Extraordinary 2021 snowstorm in Spain reveals critical threshold response to anthropogenic climate change

Attribution of extreme weather events to anthropogenic climate change (ACC) has become an increasingly important line of research in recent years. However, the potential influence of ACC on heavy snowstorms remains largely unexplored. Here we focus on studying the exceptional January 2021 snowfall event in Spain, known as Filomena. First, using observational data and flow analogs, we show that the characteristic synoptic pattern leading to the episode has not significantly changed in frequency over the past decades. Based on this, we assume a fixed dynamical pattern and focus on studying the influence of ACC on the thermodynamics of the event using an atmospheric model and a storyline attribution approach. Our simulations indicate that in northern highlands, ACC intensified snowfall by up to +40% compared with pre-industrial conditions, while in nearby southern lowlands ACC weakened snowfall by up to -80%. This characteristic shift from weakening to intensification is well defined by a critical threshold in temperature. Furthermore, we show that if Filomena were to occur at the end of the 21st century, this contrasting response to ACC would be enhanced. Altogether, our findings highlight the large but uneven impact of global warming on extreme snowstorm events.