Fifty-five million years ago, when scientists believe the Earth was in a near-runaway state, dangerously overheated by greenhouse gases, the Arctic Ocean was also a very different place. It was a large lake, connected to the greater oceans by one primary opening: the Turgay Sea.
When this channel closed or was blocked nearly 50 million years ago, the enclosed body of water became the perfect habitat for a small-leaved fern called Azolla. Imagine the Arctic like the Dead Sea of today: It was a hot lake that had become stratified, suffering from a lack of exchange with outside waters. That meant its waters were loaded with excess nutrients.
Azolla took advantage of the abundant nitrogen and carbon dioxide, two of its favorite foods, and flourished. Large populations formed thick mats that covered the body of the lake. When rainfall increased from the changing climate, flooding provided a thin layer of fresh water for Azolla to creep outward, over parts of the surrounding continents.
Azolla bloomed and died like this in cycles for roughly 1 million years, each time laying down an additional layer of the thick blanket of sediment that was finally found in 2004 by the Arctic Coring Expedition.
The fact that the fern only needs a little over an inch of water under it to grow makes the whole scenario seem just within reason—that is, until you learn how much carbon this carbon dioxide-hungry plant sucked up over the course of those million years.
"Around half of the CO2 available at the time," said Jonathan Bujak, who studies dust and fine plant particles as a palynologist. "Levels dropped from between 25,000 and 35,000 [parts per million] to between 15,000 and 16,000 ppm."
While what ended the Azolla age remains unclear, the next 49 million years saw the Earth fall into a cycle that brought even more drastic drops in CO2 levels.
The southern continents broke up, and, as South America and India migrated north, the Antarctic become isolated and increasingly cold, absorbing more CO2 and creating a conveyor-belt-like effect of cold air that perpetuated ice. A succession of ice ages was triggered once the atmosphere's CO2 dropped below 600 ppm around 2.6 million years ago, just 200 ppm shy of the Earth's current estimate.
Cyclical glacial ages began, rotating between 100,000 years of massive glaciers, followed by 10,000-year breaks. By the mid-18th century, CO2 levels were at 280 ppm.
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