If anything, the Intergovernmental Panel on Climate Change (IPCC) has a habit of understatement. The U.N. group issues reports so thoroughly scrubbed that they seem cautious in the moment and downright timid in retrospect. That gives their latest and most dire warning added force.
The new IPCC report advises global leaders that the oft-cited goal of keeping warming below 2 degrees Celsius is riskier than many imagine. A 1.5-degree goal would be far less dangerous, but the world has only about a decade to make the “rapid and far-reaching” changes required to meet that goal.
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.
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.
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.
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. Yet, at the moment, they continue to rise.
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. This would be difficult but not impossible, if we tried.