An exceptional analogue for the study of the causes and consequences of global warming occurs at the Palaeocene/Eocene Thermal Maximum, 55 million years ago. A rapid rise of global temperatures during this event accompanied turnovers in both marine^{1, 2, 3} and terrestrial biota⁴, as well as significant changes in ocean chemistry^{5, 6} and circulation^{7, 8}. Here we present evidence for an abrupt shift in deep-ocean circulation using carbon isotope records from fourteen sites. These records indicate that deep-ocean circulation patterns changed from Southern Hemisphere overturning to Northern Hemisphere overturning at the start of the Palaeocene/Eocene Thermal Maximum. This shift in the location of deep-water formation persisted for at least 40,000 years, but eventually recovered to original circulation patterns. These results corroborate climate model inferences that a shift in deep-ocean circulation would deliver relatively warmer waters to the deep sea, thus producing further warming⁹. Greenhouse conditions can thus initiate abrupt deep-ocean circulation changes in less than a few thousand years, but may have lasting effects; in this case taking 100,000 years to revert to background conditions.

1. Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92092-0208, USA

Correspondence to: Flavia Nunes¹ Correspondence and requests for materials should be addressed to F.N. (Email: fnunes@ucsd.edu).

Received 23 June 2005 | Accepted 27 October 2005

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