How 1.5 Degrees Became the Key to Climate Progress

The number has dramatically reorganized global thinking around the climate.

It’s Earth Day +51, as we near the end of President Biden’s first hundred days, and forty world leaders are scheduled to join him for a virtual summit on climate change. “For those of you who are excited about climate, we will have a lot more to say next week,” the White House press secretary, Jen Psaki, said last Thursday, which is a sweet way to think about it—better than “for those of you who are existentially depressed about climate.”

But, amid the blizzard of numbers that will come this week (such as a new report from the Lawrence Berkeley National Laboratory showing that America is “Halfway to Zero” in cutting the carbon from its electricity sector, as long as you don’t count the methane that all those new gas-fired power plants produce), it’s possible to discern the single, unlikely number that is really driving action at the moment, and it’s important to recall how it came to enter the debate. That number is 1.5 degrees Celsius, which is enshrined in Article 2 of the 2016 Paris climate accord as the world’s official goal for how much we will let the planet warm.

In the early decades of the climate era, government negotiators used two degrees Celsius as a target—that number has a tangled history, tracing back to at least the mid-nineteen-nineties, when Angela Merkel, then Germany’s environment minister, and other European officials seized on it as a benchmark. But there was never much hard science behind it, and by the mid-two-thousands the rate of damage already observable around the planet had begun to spook researchers, not to mention the residents of the most vulnerable countries. Even though the temperature had risen less than one degree Celsius, we were already seeing extensive Arctic ice melt, for instance. So the Alliance of Small Island States, with a lot of leadership from the Caribbean nations, and some of the African countries most vulnerable to drought, began to talk about a lower target. I first remember hearing chants of “1.5 to Stay Alive” at the Copenhagen climate talks, in 2009, and the Barbadian poet Adisa Andwele performed a song of that name for the assembled delegates. But those negotiations were such a mess that the short final document that the summit produced reiterated the old target of two degrees, merely mentioning the new number as a “consideration.”

Once the new number was out there, however, it took on a life of its own, and by the time the Paris climate talks met, six years later, it had become a rallying cry for movements. Paris was a mess, too—the pledges that countries made to reduce their emissions would actually lead to a world that heats by an apocalyptic three degrees Celsius—but, as a sop to campaigners, the negotiators put the 1.5-degree target in the opening section, where, one imagines, they figured it wouldn’t do much damage. They were wrong. In the years since, the number has dramatically reorganized global thinking around the climate, setting up the possibility that we might improve on the Paris timetable.

The most important result was that, in 2018, the Intergovernmental Panel on Climate Change prepared a report on the projected impact of exceeding the 1.5-degree target and the steps needed to meet it. And what it found was that, in essence, we’d need to cut emissions nearly in half by 2030, and get to net zero by 2050. The language was opaque—“in model pathways with no or limited overshoot of 1.5°C, global net anthropogenic CO₂ emissions decline by about 45% from 2010 levels by 2030 (40–60% interquartile range), reaching net zero around 2050 (2045–2055 interquartile range)”—but it was enough to reorient thinking. As the famed energy analyst Daniel Yergin told Bloomberg’s Eric Roston, in February, “I think you could say that is one of the most important sentences of the last few centuries. It has provided an incredibly powerful traffic signal to tell you where things are going.” Hence the spectacle of the biggest American banks declaring their net-zero-by-2050 plans (and the counter-spectacle of campaigners reminding them that the more important date is 2030). It was a big deal when a joint communiqué from the U.S. and China, following last week’s trip by John Kerry, Biden’s climate envoy, used the phrase “well below” two degrees. As Kerry told me, “That ‘well below’ two degrees language is important. That can’t logically mean 1.9 or 1.8.” Indeed, the White House explained that a key goal of the summit this week “will be to catalyze efforts that keep that 1.5-degree goal within reach.”

It’s entirely possible that, in fact, it’s no longer in reach; an Australian study published last week predicts that we’ll pass 1.5 degrees in the early two-thousand-thirties, and some recent research indicates that we could pass it as soon as 2024. But, by aiming for it, we clearly improve our chances of stopping closer to two degrees. And we’re only aiming for it—and this is the key point—because voices from the margins (tiny island nations, nascent movements) started demanding something that seemed, at first, mostly an annoyance to the powers that be. They asked for what they needed, and it’s made a huge difference. And it helps explain why, earlier this month, the remarkably farsighted Greta Thunberg said that she’d boycott the next global climate talks, in Glasgow, in November, if vaccine distribution hasn’t proceeded to the point where small, poor countries will be able to fully participate. It’s not that Tuvalu or Grenada or the Maldives put much carbon into the atmosphere—it’s that they put the important ideas into the debate…