Around 56 million years ago, Earth was bathed in the warmth of an endless summer. Europe and Greenland were still connected. Animals were free to roam from Asia to North America. It would have been a relatively comfy trip; even in the most northern latitudes, there was little to no ice. The Earth was warmer than ever, and it’s all because the oceans belched.

A huge surge in atmospheric carbon occurred as the Paleocene epoch and the Eocene began. For 150,000 years, a period known as the Paleocene-Eocene Thermal Maximum, that carbon blanketed the Earth and pushed global temperatures to radical highs. While extinction events caused by the PETM weren’t on the scale of the dinosaur extinctions from nine million years prior, it did result in an explosion of diversification that permanently altered the makeup of the planet’s species.

But what caused the huge carbon injection? Researchers at Rice suggest they’ve confirmed a long-controversial theory: the oceans suddenly burped up billions of tons of methane, which flooded the atmosphere with carbon compounds, namely carbon dioxide. That pushed planetary temperatures up by as much as six degrees for the entire PETM, drastically altering climate and weather patterns until that carbon was eventually reabsorbed by the oceans and plant life.

The methane was trapped in the form of methane hydrates, a flammable ice that lies trapped under the seafloor that.. It builds up as dead organisms sink to the ocean floor to decompose into base carbon molecules. The extreme pressure and cold at the ocean floor pushes methane into its crystal form.

The argument against the methane hydrate scenario—made by those who argue that the at least 2,500 gigatons of carbon that were released at the origins of the PETM came from volcanic action and peat burning—is that the oceans at that period were too warm to store enough methane hydrate to have fueled the carbon release.

Gerald Dickens, a Rice professor of Earth science and author of the study, said that calculations by the study’s lead author, graduate student Guangsheng Gu, showed that even while warmer temperatures left less of a sweet spot for methane hydrate formation under the ocean floor, they also increased concentration of the methane ice.

“If the volume – the size of the box – was less than today, how could it have released so much carbon?” Dickens said in a relase. “Gu’s solution is that the box contains a greater fraction of hydrate.”

While warmer ocean temperatures left less space where methane hydrate could form, the hotter water also pushed out oxygen. With less oceanic oxygen to fuel decomposition of dead organisms as they sunk to the ocean floor, more carbon matter ended up in the hands of the deep-ocean microbes that produce methane. With more fuel and warmer temperatures, those microbes worked faster, produced proportionately more methane hydrate.

“The critics said, ‘No, this can’t be. It’s warmer; there couldn’t have been more methane hydrate,” George Hirasaki, another study author, said. “But we applied the numerical model and found that if the oceans were warmer, they would contain less dissolved oxygen and the kinetics for methane formation would have been faster.”

What’s nerve-wracking about the PETM model is how similar it looks to our current conditions. There are similarly sized deposits of methane hydrate under the ocean floor waiting to be released. While it’s unclear what specifically triggered the methane release, already-warming temperatures were at least partly to blame

“I’ve always thought of (the hydrate layer) as being like a capacitor in a circuit,” Dickens said. “It charges slowly and can release fast – and warming is the trigger. It’s possible that’s happening right now.”

The PETM methane release then provides a valuable example as to why climate research is so key and why climate change. There’s a lack of evidence that volcano eruptions or peat burning occurred on a large enough scale to account solely for the PETM, while the sudden release of trillions of pounds of methane hydrate had to have been triggered by something. Likely, a combination of the two theories spurred the PETM in a feedback loop that took a hundred millenniums to reverse. We’re currently pumping tons and tons of carbon dioxide into the atmosphere, and the Earth is warming.

There will come a point where events begin to cascade. It’s impossible to know if those events produce an increase in the pace of evolution, as did the PETM, or if mass extinctions will occur. What research like this suggests is that we’re on a similar path to previous events that irreversibly altered the face of the Earth.