Jean-Baptiste Ladant

The slowdown of the Atlantic Meridional Overturning Circulation (AMOC) due to ongoing climate change raises concerns about its impact on the global carbon cycle. Understanding how global biosphere productivity may respond to such changes is essential for predicting the future carbon cycle, yet quantifying past global biosphere productivity remains challenging. We use triple isotope composition of air oxygen (¹⁷Δ) trapped in polar ice cores to reconstruct the global biosphere productivity over the period 40.5 to 38.6 thousand years before the present (ka B.P.), covering Heinrich Stadial 4, a period of weakened AMOC. Our data reveal a slight increase in productivity following a carbon dioxide (CO₂) rise around 39.5 ka B.P. Freshwater hosing experiments using a global climate model indicate that the CO₂ fertilization effect on the terrestrial biosphere offsets productivity decline caused by consequences of AMOC slowdown. Collectively, our results suggest that global biosphere productivity slightly increases in response to AMOC slowdown and accompanying changes.

Warm, high-CO ₂ climates of Earth's past provide an opportunity to evaluate climate models under extreme forcing, and to explore mechanisms that lead to such warmth. One such time period is the early Eocene (∼56–41 million years ago), when global mean surface temperatures were ∼15 °C higher than preindustrial, and CO ₂ concentrations were ∼1500 ppmv. In this paper we present the experimental design for Phase 2 of the Eocene component of the Deep-time Model Intercomparison project (DeepMIP-Eocene-p2). The aim is to provide a framework for modelling groups to carry out a common set of simulations, thereby facilitating exploration of inter-model dependencies. The focus is on the early Eocene Climatic Optimum (EECO, ∼53.3–49.1 million years ago). Relative to Phase 1 of DeepMIP-Eocene, we provide a new paleogeography (topography, bathymetry) derived from several recent independent reconstructions that focused on different regions, a new vegetation distribution derived by merging paleobotanical data with vegetation model simulations, and a new CO ₂ specification derived from recent re-evaluations of proxy data. The core set of simulations consists of a preindustrial control, an abrupt increase to 4× preindustrial CO ₂ concentration from this preindustrial control, a standard control EECO simulation at 5× preindustrial CO ₂ concentration, and an EECO simulation with preindustrial CO ₂ concentration. In addition to these core simulations, we suggest a suite of optional sensitivity studies, which allow the impact of various factors to be explored, such as topography/bathymetry, greenhouse gases, land-surface parameters, astronomical and solar forcings, and internal model parameters. The updated boundary conditions and guidance on initialisation and spinup in Phase 2 will allow more robust model-data comparisons, more accurate insights into mechanisms influencing early Eocene climate, and increased relevance for informing future climate change projections.

Les enregistrements climatiques globaux à l’échelle géologique entre le Crétacé et le début du Cénozoïque indiquent des variations de grande amplitude. Sur le long terme, celles-ci sont déterminées par l’équilibre entre la composition atmosphérique en gaz à effet de serre, principalement le CO2, issus du dégazage volcanique et l’altération continentale, modulée par les mouvements tectoniques des continents. Dans cette thèse, les liens entre paléogéographie et CO2 ont été étudiés dans le contexte des interactions entre climat et calottes de glace au cours d’un intervalle de temps dit de « greenhouse », entre 120 et 34 Ma. L’utilisation d’une suite de modèles impliquant un modèle couplé moyenne résolution, un modèle atmosphérique haute résolution et un modèle de calotte de glace, a permis de montrer que les changements paléogéographiques survenant au Crétacé ont régulé la présence de glace en Antarctique. Dans un second temps, une nouvelle méthode de couplage climat-calotte a été développée pour étudier la glaciation Eocène-Oligocène. Ces développements ont permis de reconstruire une évolution fidèle de celle-ci, en bon accord avec les données. Deux rétroactions liées à cette glaciation et à la chute concomitante du CO2 atmosphérique sont étudiées. En premier lieu, l’impact de la glaciation sur le Courant Circumpolaire Antarctique est abordé, montrant que celle-ci génère une intensification de ce courant. Ensuite, au sein d’une étude mêlant données et modèles pour documenter la présence de moussons en Asie dès l’Eocène moyen, il est montré que le changement climatique de la fin de l’Eocène induit une baisse d’intensité de la mousson asiatique. Enfin, dans la perspective d’analyser les conséquences des changements paléogéographiques du Cénozoïque sur la biogéochimie marine, des tests de sensibilité aux passages océaniques de Panama et de Drake ont été réalisés.

The past decades have been rich in field-based discoveries, substantially expanding both the range and resolution of geological records documenting Cenozoic Asian tectonic evolution, climate, and landscapes. Geological evidence from sedimentary, fluvial, lacustrine, and paleobotanical archives indicates that monsoon-like precipitation seasonality was likely already established during the Paleogene greenhouse period, thereby challenging the traditional view of monsoon climate emergence. Syntheses of these proxy records, combined with paleoclimate simulations spanning 40-8 Ma and testing a range of paleogeographic configurations, have enabled the evaluation of multiple working hypotheses and provided new perspectives on the drivers of South and East Asian monsoon onset. Model results suggest that summer and winter monsoons may have evolved diachronously in response to distinct forcings, and highlight the importance of paleogeographic evolution in shaping Asian climate. Increasing continentality, due to sea level drop, the retreat of the Paratethys Sea and the emergence of the Arabian Plate, appears to have enhanced summer ITCZ migration, while East African, Anatolian-Iranian, and Tibetan-Himalayan landforms contribute to the channelling of these moisture-laden winds toward Southeast Asia. In contrast, by blocking and deflecting westerly fluxes northward, the uplift of the Tian Shan-Pamir ranges and the Mongolian Plateau likely contributed to strengthening the winter monsoon. Together, these results highlight the need for integrated multi-proxy and modeling approaches to robustly constrain the timing and drivers of Asian monsoon establishment.

Tides are the main energy source for diapycnal mixing in the ocean interior. However, energy-constrained tidal mixing parameterizations are not routinely included in ocean models applied to the deep-time past of the Earth. Instead, diapycnal mixing is usually parameterised by a constant vertical diffusivity or a prescribed vertical profile of vertical diffusivity.Here, by leveraging outputs from the DeepMIP project, we compute the power effectively consumed by parameterized diapycnal mixing in each DeepMIP model and for different CO2 concentrations. We show that this power slightly increases with increasing CO2 in simulations integrated to quasi-equilibrium but skyrockets in warming, out-of-equilibrium, simulations. This reflects the increased stratification in a warming ocean, even though in principle the same amount of tidal energy is available for mixing. We find no evident relationships between the intensity of the overturning circulation and the power consumed by diapycnal mixing across the DeepMIP models. Finally, we use coupled climate-biogeochemistry simulations performed with the IPSL-CM5A2 model to show that the marine biogeochemistry is largely impacted by the vertical mixing scheme employed, even if the total power consumed by diapycnal mixing remains similar.

Among the five great extinction events of the Phanerozoic, the Late Ordovician stands out as it isconcomitant with a massive glacial event under high atmospheric pCO2. This apparent climateparadox was addressed in numerous climate modeling studies. In particular, [1] showed that underthe specific palaeogeographical conditions of the Hirnantian (445 Ma), with an ocean-dominatedNorthern Hemisphere, the climate system may undergo a “tipping point” where a small pCO2variation leads to either glacial or ice-free warm equilibrium state.Those results were obtained with the intermediate complexity Fast Ocean Atmosphere Model(FOAM). We have conducted new simulations using the state-of-the-art coupled IPSL-CM5A2-LREarth System Model [2], spanning a wide range of pCO2 for the Hirnantian. We find that the climatetipping point is entirely absent, and that the equilibrium climate sensitivity is strikingly linear in thisset of simulations.We conducted a detailed model intercomparison and we have identified major differences betweenthe models in the representation of the radiative transfer, cloud cycle and oceanic eddy dynamicswhich contribute to the qualitatively different model behaviors, enhanced under high atmosphericpCO2 content. Specifically, the FOAM tipping point corresponds to an abrupt transition from a sharpNorthern latitudinal temperature gradient at low pCO2 (cold state) to a flattened gradient with warmpolar latitudes (ice-free warm state). In contrast, the IPSL-CM5A2 temperature gradient is relativelyconstant across pCO2, with year-long sea ice confined in the Northern latitudes even under 15Xpreindustrial pCO2 level (4200 ppm).We propose a physical mechanism to link the warm FOAM flattened latitudinal temperature gradientto the dramatic sea-ice albedo feedback sensitivity via the increased stratification of the superficialocean. Since this mechanism is independent of the physical parameterizations and relativecomplexity of the models, and comparing our results with other scarce published climate simulationsof the Hirnantian [3,4], we propose that the latitudinal temperature gradient, seen as a model-dependent emerging feature, may be the main driver of the previously unveiled sea-ice albedoclimate tipping point.References:[1] Pohl et al. (2014), Climate of the Past, 10, 6[2] Sepulchre et al. (2020), Geoscientific Model Development, 13,7[3] Pohl et al. (2017), Paleoceanography, 32, 4[4] Valdes et al. (2021), Climate of the Past, 17, 4

Warm, high-CO₂ climates of Earth's past provide an opportunity to evaluate climate models under extreme forcing, and to explore mechanisms that lead to such warmth. One such time period is the early Eocene, when global mean surface temperatures were 10–17 °C higher than preindustrial, and CO₂ concentrations were ~1500 ppmv. In this paper we present the experimental design for Phase 2 of the Eocene component of the Deep-time Model Intercomparison project (DeepMIP-Eocene-p2). The aim is to provide a framework for modelling groups to carry out a common set of simulations, thereby facilitating exploration of inter-model dependencies. The focus is on the early Eocene Climatic Optimum (EECO, ~53.3–49.1 million years ago). Relative to Phase 1 of DeepMIP-Eocene, we provide a new paleogeography (topography, bathymetry) derived from several recent independent reconstructions that focused on different regions, a new vegetation derived by merging paleobotanical data with vegetation model simulations, and a new CO₂ specification derived from recent reevaluations of proxy data. The core set of simulations consists of a preindustrial control, an abrupt increase to 4x preindustrial CO₂ concentrations from this preindustrial control, a standard control EECO simulation at 5x preindustrial CO₂ concentrations, and an EECO simulation with preindustrial CO₂ concentrations. In addition to these core simulations, we suggest a suite of optional sensitivity studies, which allow the impact of various factors to be explored, such as topography/bathymetry, greenhouse gases, land-surface parameters, astronomical and solar forcings, and internal model parameters. The updated boundary conditions and guidance on initialisation and spinup in Phase 2 will allow more robust model-data comparisons, more accurate insights into mechanisms influencing early Eocene climate, and increased relevance for informing future climate change projections.

Warm, high-CO₂ climates of Earth's past provide an opportunity to evaluate climate models under extreme forcing, and to explore mechanisms that lead to such warmth. One such time period is the early Eocene, when global mean surface temperatures were 10–17 °C higher than preindustrial, and CO₂ concentrations were ~1500 ppmv. In this paper we present the experimental design for Phase 2 of the Eocene component of the Deep-time Model Intercomparison project (DeepMIP-Eocene-p2). The aim is to provide a framework for modelling groups to carry out a common set of simulations, thereby facilitating exploration of inter-model dependencies. The focus is on the early Eocene Climatic Optimum (EECO, ~53.3–49.1 million years ago). Relative to Phase 1 of DeepMIP-Eocene, we provide a new paleogeography (topography, bathymetry) derived from several recent independent reconstructions that focused on different regions, a new vegetation derived by merging paleobotanical data with vegetation model simulations, and a new CO₂ specification derived from recent reevaluations of proxy data. The core set of simulations consists of a preindustrial control, an abrupt increase to 4x preindustrial CO₂ concentrations from this preindustrial control, a standard control EECO simulation at 5x preindustrial CO₂ concentrations, and an EECO simulation with preindustrial CO₂ concentrations. In addition to these core simulations, we suggest a suite of optional sensitivity studies, which allow the impact of various factors to be explored, such as topography/bathymetry, greenhouse gases, land-surface parameters, astronomical and solar forcings, and internal model parameters. The updated boundary conditions and guidance on initialisation and spinup in Phase 2 will allow more robust model-data comparisons, more accurate insights into mechanisms influencing early Eocene climate, and increased relevance for informing future climate change projections.

Warm, high-CO₂ climates of Earth's past provide an opportunity to evaluate climate models under extreme forcing, and to explore mechanisms that lead to such warmth. One such time period is the early Eocene, when global mean surface temperatures were 10–17 °C higher than preindustrial, and CO₂ concentrations were ~1500 ppmv. In this paper we present the experimental design for Phase 2 of the Eocene component of the Deep-time Model Intercomparison project (DeepMIP-Eocene-p2). The aim is to provide a framework for modelling groups to carry out a common set of simulations, thereby facilitating exploration of inter-model dependencies. The focus is on the early Eocene Climatic Optimum (EECO, ~53.3–49.1 million years ago). Relative to Phase 1 of DeepMIP-Eocene, we provide a new paleogeography (topography, bathymetry) derived from several recent independent reconstructions that focused on different regions, a new vegetation derived by merging paleobotanical data with vegetation model simulations, and a new CO₂ specification derived from recent reevaluations of proxy data. The core set of simulations consists of a preindustrial control, an abrupt increase to 4x preindustrial CO₂ concentrations from this preindustrial control, a standard control EECO simulation at 5x preindustrial CO₂ concentrations, and an EECO simulation with preindustrial CO₂ concentrations. In addition to these core simulations, we suggest a suite of optional sensitivity studies, which allow the impact of various factors to be explored, such as topography/bathymetry, greenhouse gases, land-surface parameters, astronomical and solar forcings, and internal model parameters. The updated boundary conditions and guidance on initialisation and spinup in Phase 2 will allow more robust model-data comparisons, more accurate insights into mechanisms influencing early Eocene climate, and increased relevance for informing future climate change projections.