Constraints on hyperthermals

The abrupt warming event 56 million years ago, known as the Palaeocene–Eocene Thermal Maximum (PETM), was associated with the largescale release of 13C-depleted carbon into the ocean–atmosphere system. In sedimentary records, the event is reflected by a negative carbon isotope excursion1. Cui et al. used a carbon-cycle model to estimate the rate of carbon release during the PETM. The model assumed that the onset of the carbon isotope excursion occurred over approximately 20,000 years, an estimate based on a cyclostratigraphic model. Here we highlight several issues that weaken the conclusions of Cui et al.