17 Mar 2011

Research challenges prevailing view on the causes of past climate change

Shells of fossilised plankton

Shells of fossilised plankton

Research, led by The Open University and published today in the scientific journal Nature, is helping scientists understand how Earth’s carbon cycle may respond to the current, human-induced, interval of global warming. It has uncovered the likely cause of repeated episodes of natural global warming 50 million years ago in the Eocene epoch, when Earth last experienced the elevated temperatures and atmospheric carbon dioxide levels predicted for the end of this century.

Dr Philip Sexton, Lecturer in The Open University’s Faculty of Science, led the research, working with the Integrated Ocean Drilling Program to obtain sediment cores from beneath the deep-sea floor. Chemical analyses of the microscopic shells of fossilised plankton hosted within these sediments allowed Dr Sexton and his colleagues to establish the connections between global climate change and the carbon cycle during the warm Eocene ‘greenhouse’.

The beginning of the Eocene epoch was marked by the most dramatic natural global warming event ever known: the PETM (Paleocene-Eocene Thermal Maximum). This episode of warming lasted for about 170,000 years, with global average temperature increasing by around 6˚C and the deep sea experiencing a dramatic depletion in oxygen levels. These environmental changes were significant, not least because they led to the extinction of half of all deep-sea species, but also the major redistribution of many land dwelling species, including early primates.

It has long been accepted that the intense global warming across the PETM (known to geologists as a ‘hyperthermal’) was the result of massive release of carbon dioxide into the atmosphere. “Scientists believe that extreme PETM global warming was caused by a geologically rapid release of carbon dioxide from the huge carbon reservoirs in rocks deep below the Earth’s surface, in a manner similar to the current transfer of buried ‘fossil fuel’ carbon from rocks into our atmosphere and oceans” said Dr Sexton. “Recovery of Earth’s temperatures was likely achieved by subsequent burial of carbon back into sedimentary rocks over a long period of time, controlled by rather slow rock weathering reactions”.

Recent discoveries of a number of additional, but more modest, hyperthermals during the Eocene have led scientists to assume that they, too, were triggered by the same thing: carbon release from ‘fossil fuel’ reservoirs into Earth’s atmosphere.

Dr Sexton and his team have discovered that the relatively modest global warming events were at least 3 times more frequent than was first thought. However, with average durations of around 40,000 years, these warming episodes were much shorter-lived than the PETM, with their recoveries being particularly rapid. “We believe that the mechanisms driving these more modest, relatively short-lived hyperthermals were different from those responsible for the PETM” said Dr Sexton.

The comparatively rapid recovery of the more modest hyperthermals implicates redistribution of carbon among the readily exchangeable, active reservoirs at Earth’s surface (the ocean, land biosphere and atmosphere). “We think that large amounts of carbon dioxide were repeatedly released into the atmosphere, and subsequently rapidly taken back up again, by the ocean,” said Dr Sexton. Specifically, they implicate a much larger-than-modern, and dynamic, oceanic reservoir of dissolved organic carbon.

This study shows that past climates of high global temperatures and atmospheric carbon dioxide levels, similar to levels we are likely to experience by the end of this century, were more unstable than previously assumed. This climatic instability appears to have arisen from a more erratic shuttling around of carbon between its various reservoirs at Earth's surface.


Notes to editors

The researchers are Philip Sexton (The Open University, Scripps Institution of Oceanography, California and National Oceanography Centre, Southampton (NOC)), Richard Norris (Scripps), Paul Wilson, Heiko Pälike, Clara Bolton and Samantha Gibbs (NOC), and Thomas Westerhold and Ursula Röhl (University of Bremen).

This research used samples and data provided by the International Ocean Drilling Program (IODP). The research was sponsored by the US National Science Foundation and supported by the European Commission, the Leverhulme Trust, the UK’s Natural Environment Research Council (NERC), and the DFG-Leibniz Center for Surface Process and Climate Studies at the University of Potsdam.

Phil Sexton is a member of The Open University’s Centre for Earth, Planetary, Space & Astronomical Research (CEPSAR). The strength and excellence of the research supported by the Centre was acknowledged in the UK’s last Research Assessment Exercise (RAE 2008), with 70% of its research deemed internationally excellent and world-leading, achieving 3*/4* rankings. CEPSAR website:

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