Around 56 million years ago, there was an unmistakably warm period that appeared out of nowhere. The climate changed rapidly over just 500,000 years, a geological blink of an eye; all across the planet, temperatures shot up by as much as 8°C (14.4°F), triggering regional extinctions and overthrowing entire ecosystems’ hierarchies.

Even though the climate change signal has been clear for some time, it wasn’t clear at all what actually triggered it. Now, according to a new Nature study, we’ve finally found the culprit: a volcanic supereruption, of sorts.

An international team of researchers led by the University of Southampton decided to take another look at the so-called Palaeocene-Eocene Thermal Maximum (PETM). They were well aware that a huge release of carbon dioxide took place at the time, but where did it come from?

Carbon sources leave a signature. For example, carbon-12 is the primary variant (isotope) of that element that is trapped within the fossil fuels that we burn. Tracing this isotope is one of the primary fingerprints indicating that we are the reason there is now so much carbon dioxide in the atmosphere.

The team in this case decided to analyze the geochemistry of the remnants of tiny sea creatures known as foraminifera. Their shells are normally made of calcium carbonate, and the carbon within them can be used by researchers to see what kind of chemistry – and carbon releases – featured in the seas of the world 56 million years before now.

Combining their data with a cutting-edge climate model, the team calculated not only how much carbon was unleashed during the PETM, but where it came from. The final tally was 30 times the amount released by all the fossil fuels humanity has combusted to date – and only one event happening at the time could explain such a vast quantity.

Back then, Greenland was splitting away from Europe, and the North Atlantic Ocean was taking shape. When this type of continental rifting occurs, large upwellings of mantle material take place at the central rifting zone. This triggers a high degree of melt, which generates profound volcanism.

The PETM, then, was almost certainly caused by this gigantic melting zone, permitting a long-lasting volcanic supereruption to change the face of the Earth, for better or worse.

Ever since the PETM was discovered, researchers have had many ideas as to what may have been the cause of it. A quick look at geological history reveals that massive, prolonged, and deadly climate change often occurs because of one of three factors: volcanism, methane releases, or an asteroid impact event.

Asteroid impact events generally cool the world, as they excoriate the Earth’s crust and send plenty of sunlight-blocking particulates high up into the sky. So that option was quickly ruled out – but what about methane?

Created by various geological processes, methane is also generated biologically (by bacteria and some archaea). It builds up in the sea floor, along continental shelves, and within permafrost in Arctic environments across the surface of the world over time.

If these caches are destabilized – by oceanic warming and acidification, or by atmospheric warming – these microbes amp up their decompositional activities and produce more methane. There's a good chance it'll be subsequently released into the water column or sky above, although the extent and rate at which this methane does actually escape into the atmosphere is deeply unclear at present.

Methane’s a pretty frightening greenhouse gas. Although it breaks down in the atmosphere far quicker than carbon dioxide, it is around 36 times better at trapping in heat in the long-term. Circumstantial evidence in the geochemistry of sediment cores from the ancient seafloor suggests a huge methane release may have caused the PETM – after all, methane decomposes into carbon on occasion – but this could never really be conclusively demonstrated.

Now we have the most convincing theory to date. It’s worth noting though that, despite the enormity of the event and the rapidity of the climate change taking place, it’s still being outdone by humans. That’s right – the rate at which we are releasing greenhouse gases eclipses that of even the PETM.