Over the course of 2021 a lot of extreme weather events made headlines. The “pineapple Express” in November that dumped severe precipitation on the U.S. Northwest and Canada’s southwest, was just one good example of this. But as a matter of science, such events raise the question: can any such an event be attributed to a broader pattern of climate change. In other words: given the general understanding that “climate is not weather,” when can causal attribution cross the implied line between the two? The answer is, only in a qualified and probabilistic way.
Think of a “climate model” as a computer simulation of events across the earth’s surface. The model will have a lot of mathematical parameters, which one can think of as knobs one can turn and set at various values. One of those parameters, for example, is the level of carbon dioxide level in the atmosphere. A modeler makes a case for event attribution by saying, “an event much like this follows if we twiddle knob X.” X can be set to, say, pre-industrialization levels of CO2 and then to contemporary levels. Keeping all other knobs the same: what changes in how the events play out?
Strange New Worlds:
In an attribution study of the 2019/20 wildfires in Australia that were driven by a prolonged drought, the authors write: “We can therefore only conclude that anthropogenic climate change has made a hot week like the one in December 2019 more likely by at least a factor of two.” That is as close to event attribution as the science at present can come.