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Observation‐based and modeling studies have identified the Eastern Mediterranean and Middle East (EMME) region as a prominent climate change hotspot. While several initiatives have addressed the impacts of climate change in parts of the EMME, here we present an updated assessment, covering a wide range of timescales, phenomena and future pathways. Our assessment is based on a revised analysis of recent observations and projections and an extensive overview of the recent scientific literature on the causes and effects of regional climate change. Greenhouse gas emissions in the EMME are growing rapidly, surpassing those of the European Union, hence contributing significantly to climate change. Over the past half‐century and especially during recent decades, the EMME has warmed significantly faster than other inhabited regions. At the same time, changes in the hydrological cycle have become evident. The observed recent temperature increase of about 0.45°C per decade is projected to continue, although strong global greenhouse gas emission reductions could moderate this trend. In addition to projected changes in mean climate conditions, we call attention to extreme weather events with potentially disruptive societal impacts. These include the strongly increasing severity and duration of heatwaves, droughts and dust storms, as well as torrential rain events that can trigger flash floods. Our review is complemented by a discussion of atmospheric pollution and land‐use change in the region, including urbanization, desertification and forest fires. Finally, we identify sectors that may be critically affected and formulate adaptation and research recommendations toward greater resilience of the EMME region to climate change., Key Points: The Eastern Mediterranean and Middle East is warming almost two times faster than the global average and other inhabited parts of the world. Climate projections indicate a future warming, strongest in summers. Precipitation will likely decrease, particularly in the Mediterranean. Virtually all socio‐economic sectors will be critically affected by the projected changes., European Union Horizon 2020, https://esg-dn1.nsc.liu.se/search/esgf-liu/

Fruit d’une collaboration entre paléoclimatologues et experts de l’information scientifique, le thésaurus de paléoclimatologie décrit et structure pour la première fois près de 2000 concepts de la discipline, en français et en anglais. Destiné à offrir à la communauté scientifique un vocabulaire de référence, il facilite le partage des données de recherche et des productions scientifiques en paléoclimatologie.

International audience; The previsibility of Northern Africa dust events is quantified using daily numerical forecast simulations for the next three days. The dust concentrations fields, modeled by CHIMERE-DUST, are first evaluate by comparisons to AERONET surface data, OMI and MSG Seviri satellite measurements. The accuracy and spread between measurements and model are discussed focussing on the first short observation period of the AMMA experiment in western Africa, with a simulation period ranging from January to March 2006. Second, the previsibility of dust is estimated by comparing model results for different leads in a forecats mode. The model skill is evaluate in term of capability to forecast (i) the surface wind speed (the key process for dust emissions), (ii) the dust emissions (depending on the wind speed as well as numerous others uncertain parameters, including threshold values on the friction velocity) and (iii) the transport of aerosols from source to remote areas (depending of horizontal transport, convection etc.). It is shown that emissions forecast can vary up to 80% (close to the sources) and that final forecasted dust concentrations and relative optical thickness do not exceed 40% and 20% in forecast variability.

International audience; The precise documentation of the natural variability of the THC during the interglacials in North Atlantic is of critical interest for the understanding of its future evolution under the effect of global warming. It is one of the goals of the AMOCINT project (06-EuroMARC-FP-008). We report on detailed magnetic analyses of a Holocene marine sequence taken at the Charlie-Gibbs fracture zone (52°S), on the path of the contourite drift at the place where it turns around the Reykjanes ridge, from the Gardar drift northward. This location has been sampled twice. The first core (gravity) has been taken in 1977 with the R.V. J. Charcot from IFREMER. The second one (Casq core) has been taken during the IMAGES MD168-AMOCINT cruise on board the R. V. Marion Dufresne in 2008 (IPEV). In the first core, the apparent sedimentation rate varies between 50 and 79 cm/kyr and it obviously results from compaction as the second one known to be free of any distortion has sedimentation rates of the order of 50 to 140 cm/kyr between present and 10 kyr B.P. The two cores can be perfectly correlated using any physical property allowing to precisely transfer the 19 radiocarbon dates from the old series to the new one. Detailed magnetic analyses have been performed on both cores and will be compared. The magnetic fraction is uniformly composed during the Holocene period of magnetites with a grain size distribution in the pseudo-single domain range (a few micrometers). The magnetic concentration varies in time on both long and short terms. The long-term trends may be fitted by a 4th order polynomial with an increase of the bottom circulation strength in the mid-Holocene. We shall discuss these long term trends together with the short term features in terms of changes in the strength of the bottom current through Holocene and compare them to other proxies an other records also obtained from North Atlantic.

International audience; It is now widely recognised that the slow variations of the Earth's orbital parameters have driven the long term climate fluctuations. The orbital forcing is amplified by various feedbacks from the ocean, the cryosphere and the vegetation that are not yet fully understood. Simulations performed as part of the Paleoclimate Modeling Intercomparison Project offer the possibility to analyse how these feedbacks enhance the climate response to the insolation forcing and to test against available proxy data the realism of the simulated climate. This will be illustrated considering the Afro-Asian monsoon, for which the change in the hydrological cycle remain uncertain in future climate projections. The comparison of simulations for different climatic periods across the Eemien and the Holocene shows that changes in seasonality induced by the Earth's parameters alter differently the Indian and the African monsoon. This stresses the role of the seasonal phasing between the climate forcing and the climate phenomenon considered. The magnitude and the climatic impact of the ENSO phenomenon are also affected by the insolation forcing. Simulations of the Holocene show that ENSO strengthens across the Holocene, as suggested by coral data or lake sediments. For the mid-Holocene, model results are quite consistent. This is not the case for the Last Glacial Maximum for which there is no consensus between model results as it is the case for future climate. These examples illustrates how the analyses of past climates help to better understand the relationship between climate forcings, climate feedbacks and climate phenomenon operating at seasonal or interannual time scales.

International audience; This article was submitted without an abstract, please refer to the full-text PDF file.

International audience; Cropland ecosystem is one of the most uncertain components of the terrestrial Carbon budget at European scale. In this context, a dual effort has been initiated within the CarboEurope-IP project named as the Cropland Synthesis Activity that aims 1) to monitor and analyse C flux over crop sites and 2) to develop and refine crop model's simulations from site to continental scale. Here, we will present the results obtained with the ORCHIDEE-STICS model within the frame of this activity. In this coupled model, the agronomy-oriented model STICS provides its calculated Leaf Area Index (LAI) to the dynamic global vegetation model ORCHIDEE in order to better calculate the Growth Primary Production (GPP) and related C fluxes for crop ecosystem. With this model we performed simulations on several wheat and maize sites. Overall, ORCHIDEE-STICS agrees well with the observations especially for wheat. We will assess from the misfit between model and data, where structural improvements of the model are needed, and what is the beneficial effect of these improvements. Four main forms of model improvements will be tested : i) optimal adjustment of generic physiological parameters of ORCHIDEE, ii) adjustment of agricultural technology, iii) adjustment of crop varieties phenological parameters and iv) realistic reproduction of each site cultivation history. All this information being rather uncertain at the European scale, this assessment is particularly of importance and will help to rank priorities for future model's developments. The model data comparison will focus on NEE, GPP, Reco fluxes, but also involve latent and sensible heat flux observation.

International audience; Two mobile lidar instruments of new generation were installed on the Reunion Island in the frame of the ECLAIR (Expérience sur la Couche Limite Atmosphérique à l'Ile de la Réunion) field experiment held from 26 November to 5 December 2008. The first one was installed on a pick-up to investigate the structure and temporal evolution of the planetary boundary layer around and inside the island. The second one was located on the site of the University of Reunion Island. This study is a new approach preliminary to the development of an atmospheric observatory, OPAR, located at Piton Maïdo, a high summit (2200m) near the west coast. The field campaign aimed at understanding to what extent, and when, the future observatory will be under the influence of the boundary layer, clouds, and the upward transport of low-level air masses. Results of lidar observation periods will be presented and discussed in relation to high-resolution model outputs. For instance, the lidar instruments evidence the presence of complex vertical structures of the low troposphere above Reunion Island. Such structures remain even inside the island both in the corries and on the mountains. Moreover, some hydraulic character of the wind trade flow around the island could be highlighted from lidar measurements. Indeed, the Venturi acceleration on each side of the obstacle is accompanied by subsidence and a significant shallowing of the boundary layer along the flow.

International audience; Accurate and rapid determination of near surface wind fields in a complex area (orography, inhomogeneous surface properties) is a challenge for applications like the evaluation of wind energy production, the prediction of pollution transport and hazardous conditions for aeronautics and ship navigation, among others. This work presents a statistical downscaling approach for regional near surface wind field in the region of southern France (characterized by the presence of major mountain ranges). It is based on generalized additive models (GAM, Salameh et al. 2008), relating large scale upper air to local scale surface atmospheric fields. We apply our statistical downscaling model conditionally on regional circulation patterns defined from measurements. Hence, near-surface wind components in southern France are simulated based on large-scale information from ERA40 reanalyses (1991 to 2001) and from IPCC scenarios (1991 to 2001 and 2040 to 2050). The performances of our method are evaluated, (1) by comparing downscaled wind from ERA40 and from IPCC, and (2) by comparing them with measurements, for the period 1991-2001. Then, we evaluate the change in regional atmospheric circulations in southern France, by comparing future and present downscaled wind.

International audience; The composition of the Siberian troposphere remains highly unknown due to a lack of measurements in this area. Siberia is a key region for a quantified understanding of many land-atmosphere exchange processes. As an example, Siberian forest fire emissions are a major extratropical source of CO to the atmosphere. Fire-emitted trace gases and particles are subject to long-range transport and may contribute to pollution of nearby Arctic. However, establishing precise top-down estimates of sources strengths based on satellite or surface network measurements for species such as CO is limited by models' ability to represent sub-grid-scale dynamics associated to the wildfire (pyroconvection) and the injection height of the plume. In an experimental effort to address this issue and to increase our knowledge of the properties of the Siberian troposphere, CO, O3, CO2 and fine particles were measured onboard a research aircraft in the frame of the YAK-AEROSIB project, partially as a contribution to the Summer 2008 POLARCAT programme. Two large scale transects were established over Northern and Central Siberia between 7 and 21 July 2008. The aircraft flight pattern consisted of ramp ascents and descents so as to sample as many vertical profiles as possible. Very high CO concentrations were observed at various altitudes, essentially in Eastern Siberia near Yakutsk and Chokurdakh. The highest concentrations (up to 600ppb) were observed between 2 and 5 km (flight ceiling being at 7km) in very thin layers (few hundreds of m thick). A Lagrangian modelling analysis (FLEXPART) revealed that the aircraft sampled fire plumes from regional fire emissions, east of Yakutsk, after about 2 days of transport. The observed fire plumes are also characterized by anomalies in O3 and excess particle concentrations. These data provide new constraints on our understanding of forest fire plume transport. They also constitute a critical testbench for the models used to assess pyrogenic emissions and to predict transport of pollution to the Arctic and at the global scale.

International audience; Reconstructions of sea surface temperature (SST) from relatively stable periods in the past, such as the Last Glacial Maximum (LGM), represent one of the best means to constrain climate sensitivity and provide targets to evaluate coupled atmosphere-ocean general circulation models. The first quantitative global reconstruction of SST at the LGM was developed by the CLIMAP (Climate: Long-Range Investigation, Mapping and Prediction) project. Since then, there has not been any concerted effort to synthesize existing paleodata at the global scale, although several shortcomings of CLIMAP pioneering work have become apparent.We present a LGM global synthesis of SST reconstructions undertaken by the MARGO (Multiproxy Approach for the Reconstruction of the Glacial Ocean Surface) project. The objective has been to compile and analyse available estimates of LGM SSTs based on all prevalent microfossil-based (i.e., transfer functions based on planktonic foraminifera, diatoms, dinoflagellate cysts and radiolarians abundances) and geochemical (i.e., alkenones and planktonic foraminifera Mg/Ca) paleothermometers. The MARGO project approach is to argue that no current proxy method is objectively better than another to provide an accurate picture of past SST, and that consequently the multiproxy approach yields the least biased representation of past reality. As originally suggested by CLIMAP, the strongest annual mean cooling (up to -10Ë C) occurred in the mid-latitude North Atlantic and extended into the western Mediterranean (-6Ë C). However, in contrast to CLIMAP, MARGO data indicate that the cooling was more pronounced in the eastern than in the western basin. The magnitude and position of a steep temperature gradient between 60 and 45Ë N are supported by four different proxies. In contrast with the CLIMAP reconstruction, all proxies also agree on ice-free conditions in the Nordic Seas during glacial summer. However, large discrepancies with respect to glacial temperatures recorded by different microfossil proxies remain. The best convergence between the various proxy estimates occurs within the 30Ë N to 30Ë S band. Strong inter-basins differences as well as clear west-east gradients within each basin mark the equatorial oceans, although anomalies are smaller in the Pacific and Indian Ocean than in the Atlantic. Tropical cooling is more extensive than that proposed by CLIMAP. Large cooling of the Eastern Boundary Current (EBC) systems in the Southern hemisphere is reconstructed by all proxies, making this a very robust feature of the climate and ocean circulation during the LGM. Existing coupled atmosphere-ocean general circulation models (AO-GCMs) simulations for the LGM show significant disagreement with respect to the location and magnitude of the North Atlantic cold anomaly while exhibiting stronger glacial cooling in the western than in the eastern Atlantic (http://pmip2.lsce.ipsl.fr/). This demonstrates that the robust MARGO North Atlantic East-West SST anomaly gradient is a good target with which the skill of models can be evaluated. With the advent of the multi-proxy method, we have not only been able to produce a new reconstruction of the glacial ocean surface, but also to deliver uncertainty estimates. Taken together, this yields new observational bounds on the sensitivity of Earth's climate system, with the perspective of improving existing climate models that are being used in the assessment of ongoing and future climate change.

International audience; Understanding the relationship between the large scale variables from the Global Climatological Model (GCM) and local observations is a challenge for applications like the prediction of extremes natural hazards, of the wind energy production and among others. This study presents a statistical downscaling approach which provide local cumulative distribution functions (CDFs) of surface climate variables from large scale fields. This method is based on the probabilistic downscaling method and suggests there exits a transformation T allowing us to transform the CDFs of a GCM simulated variable into the CDF local-scale climate variable. The first step of this study is the calibration of the T transformation of the observed, from several meteorological stations located in the South of France, and ECMWF ERA-40 reanalysis wind and precipitation CDFs during the 1981-1990 period. Then, we validate this transformation with the comparison between the downscaling CDF provides by the transformations and the observed CDF during the 1991-2000 period. The same T transformation is also used with the IPCC GCM during the same period. This comparison allows us to evaluating the use of this method to predict CDFs of wind and precipitation for IPCC scenarii in the context of global change.

International audience; Studying the climate of the last millennium allows replacing the present climate change in a long term context. Since it is a relatively well-documented period, it provides an interesting base to assess the secular variability of the climate, free of anthropogenic greenhouse gas influence. Considering this, the climate of the last millennium is likely to have been driven by natural forcings, such as major volcanic eruptions or solar variability. We present here the results of the simulations performed with the IPSLCM4v2 climate model for the French ANR ESCARSEL project (reconstruction of the climate of the last millennium). In order to understand the role of the solar variability during this period, we have forced the model with a reconstruction of the Total Solar Irradiance since 1000AD (Crowley et al., 2000). The results are compared with various reconstructions based on proxy data, from the hemispheric to the continental scale. A new reconstruction of the temperature in Europe since 600AD (annual April to September mean, based on tree rings data) has been achieved within the ESCARSEL project. This dataset provides the possibility to compare the spatial response of the model to the solar forcing with the corresponding temperature patterns recorded in the proxys. As a first step we present the results on the long term variability, before focusing on selected periods to assess the spatial behaviour of the model to different value of the total solar irradiance. Crowley et al. 2000, Causes of climate change over the past 1000yrs, Science 289, 270

International audience; We present new benthic isotopic data from core MD07-3076 retrieved in the Atlantic sector of the Southern Ocean (44°09’S, 14°13’W, 3770 m water depth), and place them in the context of well-dated published Atlantic benthic foraminifera isotopic records covering the last 30 ky. Dating of core MD07-3076 was achieved by a combination of 14C AMS measurements on planktonic foraminifera and correlation of sea surface temperature signals derived from both planktonic foraminifera Mg/Ca and census counts, with Antarctic ice isotopic records (Skinner et al., submitted). Comparison of benthic isotopic records from various depths in the North and South Atlantic reveals that circulation changes over the last deglaciation did not take place simultaneously in the 1000-2000 m and in the 3000-4500 m depth ranges. Circulation changes first occurred at lower depth, causing large and relatively rapid changes in benthic δ18O and δ13C at the beginning of Heinrich Stadial 1 (HS1) and the Younger Dryas. Below 3000 m depth, North Atlantic deep water hydrology changed only gradually until a large increase in deep water ventilation took place, resulting from the resumption of North Atlantic Deep Water formation at the end of HS1. In contrast, our deep South Atlantic record indicates that Circumpolar Deep Water around 3800 m depth remained quasi-isolated from northern water masses until the end of HS1. Furthermore, our record shows that core MD07-3076 site was then flushed with better ventilated waters for a few hundred years from ~14.5 to 14 calendar ky BP, before benthic δ18O and δ13C values resumed their progression towards Holocene levels. In conclusion, this set of well-dated Atlantic records demonstrates that benthic δ18O records followed different time evolutions across the last deglaciation, depending on the site latitude and water depth, so that benthic δ18O can not be used as a global correlation tool with a precision better than 3 ky.

International audience; Within the framework of EPOCA (European Project on OCean Acidification ) and the French INSU project PHARE, we are adapting the boron isotope technique to ancient corals with the scope to reconstruct “past” ocean pH changes. In this study, we applied the technique to surface seawater pH reconstructions based on tropical 20th century corals from Fiji. Models estimated a pH drop close to 0.07 pH units in the South Western Equatorial Pacific since the onset of the industrial era (Sabine et al., 2004). To reconstruct such a change in pH, the isotopic composition of boron (δ11B) in coral material has to be determined with a precision better than ±0.2‰. This analytical criteria was meet on a Multi-Collector ICPMS Neptune. We selected a Porites coral for the reconstruction of the time dependent evolution of pH. Our results show a progressive decrease of seawater pH between 1900 and 2000 of 0.08 +/- 0.02 pH units. This decrease in pH agrees with projections of surface ocean pH for the Fiji area obtained with the biogeochemical ocean circulation model NEMO-PISCES. Our results further reveal that seawater pH changes in the Fiji area are strongly affected by regional processes such as the South Pacific Convergence Zone (SPCZ) tightly linked the Pacific Decadal Oscillation (PDO). This last observation highlights the potential of the δ11B-pH technique for studying past changes of ocean dynamics. Hönisch, B., Hemming, N. G., Grottoli, A. G., Amat, A., Hanson, G. N. & Bijma, J. (2004). Assessing scleractinian corals as recoders for paleo-pH: Empirical calibration and vital effects. Geochimica et Cosmochimica Acta, 68(18), 3675-3685. Sabine, C. L., Feely, R. A., Gruber, N., Key, R. M., Lee, K., Bullister, J. L., Wanninkhof, R., Wong, C. S., Wallace, D. W. R., Tilbrook, B., Millero, F. J., Peng, T. H., Kozyr, A., Ono, T. & Rios, A. F. (2004). The Oceanic Sink for Anthropogenic CO2. Science, 305, 367-371.

International audience; A hydrological view to the Ounianga lake region (NE Chad) evolution during the recent Holocene period is proposed. It relies upon multidisciplinary inputs combining hydrological characterization and modeling, results from climate modeling and from the recent sedimentary core analysis by Kroepelin et al. 2008 from lake Yoa. The latter shows that 4 major phases in drying have been observed in this Saharan region over the Holocene, including, (1) disappearance of tropical and altitude pollen species by 4300 cal yr BP, (2) evolution of the lake from a freshwater habitat to the present day hyper-saline oasis (around 3900 cal yr BP), (3) establishment of today's terrestrial desert ecosystem as the result of continuous vegetation succession (5600 to 2700 cal yr BP), (4) evolution in regional wind regime with onsets of major dust mobilization at roughly 4300 cal yr BP and establishment of modern, near continuous northeasterly winds by 2700 cal yr BP. The overall project strategy consists in studying the drying of the region and identifying the controlling parameters from a drastic climate change evolution, from humid (traces of tropical pollen types) to present day hyper arid environment. The interplay between climate change, vegetation, hydrological inputs from distant mountains and a regional aquifer makes this zone a complex and interesting system to study the impact of past climate changes. In a first step of the hydrological study, analysis of the topography of the zone (ArcGis with SRTM data and radar images) shows that the water catchment area of lake Yoa is large (67000 km², including the Erdi plateaux and part of the Tibesti mountain range, ie 18600 km²). A connection with the Tibesti mountains accounts for the altitude pollen source present in the core before 4300 cal yr BP. Globally, the lake is situated at a very favorable position that probably benefited from relatively large and perennial water inputs associated with distant recharge from the Tibesti massif and groundwater from the Nubian Sandstone Aquifer. In a second step, a groundwater model (Cast3M code) is developed for the SW part of the Nubian Sandstone Aquifer to quantify the slow discharge to the lake from a humid period (African Humid Period, 5000 BP roughly) until now. It is forced by precipitation and evaporation issued from climate simulations. Resulting piezometric head fields are compared with present piezometric levels. Lake levels are simulated as resulting from groundwater inputs and routing of surface water for the precipitation time history considered. The reconstruction is confronted to the evolution of salinity issued from the analysis by Kroepelin et al. 2008. Addressed herein are the issues of the level of confidence / uncertainty associated with the reconstruction of the drying phase, the identification of thresholds in the hydrological system influencing the change in the vegetation, the impact of future climate change on this system, lessons learned from the study of the paleo system.

International audience; An increasing number of tropical and subtropical ice cores has been extracted along the Andes of South America. At higher latitudes, numerous ice core studies have been completed in Antarctica. In order to close the gap between subtropical and polar environments in this unique 8000 km long latitudinal ice core records transect, a deep ice core (122 m) has been extracted in 2007 from the San Valentin glacier (3747m, 47 degrees S, 73 degrees W) located in the Northern Patagonian Icefield (Chile). We focus here on the isotopic composition (oxygen 18 and deuterium) of the ice core and its preliminary interpretation. First, we will describe the glaciological aspects of the site, the local climate and the cloud cover from satellite data and various meteorological re-analyses. Second, we will briefly present a preliminary dating for the entire ice core based on a combination of different methods: radionuclide measurements and seasonal isotopic cycles counting from the surface down to around 50 m, the isotopic composition of atmospheric oxygen in air bubbles in the deepest part of the core (lower than 60 m) and glaciological modelisation. Lastly, based on preliminary results from a recent simulation of the LMDZ iso general circulation model including water stable isotopes and nudged by atmospheric reanalyses over the period 1979 2007, along with some target chemical measurements, we will discuss the interannual to decadal isotopic composition variations in terms of local and regional climate.

International audience; Constraints on the atmospheric CO2 deglacial rise based on its δ13CO2 evolution

International audience; The relationship between CO2 variation and Neoproterozoic glaciation has deeply evolved these last years. Through the use of an innovative climate-carbon coupled model, the causes of the CO2 decrease that led to the onset of the global glaciation (Sturtian) has been shown to be strongly related to the dislocation of the Rodinia super continent, promoting CO2 consumption through silicate weathering1. Another important issue is the evolution of atmospheric CO2 during the Snowball episode itself. It appears not to be just a linear accumulation with time through the ongoing solid Earth degassing. Indeed, efficient CO2 diffusion in seawater might have promoted the oceanic crust dissolution, resulting in an asymptotic CO2 rise in the atmosphere2,3, stressing the question of the snowball melting threshold. Indeed, it has been shown that greenhouse climate induced by the storage of the CO2 in the atmosphere invoked to escape a snowball Earth was possibly not sufficient to melt the snowball Earth due to thermal inversion in vertical column4. Therefore, CO2 is may be not the only trigger for the deglaciation. Finally, the super greenhouse climate thought to have followed the snowball episode was explored. We demonstrate that, despite very high temperatures under 0.2 bars of CO2, the amount of rainfall might have been limited by the availability of latent heat which cannot be higher that the total energy provided by the sun. As a consequence of limited increase in the water cycling, CO2 consumption by continental weathering might not exceed 10 times its present day value. The return to normal climatic conditions after the snowball melting should thus have lasted several million of years, further increasing the biological perturbations linked to a snowball event5. The aim of this contribution is to revisit the issue of the role of atmospheric CO2 before, during and after a Snowball Earth and to deliver a new picture of its feedbacks with climate. 1. Donnadieu, Y., Y. Godderis, et al. (2004). "A 'snowball Earth' climate triggered by continental break-up through changes in runoff." Nature 428(6980): 303- 306. 2. Ramstein G., Donnadieu Y., Goddéris Y. Proterozoic glaciations. Comptes Rendus Geoscience 336 (7-8): 639-646 Jun 2004 3. Le Hir G., Goddéris Y., Donnadieu Y., Ramstein G. (2008). A geochemical modelling study of the evolution of the chemical composition of seawater linked to a "snowball" glaciation. Biogeosci. 5, 253-267. 4. Le Hir Guillaume , Goddéris Yves, Donnadieu Yannick , Ramstein Gilles (2008) A scenario for the evolution of the atmopsheric pCO2 during a Snowball Earth, Geology, 36 (1): 47-50 5. Pierrehumbert, R.T. (2004). High levels of atmospheric carbon dioxide necessary for the termination of global glaciation: Nature, v. 429, p. 646-649, doi: 10.1038/nature02640. 6. Le Hir Guillaume, Donnadieu Yannick, Goddéris Y, Pierrehumbert Raymond T., Halverson Galen P., Macouin Mélina, Nédélec Anne b, Ramstein Gilles. (2008).The snowball Earth aftermath: Exploring the limits of continental weathering processes, Earth and Planetary Science Letters, EPSL-09573; No of Pages 11.

International audience; A new record of a geomagnetic excursion has been obtained from Core MD07-3128 taken at (75° 34' W ; 52° 40' S) off the Pacific coast of Southern Chile, during the IMAGES XV-MD159-PACHIDERME cruise of the R/V Marion Dufresne (IPEV). Radiocarbon datings extend for the moment to 36.3 kyr BP at 18 meters. Linear extrapolation of the last 2 dates downcore, gives an age of 40.7 for the middle point of the excursion at 20.5 m. This age is virtually identical to the most precise and reliable dating of the Laschamp excursion (obtained by K/Ar and 40Ar/39Ar) at the type locality at Laschamp, where Norbert Bonhommet first discovered the excursion during his PhD research work. We therefore consider that we have obtained a new record of the Laschamp Excursion. Because of the extraordinarily high sediment accumulation rate, the directional excursion is recorded over about 2 meters of sediment (between 19,65 and 21,5) and corresponds to a prolonged marked low in the relative paleointensity record. Details of the directional and relative paleointensity changes will be discussed. The high southern latitude at which this new record was obtained and its very detailed nature make it ideal to further constrain the inverse model of the Laschamp Excursion (IMOLEe) (Leonhardt et al. EPSL in press). The results obtained when these new data are considered in the model will be discussed in terms of dipolar versus non-dipolar components of the transistional field and comparison between predicted (modeled) and observed directions at all the sites used for the construction of this last version of the inverse model (IMOLEf).

International audience; The CarboEurope Regional Experiment Strategy took place in the South West of France in 2005 and 2007 gathering several intensive observation periods to estimate the regional carbon balance. Eddy covariance fluxes, atmospheric CO2 concentrations, and the solar induced fluorescence were measured from both towers and aircraft. Additional meteorological observations were used to evaluate the performance of mesoscale models to simulate the observed concentrations. Top-down and bottom-up methods were applied and evaluated against direct ecosystem measurements, demonstrating the potential and the reliability of the carbon flux estimates over the region. Mesoscale modeling showed promising results by simulating most of the observed atmospheric variability. Inverse methodology was used to optimize the carbon balance over the area. The aircraft fluxes aggregated over large footprints (few km) showed an overall agreement with the final estimates from the inversion and the eddy flux towers. The photosynthesis activity was also correlated to the sun-induced fluorescence from ground based and aircraft measurements. Potential improvement of the continental carbon balance and recommendations for designing future regional observation networks are discussed.

International audience; Due to the lack of observations, the budgets of key chemical species in the upper troposphere/lower stratosphere region are still highly uncertain. Hence, the large scale gradient of CO/CO2/CH4 or the abundance and feature of oxygenated compounds are poorly captured by the global chemistry-transport models. The CARIBIC project relies on the use of a passenger aircraft for making frequent atmospheric chemistry measurements mainly in the tropopause region. It provides accurate and simultaneous observations of several compounds in the upper troposphere/lower stratosphere. These airborne observations, nowadays covering several years, are used in this work to evaluate the ability of two global models (TM5 and LMDz-INCA) to simulate the seasonal variations, inter-hemispheric differences and vertical distributions of CH4, CO and CO2 in this region. Furthermore, as CARIBIC provides original measurements of oxygenated compounds, the distribution and budget of such compounds which control the radical production in the UTLS, is also investigated. This work is performed in the framework of the GEOmon EU project.

International audience; The Dansgaard-Oeschger (DO) event 25 initiates the millennial-scale climatic variability of the Northern Hemisphere during the last glacial period. This first rapid event is identified in the NorthGRIP ice core (Greenland - 75.10 °N, 42.32 °W, elevation 2917 m. a. s.l., modern accumulation rate 17.5 cm w.e.yr-1) δ18Oice record as a sharp 2.3‰ increase (half of a “classical” DO interstadial magnitude) occurring at ~112 ka (EDC3 age scale). While DO event 25 remained so far poorly studied, the other DO events of the glacial period have revealed (1) large amplitudes of temperature change over Greenland (8-16°C) (2) synchronous rapid increases of methane (CH4) and Greenland temperature (3) seesaw behaviour with Antarctic temperatures increasing slowly 300-3400 yrs before the rapid Greenland abrupt temperature changes. Thus, investigating in detail the first DO event is of primary importance to understand the onset of the climatic variability of the glacial period. To provide a complete description of DO event 25, we present new results from the EPICA Dronning Maud Land (EDML, Antarctica - 75.00 °S, 0.07 °E, 2882 m a.s.l., 6.4 cm w.e.yr1) and the NorthGRIP ice cores. We use high-resolution measurements of the isotopic composition of air nitrogen (δ15N) as a temperature proxy in the gas phase to infer the amplitude of temperature changes over Greenland. These data moreover allow determining precisely the phasing between temperature and CH4 concentration in the air trapped in the ice. Finally, the combination of new high-resolution CH4 measurements on the NorthGRIP ice core and δ18Oatm records over this first rapid event permits to propose the first accurate common timescale between EDML and NorthGRIP over DO event 25 and to discuss the bipolar seesaw pattern at the onset of the last glacial period.

International audience; Classical water stable isotopes (dD and d18O) have been used for more than 50 years with the aim to understand the links between water cycle and climate. They provide information on either temperature or precipitation changes depending on the latitudes. Their combination, in the so-called d-excess, brings some information on climatic conditions occurring during non equilibrium processes along air masses histories (evaporation over the Oceans, reevaporation of droplets in convective systems, continental recycling or ice crystals formation). Recently, the possibility to measure with high precision d17O in water has enabled to introduce a new parameter, 17Oexcess, resulting from the combination of d18O and d17O. According to both observations and modeling works, this new isotopic parameter is able to decipher some of the non equilibrium processes: when measured in ice core, it is expected to be a more direct tracer of relative humidity of the oceanic evaporative regions than d-excess. In order to better understand what controls this new parameter as well as to extract the maximum climatic information from the combination of 17Oexcess and d-excess, we present different original studies combining these two parameters in several key regions. First, data collected in Niger, West Africa, at scales ranging from the convective system to the seasonal cycle confirm the strong influence of relative humidity on 17Oexcess through the rain reevaporation process. Second, seasonal cycles in the Zongo Valley (Tropical Bolivia) suggest that rain recycling along air masses trajectories have different signatures on d-excess and 17Oexcess leading to decipher the different processes. Third, we study how local processes (precipitation, sublimation) in polar region (Greenland) can affect 17Oexcess archived in ice core with respect to d-excess records through (1) isotopic measurements of vapor versus precipitation collected at the NEEM station and (2) seasonal cycles measured from snow pits.

International audience; Since the identification of millennial-scale climatic variability in Greenland and the North Atlantic (Dansgaard-Oeschger (D-O) and Heinrich (H) events) several questions remain open regarding the expression of this variability in the Mediterranean region, the oceanic and atmospheric mechanisms involved and how other forcings such as orbital parameters affect this variability. Several high-resolution pollen-rich marine cores retrieved in the framework of the IMAGES and ODP programs and covering at least the last 50,000 years have been analysed in the last decade. An array of terrestrial and marine micropalaeontological (pollen, foraminifer), sedimentary (microcharcoal, ice rafted detritus) and geochemical (d18O and d13C) tracers from the same sample set has allowed us to establish a direct correlation between the evolution of the atmospheric and oceanic reservoirs. All the D-O events have a counterpart in western Mediterranean terrestrial and marine ecosystems. Cold sea surface temperatures (SST) were synchronous with the expansion of semi-desert landscapes indicating a decrease in winter precipitation likely related with the northward displacement of the westerlies. SST increases were contemporaneous with the expansion of this forest and, therefore, the establishment of the Mediterranean climate, i.e. wet and mild winters with warm and dry summers. The best expression of the Mediterranean climate occurred during the Eemian, D-O 24, 21, 17-16, 8-7, 1 and the Holocene. These maxima occurred always during precession minima (seasonality maxima). The comparison of Mediterranean and Atlantic terrestrial and marine palaeoclimatic records of the last glacial period with Greenland temperature changes reveals that the Mediterranean region is distinctly impacted by precession. The comparison of this contrasting latitudinal climatic scenario with the global methane record which is also modulated by precession during the last climatic cycle reveals that the amplitude of methane peaks parallels the magnitude of forest cover expansion in the western Mediterranean region, in particular during the glacial interval (from 74 to 13 ka), and suggests the existence of an underlying common mechanism also affected by precession, namely the Asian monsoon, which determines both the strength of Mediterranean climate and methane emission. Precession minima would trigger, on the one hand, strongest summer monsoon which, in turn, produce maxima in Mediterranean summer dryness and increase of winter precipitation and, on the other hand, the largest Asian wetland expansion reaching 40°N. Besides, peaks in the Mediterranean forest coincide with precipitation maxima in the eastern Mediterranean region as detected by the Soreq cave d18O speleothem record. These periods of maximum rainfall, likely occurring during winter as concomitant expansion of the summer dry tolerant sclerophyllous plants are detected, are chronologically associated with sapropel S5 to S1. Therefore, sapropel formation would coincide with periods of maxima in both winter rainfall and summer monsoon which led to strong freshwater discharges over the entire year. In winter these discharges arrived by the rivers of the Mediterranean borderlands and in summer trough the strong Nile floods which distinctly affected the eastern Mediterranean basin. This work suggests that several apparently unrelated phenomena, namely the amplitude of the Mediterranean forest expansion, of monsoon enhancement and of methane emissions, and sapropel formation are all modulated by the same orbital forcing (precession).

International audience; The last deglaciation (21 ka B.P. to â 7 ka B.P.) is the most recent major climate transition that occured on Earth. During this period, changes in insolation and atmospheric greenhouse gases melted the major northern hemisphere ice-sheet leaving only Greenland as we know it today. The last deglaciation presents a unique opportunity in the climate record to test the relative impact of the changes of ice-sheet, greenhouse gases, sea-level on climate. To test the impact of the different factors acting throughout this time period, we present a suite of transient simulation covering the last deglaciation, adding one forcing after the other (Albedo, Ice-sheet elevation, Greenhouse gases, Orbital & freshwater forcing). This enables us to test the relative importance of each forcing on the local and global climate and try to decipher periods where some of the forcings are predominant. Ultimately we discuss a "full" forcing scenario that accounts for some of the features observed in the climate record and the lessons we can derive from the missing ones.

International audience; Dimethylsulfide (DMS) is a natural sulfur compound arising from algal dimethylsulfoniopropionate (DMSP) but through complex biotic and abiotic processes. It is an important natural source of atmospheric sulfur providing new and/or bigger hygroscopic particles for cloud formation over the ocean. One recent study (Vallina and Simo, Science, 2007) suggests the existence of a close quantitative link, at the global scale but also locally, between DMS and the solar radiation dose (SRD), a measure of available radiation inside the mixed layer. Joint data including CTDs, DMS concentration and solar radiation data, collected all along the year 2001 in the northeastern Atlantic during the POMME experiment are analyzed. Contrary to the observations in the Sargasso Sea and Blanes Bay (Mediterranean Sea) presented by Vallina and Simo (2007), this new data set clearly indicates that SRD and DMS are only weakly correlated, and this whatever the numerous sensitivity tests performed (i.e. the solar irradiance, the optical properties of surface waters, the mixed layer depth criteria). Additionally, the DMS versus SRD relationship appears quite sensitive to the irradiance attenuation law. Hence, it appears that SRD cannot be used to understand DMS dynamics at the scale of the north Atlantic basin.

International audience; The Mesozoic, perhaps representing the longest period of warmth during the Phanerozoic Earth history has been repeatedly affected by short-lived cold interludes lasting less than a million of years in length. The evidence for these cooling events includes glendonite abundance, stable oxygen isotope records and faunal migration. While plausible mechanisms have been proposed for the two largest ice age event of the Phanerozoic, convincing explanations for these Mesozoic cold snaps are still lacking. Thought, these events are intriguing because they occur in a greenhouse world and because of their relative short duration. Here, we investigate the climate-carbon cycle behavior during these events with a particular focus on the Middle Late Jurassic Transition using a general circulation model with coupled components for atmosphere, ocean, cryosphere and biogeochemical cycles of C, O and P. We force our climate-carbon model with geological evidences of the timing of the evolution of carbonate production in the mid and low latitudes. We show that the general drawdown of carbonate platforms is a powerful mechanism capable of explaining a fast atmospheric CO2 decrease, a moderate sea level drop associated with ice-sheet buildup and an increasing seawater d13C owing to newly fresh limestones exposed to aerial erosion. Temporary nature of the carbonate drawdown explains the relative short time of these cold events but allows accounting for ice sheet inception and dearth.

International audience; Marine Isotope Stage 11 (MIS11) is the interglacial period dated around 400 kyr ago. It has been the subject of great attention as a possible "analogue" to the Holocene due to its orbital configuration, close to that of the present period. In this study, we will present data on MIS11 and the last 30 kyr from marine sediment cores from different basins, in order to be able to have a global view of these periods and to compare timing of different climatic records in different hemispheres. The work presented here is based on new data from core MD07-3077 and MD07-3076 that were collected in the Atlantic sector of the Southern Ocean (44°09'S, 14°13'W, 3770 m water depth), and its comparison with cores ODP 980 (55°29'N, 14°42'W, 2179 m water depth) and NA87-22 (55°30'N, 14°42'W, 2161 m water depth) from the North Atlantic (Oppo et al., 1998; Waelbroeck et al., 2001). A new chronology for MIS11 will be presented, based on correlation with the age scale of EPICA Dome C ice core age scale (EDC3). This chronology is common for all cores, which allows the comparison of the phasing of events between the two hemispheres. The benthic isotopic records from these cores reveal a different timing of circulation changes in the North Atlantic than in the South Atlantic site, as well as a general increase in the benthic carbon isotopic ratio during MIS11 with respect to the Holocene. Periods of increased ventilation of deep waters in the South Atlantic are interpreted as augmentations in the strength of North Atlantic Deep Water (NADW) production. The sequence of events during Termination V shows a lead of the changes in water mass properties at the North Atlantic site with respect to the South Atlantic site. We interpret this lead as reflecting an inflow of brine-generated waters from the Nordic Seas, similarly to what is observed over the last deglaciation. Conversely, circulation changes during the glacial inception towards MIS10 first occur in the Southern Ocean, as previously observed during MIS5-4 transition (Govin et al., 2008) and could likewise be explained by progressive cooling and expansion of sea ice around Antarctica.

International audience; The link between carbon cycle and climate is at the core of the understanding of the climate system, and drives many researches. Various mechanisms have been proposed to explain the variations of atmospheric CO2 and oceanic ^13C measured during the glacial/interglacial cycles. Still, though the addition of several mechanisms does help, even the most recent scenarios are neither sufficient to fully explain the low atmospheric CO2 concentration of approximately 190 ppm observed during the Last Glacial Maximum, about 21 kyrs ago (e.g. Brovkin et al., 2007) nor producing an oceanic ^13C distribution compatible with what is inferred from sediment cores proxy data (Curry & Oppo, 2005). In this context, the ocean is believed to play a major role as it can stock huge amounts of carbon, especially in the abyss, a carbon reservoir that could widen during glacial time. To create a larger carbon reservoir in the deep ocean, one possible mechanism is to produce very dense glacial waters thereby stratifying the deep ocean and reducing the carbon exchange between the deep and surface ocean. The existence of such very dense waters was inferred in the deep Atlantic during the LGM from sediment cores data, and the deep ocean stratification has been shown as a possible mechanism to store carbon in the ocean and ultimately decrease the atmospheric CO2 concentration (Paillard & Parrenin, 2004). Based on these data we propose a new mechanism that sets up such deep stratification, relying on the formation and rapid sink of brines, very salty water rejected during sea ice formation. We investigate the impact of this mechanism on the carbon cycle using the CLIMBER-2 fully coupled intermediate complexity climate model, well suited for the long simulations we run. As the model version used explicitly computes the evolution of the carbon cycle and carbon isotopes (such as ^13C) in every reservoir, it allows us to directly compare the model output with data from sediment cores. We show that the brine induced stratification both leads to low glacial CO2 and very negative deep ^13C values, as observed in the proxy data. In particular, the LGM drop is quite large as it reaches 47 ppm with the brine induced stratification and can be further extended to 76 ppm with a simultaneous reduced oceanic vertical diffusion. REFERENCES Brovkin, V., A. Ganopolski, D. Archer, and S. Rahmstorf: Lowering of glacial atmospheric CO2 in response to changes in oceanic circulation and marine biogeochemistry, Paleoceanography, 22, PA4202, doi:10.1029/2006PA001380, 2007. Curry, W. B. and Oppo, D. W.: Glacial water mass geometry and the distribution of ^13C of ΣCO2 in the western Atlantic Ocean, Paleoceanography, 20, 1017, doi:10.1029/2004PA001021, 2005. Paillard, D. and Parrenin, F.: The Antarctic ice sheet and the triggering of deglaciations, Earth and Planetary Science Letters, 227, 263-271, 2004.

International audience; The Holocene is commonly referred to as a period of relatively stable climate conditions. It is however affected by a climate anomaly 8.2 kyr ago, postulated to be the result of weakened Meridional Overturning Circulation (MOC) triggered by a freshwater outburst. This time period offers then a good opportunity to study the relationship between intermediate water masses renewal and hydrology. We have focused our work on core MD95-2037 (37°05,23 N ; 32°01,87 W, 2100 m) located in the subtropical North Atlantic gyre, on the eastern side of the mid-Atlantic ridge. Age model has been constrained using 14C AMS measurements on planctonic foraminifera, and the studied time interval is restricted between 3 and 11 kyr. Reconstructed Sea Surface Temperature (SST), and the benthic oxygen and carbon isotopes at this site remained stable during the studied period. In order to reconstruct intermediate water mass hydrology, we used Mg/Ca of benthic foraminifera as a representation of deep water temperature. Mg/Ca measurements were performed on a single benthic species C. wuellerstorfi and a linear temperature calibration has been established from published and new core-top data. In-situ deep water temperature is 3.7°C, and is within temperature range where carbonate chemistry of ambient deep water is expected to have an increasing effect on the processes controlling the incorporation of Mg during shell growth. We show a gradual increase of benthic Mg/Ca from 11 kyr, reaching a maximum during the abrupt cold climate anomaly at 8.2 kyr. From this time the benthic Mg/Ca is steadily then decreasing towards core-top value. Using the comparison of Mg/Ca-Temperature signal with the dynamic tracer 231Pa/230Th record from the same core we show the synchrony between a decrease of the renewal rate of the intermediate water masse and change in intermediate water hydrology connected with the climate anomaly 8.2 kyr ago. However, whereas intermediate overturning seems to be drastically affected, going from a "Glacial mode" characterized with important renewal rate of intermediate water masses to the "modern mode", with slower intermediate renewal rate, hydrology appears only temporarily disturbed.

International audience; The low resolution stable isotope analysis conducted on the EPICA Dome C ice core (sampling length of 55 cm) has revealed distinct behaviours of Antarctic temperature over the past interglacials with differences in duration, intensity and trends (Jouzel et al, Science, 2007). Many questions remain however open regarding not only the mean state but also the millennial and centennial variability during warm periods. Here we present new high resolution deuterium measurements conducted on 11 cm samples and covering Marine Isotope Stage (MIS) 11 (~400ky BP) and 5 (~125ky BP). Thanks to these new measurements (~800 samples for MIS 11 and ~2400 for MIS 5), the temporal resolution has been improved from 220y to 45y for the first one and from 50y to 20y for the second one, making therefore possible a detailed comparison with the current interglacial period, the Holocene. The new resolution for the MIS 11, used to be considered as a good analogue for the present Holocene because of a quite similar orbital configuration, brings really new rich climatic information. First, the power spectra of the two interglacial periods reveal interestingly similar submillennial periodicities. This point suggests that similar processes involved in the centennial scale variability may be operating during these two warm periods. However, both the long term trend and the time evolution of the centennial to millennial variability are quite different between the Holocene and the early MIS 11. We therefore point at strong limitations in the use of MIS 11 as a simple analogue of the present warm period and its future. Measurements are still being conducted on MIS 5 in order to characterize the variability associated with the warmest of EPICA Dome C interglacial periods, occurring with an orbital context quite different from the current interglacial. At the time of preparation of this abstract, 1200 samples have already been measured. MIS 5.5 trend and variability will then be compared to the results obtained and previously described for early MIS 11 and Holocene. Moreover our results regarding not only mean Antarctic climate but also centennial climatic variability are expected to be valuable for the comparison with simulations conducted with climate models, as the phase 3 of the Paleoclimate Modelling Intercomparison Project will include MIS 5.5 amongst the target modelling periods.

International audience; The direct effect of greenhouse gas (GHG) changes is a warming of the atmosphere due to greater long-wave (LW) radiation absorption. Nevertheless, many other processes and feedbacks also take place which modify the whole climatic system and especially the surface energy budget, which determine the precise value of the surface temperature change. In this study, we decompose the surface energy fluxes to determine and quantify the role of many different processes in explaining the surface temperature response to an increase in GHG forcing in a coupled Ocean-Atmosphere General Circulation Model (AOGCM), IPSL-CM4. In particular, we show that the direct feedback effect consisting of greater backward LW radiation due to greater LW emission is particularly strong as is the effect of an increase in water vapor in the atmosphere due to greater temperatures. Nevertheless, many other terms are also important. We use this decomposition to understand the role of the different processes in the polar amplification, the warming contrast between the oceans and the continents and the differences in the surface warming under interglacial (preindustrial) and glacial (Last Glacial Maximum) conditions. This decomposition could be usefull to compare the sensitivity of different AOGCMs to a GHG forcing.

International audience; The water isotopic composition of precipitation (δ18OPrec or δDPrec) is a physical tracer of hydro-climatic conditions both today and for the past. In high-latitudes the δPrec composition is strongly linked to condensation temperatures whereas in low latitudes precipitation amount, surface temperatures in the oceanic source areas or recycling intensity affect the isotopic signal. The isotopic signal is archived in many different continental systems such as high altitude or polar glaciers, speleothems, tree ring cellulose or fresh water lake sediments. Many of the records obtained from these archives have an excellent time control with often annual resolution well suited for an analysis of millennial variability. Within this context there is considerable potential for incorporating water isotope diagnostics into a general circulation model (GCM). This can allow both to answer questions concerning the water isotope cycle within the dynamics of the model simulations and also to compare the outputs with real isotopic data. Here we analyse a simulation made with the GCM ECHAM4, equipped with water isotope diagnostics and integrated over the last millennium. This model was driven with sea surface temperatures taken from a 1000 yr forced simulation with the ECHO-G Atmosphere (ECHAM4)- Ocean (HOPE-G) GCM that did not include an isotope dynamics scheme. The ECHO-G simulation was forced by estimates of solar variability, volcanic activity and greenhouse gases through the last millennium. This work will present results of the isotope simulation focusing on: i) a description of the spatial and temporal variability of the isotopic signal in the context of the natural and anthropogenic forcing changes as simulated along the last millennium and ii) how isotopic changes are influenced by temperature and precipitation variability as well as changes in the main modes of large scale circulation (PDO, NAO, ENSO ...). The comparison and evaluation of the simulation is based on a global data base of paleo water isotope records from many different continental archives.

International audience; A fast OAVGCM called MGV has been developed at IPSL to handle multi-millennial climate simulations. It couples the ocean and sea-ice OPA-LIM model to the atmosphere-vegetation LMDZ-ORCHIDEE model. The more affordable computing cost compared to the version of the IPSL Climate Model used for the IPCC has been obtained by decreasing the resolution of the atmosphere and vegetation components down to a mesh of 44 per 43 grid points in the horizontal. In ORCHIDEE, the carbon cycle is activated (STOMATE) as well as the dynamic vegetation (based on LPJ). On the other hand, the ocean carbon cycle and atmospheric chemistry are not taken into account yet due to their prohibitive additional computing costs. Two transient runs of the middle Holocene are carried out with the newly designed model. In one of them, the dynamic vegetation is not activated to assess this biospheric feedback on climate. The transient response of the climate system to solar forcings and gas concentrations is analysed with emphasize on changes in vegetation cover (such as the greening of the Sahara) and carbon fluxes. Lake extend (such as Lake Chad) and emissions of methane by wetlands are diagnosed using offline simulations with a refined version of ORCHIDEE recently developed at LSCE.

International audience; Deep water circulation changes during the initiation of the last glacial period, and its role in the global cooling are still poorly known. We compare high-resolution oxygen and carbon isotope composition records of planktic and benthic foraminifera from Southern Ocean and North Atlantic cores for the 130-100 ka period in order to establish the deep circulation changes in both area, in relationship to surface hydrology and global climatology. A common chronostratigraphic framework has been defined between marine cores from the northern and southern latitudes over that period, assuming that temperature changes occurred simultaneaously in the surface ocean and over the nearby ice cap. Thus North Atlantic and Southern Ocean sea surface temperature records have been correlated to isotopic ice core records of NGRIP and EPICA Dome C respectively. Sea surface temperatures decrease at about the same time (122ka) in both the northern Atlantic and the Southern Ocean. About 3ka after, an expansion of a poorly ventilated Antarctic Bottom water is observed in the Southern Ocean while no change occurs in the North Atlantic deep circulation. Height thousand years more are needed to observed a shoaling of the North Atlantic deep waters (NADW). We propose that the Southern Ocean respond rapidly to orbital variations: a spatial and/or temporal increase in sea-ice extent might follow the decrease in obliquity. This positive retroaction through albedo drive a decrease in sea surface temperature and a northward movement of the water fronts. A greater seasonal extent of sea-ice will lead to a less ventilated Antarctic Bottom Water (AABW). On the opposite a decrease of sea surface temperatures in the North Atlantic is not enough to produce a change in the North Atlantic deep water formation. Only when Ice sheet have build up sufficiently, can they induce a shoaling of NADW through a change in atmospheric circulation pattern and a decrease in sea surface salinity. The influence of AABW reach the North Atlantic simultaneously to the shoaling of NADW.

International audience; The construction of the Amsterdam-St. Paul (ASP) plateau is related to the interaction between the Southeast Indian Ridge (SEIR) and the ASP hotspot. A chain of seamounts located to the NE of the Plateau has been K/Ar dated and appears too young to be the track of ASP hotspot on the Australian plate [Janin et al., 2008]. However, new age data from the area show that this chain was formed during two different stages, the first one being younger than 2 My, and the other much older. The two types of seamounts have different morphologies. The younger ones are smaller and elongated while the older ones are more massive [Maia et al., in prep] The two types of seamounts can be distinguished using their geochemical composition which is highly alkaline for both. The youngest ones are very enriched with (La/Sm)norm higher than 3. The oldest ones are also enriched but not as much with (La/Sm)norm between 2.5 and 2.8. A heterogeneous enriched mantle as the source of them could be an explanation but according to the geochronological constraints we consider that the old seamounts are related to the expression of the ASP plume on the Australian plate while the young ones result of melting due to a crustal crack over an enriched mantle. The construction of ASP Plateau is also the result of several phases in the hotspot activity. Geophysical observations established that the ASP-SEIR system is conditioned by plume flux variations and by the relative movement between the plume and the spreading center [Maia et al., 2008]. Geochemical data analyses confirm that the intensity of the interaction between the plume and the ridge fluctuated in time. The oldest lavas (about 3 +/- 0.5 My old) have a stronger hotspot signature than the 1.5 My old lavas, but weaker than the most recent ones.

Water isotope ratios of ice cores are a key source of information on past temperatures. Through fractionation within the hydrological cycle, temperature is imprinted in the water isotopic composition of snowfalls. However, this signal of climatic interest is modified after deposition when snow remains at the surface exposed to the atmosphere. Comparing time series of surface snow isotopic composition at Dome C with satellite observations of surface snow metamorphism, we found that long summer periods without precipitation favor surface snow metamorphism altering the surface snow isotopic composition. Using excess parameters (combining D,17O, and 18O fractions) allow the identification of this alteration caused by sublimation and condensation of surface hoar. The combined measurement of all three isotopic compositions could help identifying ice core sections influenced by snow metamorphism in sites with very low snow accumulation., Plain Language Summary: Water isotopes in ice core records are often used to reconstruct past climate temperature variations. Classically, the temperature signal is thought to be imprinted in water isotopes of precipitation, and then archived in the ice core as it falls, and in cold areas of Antarctica, piles up for very long period. Here, we show that the surface snow isotopic composition varies in between precipitation events, suggesting that there might be more than one contribution to the isotopic signal in ice core records. This is particularly important for low accumulation sites, where the snow at the surface remains exposed for very long time periods. The combined use of several isotopic ratios in surface snow helps us disentangle the processes that create this signal., Key Points: During summer without precipitation, intense snow metamorphism shows a strong water isotopic signature. During summer without precipitation, intense snow metamorphism shows a strong water isotopic signature. The d‐excess and 17O‐excess of the snow is a proxy of snow metamorphism for low accumulation regions., FP7 Ideas: European Research Council (FP7 Ideas) http://dx.doi.org/10.13039/100011199, Foundation Prince Albert of Monaco, Alexander von Humboldt‐Stiftung (Humboldt‐Stiftung) http://dx.doi.org/10.13039/100005156, DFG project CLIMAIC, https://doi.pangaea.de/10.1594/PANGAEA.934273

Made available in DSpace on 2022-04-29T08:31:27Z (GMT). No. of bitstreams: 0 Previous issue date: 2021-01-01 Global warming is likely to cause a progressive drought increase in some regions, but how population and natural resources will be affected is still underexplored. This study focuses on global population and land-use (forests, croplands, pastures) exposure to meteorological drought hazard in the 21st century, expressed as frequency and severity of drought events. As input, we use a large ensemble of climate simulations from the Coordinated Regional Climate Downscaling Experiment, population projections from the NASA-SEDAC dataset, and land-use projections from the Land-Use Harmonization 2 project for 1981–2100. The exposure to drought hazard is presented for five SSPs (SSP1-SSP5) at four Global Warming Levels (GWLs, from 1.5 to 4°C). Results show that considering only Standardized Precipitation Index (SPI; based on precipitation), the combination SSP3-GWL4 projects the largest fraction of the global population (14%) to experience an increase in drought frequency and severity (vs. 1981–2010), with this value increasing to 60% if temperature is considered (indirectly included in the Standardized Precipitation-Evapotranspiration Index, SPEI). With SPEI, considering the highest GWL for each SSP, 8 (for SSP2, SSP4, and SSP5) and 11 (SSP3) billion people, that is, more than 90%, will be affected by at least one unprecedented drought. For SSP5 (fossil-fuelled development) at GWL 4°C, approximately 2·106 km2 of forests and croplands (respectively, 6 and 11%) and 1.5·106 km2 of pastures (19%) will be exposed to increased drought frequency and severity according to SPI, but for SPEI, this extent will rise to 17·106 km2 of forests (49%), 6·106 km2 of pastures (78%), and 12·106 km2 of croplands (67%), with mid-latitudes being the most affected areas. The projected likely increase of drought frequency and severity significantly increases population and land-use exposure to drought, even at low GWLs, thus extensive mitigation and adaptation efforts are needed to avoid the most severe impacts of climate change. European Commission Joint Research Centre (JRC) Centro Euro-Mediterraneo sui Cambiamenti Climatici (CMCC Foundation—REMHI Division) Cooperative Institute for Research in Environmental Sciences (CIRES) and Department of Atmospheric and Oceanic Sciences Snow and Ice Data Center University of Colorado Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE) University of Copenhagen Niels Bohr Institute (NBI) Danish Meteorological Institute Abdus Salam International Centre for Theoretical Physics (ICTP) Faculty of Science University of New South Wales Helmholtz-Zentrum Geesthacht (HZG) Institute of Coastal Research The Cyprus Institute (CyI) Climate and Atmosphere Research Center (CARE-C) Climate Service Center Germany (GERICS) Helmholtz-Zentrum Geesthacht Commonwealth Scientific and Industrial Research Organisation (CSIRO) Marine and Atmospheric Research Swedish Meteorological and Hydrological Institute (SMHI) Rossby Centre Département des Sciences de la Terre et de l'Atmosphère Université du Quebec à Montréal (UQAM) University of Cape Town Climate System Analysis Group (CSAG) Department of Physics Faculty of Arts and Sciences Bogazici University Center for Climate Change and Policy Studies Bogazici University Key Laboratory of Ocean Circulation and Waves Institute of Oceanology Chinese Academy of Sciences (CAS) São Paulo State University and Bauru Meteorological Centre (IPMet/UNESP) Department of Physics Faculty of Arts and Sciences Isik University Institute of Meteorology and Climate Research Karlsruhe Institute of Technology (KIT) Departamento de Ciências Atmosféricas Universidade de São Paulo Facultad de Ciencias Exactas y Naturales Departamento de Ciencias de la Atmósfera y los Océanos (DCAO-FCEN-UBA) Universidad de Buenos Aires Universidad de Buenos Aires Centro de Investigaciones del Mar y la Atmósfera (CIMA/CONICET-UBA) School of Biological Sciences The University of Queensland Department of Earth Sciences and Environment The National University of Malaysia (UKM) National Centre for Scientific Research (CNRS) Institut Pierre-Simon Laplace (IPSL) Laboratoire des Sciences du Climat et de l'Environnement (LSCE) NORCE Norwegian Research Centre AS Centro Italiano Ricerche Aerospaziali (CIRA) São Paulo State University and Bauru Meteorological Centre (IPMet/UNESP)

International audience; In order to quantify the impact of evapotranspiration phenomena on active layer dynamics in a permafrost-dominated forested watershed in Central Siberia, we performed a numerical cryohydrological study of water and energy transfer using a new open source cryohydrogeology simulator, with two innovative features: the spatially distributed, mechanistic handling of evapotranspiration and the inclusion of the developed numerical tool in the high performance computing tool box for numerical simulation of fluid dynamics OpenFOAM®. In this region, the heterogeneity of solar exposure leads to strong contrasts in vegetation cover, which constitute the main source of variability of hydrological conditions at the landscape scale. The uncalibrated numerical results reasonably reproduce the measured soil temperature profiles and the dynamics of infiltrated waters revealed by previous biogeochemical studies. The impacts of the thermo-hydrological processes on the water fluxes from the soils to the stream are discussed through comparison between numerical results and field data. The impact of evapotranspiration on water fluxes is studied numerically, which highlights a strong sensitivity to the variability of rooting depth and corresponding evapotranspiration at the slopes of different aspects in the catchment.