An international team of climate scientists 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 statement of their model performance.

Paleoclimate evidence reaches back millions of years and can tell us about the vast range of climates the Earth has experienced in the past. These past climates can be used to understand how increasing levels of greenhouse gases, and other climate forcings affect the climate system, and can inform climate models in ways that historical data cannot. Historical records of climate typically only go back one or two centuries, and whilst these records are incredibly valuable, they offer only a snap shot of the Earth’s climate history.

“Past climates are currently under-utilized in model development. If your model can simulate past climates accurately, it is likely it will be more skillful at predicting future scenarios said Nav Sagoo, paper author and researcher at the Department of Meteorology at Stockholm University.

Figure showing paleoclimate context for future climate scenarios.
Paleoclimate context for future climate scenarios. Shared Socioeconomic Pathways (SSPs) are scenarios of projected socioeconomic global changes up to 2100. These are shown alongside different paleoclimates from the past 100 million years. Global mean surface temperature and CO2 (grey line and error in shading) are estimated for the past 100 million years from proxy data. Temperature colors are scaled relative to preindustrial conditions. Blue bars indicate when there are well-developed ice sheets (solid lines) and intermittent ice sheets (dashed lines), according to previous syntheses studies. From Jessica E. Tierney et al., Past climates inform our future, Science, Nov. 6, 2020. DOI: 10.1126/science.aay3701.

 

Climate sensitivity, a measure of how strongly the Earth’s climate responds to a doubling of greenhouse gas emissions is one of the areas that climate models differ from each other. Several of the latest generation models that are being used for the next report by the Intergovernmental Panel on Climate Change, or IPCC, have a higher climate sensitivity than previous iterations. Much of this divergence is due to how models compute the effect of clouds and better constraining a value of climate sensitivity for the climate system today has implications for the environmental impacts, socio-economic implications and mitigation timescales for anthropogenic climate change.

"Past climates should be used to evaluate and fine-tune climate models," Jessica Tierney, the papers lead author said. "We urge the climate community to test models on paleoclimates early on, while the models are being developed, rather than afterwards, which tends to be the current practice," Tierney said. "Seemingly small things like clouds affect the Earth's energy balance in major ways and can affect the temperatures your model produces for the year 2100."

In the paper, the authors applied climate models to several known past climate extremes from the geological record. The most recent warm climate offering a glimpse into the future occurred about 50 million years ago during the Eocene epoch, Tierney said. Global carbon dioxide was at 1,000 parts per million at that time and there were no large ice sheets.

"If we don’t cut back emissions, we are headed for Eocene-like CO2 levels by 2100," Tierney said.

The authors discuss climate changes all the way to the Cretaceous period, about 90 million years ago, when dinosaurs still ruled the Earth. That period shows that the climate can get even warmer with carbon dioxide levels up to 2,000 parts per million and the oceans as warm as a bathtub.

Link to the article: Tierney JE, Poulsen CJ, Montañez IP, Bhattacharya T, Feng R, Ford HL, Hönisch B, Inglis GN, Petersen SV, Sagoo N, Tabor CR. Past climates inform our future. Science. 2020 Nov 6;370(6517).