Abstract. During the Last Glacial Maximum (LGM), atmospheric
CO2 was around 90 ppmv lower than during the
pre-industrial period. The reasons for this decrease are most often
elucidated through factorial experiments testing the impact of individual
mechanisms. Due to uncertainty in our understanding of the real system,
however, the different models used to conduct the experiments inevitably
take on different parameter values and different structures. In this paper,
the objective is therefore to take an uncertainty-based approach to
investigating the LGM CO2 drop by simulating it
with a large ensemble of parameter sets, designed to allow for a wide range
of large-scale feedback response strengths. Our aim is not to definitely
explain the causes of the CO2 drop but rather
explore the range of possible responses. We find that the LGM
CO2 decrease tends to predominantly be associated
with decreasing sea surface temperatures (SSTs), increasing sea ice area, a
weakening of the Atlantic Meridional Overturning Circulation (AMOC), a
strengthening of the Antarctic Bottom Water (AABW) cell in the Atlantic
Ocean, a decreasing ocean biological productivity, an increasing
CaCO3 weathering flux and an increasing deep-sea
CaCO3 burial flux. The majority of our simulations
also predict an increase in terrestrial carbon, coupled with a decrease in
ocean and increase in lithospheric carbon. We attribute the increase in
terrestrial carbon to a slower soil respiration rate, as well as the
preservation rather than destruction of carbon by the LGM ice sheets. An
initial comparison of these dominant changes with observations and
paleoproxies other than carbon isotope and oxygen data (not evaluated
directly in this study) suggests broad agreement. However, we advise more
detailed comparisons in the future, and also note that, conceptually at
least, our results can only be reconciled with carbon isotope and oxygen
data if additional processes not included in our model are brought into
play.
Coupled climate–carbon cycle simulation of the Last Glacial Maximum atmospheric CO<sub>2</sub> decrease using a large ensemble of modern plausible parameter sets