In this study, temporal trends in daily time series data of key climatic parameters were analyzed using Mann–Kendall and Sen's slope estimator. Sensitivity analysis of each climatic parameter on reference evapotranspiration (ETo) was performed to estimate the sensitivity coefficients and to evaluate the impact of global warming on ETo in the eastern Himalayan region of Sikkim, India. Results of trend analysis showed a significant increasing trend for minimum temperature and mean temperature. Mean relative humidity and sunshine duration showed decreasing trends. Reference evapotranspiration also showed a significant decreasing trend by 0.008 mm year–1 in Sikkim state of India. Sensitivity analysis revealed that the seasonal and annual ETo were most sensitive to maximum temperature followed by sunshine hours whereas wind speed, minimum temperature and relative humidity had a fluctuating effect on mean ETo. The sensitivity coefficient indicated that ETo changes positively with maximum and minimum temperature, sunshine hour, and wind speed, while it changes negatively with relative humidity. Analysis indicated that increase in relative humidity would decrease the ETo in the study area. The findings of this study would be useful for sustainable water resources planning and management of agriculture in hilly regions of the state and for development of adaptation strategies in adverse climatic conditions.