A new paper reveals that the frozen continent of Antarctica was once a temperate rainforest.

This dramatic difference in climate was due to high levels of CO2 that managed to maintain mild weather even through months when the sun didn't shine on this part of the world.

The team of researchers is working to figure out what exactly caused the extreme flip from rainforest to frozen tundra.

Ninety million years ago, Antarctica was home to a thriving rainforest— even during extended periods of darkness in winter.

According to a paper published in Nature, the “mid-Cretaceous period was one of the warmest intervals of the past 140 million years, driven by atmospheric carbon dioxide levels (CO2) of around 1,000 parts per million (ppm) by volume.” That’s in comparison to the contemporary global average which floated around 407.4 ppm in 2018.

Still, it seems impossible that ice-covered Antarctica could ever have been anything but that. In order to find out more about the frozen continent, researchers analyzed sediment that they had obtained from nearly 100 feet underground.

The team used a seafloor drill from the University of Bremen in Germany while on an expedition aboard the Polarstern research vessel and were surprised to discover that their samples contained fossilized roots, pollen, and spores that had been preserved in mudstone from the west Antarctic shelf. X-rays and CT scans of the sediment samples showed spectacular preservation that allowed the researchers to see individual cells and their structures.

Johann Klages (right) and co-author Tina van de Flierdt work to extract sediment. Thomas Ronge / Alfred-Wegener-Institut The Polarstern beside an Antarctic glacier. J.P. Klages / Alfred-Wegener-Institut

Johann Klages, one of the paper authors, said in a press release that one of the things that caught the team’s attention was the “unusual coloration” of the sediment they had unearthed.

“It clearly differed from the layers above it. Moreover, the first analyses indicated that, at a depth of 27 to 30 meters [88.5 to 98 feet] below the ocean floor, we had found a layer originally formed on land, not in the ocean,” Klages adds.

The paper authors share that due to the “numerous plant remains…the coast of west Antarctica was a swampy landscape in which temperate rainforests grew—similar to the forests that can still be found…on New Zealand’s South Island.”

The researchers also figured out how rainforest life was sustainable during those sunless months (since Antarctica was still the South Pole).

“We now know that there could easily be four straight months without sunlight in the Cretaceous; the [CO2] concentration was so high, the climate around the South Pole was nevertheless temperate, without ice masses,” says Torsten Bickert, paper author and geoscientist.

But what caused the once tropic continent to freeze over remains a mystery.

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