Buried under thousands of feet of hard, ancient ice lies the solid earth of the Antarctic continent. For some 34 million years, vast glacial plains have ebbed and flowed over this rocky land. But the initiation of Antarctic glaciation—the point in time when conditions became right for snowfall to exceed snowmelt year after year—began suddenly and enigmatically.
The growth of glaciers on Antarctica marks the end of the geologic epoch known as the Eocene—an epoch actually known for some of the hottest global temperatures in Earth’s geologically recent history. High CO2 punctuated by extreme bursts of even more CO2 caused significant warming for the early part of the Eocene’s 22 million year span. Fossil records show that the Antarctic continent was not only ice free then, but that it supported rainforests and crocodiles!
So the transition from a lush tropical landscape to a barren ice covered wasteland is a mystery that scientists have yet to fully explain. Cooling began gradually around the middle Eocene, and it made a pronounced and sudden shift at the Eocene’s conclusion 34 million years ago.
At that time, CO2 levels plummeted. In a geological instant—400,000 years—Antarctica was covered in ice. Some sort of threshold must have been passed, geologists reason. Cooling can beget more cooling because ice reflects incoming heat from the Sun back into space. This undoubtedly happened. But something else had to have occurred to cause the drop in CO2 that allowed the world to become cool enough to form glaciers in the first place.