Is climate change scary because of “black swans”—the low-probability, high-impact tail risks that are, by definition, unlikely? Or is it about what’s well known, already quantified, and very likely to happen unless the world slams on the emissions brakes? And does the difference matter? Headlines are typically driven by extremes: droughts, floods, fires, tropical cyclones, temperature records, and other nightmares both real today and projected to happen in the near and not-so-near future. It’s easy to see these headlines and want to appear “rational” by countering the “emotional” climate “alarmism.”
That reasoning has two fundamental flaws. First, even the most middle-of-the-road predictions of what’s likely in store are bad enough, pointing to the very real need to cut CO₂ emissions yesterday. Second, the low-probability, high-impact tail risks make action now even more desirable. Uncertainty is not our friend.
This new assessment narrows the “likely” range to 2.6 to 3.9°C. Good news, bad news. It’s good news because climate change just became significantly more predictable. After all, it’s the uncertainty itself that’s costly.
It’s bad news for the rest of us. It looks like we aren’t going to get lucky. The best likely case no longer includes anything close to 1.5°C. That number increased by around a degree to 2.6°C. Under various other scenarios, the scientists who assessed the evidence did move the lower bound back to 2.3°C to cover all their statistical bases. That’s still almost a full degree up from the prior lower bound.
Either way, increasing the lower bound wasn’t much of a surprise. The 1.5°C on the low end always seemed like a reach. Temperatures, after all, have already increased by at least 1°C, even though CO₂ concentrations have not yet increased by 50%—and the goal of rapidly decarbonizing the world economy is to keep it so.