Carbon cycle dynamics during interglacials

Explaining a difference in atmospheric CO2 dynamics among interglacials is an elusive issue. Several biogeochemical mechanisms of different origin are involved in interglacial CO2 dynamics leading to a CO2 release from the ocean (carbonate compensation, coral growth) compensated by a land carbon uptake (biomass and soil carbon buildup, peat accumulation). The balance between these fluxes of CO2 is delicate and time-dependent, and it is not possible to provide firm constraints on these fluxes from proxy data. The best framework for quantification of all these mechanisms is an Earth System model that includes all necessary physical and biogeochemical components of the atmosphere, ocean, and land. To perform multi-millennial model integrations through the Holocene and Eemian, we use an intermediate complexity climate model, CLIMBER-2, coupled to the LPJ DGVM model with recently implemented boreal peatland module. The global carbon cycle is never in complete equilibrium during the glacial cycles due to changes in small but persistent fluxes such as terrestrial weathering. This complicates setting up the interglacial runs as the usual approach to start model integration from equilibrium state is not valid anymore. To by-pass this problem of non-equilibrium initial conditions, the model is initialized with the oceanic biogeochemistry state taken from a transient CLIMBER-2 simulation through the last glacial cycle. In this simulation, the CLIMBER-2 model was run through the last glacial cycle with carbon cycle in “offline mode” as interactive components of the physical climate system (atmosphere, ocean, ice sheets) were driven by concentration of greenhouse gases reconstructed from ice cores. In response to simulated climate change, the carbon cycle model was able to reproduce the main features of glacial CO2 dynamics reconstructed from ice cores. Results of the CLIMBER-LPJ model integrations through the Holocene and Eemian interglacials in terms of climate changes and atmospheric CO2 and d13CO2 dynamics will be presented.