Coupled climate-carbon cycle simulation of the Last Glacial Maximum atmospheric CO₂ decrease using a large ensemble of modern plausible parameter sets

During the Last Glacial Maximum (LGM), atmospheric CO2 was around 90 ppmv lower than during the preindustrial period. Despite years of research, however, the exact mechanisms leading to the glacial atmospheric CO2 drop are still not entirely understood. Here, a large (471-member) ensemble of GENIE-1 simulations is used to simulate the equilibrium LGM minus preindustrial atmospheric CO2 concentration difference (ΔCO2). The ensemble has previously been weakly constrained with modern observations and was designed to allow for a wide range of large-scale feedback response strengths. Out of the 471 simulations, 315 complete without evidence of numerical instability, and with a ΔCO2 that centres around −20 ppmv. Roughly a quarter of the 315 runs predict a more significant atmospheric CO2 drop, between ~ 30 and 90 ppmv. This range captures the error in the model's process representations and the impact of processes which may be important for ΔCO2 but are not included in the model. These runs jointly constitute what we refer to as the plausible glacial atmospheric CO2 change-filtered (PGACF) ensemble. Our analyses suggest that decreasing LGM atmospheric CO2 tends to be associated with decreasing 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, an increasing terrestrial biosphere carbon inventory and an increasing deep-sea CaCO3 burial flux. The increases in terrestrial biosphere carbon are predominantly due to our choice to preserve rather than destroy carbon in ice sheet areas. However, the ensemble soil respiration also tends to decrease significantly more than net photosynthesis, resulting in relatively large increases in non-burial carbon. In a majority of simulations, the terrestrial biosphere carbon increases are also accompanied by decreases in ocean carbon and increases in lithospheric carbon. In total, however, we find there are 5 different ways of achieving a plausible ΔCO2 in terms of the sign of individual carbon reservoir changes. The PGACF ensemble members also predict both positive and negative changes in global particulate organic carbon (POC) flux, AMOC and AABW cell strengths, and global CaCO3 burial flux. Comparison of the PGACF ensemble results against observations suggests that the simulated LGM physical climate and biogeochemical changes are mostly of the right sign and magnitude or within the range of observational error, except for the change in global deep-sea CaCO3 burial flux – which tends to be overestimated. We note that changing CaCO3 weathering flux is a variable parameter (included to account for variation in both the CaCO3 weathering rate and the un-modelled CaCO3 shallow water deposition flux), and this parameter is strongly associated with changes in global CaCO3 burial rate. The increasing terrestrial carbon inventory is also likely to have contributed to the LGM increase in deep-sea CaCO3 burial flux via the process of carbonate compensation. However, we do not yet rule out either of these processes as causes of ΔCO2 since missing processes such as Si fertilisation, Si leakage and the effect of decreasing SSTs on CaCO3 production may have introduced a high LGM global CaCO3 burial rate bias. Including these processes would, all else held constant, lower the rain ratio seen by the sediments and result in a decrease in atmospheric CO2 and increase in ocean carbon. Despite not modelling Δ14C(atm (DIC)) and δ13C(atm (DIC)), we also highlight some ways in which our results may potentially be reconciled with these records.