Atmospheric¹ carbon dioxide levels plunged² by 40 percent before and during the formation of the Antarctic ice sheet 34 million years ago, according to a new study. The finding helps solve a long-standing scientific puzzle and confirms the power of CO2 to dramatically alter global climate. The study by an international team, published in the journal Science, is the first multidisciplinary（有关各种学问的） research of its kind to show that CO2 was tracking global cooling at that time. It confirms 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 climbing during the Eocene to Oligocene climate transition, when ice first formed over Antarctica. This presented a conundrum³（难题） , suggesting the climate was warming at the same time as Antarctica was freezing.

But when the vastly different ancient Southern Ocean currents and temperatures of that period were factored in, it quickly became apparent that Antarctica's big freeze followed a fall in CO2 levels.

The paper, "The Role of Carbon Dioxide During the Onset⁴ of Antarctic Glaciation," reinforces carbon dioxide's place as a primary driver for global temperatures. It was lead authored by Mark Pagani, of Yale University, and co-authored by UNSW Climate Change Research Centre researcher Dr Willem Sijp.

"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 continental⁵ masses（大陆块体） ," said Dr Sijp.

"Previous research relied on different temperature estimates and had also not taken these different currents into account. This led to significant upward biases⁶（向上偏误） in calculated CO2 which made it appear that carbon dioxide was actually increasing around the Southern Ocean as the Antarctic ice sheet formed. We can now see the reverse was true.

"We found a 40 percent CO2 decrease over a three million year period, in agreement with global cooling during that time.

"This decline was a critical condition for global cooling and the emergence⁷（出现） of the Antarctic ice sheet. In short, the apparent increase of CO2 during Antarctic glaciation（冰川作用） is refuted."

The nature of ocean currents is vital in calculating atmospheric CO2, because the estimates of atmospheric carbon dioxide are derived⁸ from measurements of molecules⁹ from algae¹⁰ deposited on the ocean floor at six ocean sites millions of years ago.

The fundamentally different circulation of the Southern Ocean during the Eocene directly affected¹¹ the algae by changing the temperatures of these ocean currents and significantly lowering the nutrient¹² load. This had a direct impact on the inferred CO2.