Over the past Ice Age, Antarctica wasn’t fairly as chilly as beforehand thought – Watts Up With That?
OREGON STATE UNIVERSITY
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PICTURE: ICE NUCLEAR RESEARCHER DON VOIGT EXAMINES AN ICE CORE AT THE WEST ANTARCTIC ICE SHEET DIVIDE (WAIS DIVIDE) PROJECT. View More CREDIT: PHOTO BY GIFFORD WONG
CORVALLIS, Ore. – A study of two methods of reconstructing ancient temperatures gave climatologists a better understanding of how cold it was in Antarctica during the last ice age about 20,000 years ago.
Antarctica, now the coldest place on earth, was even colder during the last ice age. For decades, leading science assumed that Ice Age temperatures in Antarctica were on average around 9 degrees Celsius cooler than they are today.
An international team of scientists led by Christo Buizert from Oregon State University has found that parts of Antarctica were up to 10 degrees below current temperatures, but temperatures over central East Antarctica were only 4 to 5 degrees cooler, around half of previous estimates.
The results were published in Science this week.
“This is the first conclusive and consistent answer we have for all of Antarctica,” said Buizert, an expert on climate change at Oregon State University. “The surprising result is that the cooling varies a lot depending on where you are in Antarctica. This cooling pattern is likely due to changes in ice sheet height that occurred between the Ice Age and today. “
Understanding the temperature of the planet during the last ice age is critical to understanding the transition from a cold to a warm climate and modeling what could happen if the planet warms today as a result of climate change, said Ed Brook, paleoclimatologist the OSU and one of the co-authors of the paper.
“Antarctica is particularly important to the climate system,” said Brook. “We use climate models to predict the future, and these climate models have to do all kinds of things right. One way to test these models is to make sure we’re getting the past right. “
Co-authors of the study are an international team of researchers from the USA, Japan, Great Britain, France, Switzerland, Denmark, Italy, Korea and Russia. The study was supported in part by the National Science Foundation.
“International collaboration was critical to answering this question because it involved so many different measurements and methods from ice cores across Antarctica,” said co-author TJ Fudge, assistant professor of earth and space sciences at the University of Washington. “Ice cores recently drilled with the support of the National Science Foundation enabled us to gain new knowledge from previously drilled cores.”
The last ice age represents a natural experiment to understand the planet’s sensitivity to changes in greenhouse gases like carbon dioxide, the researchers said. Core samples of ice built up over hundreds of thousands of years help tell this story.
Researchers in the past have used water isotopes contained in the ice layers, which essentially act like a thermometer to reconstruct temperatures from the last ice age. In Greenland, these isotopic changes can be calibrated using other methods to ensure their accuracy. But for most of Antarctica, researchers were unable to calibrate the water isotope thermometer using any other method.
“It’s like we had a thermometer, but we couldn’t read the scale,” said Buizert, assistant professor at OSU’s College of Earth, Ocean, and Atmospheric Sciences. “One of the places we didn’t have a calibration is East Antarctica, where the oldest ongoing ice core records were drilled, making it a crucial place in understanding climatic history.”
In the new study, researchers used two methods to reconstruct 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 across the thickness of an ice sheet. The Antarctic ice sheet is so thick that it retains a memory of previous, colder ice age temperatures that can be measured and reconstructed, said Fudge, assistant professor in the Department of Earth and Space Science at the University of Washington.
The second method studies the properties of the snowpack as it builds and turns into ice over time. In East Antarctica, this snow cover can be 50 to 120 meters thick and has thickened over thousands of years in a process that is very temperature-sensitive.
The researchers found that both methods provided similar temperature reconstructions, which gave them confidence in the results.
They also found that the amount of ice age cooling was related to the shape of the ice sheet. During the last ice age, part of the Antarctic ice sheet became thinner as the amount of snow decreased, Buizert said. This lowers the surface elevation and the cooling in these areas was 4 to 5 degrees. In places where the ice sheet was much thicker during the Ice Age, the temperatures dropped by more than 10 degrees.
“This relationship between altitude and temperature is known to mountaineers and pilots – the higher you go, the colder it gets,” says Buizert.
The results are important to improving future climate modeling, but they don’t change researchers’ perceptions of how sensitive the earth is to carbon dioxide, the primary greenhouse gas produced by human activities, he said.
“This paper is consistent with leading theories about sensitivity,” said Buizert. “We are just as concerned about climate change today as we were yesterday.”
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