UK, U.S. scientists suggest Antarctic climate transition tied to decline in CO2
Posted October 9, 2009
A team of U.S. and British scientists braved lions and hyenas in East Africa to extract microfossils in samples of rocks, which helped them link declining levels of carbon dioxide in the atmosphere with the formation of an ice sheet on Antarctica about 34 million years ago.
The study’s findings, published in Nature online, confirm that atmospheric CO2 declined during the transition between the Eocene and the Oligocene epochs. The scientists say the Antarctic ice sheet began to form when CO2 in the atmosphere reached about 760 parts per million (ppm). Currently, carbon dioxide levels in the atmosphere are nearly 400 ppm.
Paul Pearson from Cardiff University’s School of Earth and Ocean Sciences , who led the mission to the remote East Africa village of Stakishari, said in a press release: “About 34 million years ago, the Earth experienced a mysterious cooling trend. Glaciers and small ice sheets developed in Antarctica, sea levels fell and temperate forests began to displace tropical-type vegetation in many areas.
“The period, known to geologists as the Eocene-Oligocene transition, culminated in the rapid development of a continental-scale ice sheet on Antarctica, which has been there ever since,” he added.
The team mapped large expanses of bush and wilderness and pieced together the underlying local rock formations using occasional outcrops of rocks and streambeds.
Eventually they discovered sediments of the right age near Stakishari. By assembling a drilling rig and extracting hundreds of meters of sediment from under the ground, they were able to obtain samples dating back to the Eocene-Oligocene transition.
Co-author Bridget Wade from the Texas A&M University Department of Geology and Geophysics said in the press release: “This was the biggest climate switch since the extinction of the dinosaurs 65 million years ago.
“Our study is the first to provide a direct link between the establishment of an ice sheet on Antarctica and atmospheric carbon dioxide levels and therefore confirms the relationship between carbon dioxide levels in the atmosphere and global climate.”
In addition to declining levels of CO2 in the atmosphere, some scientists believe the opening of certain ocean gateways around Antarctica may have played a role in the development of an ice sheet. The tectonic shift, they believe, allowed for the start of a major ocean current, the Antarctic Circumpolar Current (ACC), which helped isolate the continent from warmer water and maintain the ice sheet.
In a follow-up e-mail from The Antarctic Sun, Wade said the study did not directly address the formation of the ACC in the paper.
“Our data suggest that it was the decline in CO2 levels that led to ice accumulation at the Eocene-Oligocene transition,” she wrote, “however, the isolation of Antarctica probably allowed the ice to remain, although this was outside the scope of this project.”
The threshold of 760 ppm cited in the study doesn’t necessarily represent a definitive tipping point between ice sheet formation or dissolution, Wade added in answer to a different question.
“Probably too big a leap at this stage,” Wade wrote. “There are large errors associated with the calculation of CO2 values. However, we certainly see a trend of declining CO2 prior to ice build up. Further work is needed to model the threshold levels.”
UK Natural Environment Research Council (NERC) supported the research. The work is related to a recent National Science Foundation grant Wade received in September 2009 to examine microfossils during the Oligocene, from 34 to 24 million years ago, and the climate changes that occurred during that time period.
In an unrelated study looking at the conditions that might have been present at the Eocene-Oligocene transition 34 million years ago, University of California Santa Barbara researchers suggested that at least twice as much land existed in West Antarctica than today, increasing the total landmass of Antarctica by 10 to 20 percent. The additional land would have held more ice, helping bolster climate models developed by other scientists that illustrate when Antarctica turned into an icehouse. [See previous article: Rising up.]