Twenty degrees: Luxembourg's must weight since the 1980s

Luxembourg's vineyards lie along its bank of the Moselle, and they are small: 1,164.5 hectares were in production in 2025, down from a high of 1,355 in 1993. What makes them unusually useful is the record kept about them. STATEC, the national statistics office, publishes every year since 1980 the average must weight of the harvest in degrees Oechsle and the average total acidity in grams per litre, both from musts analysed at the state wine institute, alongside the crop in hectolitres, the crop per hectare and, from 1975, the area in production for each grape variety.

The headline is in the title. In the ten harvests 1980 to 1989 the average must weight was 61.0 degrees Oechsle, with 1984's 50 the lowest reading in the whole series and no year above 66. In the ten harvests 2016 to 2025 the average was 81.5, with no year below 76. The difference is 20.5 degrees. Taken by decade the series climbs in steps: 61.8 for 1981–1990, 67.6 for 1991–2000, 77.0 for 2001–2010, 79.9 for 2011–2020 and 80.0 for the five harvests since. The highest reading is 2003, at 87, followed by 2018 at 86 and then 2019, 2020 and 2022 at 85. Measured acidity moved the other way, from an average of 10.7 grams per litre in the 1980s to 7.8 in 2016–2025, and the crop per hectare fell from 130 hectolitres to 73 over the same two periods.

Three things changed under those numbers, and the data let us measure two of them. The first is the weather. Our ERA5-Land grid point for the Mosel sits at 49.9° N, 6.9° E, in the German part of the valley about fifty kilometres north-east of Remich, the nearest point we hold. Its mean temperature across April to September, taken as the average of each day's maximum and minimum, was 13.70 °C in 1980–1989 and 15.65 °C in 2016–2025, a rise of 1.95 degrees. By decade it reads 13.84, 14.39, 14.83 and 15.32 for the four decades from 1981, then 15.51 for 2021–2025. The 1991–2020 average, the current climatological norm, is 14.85. For the record, the same window in 2026 came in at 17.39 °C, the warmest April-to-September at this grid point since the series begins in 1950; STATEC's 2026 must weight is not yet in the file, and we say nothing about it.

The second change is what is planted. In 1980 Elbling and Rivaner, the two varieties that then dominated the vineyard, covered 875 of the 1,283 hectares in production, 68 per cent. By 2025 they covered 256 of 1,164.5 hectares, 22 per cent. Elbling alone went from 281 hectares to 47. Over the same forty-five years Pinot gris rose from 45 hectares to 189, Pinot blanc from 56 to 155, Pinot noir from 2 to 130 and Chardonnay, first recorded in 1997 at under 6 hectares, to 59. The four together went from 8 per cent of the vineyard to 46 per cent. Auxerrois stayed between about 155 and 195 hectares throughout, and Riesling between about 130 and 180. The series does not record must weight by variety, and we have not used any table that does.

The third change, how growers manage their vines and how much fruit they leave on them, is not in these files at all, and the halving of the crop per hectare is the only trace of it.

That is why the year-to-year figures matter more than the long slope. If one simply regresses the annual must weight on the season's mean temperature across the 46 harvests 1980–2025, the line rises 7.5 degrees Oechsle for each degree Celsius, with a correlation of 0.79. But that fit is carried partly by two trends running in the same direction at the same time, warmer seasons and a vineyard replanted away from Elbling and Rivaner, and it cannot tell them apart. Comparing each harvest with the one before removes most of that. Across the 45 consecutive pairs of years, a season one degree warmer than the last went with a must weight 3.8 degrees Oechsle higher, with a correlation of 0.60; 29 of the 45 pairs moved in the same direction, 12 in opposite directions and 4 saw no change in one of the two measures. The largest moves are the clearest: 2022 was 2.16 degrees warmer than 2021 and the must weight rose 9 degrees; 2021 was 2.08 degrees cooler than 2020 and it fell 9; 1984 was 2.0 degrees cooler than 1983 and it fell 12, to the series low; 2018 was 1.86 degrees warmer than 2017 and it rose 7.

What the year-to-year figures support is this: from one season to the next, about four degrees Oechsle of movement go with each degree Celsius of change in the April-to-September mean, and that relation accounts for a little over a third of the year-to-year variation. The long-run rise of two degrees Celsius would, at that rate, account for about seven and a half of the twenty degrees Oechsle gained since the 1980s. The remaining thirteen or so sit with the replanting and with practices the files do not record, in proportions these data cannot settle. We stop there.

Must weight, acidity, crop and area: STATEC, LUSTAT, Indicateurs viticoles (tables D2200 and D2201), CC0; must-weight and acidity averages are STATEC's published results of musts analysed at the Institut viti-vinicole. Weather: Contains modified Copernicus Climate Change Service information 2026.