Predicting future climate with help from ice age data

The Earth’s temperature will increase as more carbon dioxide builds up in the atmosphere, but researchers are still investigating the exact amount of warming that will result from a specific rise in CO₂ levels. The connection between CO₂ and warming, referred to as climate sensitivity, will determine the potential future outcomes as CO₂ levels continue to increase.

A recent study conducted by the University of Washington examines the most recent ice age, during which a significant portion of North America was covered in ice, in order to gain a better understanding of the correlation between CO₂ levels and global temperature. The study concludes that while the majority of projections for future warming remain unchanged, the absolute worst-case scenario is unlikely.

“The main contribution from our study is narrowing the estimate of climate sensitivity, improving our ability to make future warming projections,” said lead author Vince Cooper, a UW doctoral student in atmospheric sciences. “By looking at how much colder Earth was in the ancient past with lower levels of greenhouse gases, we can estimate how much warmer the current climate will get with higher levels of greenhouse gases.”

The doubling of CO₂ in the new study does not alter the best-case warming projection of approximately a 2-degree Celsius global average temperature increase, nor the most probable estimate of around 3 degrees Celsius.

Nevertheless, it does decrease the worst-case scenario for doubled CO₂ by a full degree, from 5 degrees Celsius to 4 degrees Celsius. For reference, current CO₂ levels are at 425 ppm, roughly 1.5 times higher than preindustrial levels, and are projected to reach double preindustrial levels by the end of this century unless emissions are reduced.

As the planet approaches a doubling of CO₂, the authors advise that the recent decades are not a reliable indicator of the future of global warming. Recent trends can’t accurately predict the remainder of this century due to shorter-term climate cycles and the impact of atmospheric pollution.

“The spatial pattern of global warming in the most recent 40 years doesn’t look like the long-term pattern we expect in the future – the recent past is a bad analog for future global warming,” said senior author Kyle Armour, a UW associate professor of atmospheric sciences and of oceanography.

Rather than focusing on modern trends, the new study examined the Last Glacial Maximum period 21,000 years ago, when the Earth was, on average, 6 degrees Celsius cooler than it is today. Records from ice cores indicate that the atmospheric CO₂ levels during that time were less than half of the current levels, approximately at 190 parts per million.

“The paleoclimate record includes long periods that were, on average, much warmer or colder than the current climate, and we know that there were big climate forcings from ice sheets and greenhouse gases during those periods,” Cooper said. “If we know roughly what the past temperature changes were and what caused them, then we know what to expect in the future.”

Researchers, including co-author Gregory Hakim, a UW professor of atmospheric sciences, have developed innovative statistical modeling techniques that enable the incorporation of paleoclimate records into computer models of Earth’s climate, resembling modern weather forecasting models. As a result, temperature maps from previous millennia are more accurately represented.

In a recent study, the authors merged ancient climate records, such as ocean sediments, ice cores, and preserved pollen, with computer simulations of Earth’s climate to replicate the weather conditions during the Last Glacial Maximum. During this period, when a significant portion of North America was covered in ice, the ice sheet not only cooled the planet by reflecting summer sunlight off the continents, which was previously believed in earlier studies.

The ice sheet modified wind patterns and ocean currents, resulting in unusually cold and cloudy conditions in the northern Pacific and Atlantic oceans. According to the new study’s analysis, these changes in cloud cover over the oceans intensified the global cooling impact of the glacier by reflecting a greater amount of sunlight.

In summary, the study indicates that CO₂ had a smaller influence on ice age temperatures than previously thought. On the other hand, this means that the most extreme predictions of warming due to increasing CO₂ levels are less probable in the coming decades.

“This paper allows us to produce more confident predictions because it really brings down the upper end of future warming and says that the most extreme scenario is less likely,” Armour said. “It doesn’t really change the lower end or the average estimate, which remains consistent with all the other lines of evidence.”

Journal reference:

1. Vincent T. Cooper, Kyle C. Armour, Gregory J. Hakim, Jessica E. Tierney, Matthew B. Osman, Cristian Proistosescu, Yue Dong, Natalie J. Burls, Timothy Andrews, Daniel E. Amrhein, Jiang Zhu, Wenhao Dong, Yi Ming, Philip Chmielowiec. Last Glacial Maximum pattern effects reduce climate sensitivity estimates. Science Advances, 2024; DOI: 10.1126/sciadv.adk9461