Trends in the annual snow melt-out day over the French Alps and the Pyrenees from 38 years of high resolution satellite data (1986–2023)

Abstract. Information on the spatial-temporal variability of the seasonal snow cover duration over long time periods is critical to study the response of mountain ecosystems to climate change. However, this information is often lacking due to the sparse distribution of in situ observations or the lack of adequate remote sensing products. Here, we combined snow cover data from ten different optical platforms including SPOT 1-5, Landsat 5-8 and Sentinel-2A&B to build a time series of the annual snow melt out day (SMOD, i.e. the first day of no snow cover) at 20 m resolution across the French Alps and the Pyrenees (43×10³ km²). We evaluated the pixel-wise accuracy of the computed SMOD using in situ snow measurements at 344 stations. We found that the residuals are unbiased (median error of 1 day) despite a dispersion (RMSE of 28 days), which suggests that this dataset can be used to study SMOD trends after spatial aggregation. We found an average reduction of 20.4 days (5.51 days per decade) over the French Alps and of 14.9 days (4.04 day per decade) over the Pyrenees over the period 1986–2023. The SMOD reduction is robust and significant in most part of the French Alps and can reach one month above 3000 m. The trends are less consistent and more spatially variable in the Pyrenees. This dataset is available for future studies of mountain ecosystems changes and is updated every year using Sentinel-2 data.