Description: The analysis of air bubbles trapped in polar ice permits the reconstruction of the evo-lution of major greenhouse gases over various timescales. This study leans on thepast behaviour of the most important human-induced greenhouse gas, carbon dioxide(CO2). The past origin of CO2 is better comprehended when studying concomitantlythe evolution of its stable carbon isotope composition, as it is affected by various frac-tionation processes in and between carbon reservoirs.The LGGE dry extraction method of gases occluded in ice was used in combinationwith a new instrumental setup to investigate the CO2 mixing ratio and its stable car-bon isotope composition (delta13CO2) in air from the last deglaciation at the EPICADome Concordia site (Antarctica). The resolution of our results (250 years in average)allows us to divide Termination I (TI) into four sub-periods, each representing differ-ent climatic features at the Earth surface (Heinrich I, Bølling/Ållerød, Antarctic ColdReversal, Younger Dryas). We observe that CO2 and delta13CO2 are not correlated.Delta13CO2 shows positive and negative excursions associated with changes in thegrowth rate of atmospheric CO2. This illustrates the dynamic character of the carboncycle and its coupling to climate change during the deglaciation. The use of two car-bon cycle box models highlight oceanic mechanisms as the major contributors to theCO2 evolution during these periods of TI, and the terrestrial biosphere for the warmBølling/Ållerød event.We will also present pioneering delta13CO2 data obtained in the course of the penul-timate deglaciation (TII); this is expected to bring some more light in the carbon cyclequestion during glacial-interglacial transitions although the existing challenge on icephysics (clathrate ice for TII vs bubbly ice for TI) should not be neglected.

Description: the onset of the African Humid Period (AHP) 14,500 to 11,000 years ago. Our analysis suggests that locally increased boreal summer solar radiation leads to a northward shift of the ITCZ. Vegetation in northern Africa adjusts to wetter conditions and provides a positive albedo-vegetation feedback that is further accelerating the northward migration of the ITCZ. Sensitivity experiments reveal that the albedo-vegetation feedback and its effect on the large-scale atmospheric circulation together with the CO2-fertilization effect provide a mechanism for generating multiple states of early to mid-Holocene vegetation over northern Africa. Our model simulations document that not only orbitally-driven insolation changes played a key role in controlling the onset of the AHP, but also the presence of the remnant ice-sheet over Europe, variations of the Atlantic Meridional Overturning circulation during the Younger Dryas and increasing levels atmospheric CO2. The model results presented here do not lend support to the notion that simple insolation thresholds govern the abrupt transitions of north African vegetation during the early to middle Holocene.

Description: The underlying causes for Earths temperature variations during glacial cycles observed in the late Pleistocene are still not fully understood. To contribute to this understanding we here compile various climate records in order to calculate the direct effect of various processes on Earths radiative budget and thus on global annual mean near surface temperature over the last 800,000 years. The importance of orbital variations, of the greenhouse gases CO2, CH4 and N2O, of the albedo of land ice sheets, sea ice area and vegetation, and of the radiative perturbation of mineral dust in the atmosphere are investigated in detail, while potential impacts from variable solar activity are only briefly discussed. Altogether we can explain with these processes a global cooling of 3 K for the Last Glacial Maximum directly out of theradiative budget. These are about two thirds of the reconstructed temperature anomaly and thus a significant improvement to earlier estimates. Increased temperatures during warmer than Holocene interglacials in Marine Isotope Stages 5.5, 7.5, 9.3 and 11.3 as recorded in Antarctic ice cores are largely unexplained. Other processes and feedbacks in the climate system are necessary to explain the still existing bias between temperature reconstructionsand our compilation.

Description: Paleo-environmental records and extensive modeling studies have demonstrated thatthe Sahara was largely covered by grass and steppe vegetation in the early to midHolocene. The orbitally controlled incoming summer insolation is the primary forcingfactor during the Holocene. It is well-documented that internal feedback-mechanismsbetween the vegetation and the atmosphere-ocean system caused a sudden shift fromthe vegetated humid Sahara state to a arid desert climate about 50004000 years ago.Proxy evidence suggests also an abrupt onset of the African Humid Period between14,000 and 11,000 yr BP. However, the attribution of the rapid onset to orbitally driveninsolation anomalies or to the Bølling-Allerød, Younger- Dryas transitions is non-trivial. Here we show in transient simulations with climate and vegetation modelsof different complexity that the abrupt change of the African Monsoon/vegetationsystem from dry/deserted glacial state to wet/green conditions is accelerated by thevegetation-albedo feedback. The non-linear response of the climate-vegetation sys-tem to precessional forcing leads to a rapid onset of the African Humid Period at∼11,000 yr BP.