4 June, 2021 News stories

A study of two alternative methods for reconstructing ancient temperatures has given climate researchers a better understanding of how cold it was in Antarctica during the last Ice Age, around 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 at present.

An international team of scientists, led by Oregon State University’s Christo Buizert, has found that while parts of Antarctica were as cold as ten 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.

“This is the first conclusive and consistent answer we have for all of Antarctica,”

said Buizert, a climate change specialist.

“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 study’s co-authors include Carlos Martin Garcia from British Antarctic Survey, and a team of researchers from 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 and EU Horizon 2020 project Beyond EPICA: Oldest Ice Core.

Ed Brook, a paleoclimatologist at OSU and one of the paper’s co-authors 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 modelling what might occur as the planet warms as a result of climate change. Antarctica is particularly important in the climate system. We use climate models to predict the future, and those climate models have to get all kinds of things correct. One way to test these models is to make sure we get the past right.”

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.

In the past, researchers 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.

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.

“The borehole thermometry method measures temperatures throughout the thickness of an ice sheet. The Antarctic ice sheet is so thick that it keeps a memory of earlier, colder Ice Age temperatures that can be measured and reconstructed.”

said Fudge, from the University of Washington.

The second method examines the properties of the snowpack as it builds up and transforms into ice over time. In East Antarctica, that snowpack can range from 50 to 120 metres thick and has compacted over thousands of years in a process that is very sensitive to temperature changes.

The researchers found that both methods produced similar temperature reconstructions, giving them confidence in the results.

The researchers found that the amount of Ice Age cooling is related to the shape of the ice sheet. During the Ice Age, some parts 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 four to five degrees. In places where the ice sheet was much thicker during the Ice Age, temperatures cooled by more than ten degrees.

Carlos Martin Garcia, a glaciologist at British Antarctic Survey said:

“We have been using new radar technologies, called ApRES, to better understand glacial processes in Antarctica. Our findings have unlocked one of the two methods presented in the paper, in the most contested area – Dome C in central East Antarctica. The key to unlock the puzzle is that previous temperature reconstructions were neglecting changes in the shape of Antarctica, its surface elevation. Climate models are sensitive to these changes in shape. This paper shows how we can leverage glaciological observations to improve climate models.”

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

“This paper is consistent with the leading theories about sensitivity,”

Buizert said.

“We are the same about of worried today about climate change as we were yesterday.”

You can read the paper “Antarctic surface temperature and elevation during the Last Glacial Maximum” online.