Avoiding two degrees of warming “is now totally unrealistic.”
Additionally, humanity would need to start removing more carbon dioxide from the atmosphere than it emitted by 2050. But even in that world, humanity would probably overshoot the 1.5-degree target for a few decades.
Nowhere are the report’s prescriptions more glaring than around coal. By 2050, it warns that coal must generate no more than 7 percent of global electricity. Today coal generates about 40 percent of the world’s power.
But more than 1,600 new coal plants are due to come online worldwide in the next few decades, most under contract from Chinese companies. The Trump administration, meanwhile, has tried to create new subsidies for coal companies. It has also moved to weaken or repeal pollution regulations limiting airborne neurotoxins, as well those reducing greenhouse-gas emissions—rules that attracted the ire of coal companies.
“Many will dismiss the [2.7-degree] target as unrealistic, if not laughable,” said Kim Cobb, a professor of climate science at Georgia Tech, in an email. “It is not our job as scientists to give the world a ‘pass’ in the face of damaging delays in tackling climate change.”
She added that the report’s authors “spent months of their lives outlining a path that is entirely justified, from a cost perspective, and urgently needed.”
The report marks the first time the IPCC has examined a 2.7-degree warming “target.” For more than a decade, every nation on the planet has pledged to limit warming to 3.6 degrees Fahrenheit, or 2 degrees Celsius. This goal remains as notional as its newer and more ambitious peer: Even if every country met its obligations as they stand today under the Paris Agreement, the world would emit too much carbon dioxide and shoot past the goal.
But that less ambitious target of 3.6 degrees Fahrenheit has started to look more and more dangerous. In particular, increasing Earth’s temperature by that amount seems like it might greatly increase the risk of destabilizing ice sheets in Antarctica and Greenland.
“We know there’s a threshold somewhere—probably in the vicinity of [3.6 degrees Fahrenheit]—where we’re very likely committed to more than [30 feet of sea-level rise] over centuries,” Field told me. It may be possible to preserve those large stores of ice at 2.7 degrees, scientists have found.
Katharine Hayhoe, a climate scientist at Texas Tech, told me that the report as a whole should be seen as a reappraisal of where we’re heading as a planet. “It’s like we had a medical issue. The physician had diagnosed it. But now they’re worried it might be worse than we thought,” she said. “So we go back and do a complete work over, every type of test we can imagine.”
The new prognosis is stirring. A world that warms by 3.6 degrees—and not 2.7 degrees—will find that its problems metastasize out of scale with that seemingly small difference. In the hotter world, the number of people affected by water scarcity will double. Twice as many corn crops will perish in the tropics. The size of global fisheries will drop by 50 percent. And 99 percent of the world’s coral reefs will perish.