Ancient global warming explained

Episodes of global warming are known to have occurred several times during the Palaeocene and Eocene epochs (65–34 million years ago). The most extreme of these ‘hyperthernals’ was the Palaeocene–Eocene Thermal Maximum (PETM) around 56 million years ago (56 Myr). This episode of warming lasted for about 170,000 years, with global average temperature increasing by 5–7°C.

“Scientists believe that the PETM resulted from massive release of greenhouse gases from carbon stored in sedimentary rocks,” said Professor Paul Wilson of SOES. “Because of its rapid onset and the estimated release of carbon dioxide into the atmosphere, the PETM has been compared to modern global warming caused by anthropogenic greenhouse gas emissions.”

Scientists have previously documented at six other more modest warming events during the Palaeocene and Eocene, which have also been linked to the release of sedimentary carbon.

Past climate can be reconstructed from the chemical isotope composition of tiny marine plankton called foraminiferans, the fossilised shells of which are preserved in marine sediments.

Professor Wilson and his colleagues did this using sediment cores drilled from the Demerara rise in the tropical western Atlantic Ocean. These new records cover a period of 2.4 million years (47.6–50.0 Myr), corresponding to the transition between the early Eocene and middle Eocene transition.

They found evidence for 13 comparatively modest hyperthermals, suggesting that such events occurred much more often than has been appreciated hitherto. However, with average durations of around 40,000 years, these warming episodes were shorter lived than the PETM, and recovery was also more rapid.

“These modest hyperthermals occurred every 100–400 thousand years during the period covered by the new records, reflecting changes in the eccentricity of Earth’s orbit at that time” said Professor Wilson. “They developed very rapidly, over a period of around 5–10 thousand years, but recovery look longer, up to 30,000 years, which is still comparatively rapid.”

“We believe that the mechanisms driving these modest, relatively short-lived hyperthermals were different from those responsible for the PETM,” said lead author Dr Philip sexton, who is now at the Open University in Milton Keynes, UK.

The PETM is believed to have involved the exhumation of carbon from the sedimentary reservoir, possibly triggered by plate tectonics and volcanism, or even a comet impact. This would have been followed by the subsequent burial of carbon back into sedimentary rocks over a long period of time.

However, the comparative rapid onset and recover of the more modest, short-lived hyperthermals is more consistent with the redistribution of carbon among Earth’s readily exchangeable surface reservoirs.

“We believe that large amounts of carbon dioxide were repeatedly released into the atmosphere due to increased ventilation of the ocean’s interior and strengthened oxidation of dissolved organic carbon,” said Professor Wilson. “When conditions permitted, this carbon dioxide would have been taken up again by the ocean, leading to recovery.”

The implication is that exchanges of carbon dioxide between the ocean surface and the atmosphere played a central role in regulating global climate during the Eocene, just as they do today.