Uniformitarian Prediction of Early‐Pleistocene Atmospheric CO₂

Abstract

The first reconstructions of atmospheric composition over multiple glacial-interglacial cycles, derived from the Vostok ice cores (Barnola et al., 1991;Petit et al., 1999), revealed a close relationship between atmospheric CO 2 and global climate over at least the past 400,000 years. Atmospheric CO 2 decreased from ∼280 to ∼180 ppm over order 100 Kyr periods before rising back to interglacial levels in order 10 Kyr, following the same sawtooth-like pattern that preceding studies had identified in the benthic δ 18 O proxy for global ice-volume and deep-ocean temperature (Hays et al., 1976;Imbrie & Imbrie, 1980). The close coupling of CO 2 and ice volume gave crucial insight, albeit over a limited time interval, into the sensitivity of past climate. Importantly, a subsequent extension of the ice-core record to 650 ka (Siegenthaler et al., 2005) revealed that the relationship between CO 2 and δD, an air temperature proxy, remained consistent through at least 650 ka, and a similar finding was made when the ice-core record was eventually extended to 800 ka (Lüthi et al., 2008), the current extent of continuous observations. The apparent stability of the CO 2 -climate relationship might suggest we could predict CO 2 levels in earlier epochs on the basis of similar coupling, but it is unclear how similar the CO 2 -climate relationships are before and after the mid-Pleistocene (