The Antarctic ice sheet was even more unstable in the past than previously thought, and at times came close to collapsing, new research shows.
The findings raise concerns that, in a warmer climate, exposing the land beneath the ice sheet as it recedes will increase precipitation over Antarctica, which could trigger processes that further accelerate ice loss.
The research is based on climate modeling and data comparisons for the Middle Miocene (13-17 million years ago), when atmospheric carbon dioxide and global temperatures reached levels similar to those expected in here the end of this century.
The study was carried out by the Met Office, the universities of Exeter, Bristol, Cardiff and Stockholm, NORCE and the Bjerknes Center for Climate Research.
“When an ice cap melts, the newly exposed ground below is less reflective and local temperatures become warmer,” said lead author Dr Catherine Bradshaw, of the Met Office and the Global Systems Institute at the University of Exeter. .
“It can drastically change the weather conditions.
“With a large ice cap on the mainland like we have today, Antarctic winds generally blow from the mainland towards the sea.
“However, if the continent warms, this could be reversed, with winds blowing from the colder sea to the warmer land – as we see with monsoons around the world.
“This would bring additional rainfall to the Antarctic continent, causing more fresh water to run off into the sea.
“Fresh water is less dense than salt water and can therefore rest on the surface of the sea, rather than sinking and flowing like salt water.
“This effectively breaks the connection between the deep ocean and the surface ocean, causing warmer water to accumulate at depth.”
The study suggests that processes triggered by increased precipitation reduce the ability of the climate system to maintain a large Antarctic ice sheet.
“Essentially, if more land is exposed in Antarctica, it becomes more difficult for a large ice cap to reform, and without favorable orbital positions in the Mid Miocene playing a role, perhaps the ice cap would have collapsed. at that time, “said Dr Bradshaw. .
During the warm period of the Middle Miocene, unusually large oscillations in both directions of deep water temperatures were recorded.
The study shows that fluctuations in the area covered by the ice sheet have been a major factor in the dramatic variation in deep water temperatures. Fluctuations in ice volume were found to be much less significant.
Variations in the Earth’s positioning relative to the Sun have moved the ice sheet forward and backward, altering weather patterns – triggering processes that can accelerate the loss or gain of ice.
Rain falling on the ice sheet can cause fractures, surface melting and excess fresh water flowing from the mainland, which in turn can cause deep water temperatures to rise – potentially influencing the ice. Antarctica from below.
The results of the new study suggest that the Antarctic ice sheet retreated significantly during the Middle Miocene, then stabilized at the end of the warm period.
Co-author Associate Professor Agatha De Boer, Stockholm University, said: “When the mid-Miocene climate cooled, the link we found between the ice cap area and water temperatures deep through the hydrologic cycle has become a finish.
“Once Antarctica was completely covered by the ice cap, the winds would still flow from land to sea and as a result precipitation would be reduced to low levels falling as snow on the continent we see today.”
Dr Petra Langebroek, principal investigator of NORCE and the Bjerknes Center for Climate Research, another co-author, added: “These results imply a change in the sensitivity of the ocean to changes in the ice sheet as the ice cap exposes land previously covered in ice. “
Professor Carrie Lear of Cardiff University, who first designed the project, concluded: “This study suggests that during a warm period around 15 million years ago, the Miocene Antarctic ice sheet was able to move forward and backward in significant ways across the continent.
“This is concerning, but more research is needed to determine exactly what this means for the long-term future of the modern Antarctic ice sheet.”
Dr Bradshaw pointed out that current conditions are not the same as in the Middle Miocene, and that the model used in the study does not include the impact of carbon cycle feedbacks or the ice sheet itself.
The study was funded by the Natural Environment Research Council and the Swedish Research Council.