Climate of the Past
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2006
10.5194/cp-2-57-2006
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57
78
2006-09-11
Simulating low frequency changes in atmospheric CO<sub>2</sub> during the last 740 000 years
P. Köhler
pkoehler@awi-bremerhaven.de
H. Fischer
Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research, P.O. Box 12 01 61, 27515 Bremerhaven, Germany
Atmospheric CO<sub>2</sub> measured in Antarctic ice cores shows a natural
variability of 80 to 100 ppmv during the last four glacial cycles and
variations of approximately 60 ppmv in the two cycles between 410 and
650 kyr BP. We here use various paleo-climatic records from the EPICA Dome C
Antarctic ice core and from oceanic sediment cores covering the last 740 kyr
to force the ocean/atmosphere/biosphere box model of the global carbon cycle
BICYCLE in a forward mode over this time in order to interpret the
natural variability of CO<sub>2</sub>. Our approach is based on the previous
interpretation of carbon cycle variations during Termination I
(Köhler et al.,
2005a). In the absense of a process-based sediment
module one main simplification of BICYCLE is that carbonate
compensation is approximated by the temporally delayed restoration of deep
ocean [CO<sub>3</sub><sup>2−</sup>]. Our results match the low frequency changes in
CO<sub>2</sub> measured in the Vostok and the EPICA Dome C ice core for the last
650 kyr BP (<i>r</i><sup>2</sup>≈0.75). During these transient simulations the
carbon cycle reaches never a steady state due to the ongoing variability of
the overall carbon budget caused by the time delayed response of the
carbonate compensation to other processes. The average contributions of
different processes to the rise in CO<sub>2</sub> during Terminations I to V and
during earlier terminations are: the rise in Southern Ocean vertical mixing:
36/22 ppmv, the rise in ocean temperature: 26/11 ppmv, iron limitation of the
marine biota in the Southern Ocean: 20/14 ppmv, carbonate compensation:
15/7 ppmv, the rise in North Atlantic deep water formation: 13/0 ppmv, the
rise in gas exchange due to a decreasing sea ice cover: −8/−7 ppmv, sea
level rise: −12/−4 ppmv, and rising terrestrial carbon storage:
−13/−6 ppmv. According to our model the smaller interglacial CO<sub>2</sub>
values in the pre-Vostok period prior to Termination V are mainly caused by
smaller interglacial Southern Ocean SST and an Atlantic THC which stayed
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