Every ten years, Météo-France updates the reference period for its “climate normals”, more commonly referred to as “seasonal”, to align with the recommendations of the World Meteorological Organization (WMO).
Over the new reference period 1991-2020, the new average annual temperature normal in France is 12.97°C, up by just over 0.4°C compared to the previous reference period 1981- 2010 (12.55°C), according to Météo-France.
The climatic normals, statistical products, make it possible to “characterize the climate” over a given period, by convention a period of 30 years, and “serve as a reference for analyzing climatic events in real time”, explain the French weather services.
It is in spring and summer that the increase in the new annual temperature normal is greatest. It is also slightly more marked on the continental east (Grand-Est and Bourgogne-Franche-Comté) and a little less on the coastal areas (Brittany and Corsica).
The average annual temperature was 11.82°C between 1961 and 1990, the period that the WMO recommends taking as a reference for communication on climate change.
Overseas, the increase in this indicator is a little less marked, and it even drops at the Dumont d’Urville polar station (Antarctica), by 0.2°C.
Compared to the previous period, the number of days of high heat (maximum temperature greater than or equal to 30°C) increased in particular in Nîmes (8 days), Figari (9) or Marignane (10).
These changes in climatic normals do not call into question the heat wave vigilance thresholds, which “are calculated in relation to bio-meteorological indices”, in collaboration with Public Health France and other agencies, explains Matthieu Sorel, climatologist at Météo- France.
The number of days of frost (temperature below 0°C) is down, from 8 days in Troyes, Poitiers, Langres or Chambéry, and up to 10 days in Lyon.
For its part, the average cumulative precipitation changes little (a range between 911 and 935 mm over the updates), except in the North-East where this average cumulative decreases more significantly.
Conversely, in PACA and Corsica, average precipitation increases, particularly during the groundwater recharge period (September to March).
– Other factors too –
Météo-France also notes a more marked drying of the soils from the Massif Central to the Grand Est, particularly in summer and autumn, but wetter soils in certain areas such as the western facade, except Poitou.
Climate normals are used in different sectors, such as agriculture or energy, and also allow the comparison of climatic conditions between different places.
Or, to make comparisons on the evolutions of the climate in the long term.
However, several parameters can cause differences between the reference periods: changes in the measurement conditions of a weather station, evolution of the fleet of stations used to calculate an aggregate indicator, evolution of WMO directives.
“The explanation we all want to have in mind is of course the evolution of the climate in this context of climate change, which is still quite significant. The difficulty we have is that all these effects are combined and, unfortunately, difficult to separate”, nuance Matthieu Sorel.
“You have to take a lot of tweezers when studying the differences with previous normals, you can’t directly attribute a difference to the evolution of climate change, even if you would still like to do it, especially since the results that we still pleaded in favor of that,” he adds.
