A dramatic period of global warming 56 million years ago that saw temperatures climb by up to five degrees and triggered extinctions of marine organisms was down to volcanic eruptions, researchers have revealed, in a study they say offers insights into the scale and possible impact of global warming today.

One of the most rapid periods of warming in Earth’s history, the Palaeocene/Eocene Thermal Maximum (PETM), occurred as Greenland pulled away from Europe.

However, details of the quantities of carbon dioxide behind the warming and where it came from had remained unclear.

Now scientists say they have solved the puzzle, revealing that the main driver of the event was a gradual release of carbon dioxide through volcanic eruptions – findings, they say, that overturn a long-held view that the PETM mirrors the rapid rise in carbon emissions seen today.

“[The PETM] was always regarded as the best natural analogue for current anthropogenic carbon emissions – but we have found that not even that event is a actually good analogue,” said Marcus Gutjahr, first author of the study from the Geomar-Helmholtz Centre for Ocean Research in Germany. “We are actually currently marching in unknown territory.”

Writing in the journal Nature, Gutjahr and colleagues from the UK and US reveal how they unpicked the conundrum by combining computer simulations with an analysis of fossil shells from microscopic single-cell organisms found within a sediment core from the north-east Atlantic Ocean.

The team focused on the ratios of different forms – or isotopes – of oxygen, carbon and boron within the shells, the latter of which offers tell-tale clues about the ocean’s acidity, a measure affected by levels of carbon dioxide that dissolved from the atmosphere into the ocean. “These shells recorded the chemistry of the ocean in which [they] grew,” said Gutjahr.

The carbon dioxide, he notes, could either have been pumped directly into the atmosphere through volcanic events or have formed from other carbon sources, such as underwater methane deposits or organic-rich sediments.

However, carbon dioxide from the different sources would have a very different impact on carbon isotope ratios. What’s more, while volcanic eruptions gradually release carbon dioxide over time, gases from methane deposits or sediments are released rapidly.

Drawing these factors together with the level and duration of the increased ocean acidity, the team found that the carbon dioxide was probably released through volcanic eruptions, with such events accounting for up to 90% of the emissions.

But, says Gutjahr, the study does not rule out the possibility of some very short, sudden releases of methane and other carbon sources.

“The key things is the acidification took 20,000 years and if we had introduced enough methane over 20,000 years to keep the pH so low, then the carbon signature would be very different to what we see,” he said.

The team were also able to calculate that overall between 10,200-12,200 petagrams of carbon were released into the atmosphere during the PETM – more carbon than is in the world’s total fossil fuel reserves – with rates of up to 0.58 petagrams of carbon released each year over 50,000 years. About 10 petagrams of carbon are currently released every year from fossil fuel emissions.

Daniela Schmidt, a professor of palaeobiology at the University of Bristol who was not involved in the study, said the study held a warning for the planet today.

“We know something which was smaller than what we are currently doing had profound biological implications,” she said. “We always assume that if something happens quicker and we have less time to adapt, the impact will be larger.”

Mark Maslin, professor of geography at University College London, said the case for volcanic eruptions being behind the PETM was compelling, adding that the research suggests underwater sources of carbon, like methane deposits, might be more stable than previously thought and play a smaller role in climate change.

But, he added, there are plenty of causes for concern, not least the rapid rate of carbon emissions today, and Earth’s current sensitivity to such changes, means the planet will not be able to adjust in the same way it did in the past. What’s more, he said, the potential loss of other carbon-rich deposits remains a serious issue.

“We should still be worried about methane stored in the permafrost in the high Arctic,” he said.