On a meadow in Laqueuille, perched at 1040 meters in the Massif Central, grass doesn't grow at the whim of weather but follows a schedule set by the ocean thousands of kilometers away. From 1959 to 2015, the PaSim pasture model, fed with hourly weather data from the SAFRAN archive, simulated grass growth while excluding pests and farmer interventions. What remained was a pure climatic signal. It turned out that the meadow's yield is higher in sunnier years, because light, not rain, limits growth in this subarctic climate. Over 57 years, the simulated yield increased by 29%, consistent with warming, rising CO2, and increased solar radiation.
The researchers compared yield anomalies with sea surface temperatures in the Mediterranean in February and September, in the tropical Atlantic from September of the previous year through January, and with El Niño (the periodic warming of the equatorial Pacific) in May-June. The links proved robust, especially for the three-cut scenario. A statistical model with cross-validation, where each year is excluded from training in turn, achieved a correlation of 0.55 and 71.43% accuracy in predicting 'above/below average' against a baseline of 50%. For optimal management, mimicking farmer practices, the numbers were more modest: correlation 0.41 and 67.86% accuracy.
This is the first time in Europe that a bridge has been built from ocean temperature to a specific agroecosystem indicator. Previously, climate services focused on grain crops, leaving pastures aside, even though they cover up to 60% of agricultural land in the Massif Central and support dairy farming. Uncertainty in forage hits the economy, so the ability to look months ahead changes planning: farmers and cooperatives can prepare for haymaking in advance, insurance companies can assess risks, and government agencies can monitor food security.
But the conductor isn't always precise. The predictive power of El Niño has varied over decades, which calls for caution: the links are non-stationary. Nevertheless, the result provides a basis for seasonal forage forecasting. The next step is validation with real data, including grazing rather than just cutting, and extending to other European grasslands.
The meadow in Laqueuille sits at an altitude of 1040 meters, and its climate is classified as subarctic: sunlight is so scarce that radiation, not precipitation, limits grass growth. So in sunny years, yields are higher, even though intuitively rain seems more important.