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The European Project for Ice Coring in Antarctica Dome C ice core enables us to extend existing records of atmospheric methane (CH[sub 4]) and nitrous oxide (N[sub 2]O) back to 650,000 years before the present. A combined record of CH[sub 4] measured along the Dome C and the Vostok ice cores demonstrates, within the resolution of our measurements, that preindustrial concentrations over Antarctica have not exceeded 773 ± 15 ppbv (parts per billion by volume) during the past 650,000 years. Before 420,000 years ago, when interglacials were cooler, maximum CH[sub 4] concentrations were only about 600 ppbv, similar to lower Holocene values. In contrast, the N[sub 2]O record shows maximum concentrations of 278 = 7 ppbv, slightly higher than early Holocene values. [ABSTRACT FROM AUTHOR]

The temperature increase at Vostok (Antarctica) from the last glacial maximum to the present warm period is about 8°C based on the deuterium isotope profile. The bore hole temperature (temperature profile in the ice sheet) indicates that the temperature difference may have been much larger, about 15°C. The temperature dependent gas occlusion process is the key to evaluate the two scenarios. Atmospheric air penetrates the porous firn layer of the ice sheet and gets trapped at the firn ice boundary. Consequently the air is younger than the surrounding ice when it gets enclosed in bubbles. This age difference (Δage) between ice and enclosed gas is temperature and accumulation rate dependent. Therefore it is possible to estimate paleotemperatures from a known Δage. We use the linkage between chronologies of CH4 and water isotopes from Byrd station and Vostok to obtain an experimental Δage for Vostok. This experimental Δage is then compared to modeled Δage for the two temperature scenarios. Our results indicate that the temperature reconstruction deduced from the water isotopic composition is the more probable one. [Copyright &y& Elsevier]

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, CO 2 remained stable during the Eemian while rising in the Holocene. We identify and review twelve biogeochemical mechanisms of terrestrial (vegetation dynamics and CO 2 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 CO 2 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 CO 2 and δ 13 CO 2 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 CO 2 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 CO 2 changes after 122 ka BP. This failure to simulate late-Eemian CO 2 dynamics could be a result of the imposed forcings such as prescribed CaCO 3 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 CO 2 dynamics – shallow water CaCO 3 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 CO 2 dynamics. [ABSTRACT FROM AUTHOR]

Abstract: Typical rapid climate events punctuating the last glacial period in Greenland, Europe and Antarctica are compared to two rapid events occurring under warmer conditions: (i) Dansgaard–Oeschger event 25, the first abrupt warming occurring during last glacial inception; (ii) 8.2 ka BP event, the only rapid cooling recorded during the Holocene in Greenland ice cores and in Ammersee, Germany. The rate of warming during previous warmer interglacial periods is estimated from polar ice cores to 1.5 °C per millennium, without abrupt changes. Climate change expected for the 21st century should however be at least 10 times faster. To cite this article: V. Masson-Delmotte et al., C. R. Geoscience 337 (2005). [Copyright &y& Elsevier]