Seasonal Transformation of the Thermal Regime in Bulgaria (1950–2025): A Spatiotemporal Analysis Using ERA5-Land data set and Google Earth Engine

This research presents a high-resolution spatiotemporal analysis of surface air temperature trends in Bulgaria over a 76-year period (1950–2025). Utilizing the ERA5-Land reanalysis dataset and the Google Earth Engine (GEE) cloud computing platform, the study quantifies seasonal warming magnitudes and identifies structural shifts in the regional thermal regime. Results demonstrate a definitive transition from a stable continental climate to an accelerated warming phase, particularly after 1987. The total warming magnitude since 1950 is most pronounced in summer (+3.2°C) and winter (+2.9°C), with spring and autumn showing significant increases of +2.4°C and +2.1°C, respectively. A comparative dual-trend analysis reveals that warming velocity has approximately doubled during the most recent 20-year window (2005–2025), with decadal slopes in summer reaching nearly +0.80°C. Statistical robustness, evidenced by a significant rise in the coefficient of determination (R2) during the 21st century, indicates that the warming signal has become a forced response that overrides natural stochastic variability. Spatially, the Danubian Plain and Upper Thracian Lowland have emerged as warming hotspots, exhibiting total magnitudes exceeding +3.8°C, effectively signaling a "Tropicalization" of the Bulgarian lowlands. These shifts are linked to critical environmental consequences, including the "False Spring" paradox – where early thermal onset increases frost vulnerability – and a destabilized hydrological cycle characterized by premature snowmelt and a transition to a drought-to-flood regime. The findings align with the other regional studies identifying the Balkan Peninsula as a high-sensitivity climate hotspot, with warming rate higher than the global terrestrial average. This study underscores the urgent necessity for climate-resilient adaptation strategies in Bulgarian agriculture, water management, and urban planning to address the high-velocity thermal transformation of the territory.