Simulating low frequency changes in atmospheric CO2 during the last 740 000 years

pkoehler [ at ]


Atmospheric CO2 measured in Antarctic ice coresshows a natural variability of 80 to 100 ppmv during the lastfour glacial cycles and variations of approximately 60 ppmvin the two cycles between 410 and 650 kyr BP. We hereuse various paleo-climatic records from the EPICA DomeC Antarctic ice core and from oceanic sediment cores coveringthe last 740 kyr to force the ocean/atmosphere/biospherebox model of the global carbon cycle BICYCLE in a forwardmode over this time in order to interpret the natural variabilityof CO2. Our approach is based on the previous interpretationof carbon cycle variations during Termination I (Kšohleret al., 2005a). In the absense of a process-based sedimentmodule one main simplification of BICYCLE is that carbonatecompensation is approximated by the temporally delayedrestoration of deep ocean [CO2− 3 ]. Our results match the lowfrequency changes in CO2 measured in the Vostok and theEPICA Dome C ice core for the last 650 kyr BP (r20.75).During these transient simulations the carbon cycle reachesnever a steady state due to the ongoing variability of the overallcarbon budget caused by the time delayed response ofthe carbonate compensation to other processes. The averagecontributions of different processes to the rise in CO2during Terminations I to V and during earlier terminationsare: the rise in Southern Ocean vertical mixing: 36/22 ppmv,rising in ocean temperature: 26/11 ppmv, iron limitation ofthe marine biota in the Southern Ocean: 20/14 ppmv, carbonatecompensation: 15/7 ppmv, the rise in North Atlanticdeep water formation: 13/0 ppmv, the rise in gas exchangedue to a decreasing sea ice cover: −8/−7 ppmv, sea levelrise: −12/−4 ppmv, and rising terrestrial carbon storage:−13/−6 ppmv. According to our model the smaller interglacialCO2 values in the pre-Vostok period prior to TerminationV are mainly caused by smaller interglacial SouthernOcean SST and an Atlantic THC which stayed before MIS 11(before 420 kyr BP) in its weaker glacial circulation mode.

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Köhler, P. and Fischer, H. (2006): Simulating low frequency changes in atmospheric CO2 during the last 740 000 years , Climate of the Past, 278, 57 .

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