Causes and consequences of global climate warming 56 million years ago revealed

Researchers are studying the causes and consequences of global climate warming that took place 56 million years ago.

Victoriano Pujalte, lecturer in the UPV/EHU's Department of Stratigraphy and Palaeontology, and lead researcher of the study, said that the fall in sea level did not unleash the emission of greenhouse gases during the Palaeocene-Eocene Thermal Maximum (PETM).

The Palaeocene-Eocene Thermal Maximum (PETM) was a brief interval (in geological terms, it "only" lasted about 200,000 years) of extremely high temperatures that took place 56 million years ago as a result of a massive emission of greenhouse gases into the atmosphere.

The global temperature increase is reckoned to have been between 5 degree Celsius and 9 degree Celsius.

The most widely accepted hypothesis suggests that it was due to the destabilising of methane hydrates that remained frozen on ocean floors. "Some authors, like Higgins and Schrag (2006), for example, proposed that a fall in sea level could have caused or co-contributed towards the unleashing of the emission of methane or CO2," pointed out Victoriano Pujalte, lecturer in the UPV/EHU's Department of Stratigraphy and Palaeontology, and lead researcher in the study.

According to this hypothesis, "the marine sediments that were submerged in the sea were exposed when the sea level fell, and were responsible for the CO2 emissions," he added. That is what, to a certain extent, prompted this study. Others not only reject that possibility but also the fall in sea level itself. "We set out to try and establish the behaviour of the sea level during that time interval, the PETM," said Pujalte.

The most useful indicators are the stable oxygen and carbon isotopes. The results obtained indicate that the PETM was in fact preceded by a fall in sea level, the size of which is estimated to have been about 20 metres and the maximum descent of which probably occurred about 75 million years before the start of the PETM.

Furthermore, the researchers observed that the rise in the sea level continued after the PETM, when the global temperature returned to normal levels. "Its origin was not only caused, therefore, by the thermal expansion of the oceans linked to the warming," said Pujalte.

The study has been published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology.