Photographer: Walter Diaz/AFP via Getty Images

The Half Degree That Will Change Earth

How a tick on the global thermometer may affect human history.

Something not so funny happened on the road to the United Nations climate summit in Paris last year.

After trying and failing for decades to reach an accord that limits global warming to less than 2 degrees Celsius (3.6 degrees Fahrenheit), world officials finally struck a deal in December cementing that goal. But the accord took an extra half step, "encouraging" an even lower target of 1.5C in the future.

As representatives of more than 150 nations gather tomorrow at the UN's headquarters to sign the agreement, it's become clear to many scientists that even the required 2C goal is aspirational at best. And that's a depressing prospect, it turns out, because a new report Thursday shows why the safer goal of 1.5C is the outer limit of what the world needs to avoid an unprecedented shift in human history. As it stands, the pact to be signed in New York implicitly acknowledges that the planet is headed into decidedly choppy waters.

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The difference between the two targets is stark, according to the study in the journal Earth System Dynamics.

While a global temperature target is a fuzzy convention around which to organize a complex debate, the scientific community spent years studying what 2C of warming would do, only to realize too late that the goal posts were further away than originally thought. That's why, according to the new study's authors, "a comprehensive overview of the difference in climate impacts at these levels is still missing." 

The difference between the two worlds will be the difference between living at the upper end of our current climate and living in one humankind has never experienced. Monthlong heat waves in a 1.5C world might drag on for six weeks in a 2C world; the rate of sea-level rise would be a third faster; 90 percent of coral reefs may be destroyed by the end of the century, as opposed to 70 percent.

Key understanding will come as researchers focus on how the half-degree difference will affect specific regions, particularly in the tropics, where the toll of the 2C compromise will be more evident. In the Mediterranean, for example, nations could see a 17 percent drop in water availability, as opposed to 9 percent in a 1.5C world. 

Any chance of staying under the 1.5C target would require the global energy system to radically transform (in a way that isn't feasible in the current geopolitical atmosphere), according to a Nature Climate Change paper from June. A 1.5C world "is strongly at odds with climate policy achievements over the past decade and thus requires a significant trend break," the authors wrote. (Two scientists, Joeri Rogelj and Michiel Schaeffer, contributed to both studies.) 

And despite phenomenal revolutions in solar, wind, natural gas, cars, materials, and even human behavior, 2C is still way out of reach. While the ceremony tomorrow on Manhattan's East Side will itself be a trend break of sorts, today's study, and evidence accumulated in the months since the deal was reached, show that it won't be enough.

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