From Frost to Heat: Changes in Thermal Winter and Winter Warm Extremes in Central Europe

Winter plays a key regulatory role in the natural environment of the temperate zone. It determines the dormancy period of plants, enables the accumulation of water in the form of snow, and limits populations of pests and pathogens. Rising temperatures during the winter season, the shortening of thermal winter, and increasingly frequent warm episodes are leading to the loss of a stable, long-lasting cold phase, which is an important factor regulating ecosystem functioning.

The aim of this study is to assess winter warm episodes and heat waves in the context of the duration of thermal winter in Central Europe. Cases of anomalously high air temperature were analyzed, taking into account the spatial extent of the extreme event, its intensity, and its duration. Variability in the frequency, affected area, and magnitude of the region’s heat load in recent decades was examined using high-resolution (2 km × 2 km) gridded data developed for the years 1961–2024.

The obtained results clearly confirm the shortening of thermal winter in Central Europe – sometimes to only a few to a dozen days – while it is increasingly interrupted by prolonged episodes of above-zero temperatures. An extended area of the region is exposed to thermal extremes (positive values of minimum air temperature and days with maximum temperature exceeding the 95th percentile), and heat surpluses anomalous for the winter months are leading to a progressive destabilization of climate seasonality.

The research findings indicate that the observed changes are not limited solely to an increase in mean temperature but also involve a transformation of the seasonal structure of the climate. The shortening and fragmentation of thermal winter, together with the increasing frequency of warm episodes, weaken the regulatory mechanisms of the natural environment, increasing ecosystem vulnerability to thermal stress and phenological disturbances. In light of projections of continued climate warming, this process may intensify further, making winter one of the key seasons in the analysis of contemporary climate change in Central Europe.

Research funded by the National Science Centre, Poland under the OPUS call in the Weave programme 2022/47/I/ST10/02144