The difference between 1.5 degrees and 2 degrees would be substantial. Coral reefs would go from mostly gone to almost entirely gone. More sea-level rise would put up to 10 million more people in danger. High heat would kill more people. It would be much hotter on land and in cities. Deadly mosquito-borne illnesses such as malaria and dengue fever would spread farther. Droughts would be more likely. So would deluges. Tropical fisheries would empty further. Staple crop yields, particularly in some of the world’s poorest nations, would decline more. Disastrous loss of the Antarctic ice sheet would be more likely. Feedback loops could push warming further than anticipated, as, for example, thawing permafrost releases gases the frozen ground has trapped for centuries. Up to nearly 1 million additional square miles of permafrost would thaw at 2 degrees of warming.

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The risk of activating such a feedback loop is one reason it would be so foolish and irresponsible to breach the 1.5-degree threshold. Extinct species and obliterated ecosystems would be impossible to revive. Workarounds that do not involve rapidly slashing emissions — such as innovative ways of sending more sunlight back into space by, for example, seeding the skies with materials that reflect solar radiation — would not stop the oceans from acidifying.

If followed through, the emissions-reduction commitments countries pledged in the Paris agreement would help avert catastrophic warming. But they would still only restrain warming to 3 degrees by the end of the century, barring more ambition in the next decade and after. Global greenhouse gas emissions must fall by 45 percent of 2010 levels by 2030 and decline sharply thereafter to avoid blowing past 1.5 degrees.

Yet, at the moment, they continue to rise.

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Radically changing the trajectory would require a combination of strategies. Humans would need to waste far less energy. Forests would have to be preserved and expanded. Emissions-free renewables would have to ramp up — to around three-quarters of global electricity demand by 2050 — with an assist from nuclear and still-nascent carbon-capture technology that sequesters emissions from traditional fossil fuel burning. Extra-dirty coal, which still produces more electricity than any other single source, would have to be finally phased out. The transition would require investment of about 2.5 percent of world GDP through 2035.

This would be difficult but not impossible, if we tried. Historians will look in absolute astonishment at an American administration that not only failed to try but actually pushed in the wrong direction.