The amount of carbon released during an abrupt warming event 55.5 million years ago was insufficient to cause the 5-9 C temperature rise at the time, as reported online this week in Nature Geoscience. The study suggests that the greenhouse effect of the atmospheric carbon dioxide must have been amplified substantially by processes that, at present, are not understood, or taken into account in estimates of future warming.

Richard Zeebe and colleagues used analyses of marine sediments and a carbon-cycle model to estimate the amount of carbon dioxide released during the Palaeocene-Eocene Thermal Maximum, which lasted for about 100,000 years. They found that, using current estimates of climate sensitivity to increasing levels of atmospheric carbon dioxide, the carbon release could only explain up to 3.5 C of the warming. They concluded that as yet unknown warming feedbacks must have caused the additional rise in temperature.

In the accompanying News & Views article, David Beerling writes "The upshot of the study by Zeebe and colleagues is that forecasts of future warming could be severely underestimating the extent of the problem that lies in store for humanity as greenhouse gases accumulate in the atmosphere."