Around 55 million years ago, a major climate event upended our planet’s weather systems in what is recognised as one of the most rapid and dramatic instances of climate change the world has ever seen.
But theories behind the cause of the 150,000-year-long event have been inconclusive.
A new study suggests the warming planet hit various ‘tipping points’ that released more carbon, triggering more warming.
Known as the Paleocene-Eocene Thermal Maximum (PETM), the geological record shows a “vast” rise in CO2 levels pushed up temperatures by as much as 5C above average, over a period of just a few thousand years.
The event was blamed on heightened levels of volcanic activity pumping huge amounts of CO2 into the atmosphere.
The new study, from the Camborne School of Mines at the University of Exeter, suggests the initial rise in volcanic activity created conditions that tapped ‘carbon reservoirs’ involved in a chain reaction of events.
These carbon reservoirs were likely to have included greenhouse gases released by melting permafrost at the poles, and extensive methane deposits escaping from warming sea floors.
Dr Sev Kender, a co-author, said: ”Greenhouse gasses such as CO2 [and] methane were released to the atmosphere at the start of the PETM in just a few thousand years.
"We wanted to test the hypothesis that this unprecedented greenhouse gas release was triggered by large volcanic eruptions. As volcanoes also release large quantities of mercury, we measured the mercury and carbon in the sediment cores to detect any ancient volcanism.
"The surprise was that we didn’t find a simple relationship of increased volcanism during the greenhouse gas release. We found volcanism occurred only at the beginning phase, and so another source of greenhouse gasses must have been released after the volcanism.”
The research team also found evidence that the increased volcanism included “thousands of hydrothermal vents on a scale far beyond anything seen today”, indicating a huge amount of the volcanic activity was taking place deep underwater.
Crucially, the rock samples taken from the North Sea also showed that in the early stages of the PETM, there was a significant drop in mercury levels – suggesting at least one other carbon reservoir – from a non-volcanic source – took over and went on to release significant additional greenhouse gases as the heating phenomenon took hold.
The research team said their findings reveal evidence of “the existence of tipping points in the Earth’s system”, which are relevant today.
The study shows how hitting such tipping points today could trigger the release of similar additional carbon reservoirs which previously drove the Earth’s climate to unprecedented high temperatures.
The research team, which includes experts from the British Geological Survey, the University of Oxford, Herriot-Watt University and the University of California at Riverside, said their study “could give a fresh understanding of how modern day climate change will affect the Earth in the centuries to come.”
Dr Kender added: “We were able to carry out this research as we have been working on exceptionally well preserved new core material with collaborators from the Geological Survey of Denmark and Greenland. The excellent preservation allowed detailed detection of both the carbon released to the atmosphere and the mercury. As the North Sea is close to the region of volcanism thought to have triggered the PETM, these cores were in an ideal position to detect the signals.
"The volcanism that caused the warming was probably vast deep intruded sills producing thousands of hydrothermal vents on a scale far beyond anything seen today. Possible secondary sources of greenhouse gases were melting permafrost and sea floor methane hydrates, as a result of the initial volcanic warming.”
The research is published in the journal Nature Communications.