Antarctica, which is almost four times the size of the United States, is almost entirely covered by a mile-thick layer of ice.
But the South Pole has not always been frozen. So when was the last time Antarctica was ice free?
This ice sheet formed relatively recently in geological terms, experts told LiveScience. “I think most people would say that 34 million years ago was when the ice sheet first formed in Antarctica,” said Erik Wolffa palaeoclimatologist at the University of Cambridge. “(In the past) most of it would have been like northern Canada today — tundra and coniferous forest.”
Global temperatures are a key factor affecting the extent of ice cover. About 50 million years ago, the world was about 25 degrees Fahrenheit (14 degrees Celsius) warmer than it is today, but temperatures fell steadily over the next 16 million years. 34 million years ago – a time period known as the boundary between the Eocene and Oligocene – 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 caps to form?
Related: Which is colder: the North or the South Pole?
“There are two factors, and probably both were at play,” Wolff told LiveScience. “One of them is a change in the concentration of carbon dioxide in the atmosphere, and the other is the movements of the continents and especially the opening of the Drake Passage,” the strait between South America and Antarctica that connects the South Atlantic with the South Pacific.
The more carbon dioxide it is in the atmosphere, the more heat is trapped and the hotter the planet.
From about 60 million to 50 million years ago, the carbon dioxide concentration in Earth’s atmosphere was really high – somewhere around 1000 to 2000 parts per million, or between 2.5 to 5 times current levelsso Tina van de Flierdta geochemist at Imperial College London.
“But we know that atmospheric CO2 came down over the Eocene-Oligocene boundary,” she told LiveScience. This reduction in atmospheric CO2 would have been accompanied by a cooling of the global climate, she added, likely tipping Earth over a threshold and allowing ice sheets to form.
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 now the Drake Passage.
“This led to what we call a circumpolar current – water that goes right 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 come across the Southern Ocean and therefore makes Antarctica colder.”
Plate tectonics also directly affected carbon dioxide levels, he added. Rock weathering and volcanic activity are both part of the carbon cycle, so over thousands of years geological processes can change the balance of gases in the atmosphere.
Although some uncertainty remains, scientists are fairly certain about this transition 34 million years ago thanks to the chemical signatures in rock sediments. Oxygen atoms exist in two forms: oxygen-16 (ordinary oxygen) and oxygen-18 (heavy oxygen). Continental ice contains a higher proportion of the lighter oxygen-16, meaning that the oceans – and therefore the shells of small marine animals – contain a higher percentage of oxygen-18 when the ice sheets are larger.
“By looking at the oxygen isotopes in the carbonate shells of small marine animals in ocean sediments, you see a jump about 34 million years ago, which people take to be because the (lighter) oxygen isotope goes into the continent of Antarctica,” Wolff explained.
As for whether Antarctica could ever become ice-free again, “It’s definitely possible,” van de Flierdt said.Planet Earth have done it before. Planet Earth could do it again.” Although 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 ice from Antarctica now, she added. “It is in our hands to avoid the worst-case scenario,” van de Flierdt said.
