The role of CO2 change in driving the ‘greenhouse-to-icehouse’ transition?
Recent research suggests that this change in Cenozoic climate state from the greenhouse of the early Cenozoic to the icehouse of the late Cenozoic was driven by a decline in the CO2 content of the atmosphere. It is thought that this was caused by a very subtle imbalance between long-term carbon fluxes from the mantle and upper crust to the atmosphere: either a decline in CO2-sources (volcanism/metamorphism), or an increase in CO2-sinks such as silicate weathering. Although the role of CO2 decline in triggering the rapid transition between climate states at the Eocene/Oligocene (E/O) boundary (~34 Ma) has recently been confirmed , there are currently two crucial limitations to the hypothesis that the more long-term Cenozoic climate change was also CO2 driven. These are:
1) Continuous records of pCO2 only indicate a broad decline over the last 50 million years that is only loosely correlated with climate variations (Fig. 1). For example, pCO2 undergoes relatively rapid and large scale oscillations in the early Cenozoic, whilst climate shows a gradual cooling. If these records are reliable, this implies that other factors such as ocean circulation may influence climate more than pCO2 during this time interval.
2) There is also very little correlation between the records of silicate weathering and/or mantle outgassing (reflected by oceanic 87Sr/86Sr and mean spreading rate, respectively) with these pCO2 records (Fig. 1). Indeed, although weathering may have increased during the early Cenozoic in response to Himalayan uplift, , spreading rates are, if anything, lower during the early part of the Cenozoic compared to the latter half, implying an increasing, not decreasing, flux of mantle CO2 during the course of the Cenozoic (Fig. 1).
Thus, we neither understand the cause of pCO2 change nor its influence on the Earth’s climate and biota, during even this most recent and best studied greenhouse-icehouse transition. These limitations strike at the heart of our understanding of what is controlling the long-term evolution of Earth’s climate and, in detail at least, challenge the widely accepted view that changes in the atmospheric CO2 concentration are crucial in driving climate on these geological timescales. Reconstructing the climate system in the past is a difficult task and the methods used undergo continuous assessment and improvement. Of particular relevance here is that the methods used to reconstruct pCO2 beyond the reach of the ice cores has matured greatly in the last decade (see below). Consequently, before the role of pCO2 in Cenozoic climate change can be fully assessed, better and more complete pCO2 records need to be generated with state-of-the-art techniques and understanding.
 Pearson, P.N., Foster, G.L., and Wade, B.S., 2009, Atmospheric carbon dioxide through the Eocene–Oligocene climate transition: Nature, v. 461, p. 1110-1113.
 Pagani, M., Huber, M., Liu, Z., Bohaty, S., Henderiks, J., Sijp, W., Krishnan, S., and DeConto, R.M., 2011, The role of carbon dioxide during the onset of Antarctic glaciations: Science, v. 334, p. 1261-1264
 Pagani, M., Freeman., K., and Arthur, M., 1999, Late Miocene atmospheric CO2 concentrations and the expansion of C4 grasses: Science, v. 285, p. 876-879
 Raymo, M.E., and Ruddiman, W.F.,1992, Tectonic forcing of late Cenozoic climate: Nature, v. 359, p. 117-122
 Misra, S., and Froelich, P.N., 2012, Lithium isotope history of Cenozoic seawater: changes in silicate weathering and reverse weathering: Science, v. 335, p. 818-823
 Conrad, C.P., and Lithgow-Bertelloni, C., 2007, Faster seafloor spreading and lithosphere production during the mid-Cenozoic: Geology, v. 35, p. 29-32
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