An abrupt climate warming of 5 to 10 °C during the Palaeocene/Eocene boundary thermal maximum (PETM) 55 Myr ago is linked to the catastrophic release of ∼1,050–2,100 Gt of carbon from sea-floor methane hydrate reservoirs1. Although atmospheric methane, and the carbon dioxide derived from its oxidation, probably contributed to PETM warming, neither the magnitude nor the timing of the climate change is consistent with direct greenhouse forcing by the carbon derived from methane hydrate. Here we demonstrate significant differences between marine2,3 and terrestrial4,5,6 carbon isotope records spanning the PETM. We use models of key carbon cycle processes7,8,9 to identify the cause of these differences. Our results provide evidence for a previously unrecognized discrete shift in the state of the climate system during the PETM, characterized by large increases in mid-latitude tropospheric humidity and enhanced cycling of carbon through terrestrial ecosystems. A more humid atmosphere helps to explain PETM temperatures, but the ultimate mechanisms underlying the shift remain unknown.
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We thank W. Cheng, G. Dickens, D. Schrag, L. Sloan and F. I. Woodward for comments. Funding was provided by a National Science Foundation Biocomplexity grant. G.J.B. was supported by the National Science Foundation Graduate Research Fellowship Program, and D.J.B. gratefully acknowledges funding from the Royal Society and the Leverhulme Trust.
The authors declare that they have no competing financial interests.
Contains Supplementary Methods and calculations. (PDF 134 kb)
Contains Supplementary Table summarizing the results of mass balance calculations of the PETM carbonate ion effect. (PDF 47 kb)
Contains Supplementary Table summarizing the results of paleosol organic carbon concentration measurements. (PDF 51 kb)
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Bowen, G., Beerling, D., Koch, P. et al. A humid climate state during the Palaeocene/Eocene thermal maximum. Nature 432, 495–499 (2004). https://doi.org/10.1038/nature03115
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