Human influence on growing-period frosts like the early April 2021 in Central France

In early April 2021 several days of harsh frost affected central Europe. This led to very severe damages in grapevine and fruit trees in France, in regions where young leaves had already unfolded due to unusually warm temperatures in the preceding month (march 2021. We analysed with observations and 172 climate model simulations how human-induced climate change affected this event over central France, where many vineyards are located. We found that, without human-caused climate change, such temperatures in April or later in spring would have been even lower by 1.2 °C [0.75 °C;1.7 °C]. However, climate change also caused an earlier occurrence of bud burst, that we characterized in this study by a growing-degree-day index value. This shift leaves young leaves exposed to more winter-like conditions with lower minimum temperatures and longer nights, an effect that over-compensates the warming effect. Extreme cold temperatures occurring after the start of the growing season such as those of April 2021 are now 2 °C colder [0.5 °C to 3.3 °C] than in pre-industrial conditions, according to observations. This observed intensification of growing-period frosts is attributable, at least in part, to human-caused climate change with each of 5 climate model ensembles used here simulating a cooling of growing-period annual temperature minima of 0.41 °C [0.22 °C to 0.60 °C] since pre-industrial conditions. The 2021 growing-period frost event has become 50 % more likely [10 %–110 %]. Models accurately simulate the observed warming in extreme lowest spring temperatures, but underestimate the observed trends in growing-period frost intensities, a fact that remains yet to be explained. Model ensembles all simulate a further intensification of yearly minimum temperatures occurring in the growing period for future decades, and a significant probability increase for such events of about 30 % [20 %–40 %] in a climate with global warming of 2 °C.