Past is key to predicting future climate

According to a new study, including paleoclimate data in the development of climate models could help scientists predict scenarios for future climate and propose strategies for mitigation.

An international team of climate scientists, including Dr Benjamin Mills from the University of Leeds, suggest that numerical models predicting future climate change should include simulations of past climates in their evaluation and statement of their model performance.

Their study, published today in Science, highlights that as more and better information becomes available about Earth's distant climate history, this insight becomes increasingly relevant for improving understanding of how key elements of the climate system are affected by high greenhouse gas levels.

Unlike historic climate records, which typically only go back a century or two – a mere blink of an eye in the planet's climate history – paleoclimate data stretches back many millions of years before humans existed.

This data covers a vastly broader range of climatic conditions that can inform climate models in ways historic data cannot. These periods in Earth’s past span a large range of temperatures, precipitation patterns and ice sheet distribution.

In order to improve our climate modelling for the future, it is important to test the models by reconstructing these warm climates of the past.

Study co-author Dr Mills is from the School of Earth and Environment at Leeds and part of the internationally respected Palaeo@Leeds and Cohen Geochemistry groups. Their work includes Earth system modelling and compiling the geological datasets to which climate models are compared.

He said: "The Earth is no stranger to high CO2 levels and warm climate, and these warmer conditions have in fact persisted for most of the time since animals evolved. It is the speed of the transition which is so concerning today - rapid warming episodes in Earth's past are linked to mass extinction.

"Our geological archives give us a detailed picture of these ancient high-CO2 worlds. In order to improve our climate modelling for the future, it is important to test the models by reconstructing these warm climates of the past."

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.

"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. Past climates should be used to evaluate and fine-tune climate models.

"Looking to the past to inform the future could help narrow uncertainties surrounding projections of changes in temperature, ice sheets, and the water cycle."

While there is no debate in the climate science community about human fossil fuel consumption pushing the Earth toward a warmer state for which there is no historical precedent, different models generate varying predictions. Some forecast an increase as large as six degrees Celsius by the end of the century.

The authors warn that while Earth's atmosphere has experienced carbon dioxide concentrations much higher than today's level of about 400 parts per million, there is no time in the geological record that matches the speed at which humans are contributing to greenhouse gas emissions.

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. 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.

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. 

For further information, view the story on the University website.