Hausfather Climate Model Paper Not What It’s Cracked up to Be

This week’s press is abuzz with a paper just published by Zeke Hausfather and others in the journal Geophysical Research Letters. It purports to show that early (1970s and ’80s) climate models are doing just fine, with a special concentration on NASA’s James Hansen and the 1988 model he showcased in front of Congress, initiating the still-raging global warming hubbub.

The Hausfather et al. paper directly confronts an op-ed that I placed in The Wall Street Journal on the thirty-year anniversary of Hansen’s Washington theater. I found that Hansen’s model was substantially over-forecasting warming.

Hansen ran the model with three emission scenarios: A, B, and C. He called Scenario A “Business As Usual,” as it maintained the accelerating emissions growth typical of the 1970s and ’80s. In Scenario B, emissions rise at the same rate in 2018 as in 1988 and produce a 30-year warming of 0.7°C. In Scenario C, a strong mitigation scenario, emissions would flatline after 2000 and so would temperatures after rising a few tenths of a degree. Hansen deemed Scenario B the “most plausible.” He considered Scenario C highly unlikely. My op-ed noted that observed temperatures in 2018 were closest to Scenario C, although emissions were increasing as projected in Scenario B.

Hausfather et al. agree with my finding. But, they object, Scenario B overestimated the increase in methane and chlorofluorocarbon (CFC) emissions, and when they adjust for this, Scenario B (barely) matches the statistical extreme of observed warming, i.e., it worked.

Not so fast. Hansen’s model also exaggerated the cooling effect from sulfate aerosols, by a factor of three. When I adjust the results for that, Scenario B becomes even hotter than his “business as usual” scenario.

The point is if you are going to adjust model output for revised science, you must adjust all the relevant variables that later science has revised. Hausfather et al. didn’t do this. If they had, it would have invalidated their result.

Interviewed by Science magazine for my comments on their paper, I pointed out that “What’s good for the CFC goose had better be good for the sulfate gander.” I doubt Science will print that, but it is what I said, and it is true.