When was the last time Antarctica was ice-free?
Antarctica, which is almost four times the size of the United States, is almost entirely covered in a layer of ice several kilometers thick.
But the South Pole wasn’t always frozen. So when was the last time Antarctic was he without ice?
This ice sheet formed relatively recently in geological terms, experts told Live Science. “I think most people would say the ice sheet first formed in Antarctica 34 million years ago,” said Eric Wolffpaleoclimatologist at the University of Cambridge. “[Previously] most of it would have been like northern Canada today: tundra and coniferous forest.
Global temperatures are a key factor influencing the extent of ice cover. About 50 million years ago, the planet was about 25 degrees Fahrenheit (14 degrees Celsius) warmer than today, but temperatures steadily declined over the next 16 million years. 34 million years ago – a period known as the Eocene-Oligocene boundary – the climate was 14.4 F (8 C) warmer than it is today.
But what triggered this drop in temperature, and was that all it took for the ice sheets to form?
Related: Which is colder: the North Pole or the South Pole?
“There are two factors, and probably both were at play,” Wolff told Live Science. “One of them is a change in the concentration of carbon dioxide in the atmosphere, and the other is the movement of the continents and, in particular, the opening of the Drake Passage,” the strait between South America and Antarctica that connects the South Atlantic to the South Pacific.
More carbon dioxide there is in the atmosphere, the more heat is trapped and the warmer the planet.
About 60 to 50 million years ago, the concentration of carbon dioxide in the Earth’s atmosphere was very high – between 1,000 and 2,000 parts per million, or between 2.5 and 5 times. today’s levelssaid Tina van de Flierdtgeochemist at Imperial College London.
“But we know that CO2 in the atmosphere has crossed the Eocene-Oligocene boundary,” she told Live Science. This decrease in atmospheric CO2 would have been accompanied by a cooling of the global climate, she added, likely tipping the Earth beyond a certain threshold and allowing the formation of ice caps.
However, there was probably also localized cooling on the Antarctic continent due to plate tectonicsWolff said. Around this time, South America and Antarctica finally separated, opening what is today the Drake Passage.
“This led to what we call a circumpolar current – water flowing around Antarctica in a circle,” Wolff said. “This isolates Antarctica from the rest of the world and makes it much more difficult for warm air masses to pass across the Southern Ocean, making Antarctica colder.”
Plate tectonics also directly influenced carbon dioxide levels, he added. Rock weathering and volcanic activity are both part of the carbon cycle. Thus, over thousands of years, geological processes can modify the balance of gases in the atmosphere.
Although some uncertainty remains, researchers are fairly confident that this transition occurred 34 million years ago thanks to the chemical signatures of rock sediments. Oxygen atoms exist in two forms: oxygen-16 (common oxygen) and oxygen-18 (heavy oxygen). Continental ice contains a higher proportion of the lighter oxygen-16, meaning that oceans – and, therefore, the shells of small sea creatures – contain a higher percentage of oxygen-18 when the ice sheets are larger.
“By looking at oxygen isotopes in the carbonate shells of small sea creatures in ocean sediments, you see a jump around 34 million years ago, which people think was because [lighter] the oxygen isotope is heading towards the continent of Antarctica,” Wolff explained.
As for whether Antarctica could one day be ice-free again, “It’s entirely possible, van de Flierdt said.”Planet Earth already did it. Planet Earth could start again. “While it is unlikely that human activity will lead to the complete melting of the ice sheet, it is important that we do everything we can to limit the loss of Antarctic ice now, she added. “It is in our hands to avoid the worst-case scenario,” van de Flierdt said.
Editor’s note: This story was originally published on September 8, 2024.
