Historical Trend and Future Projection of Summer Temperature Extremes in Nepal

ABSTRACT The monsoon dominated climate along with the complex topographic gradient of Nepal, make it highly sensitive to changes in summer temperature extremes, yet nationwide evidence combining observations and projections remains limited. This study examines historical and future changes in summer temperature indices and extreme exceedance indices across Nepal using 34 years of quality‐controlled station observations from 39 stations and bias‐corrected CMIP6 projections under SSP245 and SSP585. Historical results show statistically significant and spatially heterogeneous warming, with the strongest increases concentrated in the Middle Mountain region and a clear dominance of nighttime over daytime warming. Temperature indices exhibit more coherent elevation‐dependent warming than extreme exceedance indices, while exceedance‐based metrics show weaker, more fragmented, and less spatially consistent responses, indicating greater uncertainty in frequency‐based extremes. Nighttime indices, particularly TNn and selected TN95p‐related metrics, show stronger positive elevation dependence, especially in the mid‐ and late‐century periods. Future projections indicate continued warming throughout the twenty‐first century under both scenarios, with stronger and more persistent changes under SSP585 than SSP245. Pre‐monsoon and monsoon seasons show distinct responses in both temperature indices and extreme exceedance indices, reflecting the combined influence of elevation, monsoon dynamics, and scenario‐driven radiative forcing levels. Under SSP585, warm‐day frequencies and nighttime extremes increase more robustly and with broader spatial coherence, whereas SSP245 often shows mid‐century intensification followed by weaker late‐century changes or partial reversal in some metrics. The spatial pattern of warming also shifts from higher‐elevation areas earlier in the century toward lower‐elevation and more densely populated regions later in the century. Inter‐model spread is substantially larger for extreme exceedance indices than for temperature indices, underscoring greater uncertainty in projected frequency‐based extremes. Overall, the results highlight a warming, more spatially differentiated, and increasingly scenario‐dependent summer climate in Nepal, with important implications for heat stress, agriculture, water resources, and adaptation planning.