Just in time for the Durban climate conference we have a bit of science fiction from Yale and Purdue universities. The paper is Pagani et al., 2011, The Role of Carbon Dioxide During the Onset of Antarctic Glaciation, Science, Vol. 334 no. 6060 pp. 1261-1264. (Link to abstract)
The research team claims “A drop in carbon dioxide appears to be the driving force that led to the Antarctic ice sheet’s formation.” According to the press release:
The onset of Antarctic ice is the mother of all climate tipping points. The team found the tipping point in atmospheric carbon dioxide levels for cooling that initiates ice sheet formation is about 600 parts per million. Prior to the levels dropping this low, it was too warm for the ice sheet to form. 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 team studied geochemical remnants of ancient algae from seabed cores collected by drilling in deep-ocean sediments and crusts as part of the National Science Foundation’s Integrated Ocean Drilling program. The biochemical molecules present in algae vary depending on the temperature, nutrients and amount of dissolved carbon dioxide present in the ocean water. These molecules are well preserved even after many millions of years and can be used to reconstruct the key environmental variables at the time, including carbon dioxide levels in the atmosphere.
The team actually studied 7 cores but their study is based on computer modeling of only the two cores that seemed to support their hypothesis.
Why I think the study is science fiction
First some background. Antarctica was drifting toward the south pole since about 175 million years ago and arrived approximately 65 million years ago. It was still attached to South America and Australia. In spite of being at the south pole, Antarctica was ice-free until about 34 million years ago.
Atmospheric carbon dioxide had been dropping since the mild ice age at the Jurassic-Cretaceous boundary about 150 million years ago. Carbon dioxide continued to drop after the ice age, but global temperatures rebounded and the planet was hot and steamy until the end of the Paleocene-Eocene Thermal Maximum about 55 million years ago. The graphic below shows that there is apparently no correlation between temperature and carbon dioxide.
The drop in carbon dioxide was precipitated by the ice age which allowed cooling ocean water to absorb more carbon dioxide. The continued drop in carbon dioxide was due, at least in part, to sequestering of carbon by the extensive formation of Cretaceous coal and limestone deposits.
An alternative hypothesis of why Antarctica developed a continental ice sheet about 34 million years ago is that when Antarctica became detached from Australia and South America, a strong circumpolar ocean current developed that isolated Antarctica from the warming tropical ocean currents. This current developed in fits and starts because the connection to South America opened and closed several times. This hypothesis is supported by a recent paper:
Katz et al., 2011, Impact of Antarctic Circumpolar Current Development on Late Paleogene Ocean Structure, Science, Vol. 332 no. 6033 pp. 1076-1079. (Link to abstract)
The following graphic shows a more detailed history of Antarctic temperature history.
This shows that the initial glaciation began about 34 million years ago with the establishment of the circumpolar current. At about 26 million years ago the Drake passage connecting Antarctica to South America became restricted again and Antarctica warmed up. The carbon dioxide driver hypothesis cannot account for this. Antarctica reglaciated about 14 million years ago when the Drake passage reopened allowing the circumpolar current to block warm water again. Also near this time, a source of warm water, the Tethys sea became blocked by tectonic shifts.
Geology provides “hard” evidence; the carbon dioxide hypothesis of the Yale-Purdue team provides only computer speculation.