The appearance of the Antarctic ice sheets was triggered by a massive fall in the amount of atmospheric carbon dioxide, new research has found.
The CO2 level plunged by 40 percent before and during the formation of the ice sheet 34 million years ago, indicating that significant falls in the greenhouse gas result in global cooling, just as rises result in global warming.
Previous studies had suggested that atmospheric CO2 over the Southern Ocean was actually climbing during the Eocene to Oligocene climate transition, when the ice first formed.
But, say the researchers, when the Southern Ocean currents and temperatures of that period - vastly different from today - are factored in, it becomes apparent that Antarctica's big freeze followed a fall in CO2 levels.
"Our research recognised that the flows of deep ocean currents at the end of the Eocene were dramatically different from those of today because of the altered position and shape of continental masses," says NSW Climate Change Research Centre researcher Dr Willem Sijp.
"Previous research relied on different temperature estimates and had also not taken these different currents into account. This decline was a critical condition for global cooling and the emergence of the Antarctic ice sheet. In short, the apparent increase of CO2 during Antarctic glaciation is refuted."
Ocean currents are more of a factor than you might think in calculating atmospheric CO2, as estimates of atmospheric carbon dioxide are derived from measurements of molecules from algae deposited on the ocean floor.
"The evidence falls in line with what we would expect if carbon dioxide is the main dial that governs global climate; if we crank it up or down there are dramatic changes," says Matthew Huber, a professor of earth and atmospheric sciences at Purdue.
"We went from a warm world without ice to a cooler world with an ice sheet overnight, in geologic terms, because of fluctuations in carbon dioxide levels."
For 100 million years before the cooling, which occurred at the end of the Eocene epoch, Earth was warm and wet. Mammals and even reptiles and amphibians inhabited the North and South poles, which then had subtropical climates.
Then, over a span of about 100,000 years, temperatures fell dramatically, many species of animals became extinct, ice covered Antarctica and sea levels fell.
"The onset of Antarctic ice is the mother of all climate 'tipping points,'" says Pagani. "Recognizing the primary role carbon dioxide change played in altering global climate is a fundamentally important observation."
The team found the tipping point in atmospheric carbon dioxide levels for cooling that initiates ice sheet formation is about 600 parts per million.
At the Earth's current level of around 390 parts per million, the environment is such that an ice sheet remains, but carbon dioxide levels and temperatures are increasing.
"The system is not linear and there may be a different threshold for melting the ice sheet, but if we continue on our current path of warming we will eventually reach that tipping point," says Huber. "Of course after we cross that threshold it will still take many thousands of years to melt an ice sheet."