Around 56 million years ago a significant global warming event took place lasting several thousand years. However, its causes have long perplexed scientists.

Now, a study conducted by an international team of researchers has concluded that the Paleocene–Eocene Thermal Maximum (PETM), as it is known, was triggered largely by volcanic eruptions that occurred as Greenland separated from Europe during the breakup of the supercontinent Pangaea.

In recent years, the PETM event has been the focus of much research in the geosciences because analysts believe it to be one of the best analogues in the earth's history for understanding the implications of a global warming climate.

It is thought global temperatures rose around 5°C during the span of the event, which is roughly comparable to the increase that some scientists are predicting to occur by the end of the next century. The main difference between now and then is that current warming is taking place at a much faster rate.

The study's findings counter the established hypothesis which posited the release of carbon from reservoirs in the ground as the most likely scenario.

"While it has long been suggested that the PETM was caused by injection of carbon into the atmosphere and ocean, the mechanism has remained elusive until now," Andy Ridgwell, an author of the study and professor at the University of California, Riverside, said. "By combining geochemical measurements and a global climate model that my group has been developing for over a decade, we have shown that this event was caused almost entirely by carbon emissions from the Earth's interior."

To try and determine where all the extra carbon was coming from, the team examined the remains of tiny sea creatures called foraminifera which were living millions of years ago. Specifically, the researchers studied the shells of the animals to track how the pH of sweater changed during the PETM and then combined this information with Ridgewell's climate models. They concluded from this that volcanic activity during the breakup of Pangaea was to blame.

"The amount of carbon released during this time was vast - more than 30 times larger than all the fossil fuels burned to date and equivalent to all the current conventional and unconventional fossil fuel reserves we could feasibly ever extract." Ridgwell added.

"Studying the PETM helps us understand the mechanisms that aid recovery from global warming, thereby helping researchers reduce the uncertainties surrounding the Earth's response to global climate change. While it is encouraging that most ecosystems were able to adapt during the PETM, today's global temperature could be increasing at a rate that is too fast for plants and animals to adjust."