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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.

Last week saw the publication of a crucial new assessment on one of the most basic of climate science questions: the link between CO2 in the atmosphere and eventual global average warming. For over 40 years, the answer to the question of how much temperatures increase when atmospheric CO2 doubles has been a "likely" range of 1.5 to 4.5 degrees C. The definition of what "likely" means has changed over the years. The range itself has barely budged, and not for a lack of trying. This new assessment narrows the "likely" range to 2.6 to 3.9 degrees 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 degrees C. That number increased by around a degree to 2.6 degrees C. Under various other scenarios, the scientists who assessed the evidence did move the lower bound back to 2.3 degrees 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 degrees C on the low end always seemed like a reach. Temperatures, after all, have already increased by at least 1 degree C, even though CO2 concentrations have not yet increased by 50 percent — and the goal of rapidly decarbonizing the world economy is to keep it so.

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Sadly, we are not as lucky on the upper end. The decrease from 4.5 degrees C to 3.9 degrees C is clearly good. Even more extreme outcomes now are less likely to occur. Alas, that bound goes right back up to 4.5 degrees C under various other scenarios in the scientists' assessment. And we can't cut things off even at 4.5 degrees C. There's a chance — a small chance, but a chance nonetheless — that temperatures would rise even further due to a doubling of CO2 in the atmosphere. That's the truly scary bit.

By definition, any of these large temperature increases are unlikely and far out in time. That does not mean we can wish them away.

Even a small chance of truly catastrophic, runaway climate change is a planetary gamble nobody should be prepared to take. In fact, those low-probability, high-impact risks may well dwarf all else. The late Harvard economist Martin Weitzman dubbed it the "dismal theorem" in an academic paper. Our joint book, Climate Shock, is an explanation of just that. Surprisingly to me, our assumptions in that book were so conservative that they roughly correspond to even this new, narrower temperature range.

Climate policy has long looked toward trying to keep global average warming to below 2 degrees C — or, better, 1.5 degrees C — for good reason. Most economists' current estimates of climate damages for these temperature increases are woeful underestimates of what's in store. It's equally clear that climate damages increase relentlessly the higher temperatures go.

All of that brings us right back to how to think about climate change in the first place. Climate change is about the here and now, and about the known. The most likely outcomes of unmitigated climate change are so costly, they should have prompted the world to take much more ambitious action a long time ago.

Climate change is also about unpredictability. Even a low probability of extreme global average temperature outcomes ought to drive action today. On top of that is the crucial link between even small increases in global average temperatures and rapid increases in weather extremes. What's known and quantified is alarming enough. What's not yet known increases the need for climate policy ambition further still.

Gernot Wagner writes the Risky Climate column for Bloomberg Green. He teaches at New York University and is a co-author of "Climate Shock." The opinions expressed by columnists do not necessarily reflect the views of the Brattleboro Reformer.