The Holocene Carbon Cycle

Explaining the carbon cycle dynamics (and hence atmospheric CO2) since the last glacial maximum 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). During deglaciation, on the other hand, the magnitude of fluxes is substantially larger, with carbon release from the ocean and the regrowth of vegetation in the formerly glaciated areas contributing. The balance between these fluxes of CO2 is delicate and time-dependent, and it is not possible to provide firm constraints on many of 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, Eemian, and MIS11, we use an earth system model of intermediate complexity, CLIMBER-2, coupled to the dynamic global vegetation model LPJ with a recently implemented module for peatland dynamics. During glacial-interglacial cycles, the carbon cycle never is in complete equilibrium due to a number of small but persistent fluxes such as terrestrial weathering. This complicates setting up interglacial experiments as the usual approach to start model integrations from an equilibrium state is not valid any more. In order to circumvent the problem of non-equilibrium initial conditions, the model is initialised with the oceanic biogeochemistry state taken from a transient simulation through the last glacial cycle with CLIMBER-2 only. In CLIMBER-2, the CO2 release after deglaciation mainly stems from ocean circulation changes and changed marine productivity. Using these initial conditions, we performed coupled climate carbon cycle experiments for the Holocene, as well as previous interglacials, driven by orbital forcing. Contrary to the results we published previously (Kleinen et al., GRL, 2010), peat accumulation was not prescribed, but rather determined dynamically. For the Holocene, our results resemble the carbon cycle dynamics as reconstructed from ice cores quite closely, both for atmospheric CO2 and d13CO2. These experiments will be presented and compared to previous interglacials, analysing the role of different forcing mechanisms.