Climate doubters lose one of their last remaining arguments

The United Nations’ Intergovernmental Panel on Climate Change is the keeper of the expert consensus on global warming, issuing a painstaking, authoritative report on the state of the science roughly every five years. The body released its sixth assessment on Monday. As with those before it, the latest appraisal was both alarming and unsurprising. Humans are warming the planet. This warming threatens civilization. The worst effects are avoidable, but the problem gets harder to address the longer world leaders wait.

This essential picture has not changed in decades. Yet scientists’ confidence has. Experts are more certain than ever that dire consequences are coming. For decades, climate change doubters clung to scientists’ acknowledgment that there is some give in their numbers — in particular, a key measure known as “climate sensitivity,” which refers to how much the planet will warm given a doubling of carbon dioxide in the atmosphere. The U.N. panel had previously offered a wide range of likely scenarios, 1.5 to 4.5 degrees Celsius. Doubters argued that warming might end up reflecting the low end of this range. Why force economic disruption to stave off warming that experts admit might not be as bad as some fear?

This view never took into account the fact that uncertainty works in two directions: Things could also turn out far worse than scientists’ median estimates. Now the United Nations has voided this argument. In its latest report, the panel narrowed its climate sensitivity range to 2.5 to 4 degrees Celsius, ruling out the benign warming scenarios doubters insisted were still possible.