The most rapid and extreme natural global warming event of the last 66 million years was driven by massive carbon dioxide (CO2) emissions from volcanoes during the formation of the North Atlantic Ocean.

During the period known as Palaeocene-Eocene Thermal Maximum (PETM) which lasted for around 150 thousand years, global temperatures increased by at least five degrees Celsius — a temperature increase comparable with projections of modern climate beyond the end of this century.

The PETM period occurred some 56 million years ago and is believed to be the most rapid and extreme natural global warming event of the last 66 million years.

The study, published in the journal Nature, used a combination of new geochemical measurements and novel global climate modelling to show that PETM was associated with a geologically rapid doubling of atmospheric CO2 in less than 25 thousand years — with volcanoes squarely to blame.

While it has long been suggested that the PETM event was caused by the injection of carbon into the ocean and atmosphere — the ultimate trigger, the source of this carbon, and the total amount released, have up to now all remained elusive.

“In order to identify the source of carbon we first generated a new record of the change in ocean pH (a measure of its acidity) through the PETM, by measuring changes in the balance of isotopes of the element boron in ancient marine fossils called foraminifera,” said Marcus Gutjahr, who led the study while a post-doctoral fellow at University of Southampton in Britain.

Gutjahr is now with the GEOMAR Helmholtz Centre for Ocean Research in Kiel Germany.

“Ocean pH tells us about the amount of carbon absorbed by ancient seawater, but we can get even more information by also considering changes in the isotopes of carbon, as these provide an indication of its source,” Professor Andy Ridgwell from University of California, Riverside in the US said.

“When we force a numerical global climate model to take into account both sets of changes, the results point to the large-scale volcanism associated with the opening of the North Atlantic as the primary driver of the PETM,” Ridgwell added.

The team found that the PETM was associated with a total input of more than 10,000 petagrams of carbon from a predominantly volcanic source.

This is a vast amount of carbon — some 30 times larger than all the fossil fuels burned to date and equivalent to all current conventional and unconventional fossil fuel reserves, the study said.

“How the ancient Earth system responded to this carbon injection at the PETM can tell us a great deal about how it might respond in the future to man-made climate change,” Gavin Foster, Professor at University of Southampton, said.