Early climate change models were pretty accurate, study finds

Early climate models get a bad wrap for being imprecise, but new research suggests they were surprisingly accurate.

Researchers at the University of California, Berkeley, Massachusetts Institute of Technology and NASA reviewed 17 climate models described in scientific papers over the last several decades. The earliest were developed in the 1970s, while the most recent were created in late 2000s.

Of the 17 models, analysis showed 14 were very accurate in predicting the average global temperature in the years following their publication.

Scientists published their review this week in the journal Geophysical Research Letters.

"The real message is that the warming we have experienced is pretty much exactly what climate models predicted it would be as much as 30 years ago," lead study author Zeke Hausfather, a doctoral student in UC Berkeley's Energy and Resources Group, said in a news release. "This really gives us more confidence that today's models are getting things largely right as well."

Scientists looked at each model's prediction for the Earth's average temperature based on greenhouse gas concentrations in the atmosphere, and compared the predictions to actual global temperatures recorded at similar greenhouse gas levels.

In effect, the study's authors looked at how well models simulated the relationship between emissions and temperature increases, not at how well models predicted changes in global emissions.

"We did not focus on how well their crystal ball predicted future emissions of greenhouse gases, because that is a question for economists and energy modelers, not climate scientists," Hausfather said. "It is impossible to know exactly what human emissions will be in the future. Physics we can understand, it is a deterministic system; future emissions depend on human systems, which are not necessarily deterministic."

One of the first models to draw attention to the problem of global warming, developed by James Hansen of NASA in 1988, overestimated methane emissions and underestimated the impact of the Montreal Protocol treaty, which banned chlorofluorocarbons. As a result, the model predicted inflated temperature increases.

But when researchers accounted for these missteps and looked only at the model's understanding of the relationship between greenhouse gas emissions and temperature increases -- the physics of global warming -- they found Hansen's work was quite accurate.

"If you account for these and look at the relationship in his model between temperature and radiative forcing, which is CO2 and other greenhouse gases, he gets it pretty much dead on," Hausfather said. "So the physics of his model was right. The relationship between how much CO2 there is in the atmosphere and how much warming you get, was right. He just got the future emissions wrong."

When scientists plugged real-world greenhouse gas levels into most of the surveyed models, they spit out temperatures close to what weather stations actually recorded.

Climate models continue to improve, but the accuracy of most models is judged on their ability to predict -- or replicate -- past climate scenarios. The latest is one of the first to take a shorter look back to see how climate models predicted the future of global warming. The study's authors suggest their work can help climate modelers continue to improve their model's predictive powers.

"Climate models are a really important way for us to understand how the climate could change in the future, and now that we have taken a detailed look at how well past climate models have held up in terms of their projections, we are far more confident that our current generation of models are getting it right," Hausfather said.

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