How much should the world limit warming?

The problem: In 2009, the world’s nations agreed: The global temperature average should not be allowed to rise more than two degrees Celsius over pre-industrial levels.

Since then, study after study has emphasized the dangers of a two-degree-warmed world and the simultaneous difficulty of actually halting climate change there. When Paris began, some observers expected the UN to abandon the two-degree limit in favor of something more realistic.

Instead, nations began endorsing a more ambitious path. More than 100 countries, including the United States, announced their support for limiting warming to 1.5 degrees Celsius. Many island nations declared they could not accept anything more. Even China signaled its endorsement.

The solution: The final Paris agreement retains the two-degree target, while recognizing the importance of pursuing 1.5 degrees. Article 2, Section 1 deals most directly with temperature limits:

Including the 1.5 degree target is a triumph for both green activists and the most climate-vulnerable nations. But Rachel Cleetus, the lead economist and climate-policy manager at the Union of Concerned Scientists, said that the phrase “well below 2 degrees Celsius” was also noteworthy. It’s “the first time we’ve ever seen this kind of language in a global context,” she told me.

The agreement also directed the Intergovernmental Panel on Climate Change—the climate-science arm of the UN—to draw up a report by 2018 on how to reach 1.5 degrees Celsius.

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How quickly will the world abandon fossil fuels?

The problem: If humanity hopes to keep warming below 1.5 degrees Celsius by 2100, it must essentially stop emitting greenhouse gases into the atmosphere by 2060, according to a recent study from Nature Climate Change. (Then it must start pulling carbon out of the atmosphere—no easy feat.)

But would the Paris agreement come out and say that? Delegates could not agree. Small island states wanted forceful language, calling for draft phrases like “zero global [greenhouse-gas] emissions by 2060” or “decarbonization as soon as possible after mid-century.” But Saudi Arabia said that such a goal was a “threat to sustainable development”—which some interpreted as meaning a threat to its oil production. And according to The New York Times, petroleum-pumping Venezuela was also skeptical of long-term decarbonization language.

The solution: The final version of the Paris text, Article 4, Section 1, resolves to peak global greenhouse-gas emissions as soon as possible. Then, after 2050, it says that all anthropogenic emissions should be balanced with “removal by sinks”:

“That’s essentially a net-zero goal,” said Cleetus. She hailed the wording, saying it is “the language we need to deliver on this type of an ambition.”

Some have noted the technological ambiguity in this article. Forests and oceans can both serve as carbon “sinks”—that is, both absorb carbon dioxide out of the atmosphere. But one day, humanity might be able to deploy negative-emissions technologies at scale, which could absorb and thus “neutralize” any additional carbon emissions. Indeed, the European Union’s climate chief said after the conference that the 1.5-degree target will require use of such tech.