Climate skeptics have long questioned the accuracy of computer models that predict global warming, but most early climate models turn out to be spot-on, according to a flashback from University of California climatologists at Berkeley, Massachusetts Institute of Technology and NASA.
Of 17 climate models published between the early 1970s and late 2000s, 14 were accurate enough to predict average global temperature in the years after publication, said Zeke Hausfather, a doctoral student in the Energy and Resources group at the ‘UC Berkeley and lead author of a new paper analyzing patterns.
“The real message is that the warming we’ve been experiencing is pretty much exactly what climate models predicted 30 years ago,” he said. “It really gives us more confidence that today’s models are also doing things largely correctly.”
The results were published today in the journal Geophysical Research Letters and will appear, with Hausfather as contributing author, as part of the first chapter of the Intergovernmental Panel on Climate Change’s upcoming climate assessment, scheduled for 2021. .
Hausfather and his colleagues at the Massachusetts Institute of Technology and the NASA Goddard Institute for Space Studies evaluated the models on their ability to predict the actual global average temperature – the Earth’s average temperature – based on carbon dioxide levels, of methane and other greenhouse gases. in the air. Greenhouse gases, mostly from the burning of fossil fuels, have been responsible for an increase of nearly 1.4 degrees Fahrenheit in global average temperatures since 1880, two-thirds since 1975.
The researchers looked at both how well the models predicted future temperatures and how well they fit the relationship between warming and changes in greenhouse gas levels after they were released. This second approach takes into account that climate modellers cannot necessarily predict future emissions, which are determined by human behavior rather than atmospheric physics.
“We didn’t focus on their crystal ball’s ability to predict future greenhouse gas emissions, because that’s a question for economists and energy modellers, not climate scientists,” Hausfather said. . “It is impossible to know exactly what the human emissions will be in the future. Physics, one can understand, is a deterministic system; future emissions depend on human systems, which are not necessarily deterministic.
Most models have the right physics
One of the iconic climate models, and the one that first brought the issue of climate change to the public’s attention, was published by NASA’s James Hansen in 1988. However, his predictions for temperatures after 1988 were 50% higher than actual global average temperatures. in those years.
That’s partly because Hanson hadn’t anticipated the Montreal Protocol, a treaty that came into effect in 1989 that banned chlorofluorocarbons, which are potent greenhouse gases. His estimates of future methane emissions were also wrong, Hausfather said.
“If you factor that in and look at the relationship in his model between temperature and radiative forcing, which is CO2 and other greenhouse gases, he pretty much gets it all,” he said. “So the physics of his model was correct. The relationship between the amount of CO2 in the atmosphere and the amount of warming you get was correct. He was just wrong about future shows.
Most other models also accurately predicted average temperature when given actual greenhouse gas levels.
Climate models continue to improve, Hausfather said, as they increasingly incorporate the physics and chemistry of the atmosphere, clouds, oceans and land. But it’s too early to assess how well current models predict future temperatures because the global average temperature has a natural variation that can obscure the overall trend. It is even too early to judge the fourth assessment of the IPCC, published in 2007.
Nevertheless, these models were tested in other ways, including how well they would have predicted past climate variations: what is called retrospective analysis.
“Climate models are a very important way for us to understand how the climate might change in the future, and now that we have looked in detail at how well past climate models have held up in terms of projections, we are much more confident. .that our current generation of models are doing well,” Hausfather said.
Assess the performance of past climate model projections. Zeke Hausfather, Henri F. Drake, Tristan Abbott, Gavin A. Schmidt. Geophysical Research Letters, 04 December 2019, https://doi.org/10.1029/2019GL085378.
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