ALBERT

Description: Results of two studies of the effect of changing CH4 fluxes on global tropospheric oxidant levels, O3, OH, and H2O2, performed with a multibox photochemical model, are presented. A sensitivity study is conducted by scaling back CH4, CO, and NO emissions relative to the present-day budget. When the CH4 ice core record is compared to calculated CH4 abundances, corresponding CH4 fluxes for the preindustrial Holocene (PIH) and Last Glacial Maximum (LGM) are fairly well-constrained: 175-225 Tg CH/4/yr for PIH and 100-130 Tg CH4/yr for LGM. Specific scenarios for CH4/CO/NO are selected to represent sources for the PIH and LGM. The CH4 budget is taken from an evaluation of wetlands and other natural sources.

Description: An estimate of the distribution of wetland area and associated CH4 emission is presented for the Last Glacial Maximum (LGM, 18 kyr BP, kiloyear Before Present) and the Pre-Industrial Holocene (PIH, 9000-200 years BP). The wetland source, combined with estimates of the other biogenic sources and sink, yields total source strengths of 120 and 180 Tg CH4/yr for LGM and PIH respectively. These source strengths are shown to be consistent with source estimates inferred from a photochemical model, and point to changes in wetland CH4 source as a major factor driving the atmospheric CH4 increase from LGM to PIH.

Description: Highlights: • We provide comprehensive discussion of carbon cycle forcings in interglacials. • We compare transient simulations of climate-carbon cycle models through Holocene and Eemian interglacials. • We synthesyze role of forcings in previous and current study in one summary figure. 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 δ13CO2 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 – 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.