Researchers suggests Past is Key to Predicting Future Climates

An international team of climate scientists, including academics from the School of Ocean and Earth Science at the University of Southampton, suggests that research centres 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.

In a review paper published in the journal Science, Southampton researchers Dr Gordon Inglis, Royal Society Dorothy Hodgkin Fellow, and Gavin Foster, Professor of Isotope Geochemistry, and their research colleagues make the case for including paleoclimate data in the development of climate models. Such models are used to predict future climate change scenarios and propose strategies for mitigation.

“We urge the climate model developer community to pay attention to the past and actively involve it in predicting the future," said Jessica Tierney, the paper's lead author and an Associate Professor in the University of Arizona's Department of Geosciences. "If your model can simulate past climates accurately, it likely will do a much better job at getting future scenarios right.”

As more and better information becomes available about climates in Earth's distant history, reaching back many millions of years before humans existed, past climates become increasingly relevant for improving our understanding of how key elements of the climate system are affected by greenhouse gas levels, according to the study's authors. Gordon Inglis said: “Past climates are different to modern, but they provide us with rich evidence of how climate processes operate across the range of carbon dioxide concentrations associated with future emission scenarios.”

Past climates can also improve our estimates of climate sensitivity, the global temperature change for a doubling in carbon dioxide concentrations. Gavin Foster adds: “Previous studies suggest that this was about the same in the past as anticipated for the future - about 3 to 4°C. However, we have recently shown that this value may increase in past warm climates. This could be of particular importance to our future climate as CO2 increases and the Earth continues to warm.”

In the paper, the authors also 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. Global carbon dioxide was at 1,000 parts per million at that time and there were no large ice sheets.

“The Eocene epoch was also characterised by short-lived perturbations to the Earth system”, Dr Inglis continued. “This includes the Paleocene-Eocene Thermal Maximum (PETM), which occurred about 56 million years ago. This event was triggered by the sudden and rapid emission of greenhouse gases and may foreshadow changes that the Earth will experience owing to anthropogenic emissions. During the PETM, global temperatures spiked between 4 and 6°C and had profound impacts on the environment, including rainfall patterns, vegetation distributions and soil erosion.”

Typically, climate scientists 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 mesh with the observed climate records. Thus, if a computer simulation produces a historically accurate climate based on the observations made during that time, it is considered fit to predict future climate with reasonable accuracy.

Some models are much better than others at producing the climates seen in the geologic record, which underscores the need to test climate models against paleoclimates, the authors said. In particular, past warm climates such as the Eocene highlight the role that clouds play in contributing to warmer temperatures under increased carbon dioxide levels.

“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.”

The paper, Past climates inform our future, is published in Science. DOI: 10.1126/science.aay3701