Spatiotemporal evolution of temperature extremes across India’s agro-climatic zones (1951–2025)

Long-term changes in temperature extremes are a robust signature of anthropogenic climate change, yet their spatial structure across India’s agro-climatic zones (ACZs) remains insufficiently resolved at policy scales. Here, we quantify changes in temperature extremes across 14 mainland ACZs during 1951–2025 using the India Meteorological Department 1°×1° gridded daily dataset. We compute 22 indices, including 11 ETCCDI metrics, 7 India-specific thresholds, and 4 crop-specific growing degree day (GDD) indices, and assess trends using the Mann–Kendall test with trend-free pre-whitening and Sen’s slope estimator. Tropical Nights (TR20; Tmin ≥ 20 °C) exhibit the most spatially coherent increase, with significant trends in 10 of 12 Tier A/B zones, peaking in the Western Dry Region (ACZ-14; +3.182 days decade⁻¹; p < 0.05). Warm-day frequency (TX90p) increases significantly in 9 zones, with strongest trends in the Southern Plateau (+2.962% decade⁻¹), East Coast Plains (+2.940% decade⁻¹), and West Coast Plains and Ghats (+2.917% decade⁻¹). Diurnal temperature range declines in 8 zones, indicating faster warming of minimum than maximum temperatures across Gangetic, arid, and interior plateau regions, while coastal and southern plateau zones show daytime-dominant warming. Seasonal thermal accumulation intensifies, with significant increases in rabi wheat GDD in the Trans-Gangetic Plain (+11.130 GDD decade⁻¹) and Gujarat Plains (+21.625 GDD decade⁻¹). Temporal analysis identifies 2001–2025 as the warmest 25-year period in the 1951–2025 record across most ACZs. These results provide the first comprehensive ACZ-level ETCCDI characterisation of India's 75-year thermal extreme record, revealing spatially heterogeneous warming with pronounced nocturnal intensification across the Gangetic and arid zones and daytime-dominant warming in coastal and southern plateau zones, with direct relevance for agricultural adaptation planning.