Soirée de rencontre autour du livre « Tout comprendre (ou presque) sur le climat »

10/11/2023 19:00

Keep In Touch 2023 ! Le 2e RDV Alumni de l’IPSL-Climate Graduate School

09/11/2023 18:30

Festival Explor'Espace

03/11/2023 10:00

The art of climate model evaluation : example of ENSO

18/07/2023 11:00

Climate models help us understand the complexity of Earth’s climate, forecast the next seasons and predict the influence of anthropogenic forcings. It is therefore important to evaluate the performance of these models relative to observational datasets, to build confidence and to improve them.

Sea level extremes and compounding marine heatwaves in coastal Indonesia

18/07/2023 11:00

Low-lying island nations like Indonesia are vulnerable to sea level Height EXtremes (HEXs). When compounded by marine heatwaves, HEXs have larger ecological and societal impact. Here we combine observations with model simulations, to investigate the HEXs and Compound Height-Heat Extremes (CHHEXs) along the Indian Ocean coast of Indonesia in recent decades.

Forests in the Earth System

04/07/2023 11:00

Séminaire du LGENS par Benjamin Quesada (Universidad del Rosario).

On the role of multiscale atmospheric circulations in the organization of tropical convection

18/06/2025 10:00

Convective clouds can arrange into harmonious patterns from the kilometer-scale to the planetary scale. This results from a complex interplay between the atm

ospheric circulation, convection, and the condensation of water vapor, of which our understanding remains elusive. Despite major advances during the last decade, various theories are still proposed to describe this phenomenon, which remains a key challenge for improving weather forecasts and anticipating both the magnitude and the impacts of future global warming.

In this thesis, we explore this question by focusing on the clear air surrounding the clouds, that is governed by simple and well-established physical laws.

We first show that it is possible to measure the vertical velocity of this clear air, and present an archive of such measurements based on geostationary satellites and infrared sounders. The observations reveal the rich wave activity of the clear-air tropical atmosphere. They also show strong subsidence in the vicinity of deep convective systems.

To explain this subsidence, we propose a conceptual model, the dipole model, in which thermals, shallow, and deep convective clouds are assimilated to hydrodynamic dipoles that transport air upwards in the atmosphere. We study the theoretical implications of this mass transport on the atmospheric circulation in the surrounding clear air. The model is able to explain some characteristic features of tropical convection, such as the formation of moist halos around clouds and the spontaneous clustering of clouds. It also highlights the importance of cloud geometry, and suggests that the depth of clouds controls a range of variables from the average relative humidity profile in the tropics to the characteristic horizontal scale of the cloud patterns.

We also propose a simple model of the dry boundary layer that leads to the propagation of non linear waves. We interpret the intertropical convergence zone (ITCZ) as a stationary shock wave. Conversely, we suggest that the doldrums, ubiquitous areas of weak and variable winds, are rarefaction waves. By coupling the boundary layer with the dipole model, we derive a dimensionless number that controls the transition between a single and a double ITCZ.

Our theoretical analysis is evaluated against space observations, as well as data from various airborne field campaigns. The observations have striking similarities with the theory, but also notable discrepancies, raising new questions and highlighting possible avenues for future work.

Organic and inorganic carbon dynamics at the soil-roots interface

21/05/2025 13:00

To mitigate climate change, increasing soil organic carbon (C) stocks and enhancing chemical weathering in croplands have been proposed as CO₂ removal strategies. Here, we investigate the role of belowground organic C inputs, namely roots and rhizodeposition, in these approaches. First, we quantified the rhizodeposition of 12 crop species and assessed its decomposition in soil. We found that rhizodeposition accounts for a significant C pool (42% of root inputs) and decomposes more slowly than roots, making it a relevant C input to consider.

Next, we examined its role in regulating the chemical weathering of crushed basalt using a new experimental setup with 15 lysimeters in a climate chamber. Our results showed that while plants enhanced solute release, they also reduced seepage, limiting dissolved inorganic C export. Altogether, this highlights the need for a thorough understanding of belowground C inputs to optimize CO₂ removal strategies.

 

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Terrestrial ecosystems contain vast amounts of carbon (C) and are a hub for C exchanges. They remove CO₂ from the atmosphere by sequestrating organic C in biomass and soils via photosynthesis. They also consume CO₂ through chemical weathering of minerals. To mitigate climate change, it has been proposed to increase these 2 fluxes in agricultural ecosystems. Belowground organic C inputs, namely roots and rhizodepostion, represent around 46% of net primary production, and greatly contribute to shape their surroundings and to drive the processes controlling organic C sequestration and chemical weathering. This thesis explores how belowground carbon inputs should be considered when addressing CO₂ removal through increased plant inputs and weathering.

We carried out a first 13C-CO₂ labelling mesocosm experiment in a climate chamber to quantify the C inputs attributed to aboveground biomass, roots and net rhizodeposition for 12 crop species. We highlighted a positive correlation between rhizodeposition and aboveground biomass and found no negative correlation among any of the 3 pools. This suggests that increasing inputs by targeting a specific C source will not be at the extent of the others. We then assessed root decomposition and net rhizodeposition through a litterbags incubation experiment in the field over a year. We found that rhizodeposition had a decomposition rate slightly smaller to that of roots. These 2 experiments suggest that net rhizodeposition, that accounted for 22 to 38% of the C allocated belowground, is a significant C input to consider for soil organic carbon increase. We conducted a second experiment designed to facilitate the simultaneous study of the organic and inorganic C cycles. For this purpose, we constructed a new experimental platform composed of 15 instrumented lysimeters in a climate chamber. This notably enabled the monitoring of water flow, which connects most biogeochemical processes in the critical zone. We used this setup to study the chemical weathering of a crushed basalt substrate, on which we grew different genotypes of alfalfa. We found that plants, mostly by increasing the pore CO2 concentration, increased the concentration of solutes in the discharge water.

However, through evapotranspiration, they significantly reduced seepage, thereby limiting the export of dissolved inorganic carbon from chemical weathering. This highlighted a duality between C storage strategies and water management. Altogether, our results confirm that belowground C inputs are a major lever for sequestering organic carbon and that their interaction with the inorganic carbon cycle should also be considered.

Les isotopes de l’eau pour l’étude du climat dans les régions polaires

14/05/2025 10:00

Mes travaux se concentrent sur la mesure et l’utilisation des isotopes de l’eau comme proxy de la variabilité climatique pour la reconstruction du climat du passé.

Une partie de mon projet est d’obtenir de nouvelles séries temporelles d’isotopes de l’eau dans des carottes de glace pour reconstruire la variabilité climatique à haute fréquence (inter-annuelle). Du coté analytique, ce travail se base sur l’implémentation de nouvelles techniques de spectroscopie infrarouge pour la mesure de la composition isotopique triple de l’eau (H₂¹⁸O, H₂¹⁷O, et HDO). Sur la ligne d’analyse des carottes de glace, le but est de repousser les limites de la mesure des isotopes de l’eau afin de pouvoir mesurer de manière précise et rapide les échantillons de carotte de glace. En effet, mes travaux ont montré qu’une limite de la résolution effective à laquelle les isotopes de l’eau dans les carottes de glace peuvent être interprété est la précision de mesure.

L’interprétation de ces carottes de glace utilise des techniques statistiques et spectrales avancées afin de pouvoir relier un maximum du signal isotopique archivé à la variabilité climatique sous-jacente, en évaluant les impacts des différents processus d’archivage sur le signal isotopique. Cette approche est complétée par une approche mécanistique basée sur des mesures sur le terrain des isotopes dans la vapeur, dans la précipitation et dans la neige de surface afin de comprendre comment les échanges locaux affectent le signal par rapport aux apports lointains.

Tout ce travail est complété par des études fondamentales de la physique des isotopes en laboratoire, en évaluant les propriétés thermodynamiques des isotopes, par exemple les coefficients du fractionnement isotopiques, la diffusivité, ou la fonction de la partition de chaque type de molécules d’eau. L’évaluation de ces grandeurs physiques de manière précise est clef pour améliorer les paramétrisations des isotopes de l’eau dans les modèles climatiques, aussi bien de grande échelle (GCM) que conceptuels.