In early April 2021 several days of severe frost affected central Europe following an anomalously warm March. This led to very severe damages in grapevine and fruit trees, particularly in France, where young leaves had already unfolded in the warm early spring. Using published peer-reviewed methods we analysed how human-induced climate change affected the temperatures as extreme as observed in spring 2021 over central France, where many vineyards are located. We found that although climate change made the temperatures of the observed event less cold than they would have been without the burning of fossil fuels over the last centuries, the fact that climate change has also led to an earlier start of the growing season means that frost damage in young leaves has become more likely due to human-induced climate change.
Main findings
- By analysing temperature observations and climate model simulations we found that without human-caused climate change, such temperatures in April would have been even lower by approx. 1.2°C.
- However, human-caused warming also affected the earlier occurrence of bud burst, characterized here by a growing-degree-day index value. This effect is stronger than the decrease in spring cold spells, thus exposing young leaves to more winter-like conditions with lower minimum temperatures and longer nights.
- The climate models used here simulate a cooling of growing-period annual temperature minima of about 0.5°C. This effect is larger in the observations based analysis than in climate model simulations, this number thus serves as a lower bound rather than an exact quantification.
- Overall, we conclude that human-caused climate change made the 2021 event 20% to 120% more likely.
- In a climate with global warming of 2°C (compared to the present day level of global warming of about 1.2°C) growing-period frost events such as observed in 2021 are projected to further intensify by about 0.2°C to 0.5°C.
Figure 1: Stations with March (left) high records broken and April (right) low records broken in 2021 in France. Source: Météo-France.
Background
Frost days and cold spells are decreasing in frequency in the northern latitudes. Yet, due to the natural chaotic variability of the atmosphere, severe cold spells continue to occur across the mid-latitudes. When occurring after the start of the growing period, the invasion of polar air into central and Southern Europe can create devastating frost damages in young leaves such as happened in early April 2021.
From 6 to 8 April 2021 exceptionally low daily minimum temperatures, below -5°C, were recorded in several places, leading to severe damages in grapevines and fruit trees in these places. The temperatures broke negative records at many weather stations (see Figure 1). To make matters worse, the cold event happened a week after an episode of record-breaking high March temperatures over France and Western Europe (Figure 1), leading to the growing season to start early and leaving new leaves exposed to the deep frost episode that followed.
The resulting damages affected “several hundreds of thousands of hectares” according to the French Ministry of Agriculture that also called the event “probably the biggest agricultural disaster in the beginning of the 21st century”.
To investigate the role of climate change in this event we focussed on central/northern France [-1°- 5°E; 46°-49°N] in order to investigate a relatively homogeneous, low-elevation area. It covers most of the grapevine agriculture areas of Champagne, Loire Valley and Burgundy which were identified as specifically vulnerable.
The event is further defined as the minimum temperature (TNn) conditioned on the so-called Growing Degree Day (GDD). The GDD is the cumulative temperature above 5°C after the winter solstice that leads to buds bursting. This cumulative temperature threshold is species dependent and was between 150°C and 350°C the day before the frosts started. This is high for the season, but not a record, which was in fact set in 2020. Given we are not interested in a specific species but the overall frost damage to agriculturally important species we consider three different GDD thresholds: 150°C, 250°C and 350°C. We also considered minimal temperatures in a range of GDDs : between 250 and 350, in order to characterize the early growing season when leaves are vulnerable. For each of these three thresholds or ranges we calculate the minimum temperature in April-July after the GDD threshold has been reached, denoted as TNnGDD150, TNnGDD250, TNnGDD350 and TNnGDD250-350, averaged over the region of central France. We also looked at the change in intensity and frequency for the minimum April-July temperatures over the same region alone, finding that the event would have been even colder without human-caused climate change.
Conditioning the assessment on the four GDD-indices we found instead that frost intensities on bursted buds increased due to climate change. While the exact quantitative results differ between the indices, the results are qualitatively the same.
The differences between the changes in TNnGDD-indices simulated in the models and those calculated based on observations are large, we thus have confidence in the qualitative results but not the exact quantification.
