Research Stories

New evidence says carbon dioxide was a major factor in climate change.

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.

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

Dr Liu Zhonghui

The full version of this article was originally published in Bulletin. Please click here to view this HKU publication.