Climate-carbon cycle feedback during glacial cycles

Paleoclimate records reveal a close link between global ice
volume and atmospheric CO2 concentration, at least, through
the last 800,000 years. Despite many efforts over the last two
decades, mechanisms of glacial-interglacial CO2 variability
and its role for the glacial cycles remain elusive. Here using
the Earth system model of intermediate complexity
CLIMBER-2 which includes all major components of the
Earth system – atmosphere, ocean, land surface, ice sheets,
terrestrial biota, eolian dust and marine biogeochemistry – we
performed simulations of the last glacial cycles employing
variations in the Earth’s orbital parameters as the only
prescribed climatic forcing.
In the experiments with constant CO2 concentration,
temporal dynamics of the simulated glacial cycles strongly
depend on the CO2 level. For CO2 concentrations about and
above preindustrial one, the model simulates only short glacial
cycles with precessional and obliquity frequencies. However,
for lower CO2 concentrations the model simulates long glacial
cycles with dominant 100 kyr periodicity. Simulated glacial
cycles agreed favorably with paleoclimate reconstructions, but
their amplitude is underestimated compared to those of the
simulations with time-dependent CO2 concentration. These
results confirm that the positive climate-carbon cycle feedback
plays an important role in amplification of long glacial cycles.
Experiments with fully interactive CO2 shed some light on the
mechanism of climate-carbon cycle feedback during glacial
cycles. Forced by orbital variations only, the model is able to
reproduce the main features of CO2 changes: the 40 ppmv CO2
drop during glacial inception, the minimum concentration at
the last glacial maximum being 80 ppmv lower than the
Holocene value, and the relatively abrupt CO2 rise during the
deglaciation. The main drivers of atmospheric CO2 evolve
with time: changes in sea surface temperature and volume of
bottom water of southern origin exert CO2 control during
glacial inception and deglaciation, while changes in carbonate
chemistry and marine biology are dominant during the first
and second parts of the glacial cycles, respectively.