The rate of carbon emissions during a period of abrupt warming 55.8 million years ago was ten times slower than current anthropogenic emissions, reports a paper published online in Nature Geoscience. The study shows that the current rate at which carbon is being released into the atmosphere is unprecedented in at least the past 66 million years.

During the abrupt warming known as the Palaeocene-Eocene Thermal Maximum (PETM), massive emissions of greenhouse gases caused a global warming of at least 5 °C. The PETM has been considered the closest analogue to current climate change, but the actual magnitude and rate of carbon emissions during the PETM have been difficult to determine.

Richard Zeebe and colleagues compared the timing of climate change with the timing of carbon emissions as recorded by marine sediments, and found that they occurred at essentially the same time. Using a climate and carbon cycle model, they show that such a near-concurrent change suggests the PETM carbon emissions occurred over a period of at least 4,000 years, at a rate of between 0.6 to 1.1 billion tonnes of carbon per year, compared with modern emissions that are on the order of 10 billion tonnes of carbon per year.

In an accompanying News & Views article, Peter Stassen writes: “if PETM emissions occurred over a slower timescale as proposed by Zeebe et al., pelagic marine ecosystems may have had sufficient time to adapt to environmental changes through migration or evolution. It therefore remains possible that the current rates of change might exceed the adaptive capacity of modern marine ecosystems and their constituents.”