The past is the key to predicting the future climate: models should be tested by simulating past climates

Past carbon dioxide concentrations (left) versus possible future emissions scenarios (right): the current emissions rate is much faster – occurring over decades – unlike geological changes, which occur in millions of years. If emissions continue unabated, carbon dioxide levels could reach or exceed values ​​associated with past warm climates, such as the Cretaceous period (100 million years ago) or the Eocene era (50 million years ago), by 2300. Credit: Jessica Tierney / University of Arizona

Including paleoclimatic data in the development of climate models could help climate scientists predict scenarios for future climate and propose mitigation strategies.

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 assessment and statement of their 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 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 where historical data cannot. These periods of Earth’s past span a wide range of temperatures, precipitation patterns, and ice sheet distribution.

“Past climates should be used to evaluate and fine-tune climate models,” Tierney said. “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.”

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 the future climate with reasonable precision.

“We find that many models perform very well with historical climates, but not so well with the climates of the Earth’s geological past,” Tierney said.

One reason for the discrepancies is differences in how models calculate cloud effects, which is one of the big challenges in climate modeling, Tierney said. Such differences cause different models to diverge from each other in terms of what climate scientists call climate sensitivity – a measure of how strongly the 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 report, or IPCC, have higher climate sensitivity than previous iterations, Tierney explained.

“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.

“The models get more complex and, in theory, they get better, but what does that mean?” Tierney said. “You want to know what’s going to happen in the future, so you want to be able to trust the model as to what happens in response to higher carbon dioxide levels.”

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 expect an increase of up to 6 degrees Centigrade by the end of the century.

Tierney said that while Earth’s atmosphere has experienced carbon dioxide concentrations much higher than its 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 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 era, 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 cut emissions, we’re headed for Eocene-like CO₂ levels by 2100, ”Tierney said.

The authors discuss climate change up to Cretaceous period, about 90 million years ago, when dinosaurs still 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.

“The key is CO₂“Tierney said.” Whenever we see evidence of warm weather in the geological record, CO₂ it is also high. “

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.”

Reference: November 5, 2020, Science.
DOI: 10.1126 / science.aay3701