Global climate and the concentration of atmospheric carbon dioxide (CO2) are correlated over recent glacial cycles1,2. The combination of processes responsible for a rise in atmospheric CO2 at the last glacial termination1,3 (23,000 to 9,000 years ago), however, remains uncertain1,2,3. Establishing the timing and rate of CO2 changes in the past provides critical insight into the mechanisms that influence the carbon cycle and helps put present and future anthropogenic emissions in context. Here we present CO2 and methane (CH4) records of the last deglaciation from a new high-accumulation West Antarctic ice core with unprecedented temporal resolution and precise chronology. We show that although low-frequency CO2 variations parallel changes in Antarctic temperature, abrupt CO2 changes occur that have a clear relationship with abrupt climate changes in the Northern Hemisphere. A significant proportion of the direct radiative forcing associated with the rise in atmospheric CO2 occurred in three sudden steps, each of 10 to 15 parts per million. Every step took place in less than two centuries and was followed by no notable change in atmospheric CO2 for about 1,000 to 1,500 years. Slow, millennial-scale ventilation of Southern Ocean CO2-rich, deep-ocean water masses is thought to have been fundamental to the rise in atmospheric CO2 associated with the glacial termination4, given the strong covariance of CO2 levels and Antarctic temperatures5. Our data establish a contribution from an abrupt, centennial-scale mode of CO2 variability that is not directly related to Antarctic temperature. We suggest that processes operating on centennial timescales, probably involving the Atlantic meridional overturning circulation, seem to be influencing global carbon-cycle dynamics and are at present not widely considered in Earth system models.
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This work is supported by the US National Science Foundation (NSF) (grants 0739766-ANT, 1043518-ANT, 1043092-ANT, 0839093-ANT and 1142166-ANT). We appreciate the support of the WAIS Divide Science Coordination Office at the Desert Research Institute (DRI) of Reno, Nevada, and the University of New Hampshire for the collection and distribution of the WAIS Divide ice core and related tasks (NSF grants 0230396, 0440817, 0944348 and 0944266). Additional support for this research came from the NSF Office of Polar Programs through their support of the Ice Drilling Program Office and the Ice Drilling Design and Operations group; the US National Ice Core Laboratory, for curation of the core; Raytheon Polar Services, for logistics support in Antarctica; the 109th New York Air National Guard, for airlift to Antarctica; and the Korea Meteorological Administration Research and Development Program (CATER 2012-7030). We thank T. Alig, J. Edwards and J. Lee for assisting with CO2 and CH4 measurements; the DRI ultratrace ice-core lab, including D. Pasteris, M. Sigl and O. Maselli for their contribution to the aerosol records; and I. Fung for discussions and providing software for carbon uptake calculations.
The authors declare no competing financial interests.
Extended data figures and tables
a, Borehole calibrated surface temperature reconstruction derived from δ18O measurements from the ice1. b, Accumulation rates reconstructed with the firn-densification inverse model (red curve) and from layer thickness observations (black curve). c, δ15N-N2 data for the upper 2,800 m (black dots) with model fit (green curve). d, Modelled age using firn-densification model (orange curve) and Δage estimate using the depth-difference technique from Parrenin et al.5 (black curve).
WDC CO2 concentrations (blue) plotted against non-seasalt calcium (nssCa) concentrations (black) and hydrogen peroxide (H2O2, red) at multiple depths in the core where we observe abrupt changes in carbon dioxide. Hydrogen peroxide concentrations have been smoothed (2 m centred average) from original data to improve clarity.
a, The red line is the Green’s function (smoothing function) produced by a firn model using an assumed EDC accumulation rate of 0.015 m yr−1 and a temperature of 209 K. b, CO2 data from WDC (dots) and EDC (dots) plotted against artificially smoothed CO2 data from WDC using the EDC firn smoothing function (red line in both plots). WDC data have been systematically lowered by 4 p.p.m. for direct comparison with EDC.
Extended Data Figure 6 Simple box model source history and atmospheric CO2 response compared to measured data from WDC.
a, Applied source history used in the modelling experiment. b, Atmospheric CO2 record from WDC (5-point weighted average; blue) and the model derived atmospheric history (black). Box model from ref. 55.
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Marcott, S., Bauska, T., Buizert, C. et al. Centennial-scale changes in the global carbon cycle during the last deglaciation. Nature 514, 616–619 (2014). https://doi.org/10.1038/nature13799
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