When was the last time there was no ice in Antarctica?

Almost four times the size of the United States, Antarctica is almost completely covered in a layer of ice several miles thick.

But Antarctica isn’t always frozen. So when was the last time there was no ice in Antarctica?

Experts told Live Science that this ice sheet formed relatively recently geologically. “Most people would say that 34 million years ago was when the first ice sheets formed in Antarctica,” said Eric Woolf, a paleoclimatologist at the University of Cambridge. ”[Previously] Most of it would have been tundra and coniferous forests, much like northern Canada today. ”

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Earth’s temperature is an important factor influencing ice area. About 50 million years ago, the world was about 25 degrees Fahrenheit (14 degrees Celsius) warmer than it is today, but temperatures steadily declined over the next 16 million years. By 34 million years ago (a period known as the Eocene-Oligocene boundary), the climate was 14.4 degrees Fahrenheit (8 degrees Celsius) warmer than it is today.

But what triggered this temperature drop? Was that all that was needed for ice sheets to form?

Related: Which is colder, the North Pole or the South Pole?

“There are two factors, and both are probably at play,” Wolff told Live Science. “One is changes in the concentration of carbon dioxide in the atmosphere, and the other is the movement of continents, particularly the opening of the Drake Passage, the strait between South America and Antarctica that connects the South Atlantic and South Pacific.”

The more carbon dioxide there is in the atmosphere, the more heat is trapped and the Earth becomes warmer.

About 60 million to 50 million years ago, carbon dioxide concentrations in Earth’s atmosphere were extremely high, around 1,000 to 2,000 parts per million, or 2.5 to 5 times today’s levels, says geochemist Tina van de Vryld of Imperial College London.

“But we do know that atmospheric CO2 came down across the Eocene-Oligocene boundary,” she told Live Science. This decrease in atmospheric carbon dioxide was accompanied by a cooling of the global climate, and likely would have pushed the Earth above a threshold, allowing the formation of ice sheets, she added.

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However, Wolff said there may also have been localized cooling of Antarctica due to plate tectonics. Around this time, South America and Antarctica finally separated, opening what is now the Drake Passage.

“This created what we call a circumpolar flow, where water goes all the way around Antarctica,” Wolff said. “This isolates Antarctica from the rest of the world and makes it very difficult for warm air masses to pass through the Southern Ocean, making Antarctica colder.”

Plate tectonics also has a direct impact on carbon dioxide levels, he added. Rock weathering and volcanic activity are both part of the carbon cycle, so geological processes over thousands of years can change the balance of gases in the atmosphere.

Although some uncertainty remains, researchers are fairly certain about this change 34 million years ago thanks to chemical signatures in the rock deposits. Oxygen atoms exist in two forms: oxygen-16 (common oxygen) and oxygen-18 (heavy oxygen). Continental ice has a high proportion of light oxygen-16. In other words, the larger the ice sheet, the higher the proportion of oxygen-18 in the ocean, and therefore in the shells of small marine organisms.

“If you look at the oxygen isotopes in the carbonate shells of small marine organisms in marine sediments, you can see that there was a sudden change about 34 million years ago, and people think that’s why. [lighter] “The oxygen isotopes are heading towards Antarctica,” Wolff explained.

As for whether Antarctica could become ice-free again, Van de Vryld said: “It’s definitely possible. Planet Earth has done it before.” “The Earth could do the same again. Although it is unlikely that human activities will lead to a complete melting of the ice sheets, it is important to take all possible steps now to limit the loss of Antarctic ice. It is up to us to avoid the worst-case scenario,” van de Vlield added.

Editor’s note: This article was originally published on September 8, 2024.