Compound Hot–Dry Extremes in Southeastern Europe: From Observed Trajectories to Future Projections and Agricultural Vulnerability in Romania

Compound hot–dry extremes (CHDEs)—manifested through the concurrent occurrence of extreme daily atmospheric temperatures and precipitation deficits (D-CHDEs: daily maximum temperature Tmax ≥ calendar-day 90th percentile, TX90p, and daily precipitation RR < 1.0 mm), compounded by multi-scalar seasonal evaporative drought (SPEI-3, SPEI-6) and root-zone soil moisture depletion—represent one of the most destructive hydrometeorological hazards facing contemporary European agriculture, freshwater security, and socioeconomic infrastructure. While pan-European climate risk assessments predominantly focus on Western and Mediterranean domains, Southeastern Europe and the Lower Danube Basin (predominantly Romania) have emerged as acute, fast-accelerating climate change hotspots characterized by amplified summer warming, rapid evaporative drying, and heightened vulnerability of rainfed agricultural systems. In this study, we provide a comprehensive empirical and modeled assessment of multi-decadal historical trajectories (1970–2025) and near-future projections (2030–2060) of thermal extremes (ETCCDI: SU30, SU35, TR20, TX90p), multi-scalar agricultural drought (SPEI-3, SPEI-6, CDDs), and compound hot–dry frequencies across five agro-climatic sub-regions of Romania benchmarked against pan-European reference domains (MED, CEU, WEU, and EU-27 average) using high-resolution observational datasets (E-OBS v30.0e extended through 2025 via official C3S monthly updates, ERA5-Land reanalysis) and the EURO-CORDEX multi-model regional climate ensemble (EUR-11) under moderate stabilization (RCP4.5) and high-emission (RCP8.5) representative concentration pathways. Our empirical results demonstrate a profound geographical disparity in warming and extreme accumulation: over the 1970–2025 period, summer maximum temperatures (Tmax JJA) in the Romanian Plain and Southeastern Romania increased at +0.62 °C per decade (p < 0.001), substantially outpacing the EU-27 average (+0.38 °C/decade) and Central Europe (+0.41 °C/decade). Pettitt’s changepoint tests identify an abrupt structural regime shift in 1998/1999, after which extreme heat accumulation accelerated dramatically. Extreme Heat Days (SU35: daily Tmax ≥ 35.0 °C, traditionally termed canicular days) in the Romanian Plain surged six-fold, from 3.2 days/year (1971–1990) to 18.4 days/year (2011–2025), culminating in unprecedented peaks of 27–34 days during recent megadrought years (2007, 2012, 2022, 2024). Concurrently, the seasonal frequency of daily compound hot–dry days increased by +264% in southern agricultural zones. Joint Clayton–Gumbel copula modeling indicates that historical 1-in-35-year compound events have compressed into 1-in-5-year return intervals. Under near-future EURO-CORDEX projections (2030–2060), compound event frequencies are projected to surge by +125% to +150% under RCP4.5 and by +240% to +290% under RCP8.5 across the Romanian lowlands. Empirical crop–climate response functions reveal that severe compound events induce grain maize yield losses of −38% to −52%, far exceeding the EU average (−12% to −22%) due to structural vulnerability and low irrigation coverage (>85% rainfed). These findings underscore the critical imperative for modernized pressurized irrigation networks, agro-genetic adaptation, and targeted policies within the EU Common Agricultural Policy (CAP).