Climate models are accurate, study says

Majority shown to predict results that ‘actually occurred’

WASHINGTON — The computer models used to simulate what heat-trapping gases will do to global temperatures have been spot-on in their predictions, a new study found.

After years of hearing critics blast the models’ accuracy, climate scientist Zeke Hausfather decided to see how good they have been. He tracked down 17 models used between 1970 and 2007 and found the majority of them predicted results that were “indistinguishable from what actually occurred.”

“By and large our models have gotten it right, plus or minus a little bit,” said Hausfather, a University of California, Berkeley scientist who is climate and energy director at the Breakthrough Institute. “If they get it wrong, it’s slightly on the warm side, but I wouldn’t read too much into that.”

Ten of the 17 were close to the temperatures that happened, said Hausfather, lead author of a study in this week’s journal Geophysical Research Letters.

But scientists got the physics right even more than that, Hausfather said. That’s because they make two main assumptions when they model what will happen in the future.

One is the physics of the atmosphere and how it reacts to heat-trapping gases. The other is the amount of greenhouse gases put into the air.

A few times, scientists were wrong in their predictions about the growth of carbon pollution, saying there would be more of the gases than there were, Hausfather said. If they got the heat-trapping gases wrong, they then got the temperatures wrong.

So Hausfather and colleagues, including NASA climate scientist Gavin Schmidt, looked at how well the models did on the pure science, taking out the emissions factor. On that count, 14 of the 17 computer models accurately predicted the future.

The scientists also gave each computer simulation a “skill score” that gave a percentage grade to each one. The average grade was a 69%.

One of the earliest computer models, made in 1970, got a 91%. What’s so impressive is that at the time, climate change wasn’t noticeable in the yearly temperature records like it is now, Hausfather said.

Stanford University climate scientist Noah Diffenbaugh, not part of the study, called the work creative and the results striking.

“Even without knowing what the current level of greenhouse gas concentrations would be, the climate models predicted the evolution of global temperature quite well,” Diffenbaugh said in an email.

It’s crucial that these models are accurate because “we have one planet Earth, so we can’t conduct controlled experiments on the actual climate system,” Diffenbaugh said.

The study was released as officials from almost 200 countries meet in Madrid for climate talks.