ALBERT

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.

Description: The carbonate chemistry of the world’s oceans, including their pH, has been remarkably constant for hundreds of thousands of years (Pearson and Palmer, 2000), with typical surface ocean variations between ice ages and warm phases of no more than 0.2 pH units ([Sanyal et al., 1995], [Hönisch and Hemming, 2005] and [Foster, 2008]). However, since the beginning of the industrial revolution, the oceans have taken up approximately 30% of the CO2 produced from fossil fuel burning, cement manufacture and land use changes (Sabine et al., 2004). While the invasion of CO2 into the ocean removes this greenhouse gas from the atmosphere and thereby dampens global warming, it forms carbonic acid in seawater and lowers ambient surface ocean pH (Broecker and Peng, 1982). Ocean acidification is the direct consequence of the excessive addition of CO2 to seawater (Broecker and Takahashi, 1977) and is therefore inherently more predictable than temperature and precipitation changes due to rising CO2 in the atmosphere. Changes are already measurable today ([Bates, 2001], [Bates et al., 2002], [Takahashi et al., 2003], [Keeling et al., 2004] and [Santana-Casiano et al., 2007]) and will become more pronounced as humankind emits more CO2 into the atmosphere, with surface ocean pH expected to decline by a further 0.3 pH units by the end of the century, corresponding to an approximately 100% increase in ocean acidity (hydrogen ion concentration [H+]), on top of the not, vert, similar0.1 pH unit decline to date ([Caldeira and Wickett, 2003], [Orr et al., 2005] and Solomon et al., 2007 In: S. Solomon et al., Editors, Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Fourth Assessment Report of the IPCC, Cambridge University Press, Cambridge (2007).[Solomon et al., 2007]) (Fig. 1). Such a rapid change in ocean pH has very likely not happened since the time the dinosaurs went extinct 65 million years ago ([van der Burgh et al., 1993], [Pearson and Palmer, 2000] and [Pagani et al., 2005]). While the dissolution of carbonate sediments on the bottom of the ocean and the weathering of rocks on land coupled with mixing of surface and deeper waters will eventually restore ocean pH to its pre-industrial state, this process will take up to a million years to complete ([Archer, 2005] and [Ridgwell and Zeebe, 2005]).