Grass in a French meadow doesn't grow in isolation. Its yield depends on what happens in oceans thousands of kilometers away. Scientists took the PaSim pasture model and fed it weather data from 57 years, from 1959 to 2015. The model simulated grass growth, stripping away all the extras: pests, diseases, and farmer decisions. Only the pure climatic signal remained.
It turned out that grass yield is linked to Mediterranean temperatures in February and September, tropical Atlantic temperatures from September to January, and El Niño (the periodic warming of the equatorial Pacific) in May and June. Based on these links, they built a forecast that correctly predicts whether yields will be above or below average in 71 cases out of 100, while random guessing would give only 50. This is the first such study for Europe.
Previously, forecasts were made for grain crops, while pastures were left out. But in France's Massif Central, meadows cover up to 60% of agricultural land and feed dairy cows. If a farmer knows in advance that grass will be scarce, he can stock up on hay or adjust his herd size. Now he has a tool to look several months ahead. However, the links are not always stable: El Niño's strength has varied over decades, so the forecast requires caution. But the first step is taken: the ocean dictates, and we are learning to listen.
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.