the September cold snap, a “banal” fluctuation in a very hot year

It’s a feeling that we had almost forgotten at the end of a scorching summer. It has been cold in France in recent days, colder than normal for the season, with a difference of -4.9°C recorded on Sunday September 18. In a context of global warming, where it is getting hotter and hotter as a result of human activities (transport, housing, industry, agriculture, energy), this early coolness came as a surprise. However, “there’s really nothing very exceptional. It’s quite common at this time of year even if it may surprise given the temperatures encountered in recent weeksanalyzes Samuel Morin, researcher at Météo France and director of the National Center for Meteorological Research.

The climatologist considers the reactions to this first autumn offensive more unusual. “In a warming climate, cool episodes are more and more surprising, because we are acclimating to change”, he continues. We thus collectively lose our meteorological memory and we are surprised by occasional episodes which would not have triggered any reaction ten years ago. A US study conducted in 2019 (in English) concluded that each of us forms an idea of ​​”normal” weather based on the past two to eight years.

The data from Météo France are clear. These episodes of freshness at the start of the school year have become less and less frequent in recent years. The very cool months of September, like that of 1972, with 14 days below -3°C compared to the 1991-2020 normal, are only a distant memory. The average was two days over the period 2011-2021, compared to 4.6 over the period 1947-2010.

Conversely, abnormally hot days (+3°C compared to normal) are more frequent in September. From 3.9 days over the period 1947 to 2010, we drop to 8.4 over the decade 2011-2021.

Generally, “there is an asymmetry between hot days, which are increasingly hot and increasingly frequent, and cool, even cold days, which can lead to breaking records, but in proportions five to ten times less frequent than heat records”, recalls Samuel Morin. The year 2022 illustrates this situation, with these numerous heat waves.

As with every cold snap, some climatosceptics wanted to see in these few days of September proof of the non-existence of global warming. The opportunity for Samuel Morin to do a little pedagogy. “When we look at the evolution over several decades, we have a marked warming, of the order of 1.7°C in France since the beginning of the 20th century.he recalls. An increasingly hot climate, however, does not prevent weather conditions from fluctuating from day to day, as they always have and will continue to do”. An isolated cold snap therefore does not call into question the warming trend.

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