A. K. Tripati, C. D. Roberts & R. A. Eagle, Science 326 (2009), Coupling of CO2 and ice sheet stability over major climate transitions of the last 20 million years

Science, (4 December 2009) Vol. 326, No. 5958, pp. 1394-1397; DOI: 10.1126/science.1178296

Coupling of CO₂ and ice sheet stability over major climate transitions of the last 20 million years

Aradhna K. Tripati* (Departments of Earth and Space Sciences and Atmospheric and Oceanic Sciences, and Institute of Geophysics and Planetary Physics, University of California, Los Angeles (UCLA), Los Angeles, CA 90095, U.S.A.;  Department of Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ, U.K.), Christopher D. Roberts (Department of Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ, U.K.), and Robert A. Eagle (Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, U.S.A.)

Abstract

The carbon dioxide (CO₂) content of the atmosphere has varied cyclically between ~180 and ~280 parts per million by volume over the past 800,000 years, closely coupled with temperature and sea level. For earlier periods in Earth’s history, the partial pressure of CO₂ (pCO₂) is much less certain, and the relation between pCO₂ and climate remains poorly constrained. We use boron/calcium ratios in foraminifera to estimate pCO₂ during major climate transitions of the past 20 million years. During the Middle Miocene, when temperatures were ~3° to 6°C warmer and sea level was 25 to 40 meters higher than at present, pCO₂ appears to have been similar to modern levels. Decreases in pCO₂ were apparently synchronous with major episodes of glacial expansion during the Middle Miocene (~14 to 10 million years ago) and Late Pliocene (~3.3 to 2.4 million years ago).

*Correspondence e-mail: ripple@zephyr.ess.ucla.edu

Link to abstract:  http://www.sciencemag.org/cgi/content/abstract/326/5958/1394

Excerpt:

These results provide some constraints on pCO2 thresholds for the advance and retreat of continental ice sheets in the past, which is also relevant in the context of anthropogenic climate change because it is uncertain how continental ice sheets will respond over the coming centuries to increased levels of pCO2 (1). By comparing our reconstruction to the published data sets described above, we are able to estimate past thresholds for the buildup of ice in different regions. When pCO2 levels were last similar to modern values (that is, greater than 350 to 400 ppmv), there was little glacial ice on land or sea ice in the Arctic, and a marine-based ice mass on Antarctica was not viable. A sea ice cap on the Arctic Ocean and a large permanent ice sheet were maintained on East Antarctica when pCO2 values fell below this threshold. Lower levels were necessary for the growth of large ice masses on West Antarctica (~250 to 300 ppmv) and Greenland (~220 to 260 ppmv). These values are lower than those indicated by a recent modeling study, which suggested that the threshold on East Antarctica may have been three times greater than in the Northern Hemisphere (35).

This work may support a relatively high climate sensitivity to pCO2. pCO2 values associated with major climate transitions of the past 20 Ma are similar to modern levels. During the Mid-Miocene, when pCO2 was apparently grossly similar to modern levels, global surface temperatures were, on average, 3-6 °C warmer than in the present (2, 25). We suggest that the Mid-Miocene may be a useful interval to study to understand what effect sustained high pCO2 levels (i.e., a climate in equilibrium with near-modern pCO2 values) may have on climate.