Drought risk conditions determine the recent evolution in crop insurance claims in France

Farmers have always lived with the weather, but in recent decades, the stakes have risen as climate change makes extreme events more frequent and severe. When a summer is too dry or a spring brings unexpected frost, crops can fail, threatening the livelihood of those who grow them. To manage this uncertainty, many farmers in France have turned to crop insurance, a financial safety net that pays out when nature delivers a blow. However, for the insurance companies that underwrite these policies, the math is becoming increasingly difficult. As the climate shifts, the frequency and intensity of droughts are changing, potentially making past insurance models obsolete. If the risk of crop failure rises faster than the premiums collected, the entire system can become unstable. Understanding exactly how these changing weather patterns translate into financial losses is no longer just an academic exercise; it is a necessity for keeping food production secure and insurance markets viable.

In a new study, researchers from a French university and a major insurance company joined forces to untangle this complex relationship. They focused on maize, one of the most widely grown crops in France and one that is particularly sensitive to a lack of water. The team wanted to see if they could build a digital model that tracks the severity of droughts across the country and compare it directly to the actual money paid out by insurance companies. By doing this, they hoped to determine if their model could accurately predict the financial impact of dry spells, providing a clearer picture of how climate change is reshaping the risk landscape for agriculture.

The researchers began by constructing a detailed picture of the French climate over the last sixty years. They used a sophisticated computer system that combines historical weather data with an understanding of how soil holds water. This system allowed them to simulate the daily water balance for maize crops across the entire country, grid by grid. They accounted for how much water the soil could hold, how much water the plants needed at different stages of their growth, and how much water evaporated into the air. From this simulation, they created a single, easy-to-read number for every year and every region: a drought index. This number essentially measures the total water deficit a crop experiences during its growing season, acting as a scorecard for how dry the conditions were.

With this drought scorecard in hand, the team turned to real-world financial records. They obtained data from their insurance partner covering the years 2011 to 2022. This data included the total value of maize crops insured in each region and the total cost of claims paid out when those crops suffered damage. To make a fair comparison, they calculated a "destruction rate," which is simply the ratio of money paid out to the total value of the crops insured. This metric strips away the influence of crop prices and focuses purely on the physical loss of the harvest. They then compared this financial destruction rate against their drought index for the same years and regions.

The results were striking. At the level of major maize-growing regions, the drought index and the insurance payout rates moved in lockstep. In areas like Brittany and the central west of France, the correlation was exceptionally strong, meaning that when the model predicted a severe drought, the insurance companies paid out significantly more. In the northeast, the link was also clear, though slightly less pronounced. The only region where the connection was weak was the southwest, where the researchers suspect the data might be clouded by irrigation. Because their model did not account for water pumped from rivers or wells, it could not fully explain the losses in areas where farmers might have been irrigating their fields, leading to a mismatch between the dryness score and the actual insurance claims.

To ensure their findings were robust, the team tested how different settings in their model affected the results. They tried changing the assumed planting dates, the length of the growing season for different corn varieties, and the specific threshold at which a plant is considered "stressed" by lack of water. They found that the planting dates and crop varieties made very little difference to the overall accuracy. However, the threshold for stress was critical. The model worked best when it assumed that maize plants begin to suffer significant damage even when the soil still retains a small amount of water, rather than waiting until the soil is completely bone dry. This suggests that the plants are more vulnerable to water shortages than a simple "dry soil" definition might imply.

The study also looked at how drought conditions have evolved over time. By comparing the drought scores from the 1960s and 1970s with those from the last decade, the researchers observed a clear upward trend in drought severity across most of France. This increase is largely driven by a rise in evaporation caused by warmer temperatures, which pulls more moisture from the soil and the plants. The most dramatic shifts occurred after the late 1980s, a period marked by a sudden jump in temperatures. Since 2011, the drought index has continued to climb, indicating that the risk to maize crops is intensifying just as the insurance contracts have been in place.

This work demonstrates that a computer model based on soil water and plant needs can serve as a reliable predictor for insurance losses. It provides a tangible link between the physical reality of a drying climate and the financial reality of insurance claims. For the insurance industry, this offers a new tool to better understand and price risk in a changing world. For the agricultural sector, it highlights the growing vulnerability of key crops to water deficits. While the model is currently tuned for maize, the researchers believe the approach could be adapted for other crops and other weather risks, such as frost or heavy rain. The ultimate goal is to create a more resilient system where the financial protection farmers rely on keeps pace with the evolving climate, ensuring that agriculture can continue to thrive despite the increasing unpredictability of the weather.