Linking meteorological extremes to clay shrink–swell hazard: Insights from 65 years of climate data

Clay shrink-swell (CSS) behavior arises from fluctuations in soil moisture driven by seasonal cycles of rainfall and drought. This phenomenon causes ground movements that can damage building foundations and infrastructure. In France, where approximately 54% of constructions are exposed to this hazard, CSS ranks as the second most significant category of natural disaster insurance claims.

The French central reinsurance fund reports that the average annual cost, calculated over a five-year sliding window, remained below €300 million in 2016. Since 2017, this figure has increased, reaching about €1.35 billion as of 2025. Climate change is expected to amplify droughts, heatwaves, and precipitation extremes, further intensifying CSS processes and potentially rendering their financial burden unsustainable for insurers.

To address this issue, we analyze meteorological data from the SAFRAN reanalysis provided by Météo-France, which offers daily observations at an 8 km spatial resolution across France since 1958. Our study applies geostatistical and multifractal techniques to characterize spatiotemporal variability, identify scale breaks, estimate extreme values, and examine spectral properties of key climatic variables. Specifically, we compute:

• Multifractality index (α): It measures the speed of change in intermittency;
• Mean singularity (C₁): Average singularity, characterizes intermittency;
• Maximum probable singularity (γₛ): maximum probable singularity.

Tracking these parameters from 1958 to 2025 enables us to identify regions most affected by changes in extremes. Analyses focus on variables influencing CSS behavior, including precipitation, temperature, evapotranspiration, and soil moisture index.

Finally, we compare the evolution of extremes in these climatic parameters with trends in CSS occurrence, quantified through insurance claims. This spatial and temporal comparison between multifractal indicators and affected areas provides insights into the relationship between the intensification of extreme meteorological events and the dynamics of clay shrink-swell processes.

This work is part of the IRGAK (inhibition of clay shrinkage-swelling by K⁺ ion injection) project, founded by the French Agency for Ecological Transition (ADEME). Its objective is to model the link between climate variability and CSS, and to propose adaptation strategies to mitigate a risk that is expected to increase significantly with climate change, leading to escalating insurance costs and growing socio-economic impacts.