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.

Representation and analysis of climate-carbon-nitrogen interactions over permafrost regions in the IPSL Earth system model

12/12/2024 14:00

Permafrost soils, found in cold regions of the globe, contain large amounts of organic carbon. These carbon stocks are threatened by the strong Arctic warming, which causes permafrost to thaw, exposing previously frozen organic matter to decomposition. This results in CO₂ and CH₄ emissions that amplify global warming through the so-called permafrost carbon-climate feedback, with implications for carbon budgets and emission reduction pathways. However, both the timing and magnitude of this feedback remain highly uncertain. In fact, the future dynamics of the permafrost carbon cycle have only been assessed using land surface models (LSMs) or models of intermediate complexity, with contrasting responses.

Among the models of the Coupled Model Intercomparison Project Phase 6 (CMIP6), launched in 2014 to better understand the responses of climate system to anthropogenic forcings, only two Earth System Models (ESMs) include permafrost carbon, and both share the same land component. This thesis sheds new light on the subject by incorporating permafrost mechanisms into another ESM.

As part of my thesis, I developed a new Earth system model, called IPSL-Perm-LandN, building on physical and biogeochemical permafrost processes originally developed at the Institut Pierre-Simon Laplace (IPSL) in the early 2010s. This involved incorporating additional features, such as soil insulation by groundcover, into a version of the land surface model that already included an explicit representation of the land nitrogen cycle.

As a first step towards improving the realism of the representation of the Arctic climate, the latent heat of soil water phase change and soil insulation by soil organic carbon and a surface organic layer (i.e. mosses, lichens, litter) are shown to strongly influence the surface air temperature and snowfall fraction. Their inclusion in the model leads to an improvement of the high-latitude climate simulations and of the thermal state of permafrost, which is consistent with observations and satellite products. IPSL-Perm-LandN is evaluated against observations and data-driven products over the historical period (1850-2014) and consistently simulates much larger permafrost soil carbon stocks than the previous version of the IPSL ESM. The permafrost region is found to be a net carbon sink in recent decades with a net land-atmosphere carbon flux consistent with the upscaling of flux measurements.

Under future increasing atmospheric CO₂ concentrations, the permafrost region remains a carbon sink in IPSL-Perm-LandN despite significant soil carbon losses caused by permafrost thaw, due to counteracting negative feedbacks. In particular, the increased nitrogen availability following permafrost thaw is found to reduce vegetation nitrogen limitation and thus to increase land carbon uptake, although this effect is likely to be overestimated. Finally, the model simulates irreversible land and ocean carbon changes under atmospheric CO₂ overshoot pathways. In aggressive mitigation scenarios, land and ocean turn into carbon sources, partially offsetting mitigation efforts and highlighting the need to minimise temperature overshoots as much as possible.

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.