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How hot is too hot for planet Earth? For years, there’s been a consensus in the climate movement: no more than 1.5 degrees Celsius above preindustrial levels. The figure comes from the Paris Agreement, a climate treaty ratified in 2016, and world leaders such as President Joe Biden bring it up all the time: “If we’re going to win this fight, every major emitter nation needs [to] align with the 1.5 degrees,” he said in November. Youth activists at the Sunrise Movement call 1.5 degrees a “critical threshold.” Even the corporate world is stuck on 1.5 degrees. Companies including Apple, Google, and Saudi Aramco—the world’s largest oil company—claim to be transitioning their operations in alignment with the 1.5 goal.
But here’s the thing: 1.5 degrees, or 2.7 degrees Fahrenheit, isn’t based on any scientific calculation. It doesn’t represent a specific planetary threshold or ecological tipping point. It was first proposed during international climate negotiations as a moral statement, a rebuke of the idea that the world could accept some disruption and suffering in order to burn fossil fuels just a bit longer. That’s the takeaway of a new study on the history of the target from two French academics, Béatrice Cointe from the Centre for the Sociology of Innovation and Hélène Guillemot from the Centre Alexandre Koyré, both funded by the French National Centre for Scientific Research. From the perspective of the present, it’s a relief that 1.5 degrees doesn’t represent a scientific threshold, because we are almost certainly going to blow past it. As a rebuke, however, it may live on.
Nothing about the 1.5-degree target was inevitable. For decades, the number on the lips of most climate negotiators was 2 degrees Celsius, or about 3.6 degrees Fahrenheit. And you do still hear that number as the go-to target in some climate circles. But in the late 2000s, a negotiating bloc called the the Alliance of Small Island States argued that this was simply too much warming for their vulnerable nations. Their atolls would be overtopped by the sea; their coastal cities would flood. So they called for a lower target, and 1.5 seemed like a reasonable half-step down from 2 degrees.
From there, 1.5 degrees gained momentum in diplomatic back channels and in conversations within think tanks, NGOs, and a group called the Climate Vulnerable Forum. But there was at that time very little science on the target; scientists were busy modeling higher levels of warming, which they considered more likely. A 2015 scientific panel hosted by the UN concluded that although the science on 1.5 was “less robust,” “efforts should be made to” set warming targets as low as possible. That year, after what Cointe and Guillemot characterize as “intense and difficult negotiations,” the new target was folded into the Paris Agreement, which calls for “pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels, recognizing that this would significantly reduce the risks and impacts of climate change.”
Science informs policy. But policy shapes science too. Most climate scientists thought staying under 1.5 degrees was unrealistic. But climate diplomats nevertheless asked the UN’s Intergovernmental Panel on Climate Change (IPCC) for a special report on what the planet would look like with 1.5 degrees of warming. A report was duly delivered in 2018, and it unsurprisingly suggested that, all things considered, 1.5 degrees Celsius would be less bad than 2 degrees Celsius. Which, duh. More warming is always worse.
Staying below 1.5 degrees, the IPCC scientists concluded, would be an extremely heavy lift that would require, among other things, slashing emissions about 45 percent from 2010 levels by 2030. This is the origin of the common idea that we have “12 years left” to stop climate change. The IPCC puts out lots of reports, but its report on 1.5 degrees remains its undisputed chart-topping banger. You can feel its influence in this speech that Greta Thunberg gave to the U.K.’s houses of Parliament in 2019: “Around the year 2030,” she said, “10 years 252 days and 10 hours away from now, we will be in a position where we set off an irreversible chain reaction beyond human control, that will most likely lead to the end of our civilization as we know it.”
In 2023, limiting warming to 1.5 degrees Celsius is a fantasy. It is not happening. We have already warmed the planet more than 1.1 degrees Celsius (2 degrees Fahrenheit). Climate scientists say we could pass 1.5 degrees Celsius within a decade. A December analysis by The Washington Post suggested that ending the century under 1.5 degrees Celsius without substantial mid-century overshoot would require reforestation on a mind-boggling scale, plus massive deployment of machines to suck carbon out of the air and cache it underground—technology that does not yet exist on a widespread scale—along with a near-total abandonment of fossil fuels like five minutes ago.
That we’re going to warm more than 1.5 degrees Celsius is not good, of course. The IPCC’s report on 1.5 degrees Celsius predicted more extreme heat, altered rain patterns, sea-level rise, increased wildfire, ocean acidification, and major hits to ecosystems such as arctic tundra and coral reefs as temperatures rise. But it is also probably not “the end of our civilization as we know it.” Waking up to 1.6 degrees Celsius won’t feel like we’ve crossed a threshold, because it isn’t one. It will feel similar to the hot, disrupted world we already inhabit—just worse.There are many possible futures inside the realistic range of warming possibilities; their contours depend not just on the level of warming as measured in degrees but on how we adapt.
The legacy of 1.5 degrees is complicated. The target seems to have prompted many people, such as Thunberg, to start or join activist groups to press for change. Some activists framed 1.5 degrees as a terrifying precipice that we are driving toward at top speed, and to the extent that it has felt like a point of no return, it has caused a fair amount of eco-anxiety, which research shows can lead to paralysis and apathy—the opposite of action. If people give up in despair when we cross the mark, the figure will have been counterproductive for the climate movement. For now, knowing whether it’s done more harm or good to the cause is impossible. Either way, 1.5 degrees will likely soon cease to be a target and become a historical fact.
But even then, the 1.5-degree target won’t be entirely obsolete. It has another function—governments that promised in the Paris Agreement to “pursu[e] efforts” to limit warming to 1.5 degrees can now be held accountable for breaking their promise. Enshrined in a legally binding treaty, 1.5 now represents what humanity should have accomplished. The target can be used as a basis for measuring the rich world’s moral failures—and justifying reparations (or “loss and damage,” as they are now referred to in climate-diplomacy circles) to the rest of the world. Today, 1.5 degrees is less a feasible target than a “diplomatic weapon,” Cointe and Guillemot write. It is also already being used in court to sue governments and force them to take more drastic measures to limit emissions. “In that sense, it has a use,” Cointe told me.
1.5 degrees is just a number: a little better than 1.6 degrees, a little worse than 1.4 degrees. But as a reference against which humanity’s failures can be judged, it will remain powerful.
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