“Under careful re-examination of this interval, we discovered that the CO2 drop was in phase with, and probably slightly prior to, temperature changes.”

Scientists have long been puzzled by studies that seemed to indicate the apparently impossible: 34 million years ago at the same time the Antarctic ice sheet was forming, the Earth’s atmospheric CO2 was rising. Now an international team of scientists, including Dr Liu Zhonghui of the Department of Earth Sciences, have come up with new evidence disputing this and showing that a plunge in carbon dioxide levels happened at the same time as the climate dramatically cooled. The available evidence supports a fall in CO2 as a critical condition for global cooling and cryosphere evolution at that time.

“Previous research suggested that CO2 lagged behind temperature drop and ice sheet formation by two to three million years, during the Eocene-Oligocene climate transition, around 33.7 million years ago,“ says Dr Liu. “However, under careful re-examination of this interval, we discovered that the CO2 drop was in phase with, probably slightly prior to, temperature changes. Therefore CO2 should have played some role in this important climate transition.“

Their paper, ‘The Role of Carbon Dioxide During the Onset of the Ice Age’, shows that atmospheric CO2 plunged by 40 per cent before and during the formation of the ice sheet, confirming that significant falls in greenhouse gas result in global cooling, in the same way that rises lead to global warming.

The new evidence was uncovered when the team generated CO2 records from multiple different geographical/oceanic regions, and found that only CO2 records from high southern latitudes showed abnormal patterns, while those from other regions showed patterns consistent with temperature changes.

Altered nutrient levels

“Because Drake Passage [the body of water between the most southerly tip of South America at Cape Horn and the most northerly tip of the Antarctic peninsular] was opened up at about the same time, we suspect that the development of circum-Antarctic circulation significantly altered nutrient levels in the region, which affects the reconstruction of CO2 levels,” says Dr Liu.

It became apparent that the flows of the deep ocean currents at the end of the Eocene period were very different from those of today because the shape and position of the continental masses were different. “It is commonly believed that on such long timescales, tectonic activities and changes in weathering carbonate and silicate rocks caused the drop in CO2,” he adds.

Implications for today?

At a time when public awareness of climate change is at an all-time high because of worries over global warming, it would be easy to try and make a link. However Dr Liu urges caution. “The implication is that CO2 as a greenhouse gas contributes to global warming. However, careful distinction should be made between this study and the current warming issue. The current CO2 rise has only been happening for about 100 years, which in the context of geological time, is just a snapshot. The amplitude of recent global warming – again in a geological sense – is still quite small.”

“At the current stage, it is still very difficult to separate anthropogenic versus natural forcings. The role of anthropogenic CO2 in the recent global warming has yet to be determined.”

But he also points out that atmospheric CO2 is not always a bad thing anyway – when their levels are high, plants grow better. “Some believe the next Ice Age should have started already, but levels of CO2 are keeping it at bay.”

For the new paper, Dr Liu was part of a team of scientists from universities in the US, the UK, Sweden and Australia. They also came from different disciplines and included geochemists, palaeontologists, and modellers. “The nature of global climate change requires interdisciplinary research,” he says. “We’ve been discussing the topic for many years, and are still continuing our collaborations now.”

Dr Liu, who is currently looking at climate change on the Tibetan plateau, has been working in this discipline for about 14 years. “I’m interested in past climate changes and the link to CO2 changes because such research provides clues to future climate changes.” He smiles: ”This is not only a scientist’s curiosity, as Winston Churchill said, ‘to know the future, you must know your past.’”