Atelier national sur les nuages polaires

24/06/2025 09:00

SIRTA / ICEO : Journée Scientifique 2025

24/06/2025 09:00

Evénement de clôture projet FAIR-EASE

12/06/2025 09:00

Politique de l’adaptation, enjeu d'action publique national et international pour la France

19/01/2024 14:00

Nouvelle séance du séminaire « Changement Climatique : Sciences, Sociétés, Politique » co-organisé par le Centre Alexandre-Koyré (EHESS-CNRS) et l’ENS (CERES).

Renewables for decarbonizing the electricity sector: which efficient policies

17/01/2024 16:00

Séminaire du cours « Transition énergétique » du CERES (ENS).

Climate science in court: how scientific evidence can clarify states’ and companies’ legal responsibility for climate change

16/01/2024 11:00

Séminaire du département de Géosciences de l’ENS.

Are the ocean fine-scales affecting latent heat flux spatial variability in the Northwest Tropical Atlantic?

29/11/2024 16:00

The ocean and the atmosphere are two key components of the climate system playing an essential role in heat redistribution and storage. Heat is exchanged between these components across the air-sea interface in the form of radiative and turbulent heat fluxes. The former are associated with the presence of electromagnetic waves whose ultimate source is a radiating body at a certain temperature. The latter occur because heat is transported from one medium to the other by an imbalance between a given property and a subsequent atmospheric motion. Among the turbulent heat fluxes, we find latent heat flux which results from water evaporation heat release.

Thus, latent heat flux is tightly linked to the evaporation rate and strongly affects cloud formation, precipitation and the large-scale atmospheric and ocean circulations. In this presentation, we will assess the different mechanisms by which the ocean fine-scales affect latent heat flux variations in the Northwest Tropical Atlantic, an a priori relatively quiescent region in terms of air-sea exchanges within the trades. We will perform a multi-dataset approach involving the analysis of remote sensing and in-situ observations, the ERA5 reanalysis and high-resolution coupled simulations. We will focus on three variables: sea-surface temperature, surface currents and sea surface salinity.

Évaluation des impacts régionaux du changement climatique et soutien à l'élaboration de stratégies d'adaptation : le cas du risque de feux de forêt dans le sud-ouest de la France

25/11/2024 14:00

The reality of climate change is no longer in question. It is now widely accepted that we must not only reduce greenhouse gas emissions but also adapt to climate change, as some effects are unavoidable. However, these two goals can be achieved through different ways. Mitigating climate change can be effectively addressed through a top-down approach, such as by international agreements. Adaptation, on the other hand, is more effective through a bottom-up approach. Adapting a region requires considering a wide range of environmental, social, and economic interests. It is therefore essential to co-develop measures with stakeholders that account for local specificities. We need to narrow the gap between climate science and information needed for local adaptation. The aim of this thesis is to develop a bottom-up approach to pursue research that can provide local stakeholders useful climate information for adaptation.

We chose the Nouvelle-Aquitaine Region in France as our case study. In addition to traditional literature review to identify regional impacts of climate change, we also interviewed stakeholders to understand the local specificities, and the information that decision-makers would need to better anticipate climate risks. The pronounced seasonal hydrological cycle presents a significant challenge, particularly in the Landes Forest. The region experiences winter flooding and summer droughts. However, the interests of local stakeholders diverge regarding the strategies to adopt. This led us to develop a comprehensive analytical framework to assess the evolution of climate conditions, enabling the anticipation of various impacts affecting different stakeholders. It provides a common knowledge base from which we can initiate discussions to co-construct compromise solutions. Applied to southwestern France, this analytical framework reveals that extreme hot and dry conditions could become the norm by the end of the century. Such conditions are particularly conducive to the development of wildfires. For this reason, and because fire risk is one of the region’s major concerns, the second part of this thesis focuses on understanding and assessing the evolution of wildfire risk.

We studied firstly the role of climate change in the occurrence of the exceptional wildfires during the summer of 2022 in the region. We developed an index specifically designed to assess long-term climate drivers of wildfires, particularly compound hot and dry conditions. This index complements the commonly used Fire Weather Index (FWI). We found that anthropogenic climate change has doubled the likelihood of conditions such as those experienced in the summer of 2022. Finally, we assessed the evolution of fire risk using the newly developed index and the FWI under three climate warming scenarios. Results demonstrate the benefits of mitigating global warming. In the most pessimistic scenario, extreme hot and dry conditions that are currently propitious to wildfires could become the norm by the end of the century. In the most optimistic scenario, the probability of occurrence of such conditions increases to a much lesser extent. These findings also highlight the importance of implementing adaptation strategies, as the probability of conditions favourable to wildfires increases in all scenarios.

Overall, this PhD thesis offers a dual contribution. Methodologically, we developed research directions based on the need for climate information expressed by local stakeholders. We developed indices for the specific case of wildfires, which could also be adapted to other impacts. From an operational perspective, we provide local decision-makers with climate information to help them better anticipate the impacts of climate change. This work will be further developed, as several avenues for future research have been identified.

Vers une représentation à l'échelle globale du microclimat forestier dans le modèle de surfaces continentales ORCHIDEE

29/11/2024 14:00

Les dynamiques temporelles et spatiales des échanges entre les surfaces continentales et l’atmosphère sont en grande partie contrôlées par la végétation. Dans un contexte de changement climatique, la précision de la modélisation des bilans d’énergie, d’eau et de dioxyde de carbone des écosystèmes dans les modèles de surfaces continentales revêt ainsi d’un double enjeu : elle permet d’améliorer la représentation des échanges entre les surfaces et l’atmosphère et, par conséquent, d’améliorer la fiabilité des modèles de climats ; et elle permet également de comprendre et de quantifier l’impact du changement climatique sur le fonctionnement des écosystèmes végétaux.

Dans la majorité des modèles, la structure de la végétation est simplifiée, considérée équivalente à une surface d’épaisseur infinitésimale échangeant de l’eau, de l’énergie et des composés avec l’atmosphère (modèle de type « grosse-feuille »). La dynamique complexe des échanges au sein des écosystèmes végétaux, et particulièrement des forêts, incluant le microclimat intra-canopée, reste très mal ou pas représentée dans les modèles actuels. Ce microclimat joue cependant un rôle important dans la régulation des échanges d’énergie et de masse entre la végétation et l’atmosphère et son évolution dans un contexte de changement climatiques est méconnue.

Cette étude présente les premières étapes effectuées dans le modèle ORCHIDEE (composante de surface du modèle de climat de l’IPSL) pour l’étude de ce microclimat intra-canopée à l’échelle globale. La représentation simpliste de type « grosse-feuille » utilisée dans ORCHIDEE est remplacée par un modèle d’échanges d’eau et d’énergie au sein de la canopée (suivant une discrétisation verticale). L’intégration de ce modèle est effectuée en deux étapes. La première s’attache à la représentation du transport de l’eau dans le continuum sol-plante-atmosphère et a pour objectif de contraindre les échanges feuille-atmosphère grâce à l’état hydrique de la végétation. Ce travail s’appuie sur une représentation du potentiel hydrique dans les différents compartiments de la plante (i.e. architecture hydraulique). Cette intégration est étudiée de manière détaillée à l’échelle du site avant une étude d’impact globale.

La seconde étape consiste à la mise à jour, la mise à niveau et l’amélioration d’un modèle d’échanges d’eau et d’énergie multi-couches entre la végétation et l’atmosphère précédemment implémenté dans une branche d’ORCHIDEE. L’évaluation de ce modèle est effectuée à l’échelle des sites forestiers en comparaison du modèle d’écosystèmes MuSICA sur une base de données crée à cet effet. La comparaison des gradients de température intra-canopée simulés et observés est très encourageante. Elle a aussi permis d’identifier des pistes pour l’amélioration globale du modèle. Enfin, des perspectives sont discutées pour une utilisation de ces modèles à l’échelle globale et notamment pour simuler l’évolution du microclimat sous une canopée forestière en fonction du changement climatique et des pratiques forestières.

 

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The temporal and spatial dynamics of exchanges between continental surfaces and the atmosphere are largely controlled by vegetation. In the context of climate change, accurately modeling the energy, water, and carbon dioxide balances of ecosystems in land surface models presents a dual challenge: it improves the representation of exchanges between surfaces and the atmosphere, thereby enhancing the reliability of climate models; and it also helps to understand and quantify the impact of climate change on the functioning of plant ecosystems. In most models, the structure of vegetation is simplified, treated as equivalent to an infinitesimal thickness surface exchanging water, energy, and compounds with the atmosphere (a « big-leaf » model). The complex dynamics of exchanges within plant ecosystems, particularly forests, including the intra-canopy microclimate, remain poorly represented or not represented at all in current models. However, this microclimate plays a crucial role in regulating energy and mass exchanges between vegetation and the atmosphere, and its evolution in the context of climate change is not well understood.

This study presents the first steps taken in the ORCHIDEE model (the land surface component of the IPSL climate model) to study this intra-canopy microclimate at a global scale. The simplistic « big-leaf » representation used in ORCHIDEE is replaced by a model of water and energy exchanges within the canopy (following vertical discretization). The integration of this model is carried out in two stages. The first focuses on representing water transport in the soil-plant-atmosphere continuum and aims to constrain leaf-atmosphere exchanges based on the water status of the vegetation. This work relies on a representation of water potential in the different compartments of the plant (i.e., hydraulic architecture). This integration is studied in detail at the site scale before conducting a global impact study.

The second step involves updating, upgrading, and improving a multi-layer model of water and energy exchanges between vegetation and the atmosphere previously implemented in a branch of ORCHIDEE. The evaluation of this model is conducted at the scale of forest sites in comparison to the MuSICA ecosystem model based on a database created for this purpose. The comparison of simulated and observed intra-canopy temperature gradients is very encouraging. It has also helped to identify avenues for the overall improvement of the model. Finally, prospects are discussed for using these models at a global scale, particularly to simulate the evolution of microclimate under a forest canopy in relation to climate change and forestry practices.