Global warming driven by volcanic CO2 56 million years ago?

About 56 million years ago, global mean temperatures rose by at least five degrees Celsius within a few thousand years. At that time, Earth experienced one of the fastest and most extreme climatic changes in its recent history. Since this warming marks the transition between the Paleocene and Eocene era, it is commonly referred to as the Palaeocene-Eocene Thermal Maximum (PETM). About 150,000 years later, global temperatures returned to values comparable to those before the start of the PETM. Since this temperature rise at the beginning of the PETM is comparable to current climatic warming scenarios, it is also used as a reference for near future climate projections. However, scientists still debate different theories of the exact causes of the PETM. 

New investigations by an international team of scientists led by the University of Southampton (UK) and the GEOMAR Helmholtz Centre for Ocean Research Kiel traced the PETM back to massive carbon dioxide emissions from volcanism as a result of the opening of the North Atlantic. In the study now published in the international journal Nature, atmospheric CO₂ concentrations more than doubled within less than 25,000 years during the PETM.

“There are numerous theories about the cause of the PETM ranging from meteorite impacts to the dissolution of gas hydrates”, explains the first author of the study, Dr. Marcus Gutjahr from GEOMAR. “It has long been recognised that large amounts of carbon entering the ocean and the atmosphere could have triggered the PETM”. However, the exact source of this carbon and the total amount released has been difficult to assess.

It had been known that the PETM roughly coincided with the formation of massive ‘flood basalts’ – large stretches of ocean floor and the continents coated in lava, resulting from of a series of huge eruptions. These occurred as Greenland and North America first started separating from north-western Europe, thereby creating the North Atlantic Ocean, the vestiges of which are still continuing in miniature in Iceland today. “But so far there has been no direct evidence for the causal link between these two processes”, says Dr. Gutjahr, who started the study while a post-doctoral fellow at the University of Southampton.

In order to identify the CO₂ source, he and his colleagues first reconstructed the changes in the pH value of the ocean during the PETM by applying a new analytical approach. “This method is based on the measurement of different isotopes of the elements boron and carbon in microscopically small marine fossils, so-called foraminifera”, explains Dr. Gutjahr. Southampton and Kiel are among few laboratories worldwide where such investigations are carried out.

“Ocean pH tells us about the amount of carbon absorbed by ancient seawater, but we can get even more information by also considering changes in the carbon composition, as this provides an indication of its source.”,  explains Professor Andy Ridgwell of the University of California, Riverside, co-author of the study. “If we consider both analyses in a global climate model, only large-scale volcanism during opening of the North Atlantic could have been the primary driver of the PETM”.

In detail, the analyses showed that during the PETM, more than 12,000 billion tonnes of carbon predominantly from a volcanic source were released into the atmosphere. This is about 30 times more than all the fossil fuels burned to date plus all remaining reserves for fossil fuels. In the climate model output, the atmospheric CO₂ levels increased from about 800 ppm to over 2000 ppm (currently the carbon dioxide content of Earth's atmosphere is 400 ppm).

“How the ancient Earth system responded to this carbon injection at the PETM can tell us a great deal about how it might respond in the future to man-made climate change.”, says co-author Professor Gavin Foster from the University of Southampton. However, in the models the emission and temperature evolution during the PETM was very different to the current climate: “Compared with today’s human-made carbon emissions, the rate of carbon addition during the PETM was much slower, at least by about a factor of 20," adds Dr. Gutjahr. Thus, the current climate change is much faster than any climate events of the past 56 million years, with probably far-reaching consequences and uncertain results.

Original work:

Gutjahr, M., A. Ridgwell, P.F. Sexton, E. Anagnostou, P.N. Pearson, H. Pälike, R.D. Norris, E. Thomas and G.L. Foster, 2017: Paleocene-Eocene Thermal Maximum. Nature, http://dx.doi.org/10.1038/nature23646

Note:

In addition to the above mentioned institutions, scientists from the Open University at Milton Keynes, the University of Bristol, the University of Cardiff, the University of Bremen, the University of California San Diego and Yale University participated in this study. The project was funded by the British Natural Environment Research Council (NERC), Department for Environment Food and Rural Affairs (DEFRA), and Department of Energy and Climate Change (DECC) as part of the UK ocean acidification research programme.