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Carbon dioxide pulses are a common feature of the carbon cycle

last modified Aug 25, 2020 08:37 AM
A multi-institutional study, involving researchers at the Department of Earth Sciences, University of Cambridge, has found that pulse-like releases of carbon dioxide to the atmosphere are a pervasive feature of the carbon cycle and that they are closely connected to major changes in Atlantic Ocean circulation.

Ice cores from Antarctica show that, in the span of less than two centuries, atmospheric levels of carbon dioxide jumped repeatedly at the end of the last ice age, when the Atlantic was continuously disturbed by melting ice sheets. 

Whether these CO2  jumps might occur in today’s warmer interglacial conditions, when we are already seeing the impact of human-driven CO2 emissions and rapidly melting polar ice sheets, has remained unknown.

The study, out today in Science and led by Dr Christoph Nehrbass-Ahles, reveals that rapid CO2 jumps also occurred during a period from 450,000 to 330,000 years before the present, a key time in Earth’s history when the climate transitioned from glacial to interglacial conditions. 

“By looking back further in time, to previous glacial and interglacial conditions, we find the same CO2 jumps - irrespective of whether the climate was cold or warm,” said first author Dr Christoph Nehrbass-Ahles, who is now based Cambridge’s Department of Earth Sciences, and conducted the research while based at the University of Bern and in collaboration with the Université Grenoble Alpes

These rapid CO2 rises seem to be a common feature of the carbon cycle in the past. But human activities are releasing carbon a rate ten times faster than during these past CO2 increases, “what is unclear is how a future jump in carbon may interact with, or exacerbate, anthropogenic carbon emissions”, says Nehrbass-Ahles. 

Central to the team’s finding was their detailed analysis of Antarctic ice from the EPICA (The European Project for Ice Coring in Antarctica) Dome C core. 

“Our previous understanding of rapid CO2 changes has been hampered by a lack of data over this interval – so these events were often missed” said Nehrbass-Ahles. Thanks to a novel gas extraction method and detailed sampling campaign, Nehrbass-Ahles and co-authors were able to identify even subtle changes occurring at centennial timescales. 

Image of cross section of ice core, credit: Christoph Nehrbass-Ahles, University of Cambridge

Cross section view of an ice core retrieved from the Skytrain Ice Rise (Antarctica) by the ERC-funded WArm Climate Stability of West Antarctic ice sheet in the last INterglacial (WACSWAIN) Project. Bubbles of ancient air trapped in the ice (image to the right) can be used to reconstruct the past atmospheric composition. Image copyright, Christoph Nehrbass-Ahles, University of Cambridge.

The study marks an important step in understanding what causes these abrupt increases. “Scientists are uncertain as to the mechanism behind the CO2 jumps, but think a combination of factors, including ocean circulation, changing wind patterns, and terrestrial processes, are likely responsible” said Professor David Hodell, also from the Department of Earth Sciences, Cambridge. 

In a collaboration between Bern and Cambridge, the team combined the new ice core data with detailed information on ocean circulation from marine sediments collected off the coast of Portugal. The site, which was drilled as part of the International Ocean Discovery Program (IODP), is unique for its high sediment accumulation rate and ideally situated for monitoring past changes in ocean circulation triggered by collapsing ice sheets.  

The isotopic signal of the marine sediments showed the same pattern as the ice cores. “The abrupt changes are clearly represented in both the marine and ice records, telling us that they must be connected to major changes in the surface and deep circulation of the Atlantic Ocean”, said Nehrbass-Ahles.

According to Hodell “the key here is the fine-grained resolution of the ice and marine sediments, making observations of these rapid changes in both records possible”. 

“Understanding these centennial-scale changes is crucial because they operate at a similar pace to the anthropogenic changes altering our planet,” said Nehrbass-Ahles. 

C. Nehrbass-Ahles et al. ‘Abrupt CO2 release to the atmosphere under glacial and early interglacial climate conditions.’ Science (2020). DOI: 10.1126/science.aay8178