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Are cities on track to net-zero emissions? The next generation of greenhouse gas information systems
06/09/2021 00:00
Metropolitan areas represent a large fraction of the global fossil fuel emissions. Supported by international consortiums, local governments play an active role in reducing GHG emissions at the local level, but tracking progress of mitigation actions is more tedious and more prone to systematic errors than previously thought.
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Unique thermal expansion properties of water key to the formation of sea ice on Earth
03/04/2023 11:00
The formation of sea ice in polar regions is possible because a salinity gradient or halocline keeps the water column stable despite intense cooling. Here, we demonstrate that a unique water property is central to the maintenance of the polar halocline, namely, that the thermal expansion coefficient (TEC) of seawater increases by one order of magnitude between polar and tropical regions.
Unravelling changes in global freshwater nitrogen cycling in the Anthropocene
31/03/2023 12:00
Junjie Wang, post-doctorante à l’université d’Utrecht est actuellement en visiting à METIS.
Navigating on an iceless Bellingshausen Sea on board of the Polarstern
28/03/2023 11:00
Séminaire du LOCEAN.
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The radiative effects of absorbing aerosols
19/09/2025 14:30
Atmospheric aerosols are a key component of the climate system and major contributors to the Earth’s radiation budget. Understanding the role of aerosols in the radiative transfer of solar and infrared radiation in the atmosphere is crucial to understand their implication in past, present and future climate and climate changes. Currently, knowledge of the climatic properties of aerosols and their variability in link to varying emission sources and atmospheric ageing remains limited, particularly for natural and anthropogenic absorbing species such as mineral dust and carbonaceous aerosols containing black carbon (BC) and brown carbon (BrC). As a matter of fact, the spectral optical properties of aerosols, governing their direct interaction with radiation (scattering and absorption), as well as their ability to act as condensation nuclei for liquid (CCN) and ice (IN) clouds, are still largely unknown. Consequently, the representation of absorbing aerosols in climate models and satellite retrieval algorithms remains inadequate, which severely limits the ability to constrain their role in the radiation budget and its implications for regional and global climate.
My research activity over the last 15 years has been dedicated to study absorbing aerosols, their interactions with solar (SW) and infrared (LW) radiation and their direct radiative effect (DRE). The aim of my research is to quantify the intensity and spectral signature of aerosol absorption and scattering properties, such as the complex refractive index, and to understand their variability as a function of the emission source and atmospheric ageing of the particles. In particular, I aim to estimate how composition, size, shape and mixing state influence the aerosol absorption and scattering, and to produce parameterisations that describe this variability, so to provide usable informations to improve aerosol description in climate models and remote sensing algorithms. The methods I use in my research include experimental measurements in the CESAM large atmospheric simulation chamber (Chambre Expérimentale de Simulation Atmospherique Multiphasique) at LISA, which is my tool of predilection, as well as field observations, laboratory analytical techniques, analysis of ground-based and satellite remote sensing measurements, and regional and global models.
In my research I have set up original experiments in the CESAM chamber to study the spectral optical properties of mineral dust. These experiments have produced new information on the absorption properties and complex refractive index of global dust in the visible and mid-infrared range (0.4-15 µm), which paved the way for significant advances in the evaluation of the DRE and remote sensing of dust. I have subsequently developed a new line of research at CESAM on the study of the optical properties of BC and BrC, mostly looking at primary combustion emissions. I have also conducted studies aimed at investigating the optical and DRE properties of aerosols in mixing zones, where primary and secondary species, anthropogenic and biogenic, cohexist and interact. The results to date have highlighted the power of the laboratory simulation approach, and its synergy with field observations and modelling tools, to elucidate the processes and document the properties that are needed to understand the role of absorbing aerosols on the radiation budget and the climate system. In fact, the realism of aerosol generation and ageing that can be reproduced in a chamber, together with the controlled conditions that the laboratory can provide, are fundamental to producing relevant data for guiding the improvement of climate models and remote sensing algorithms.
My future research activity will be dedicated to further progress in the investigation of the climate-relevant properties of dust, BC, and BrC via the laboratory simulation approach. In particular, I will work to extend the spectral range of documented optical properties from the ultraviolet to the far infrared, including the study of key properties for remote sensing, such as polarisation. I will also work to improve the representation of the aerosol shape in the restitution of optical properties. I will strengthen the activity to study the link between optical properties, composition and mixing state. To do so, I will target both well-established dust sources but also encompass to emerging ones, such as dust from high latitude areas, and I will explore the range of formation pathways of BrC and the impact of ageing on the absorption of BC- and BrC-containing aerosols. Finally, I will extend my activity towards the study of the IN properties of dust, BC and BrC, which is particularly important for understanding the whole radiative effects of aerosols, particularly in polar environments. In order to make progress on all these subjects, the key aspect will be to design laboratory experiments reproducing increasingly realistic and complex conditions (humidity, mixture of species, multiphase chemistry, ageing time) representative of the diversity of situations encountered in the atmosphere.
Interactions nuages-poussières-dynamique atmosphérique : le rôle des Ondes d'Est Africaines dans la formation des cyclones tropicaux dans l'Atlantique
11/09/2025 14:00
Les Cyclones Tropicaux (TCs) comptent parmi les phénomènes météorologiques les plus mortels et ils provoquent des impacts socio-économiques majeurs. Pourtant, leur prévision est toujours un défi. Dans l’Atlantique, 75% des TCs se forment à partir d’Ondes d’Est Africaines (AEWs), mais seule une fraction de ces ondes donne naissance à un cyclone. Cette thèse vise à mieux comprendre la dynamique des AEWs, leurs interactions avec les autres ondes tropicales et avec les poussières sahariennes dans le contexte de la cyclogenèse.
L’analyse s’appuie sur des données de réanalyses, des données satellites, des mesures in-situ et sur les données issues de la campagne CADDIWA (Cap Vert, Septembre 2021). Une nouvelle méthode de filtration a été développée pour permettre de mieux isoler les AEWs qui se propagent au nord du jet d’est Africain de celles qui se propagent au sud. Cette méthode améliore leur détection et révèle des interactions fréquentes qui favorisent le développement de TCs par la formation de structures verticales cohérentes.
L’interaction entre les AEWs et la mousson africaine a aussi été mise en évidence comme un chemin de cyclogenèse potentiel. Tous ces processus influencent l’admission de poussières dans les tempêtes naissantes. Des analyses de sensibilité ont été conduites sur des simulations de la Tempête Tropicale Rose pour étudier l’impact des poussières sur la phase d’intensification. Sans être le facteur dominant, les poussières influencent la microphysique nuageuse de manière complexe et non linéaire.
Des études complémentaires sont nécessaires pour améliorer la compréhension de l’impact des poussières sur la cyclogenèse.
Intégration de l’identité et de la diversité fonctionnelle dans un modèle de surface terrestre : une étude de la productivité et de la stabilité d’un écosystème dans un contexte de changement climatique
19/09/2025 09:30
La végétation terrestre joue un rôle clé dans la régulation du climat via l’absorption de CO₂ atmosphérique. Les modèles de surface terrestre, tels que le modèle ORCHIDEE, représentent la végétation via des paramètres fixes dans l’espace et dans le temps selon les types fonctionnels de plantes. Cette représentation néglige l’identité fonctionnelle et la diversité au sein des écosystèmes. Cette limite est marquée pour les prairies permanentes, riches en biodiversité.
Dans cette thèse, j’ai intégré des métriques fonctionnelles à l’échelle des communautés végétales afin d’améliorer la représentation de la productivité et de la stabilité des prairies permanentes françaises pour la période 1960-2099 :
- Variabilité spatiale des traits: j’ai étudié l’effet de cartes de traits clés (SLA, azote foliaire, durée de vie des feuilles) sur la productivité simulée.
- Identité fonctionnelle et stabilité temporelle: j’ai analysé l’effet des traits moyens (CWM) sur la productivité et la stabilité des prairies, dans le passé et le futur, selon différents scénarios climatiques.
- Adaptation des traits dans le temps: j’ai testé différentes hypothèses d’évolution des traits physiologiques (SLA, Vcmax).
Ce travail de doctorat démontre qu’il est possible d’intégrer l’identité et diversité fonctionnelle dans ORCHIDEE. Ces avancées permettent d’améliorer la prédiction de la productivité et de la stabilité des prairies permanentes, contribuant ainsi à réduire les incertitudes sur l’évolution des écosystèmes dans un contexte de changement climatique.
Terrestrial vegetation plays a key role in climate regulation through the absorption of atmospheric CO₂. Land surface models, such as the ORCHIDEE model, represent vegetation using fixed parameters in space and time based on plant functional types. This approach neglects the functional identity and diversity within ecosystems. This limitation is particularly pronounced for permanent grasslands, which are hotspots of biodiversity.
In this thesis, I introduced functional metrics at the plant community level to improve the representation of productivity and stability of French permanent grasslands over the period 1960–2099:
- Spatial variability of traits: I studied the impact of trait maps (SLA, leaf nitrogen, leaf lifespan) on simulated productivity.
- Functional identity and temporal stability: I analyzed the influence of community-weighted mean traits (CWM) on grassland productivity and stability, both in the past and under future climate scenarios.
- Temporal Trait adaptation: I tested different hypotheses regarding the evolution of physiological traits (SLA, Vcmax).
This PhD work demonstrates the feasibility of integrating functional identity and diversity into ORCHIDEE. These advancements enhance the prediction of productivity and stability in permanent grasslands, thereby helping to reduce uncertainties regarding ecosystem responses in the context of climate change.
monstrates that it is possible to integrate functional identity and diversity into ORCHIDEE. These advances improve predictions of grassland productivity and stability, by reducing uncertainties about ecosystem responses and supporting sustainable land management and biodiversity conservation.
