The recent Report by B. Hönisch et al. ("Atmospheric carbon dioxide concentration across the mid-Pleistocene transition," Reports, 19 June 2009, p. 1551) presented new data on CO2 over the past 2 million years (myr) based on Boron isotope measurements. The results are used to assess the role of CO2 during the Mid-Pleistocene Transition (MPT) about 1 myr ago, when the dominant glacial cyclicity changed from 40 to 100 thousand years (ky) for poorly understood reasons. They show that the CO2 decrease over the MPT is limited, about 30 parts per million per volume (ppmv) for glacial periods, and negligible for interglacial periods. This small decrease is interpreted as lack of support for theories that invoke a long-term drawdown of CO2 as the main cause of this climate transition, leaving the question of what caused the transition open. Their conclusions are based on the equilibrium sensitivity of present-day climate for doubling CO2 concentrations and the temperature difference between the Last Glacial Maximum and preindustrial climate. We believe that even a seemingly small decrease of 30 ppmv might have been sufficient to cause the transition from 40-ky to 100-ky glacials.
Instead of using the equilibrium sensitivity concept, we calculate the sensitivity based on the temperature derived by the ice sheet modeling approach constraint by marine benthic δ18O (1). By doing so it implicitly contains all (albedo) feedbacks, which are obviously important during a period with major glaciations. This sensitivity turns out to be about 0.15 K for a change of 1 ppmv in CO2. Over the MPT, the long-term change in Northern Hemisphere temperature is estimated to be only 4.5˚C (1), implying that a CO2 change of 30 ppmv was indeed sufficient to cause the observed climate changes across the MPT. In other words, there is no need for a large change in CO2 during the MPT in order to be the key driving mechanism. Thus, the derived transient climate sensitivity for a glacierized world suggests a much higher sensitivity than what is currently assumed for the present-day climate (2), presumably due to a stronger albedo feedback. Hence the climate shift from the 40 ky world to the 100 ky world can be understood by invoking CO2 as the driving mechanism, undoubtedly aided by other climate feedback mechanisms (1, 3–7). Rather than a limited role for CO2 during the transition as suggested by Hönisch et al., we conclude that the increased sensitivity compared to the present-day sensitivity during glacial periods might well explain that a small change of CO2 is responsible for the climate transition during the MPT.
Roderik S. W. van de Wal
IMAU, Utrecht University, Princetonplein 5, NL 3584 CC Utrecht, Netherlands.
Richard Bintanja
KNMI, De Bilt, Utrecht 3732 GK, Netherlands.
References
1. R. Bintanja, R. S. W. van de Wal, Nature 454, 869 (2008).
2. G. C. Hegerl et al., in Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, S. Solomon et al., Eds. (Cambridge Univ. Press, 2007).
3. P. U. Clark et al., Quat. Sci. Rev. 25 3150 (2006).
4. J. Oerlemans, Nature 287, 430 (1980).
5. D. Pollard, Nature 296, 334 (1982).
6. P. U. Clark, D. Pollard, Paleoceanography 13, 1 (1998).
7. E. Tziperman, H. Gildor, Paleoceanography 18, 1001 (2003).
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