global temperatures will have risen by almost four degrees Celsius (nearly seven degrees Fahrenheit). In this context, the Administration argued, why would anyone care about a mere six billion tons? Come the apocalypse, it seems, we’ll all want to be driving S.U.V.s.

The Supreme Court, for its part, appears unlikely to challenge the Administration’s baleful reasoning. Last week, it declined to hear an appeal to a lower-court ruling on hydrofluorocarbons, chemicals that are among the most potent greenhouse gases known. The lower court had struck down an Obama-era rule phasing out HFCs, which are used mostly as refrigerants. The author of the lower-court decision was, by the dystopian logic of our times, Brett Kavanaugh.

Even as the I.P.C.C. warned that 1.5 degrees of warming would be calamitous, it also indicated that, for all intents and purposes, such warming has become unavoidable. “There is no documented historical precedent” for the changes needed to prevent it, the group wrote. In addition to transforming the way that electricity is generated and distributed around the world, fundamental changes would be needed in transportation, agriculture, housing, and infrastructure. And much of this would have to be accomplished by the time today’s toddlers hit high school. To have a reasonable chance of limiting warming to 1.5 degrees, the I.P.C.C. said, global CO 2 emissions, now running about forty billion tons a year, would need to be halved by 2030 and reduced more or less to zero by 2050. And this would still not be enough. All the scenarios that the I.P.C.C. could come up with to limit warming to 1.5 degrees rely on some kind of “carbon-dioxide removal”: essentially, technologies to suck CO 2 out of the air. Such technologies exist, but so far only in the sense that flying cars exist—as expensive-to-produce prototypes. A leaked draft of the report noted that there was a “very high risk” of exceeding 1.5 degrees; although that phrase was removed from the final report, the message is clear.

Thus, it is tempting, following the Trump Administration’s lead, to simply give up. But, as Edgar puts it in “King Lear,” the “worst is not, so long as we can say, ‘This is the worst.’ ” Perhaps the most important takeaway from the report is that every extra half a degree is world-altering. According to the I.P.C.C., between 1.5 degrees and two degrees of warming, the rate of crop loss doubles. So does the decline in marine fisheries, while exposure to extreme heat waves almost triples. As always, it’s the poor who are apt to suffer most. Friederike Otto, the acting director of Oxford’s Environmental Change Institute, recently told the Web site Carbon Brief that “half a degree of additional warming makes a huge difference. For people who are already marginalised, this can be an existential difference.”

Meanwhile, two and a half degrees, three degrees, or even, per the Trump Administration, four degrees of warming are all realistic possibilities. Indeed, based on recent trends, the last figure seems the most likely. Globally, emissions rose last year, and they’re expected to rise still further this year. This disaster is going to be as bad—as very, very bad—as we make it. ♦