Changes in climate-related hazards conducive to wildfires in metropolitan France under global warming. Risk evolution and insurance loss modeling

Started in april 2025

Funding: Cifre grant, in partnership with Covéa Group

Sujpervisors: Albin Ullmann and Thierry Castel

Abstract

Forest fires are primarily caused by human activity, whether accidental or intentional (Ganteaume et al., 2013; Ganteaume & Jappiot, 2013; Curt et al., 2016; Ganteaume & Guerra, 2018), while natural causes such as lightning account for only a small fraction—around 5% globally. Climate change, by intensifying droughts and heatwaves (Ferranda et al., 2023; Seneviratne et al., 2006), contributes to vegetation drying and increases the risk of wildfires in France (Jones et al., 2022). Although the expansion of burned areas may appear to stem from a higher number of fires, the year 2022—with over 60,000 hectares burned despite a similar number of fires to 2019—mainly highlights the emergence of “mega-fires.” This trend reflects the influence of global warming, linked to changes in the water cycle (Laurent et al., 2024), vegetation water balance (Laurent et al., 2023), and the increasing duration and severity of dry spells since the 1950s (Raymond & Ullmann, 2021).

Since 2014/2015, an increase in potential evapotranspiration (PET) appears to signal a new phase in the progression of global warming. In contrast to the warming observed in the late 1980s, this recent rise in PET has not been accompanied by a corresponding increase in actual evapotranspiration (AET), resulting in an elevated PET/AET ratio, particularly during the spring season. This growing imbalance reflects an intensification of hydric stress, thereby increasing the physiological vulnerability of vegetation and contributing to a structural transformation of drought risk. The increase in PET is primarily attributable to the combined effects of rising surface temperatures and enhanced solar radiation, further reinforced by persistent large-scale anticyclonic conditions (Farranda et al., 2023) and by regional and local climatic and physiographic characteristics (Laurent, 2024; Petit et al., 2023). The recurrent nature of droughts, which vary in severity across territories, necessitates the integration of both long-term climatic trends (on the scale of several decades) and short-term extreme events—such as heatwaves and dry spells—at spatial resolutions appropriate for the assessment and insurance-related management of wildfire risk. Repeated water deficits promote the progressive desiccation of plant biomass, which, when combined with strong winds, creates highly conducive conditions for the ignition and rapid propagation of wildfires. Concurrently, the expansion of built environments at the wildland–urban interface increases the incidence of anthropogenic fire ignitions, thereby exacerbating insured losses.

A central scientific objective of this thesis is to document regionalized changes in climate indicators (e.g., dry spells, heat waves) and bioclimatic indicators (e.g., ETP/ETR ratio, forest water balance) across nested temporal scales, ranging from multi-annual to intra-seasonal. The anticipated detailed characterization of these hazards is intrinsically linked to the second challenge, which concerns both environmental and insurance vulnerability. By articulating and integrating these two dimensions of vulnerability, this thesis seeks to contribute value by enabling the implementation of risk modeling. This modeling approach will be validated through a case study employing ensemble simulations based on the WRF-Fire model (Coen et al., 2013).

Keywords

Wildfire ; Climate hazard ; bioclimatic precursors ; insurance risk

extrait:

lien_externe:

titre:

Modifications des aléas climatiques favorables aux feux de forêt en France métropolitaine en contexte de réchauffement. Évolution et modélisation des risques et des pertes assurantielles

date_de_debut_these:

avril 2025

nom:

Prunot

date_de_debut_these_numerique:

20250401

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kc_raw_content:

Changes in climate-related hazards conducive to wildfires in metropolitan France under global warming. Risk evolution and insurance loss modeling

Started in april 2025

Funding: Cifre grant, in partnership with Covéa Group

Sujpervisors: Albin Ullmann and Thierry Castel

 

Abstract

Forest fires are primarily caused by human activity, whether accidental or intentional (Ganteaume et al., 2013; Ganteaume & Jappiot, 2013; Curt et al., 2016; Ganteaume & Guerra, 2018), while natural causes such as lightning account for only a small fraction—around 5% globally. Climate change, by intensifying droughts and heatwaves (Ferranda et al., 2023; Seneviratne et al., 2006), contributes to vegetation drying and increases the risk of wildfires in France (Jones et al., 2022). Although the expansion of burned areas may appear to stem from a higher number of fires, the year 2022—with over 60,000 hectares burned despite a similar number of fires to 2019—mainly highlights the emergence of “mega-fires.” This trend reflects the influence of global warming, linked to changes in the water cycle (Laurent et al., 2024), vegetation water balance (Laurent et al., 2023), and the increasing duration and severity of dry spells since the 1950s (Raymond & Ullmann, 2021).

Since 2014/2015, an increase in potential evapotranspiration (PET) appears to signal a new phase in the progression of global warming. In contrast to the warming observed in the late 1980s, this recent rise in PET has not been accompanied by a corresponding increase in actual evapotranspiration (AET), resulting in an elevated PET/AET ratio, particularly during the spring season. This growing imbalance reflects an intensification of hydric stress, thereby increasing the physiological vulnerability of vegetation and contributing to a structural transformation of drought risk. The increase in PET is primarily attributable to the combined effects of rising surface temperatures and enhanced solar radiation, further reinforced by persistent large-scale anticyclonic conditions (Farranda et al., 2023) and by regional and local climatic and physiographic characteristics (Laurent, 2024; Petit et al., 2023). The recurrent nature of droughts, which vary in severity across territories, necessitates the integration of both long-term climatic trends (on the scale of several decades) and short-term extreme events—such as heatwaves and dry spells—at spatial resolutions appropriate for the assessment and insurance-related management of wildfire risk. Repeated water deficits promote the progressive desiccation of plant biomass, which, when combined with strong winds, creates highly conducive conditions for the ignition and rapid propagation of wildfires. Concurrently, the expansion of built environments at the wildland–urban interface increases the incidence of anthropogenic fire ignitions, thereby exacerbating insured losses.

A central scientific objective of this thesis is to document regionalized changes in climate indicators (e.g., dry spells, heat waves) and bioclimatic indicators (e.g., ETP/ETR ratio, forest water balance) across nested temporal scales, ranging from multi-annual to intra-seasonal. The anticipated detailed characterization of these hazards is intrinsically linked to the second challenge, which concerns both environmental and insurance vulnerability. By articulating and integrating these two dimensions of vulnerability, this thesis seeks to contribute value by enabling the implementation of risk modeling. This modeling approach will be validated through a case study employing ensemble simulations based on the WRF-Fire model (Coen et al., 2013).

 

Keywords

Wildfire ; Climate hazard ; bioclimatic precursors ; insurance risk