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Abstract:
Changes in temperature and carbon dioxide during glacial cycles recorded in Antarctic ice cores are tightly coupled. However, this relationship does not hold for interglacials. While climate cooled towards
the end of both the last (Eemian) and present (Holocene) interglacials, CO2 remained stable during the
Eemian while rising in the Holocene. We identify and review twelve biogeochemical mechanisms of
terrestrial (vegetation dynamics and CO2 fertilization, land use, wildfire, accumulation of peat, changes in
permafrost carbon, subaerial volcanic outgassing) and marine origin (changes in sea surface temperature,
carbonate compensation to deglaciation and terrestrial biosphere regrowth, shallow-water carbonate
sedimentation, changes in the soft tissue pump, and methane hydrates), which potentially may have
contributed to the CO2 dynamics during interglacials but which remain not well quantified. We use three
Earth System Models (ESMs) of intermediate complexity to compare effects of selected mechanisms on
the interglacial CO2 and d13CO2 changes, focusing on those with substantial potential impacts: namely
carbonate sedimentation in shallow waters, peat growth, and (in the case of the Holocene) human land
use. A set of specified carbon cycle forcings could qualitatively explain atmospheric CO2 dynamics from 8
ka BP to the pre-industrial. However, when applied to Eemian boundary conditions from 126 to 115 ka
BP, the same set of forcings led to disagreement with the observed direction of CO2 changes after 122 ka
BP. This failure to simulate late-Eemian CO2 dynamics could be a result of the imposed forcings such as
prescribed CaCO3 accumulation and/or an incorrect response of simulated terrestrial carbon to the
surface cooling at the end of the interglacial. These experiments also reveal that key natural processes of
interglacial CO2 dynamics e shallow water CaCO3 accumulation, peat and permafrost carbon dynamics -
are not well represented in the current ESMs. Global-scale modeling of these long-term carbon cycle
components started only in the last decade, and uncertainty in parameterization of these mechanisms is a main limitation in the successful modeling of interglacial CO2 dynamics.