The past is the key to predicting the future climate, scientists say

An international team of climate scientists, including UConn Department of Geosciences assistant professors Ran Feng and Clay Tabor, suggests that research centers around the world using numerical models to predict future climate change should include simulations of past climates in their evaluation and declaration of the performance of their model.

“We urge the community of climate model developers to pay attention to the past and actively involve it in forecasting the future. If your model can accurately simulate past climates, it will likely do a much better job of defining future scenarios, ”says Jessica Tierney, lead author of the article and associate professor at the University of Arizona.

Feng, who runs climate simulations in his research, adds that “A skilled model should be able to faithfully simulate both current and past climates, with the same set of parameterizations and dynamic equations.”

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 authors. Unlike historical climate records, which typically only date back a century or two – a mere blink of an eye in the planet’s climate history – paleoclimates cover a much wider range of climatic conditions that can inform climate models in ways that which historical data cannot. These periods in Earth’s past span a wide range of temperatures, precipitation patterns, and ice sheet extent.

“Paleoclimatic comparisons between geological records and climate models provide an effective way to validate models, however such comparisons remain underused during model development,” says climate modeler Tabor.

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.

Many of the next-generation models used for the next report from the Intergovernmental Panel on Climate Change, or IPCC, have greater climate sensitivity than previous iterations, Tierney explains.

“This means that if you double your carbon footprint, they produce more global warming than their previous counterparts, so the question is, how confident are we in these very sensitive new models?”

Among the IPCC reports, which are typically released every eight years, climate models are updated based on the latest research data. 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.

In the paper, the authors applied climate models to several past hot intervals known from the geological record.

“We have to go back to ~ 3 million years ago to find a CO₂ similar to the current level, “says Feng.

At the time, the Earth was several degrees warmer than today, suggesting that warming will continue in the future even if we immediately stop CO₂ emissions.

“But if we continue to emit at our current rate, we will reach CO by the end of the century₂ levels not seen in 50 million years, when the Earth had few or no polar caps, ”adds Tabor.

The authors discuss climate change up to the Cretaceous period, around 90 million years ago, when dinosaurs still ruled the Earth. That period shows that the climate can get even warmer, a scenario Tierney describes as “even scarier,” with carbon dioxide levels up to 2,000 parts per million and oceans as hot as a bathtub.

He adds that while the Earth’s atmosphere has experienced CO₂ concentrations much higher than the current level of around 400 parts per million, there is no time in the geological record that matches the rate at which humans contribute to greenhouse gas emissions.

For a complete list of authors and funding information, see the paper “Past climates inform our future”, Science, November 6, 2020. DOI: 10.1126 / science.aay3701