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An international team of climate scientists, including two from the University of Michigan, suggests that researchers using numerical models to predict future climate change should include simulations of past climates when evaluating the model’s performance.
“We urge the climate modeling community to pay attention to the past and actively involve it in predicting the future,” said Jessica Tierney, lead author of the paper and associate professor in the University of Arizona Department of Geosciences. “If your model can accurately simulate past climates, it will probably do a much better job of getting future scenarios correct.”
As more and better information becomes available about climates in Earth’s distant history, which date back many millions of years before humans existed, past climates become increasingly relevant to improve our understanding of how key elements of the system climate are affected by greenhouse gas levels, according to the study was published Nov. 6 in the journal Science.
Unlike historical climate records, which typically only date back a century or two – a mere blink of an eye in the planet’s climatic history – paleoclimates cover a much wider range of climatic conditions that can inform climate models in ways that historical data cannot. These periods of Earth’s past span a wide range of temperatures, precipitation patterns, and ice sheet distribution.
“Looking to the past to inform the future could help reduce the uncertainties surrounding the projections of temperature changes, ice caps and the water cycle,” Tierney said.
Typically, climatologists evaluate their models with data from historical weather records, such as satellite measurements, sea surface temperatures, wind speeds, cloud cover, and other parameters. The model’s algorithms are then adjusted and tuned until their predictions fit the observed climate records. Therefore, if a computer simulation produces a historically accurate climate based on observations made during that period, it is considered appropriate to predict future climate with reasonable accuracy.
“Models tend to simulate historical weather well, but they don’t work well for past climates that were much colder or warmer,” said study co-author Chris Poulsen, professor in the Department of Earth and Environmental Sciences. of UM.
“This is worrying if the models are used to predict future climates that will be well outside the historical climate range,” Poulsen said.
Previous work by Poulsen and his former postdoctoral researcher Jiang Zhu, a co-author of the Science paper who is now at the National Center for Atmospheric Research, showed that one reason for the discrepancies is differences in how models calculate the effect of clouds, which is one of the great challenges in climate modeling. The models diverge in terms of what climate scientists call climate sensitivity, a measure of how strongly Earth’s climate responds to a doubling of greenhouse gas emissions.
Many of the latest generation models used for the next Intergovernmental Panel on Climate Change (IPCC) report have greater climate sensitivity than previous iterations.
“The increased sensitivity to climate means that the newer models produce more global warming in response to a doubling of carbon dioxide emissions than their previous counterparts,” said Poulsen. “The question is, which iteration is closest to the truth?”
Among the IPCC reports, which are typically released every eight years, climate models are updated in response to the latest research and data.
“With each generation, climate models are becoming more complex, but that doesn’t necessarily mean they are better able to predict the correct climate response to rising atmospheric carbon dioxide,” said study co-author Sierra Petersen, assistant professor. at the UM Department of Earth and Environmental Sciences.
“The only way to assess the capabilities of these models is to test whether they can reproduce past temperature data from past hot and cold periods,” Petersen said.
While there is no debate in the climate scientific community about the consumption of human fossil fuels pushing the Earth towards a warmer state for which there is no historical precedent, different models generate varying predictions. Some predict an increase of up to 6 degrees Celsius by the end of the century.
Tierney said that while Earth’s atmosphere has experienced carbon dioxide concentrations much higher than its current level by around 400 parts per million, there is no time in the geological record that matches the rate at which humans contribute to gas emissions. greenhouse.
In the paper, the authors applied climate models to several past climatic extremes known from the geological record. The most recent warm climate that offers a glimpse into the future occurred about 50 million years ago during the Eocene, Tierney said. At that time, global carbon dioxide was 1,000 parts per million and there were no large sheets of ice.
“If we don’t reduce emissions, we are heading towards Eocene-like CO2 levels by 2100,” Tierney said.
The authors discuss climate change up to the Cretaceous period, about 90 million years ago, when dinosaurs ruled the Earth. That period shows that the climate can get even warmer, a scenario Tierney described as “even scarier”, with carbon dioxide levels up to 2,000 parts per million and oceans as hot as a bathtub.
“Past greenhouse ranges such as the Eocene and Cretaceous, when CO2 was very high, give an idea of what we might be headed for,” UM’s Petersen said.
Some models are much better than others at producing the climates seen in the geological record, which underscores the need to test climate models against paleoclimate, the authors said. In particular, past warm climates such as the Eocene highlight the role clouds play in contributing to warmer temperatures with increased carbon dioxide levels.
“We urge the climate community to test paleoclimate models early on, as models are developed, rather than later, which tends to be current practice,” Tierney said. “Apparently small things like clouds affect the Earth’s energy balance in important ways and can affect the temperatures your model produces for the year 2100.”
The full list of authors and funding information are available in the paper “Past climates inform our future”, Science, November 6, 2020. DOI: 10.1126 / science.aay3701.
This press release was written by Daniel Stolte of the University of Arizona, with additional information provided by Jim Erickson of the University of Michigan.
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