Abstract
Abstract. Trends in the atmospheric concentration of CO2 during three recent interglacials – the Holocene, the Eemian and Marine Isotope Stage (MIS) 11 – are investigated using an earth system model of intermediate complexity, which we extended with process-based modules to consider two slow carbon cycle processes – peat accumulation and shallow-water CaCO3 sedimentation (coral reef formation). For all three interglacials, model simulations considering peat accumulation and shallow-water CaCO3 sedimentation substantially improve the agreement between model results and ice core CO2 reconstructions in comparison to a carbon cycle set-up neglecting these processes. This enables us to model the trends in atmospheric CO2, with modelled trends similar to the ice core data, forcing the model only with orbital and sea level changes. During the Holocene, anthropogenic CO2 emissions are required to match the observed rise in atmospheric CO2 after 3 ka BP but are not relevant before this time. Our model experiments show a considerable improvement in the modelled CO2 trends by the inclusion of the slow carbon cycle processes, allowing us to explain the CO2 evolution during the Holocene and two recent interglacials consistently using an identical model set-up.
Highlights
The atmospheric concentration of carbon dioxide (CO2) increased from 260 to 280 ppm CO2 during the Holocene between 8 ka BP and the pre-industrial period
In experiment HOL_MPT, atmospheric CO2 increases slightly stronger than in HOL_NAT during the early Holocene and keeps increasing until 2.5 ka BP, after which it stays constant at 273 ppm CO2 (Fig. 2a, cyan line)
The latter process leads to an increase in atmospheric CO2 during periods of constant or slowly rising sea level, while the former process leads to a decrease in atmospheric CO2
Summary
The atmospheric concentration of carbon dioxide (CO2) increased from 260 to 280 ppm CO2 during the Holocene between 8 ka BP and the pre-industrial period. This trend in CO2 has to be seen in the context of previous interglacials, since all processes affecting the atmospheric concentration, with the exception of possible human influences, are likely to have been active during all interglacials. While the Holocene CO2 trend has generated considerable interest previously (Ruddiman, 2003), the context of previous interglacials has been neglected in process-based carbon cycle model studies. Investigations of the Holocene trend in CO2 can be classified into two basic approaches: an inverse modelling approach and a forward or process-based modelling approach. The inverse modelling approach takes the ice core record of CO2 and δ13CO2 as a starting point and aims at deriving the sources and sinks of CO2 from this record, while the forward modelling approach starts from the carbon cycle processes and aims at determining a CO2 trajectory from combinations of these
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