Antarctica wasn’t as cold during the last ice age as previously thought, new study shows

John Fegyveresi

A study of two alternative methods for reconstructing ancient temperatures has given climate researchers a better understanding of just how cold it was in Antarctica during the last ice age about 20,000 years ago.

Antarctica, the coldest place on Earth today, was even colder during the last ice age. For decades, the leading science suggested Ice Age temperatures in Antarctica were on average about nine degrees Celsius cooler than at present. By comparison, temperatures globally at that time averaged five to six degrees cooler than the present.

An international team of scientists, which included Northern Arizona University researcher John Fegyveresi, found that while parts of Antarctica were as cold as 10 degrees below current temperatures, temperatures over central East Antarctica were only four to five degrees cooler, about half of the previous estimates.

The findings were published this week in Science. Christo Buizert, a climate change specialist at Oregon State University, was the lead author; other co-authors included researchers from throughout the United States, Japan, the United Kingdom, France, Switzerland, Denmark Italy, Korea and Russia. The study was supported in part by the National Science Foundation.

“This is the first conclusive and consistent answer we have for all of Antarctica,” Buizert said. “The surprising finding is that the amount of cooling is very different depending on where you are in Antarctica. This pattern of cooling is likely due to changes in the ice sheet elevation that happened between the ice age and today.”

The total air content trapped in polar ice can provide information on past atmospheric pressure, and thus past ice sheet elevation. For his part of the research, Fegyveresi, an assistant professor of practice in the School of Earth and Sustainability at NAU, developed new calibrations for an automatic ice-core air extraction device, and completed more than 2,000 measurements from the various Antarctic ice cores used in this study.

“This incredibly collaborative effort really sheds new light on how the climate was behaving in different parts of Antarctica during the last ice age, and will help to better inform our climate models for future projections,” Fegyveresi said.

Understanding the planet’s temperature during the last ice age is critical to understanding the transition from a cold to a warm climate and to modeling what might occur as the planet warms as a result of climate change, said Ed Brook, a paleoclimatologist at OSU and one of the paper’s co-authors. The last Ice Age represents a natural experiment for understanding the planet’s sensitivity to changes in greenhouse gases such as carbon dioxide, the researchers said. Core samples taken from ice that has built up over hundreds of thousands of years help tell that story.

Researchers in the past have used water isotopes contained in the layers of ice, which essentially act like a thermometer, to reconstruct temperatures from the last ice age. In Greenland, those isotope changes can be calibrated against other methods to ensure their accuracy. But for most of Antarctica, researchers have not been able to calibrate the water isotope thermometer against other methods.

“It is as if we had a thermometer, but we could not read the scale,” said Buizert, an assistant professor in OSU’s College of Earth, Ocean, and Atmospheric Sciences. “One of the places where we had no calibration is East Antarctica, where the oldest continuous records of ice cores have been drilled, making it a critical location for understanding climate history.”

In the new study, the researchers used two alternative methods for reconstructing ancient temperatures, using ice cores from seven locations across Antarctica—five from East Antarctica and two from West Antarctica. Both methods produced similar temperature reconstructions.

The researchers found that the amount of Ice Age cooling is related to the shape of the ice sheet.

During the Ice Age, some part of the Antarctic ice sheet became thinner as the amount of snowfall declined, Buizert said. That lowers the surface elevation and cooling in those areas was by four  to five degrees. In places where the ice sheet was much thicker during the Ice Age, temperatures cooled by more than 10 degrees.

The findings are important for improving future climate modeling but they do not change researchers’ perception of the how sensitive the Earth is to carbon dioxide, the primary greenhouse gas produced through human activity.