Sensitivity analysis of reference evapotranspiration (ET_o) is essential to determine the expected fluctuations of ET_o in response to a known change in a specific climate variable. This study analyzed the sensitivity of ET_o to maximum temperature (T_max), minimum temperature (T_min), relative humidity (RH), wind speed (u₂), and sunshine hours (n) for command area of Upper Chenab Canal in Punjab, Pakistan. FAO-Penman-Monteith (FAO-PM) equation was used to estimate ET_o by using a weather data set of 32-years from 1980-2012. Long term average value of each climate variable was varied with in a possible range of -20% to ＋20% with an interval of 5% and resulting ET_o variations were plotted as sensitivity curves. ET_o showed maximum response to change in T_max followed by n, T_min, u₂ and RH. Dimension less sensitivity coefficients were also derived for each climate variable to mathematically define the sensitivity of ET_o. Sensitivity coefficients of n and T_max were highest showing that their influence on ET_o variations would be maximum. Sensitivity coefficients of RH were negative showing that ET_o would decrease as RH increase and vice versa. In general, ET_o showed more sensitivity to climate variability in summer season. The influence of T_max, n and T_min was higher during summer months whereas influence of u2 and RH was higher during winter months. Results suggests that in context of latest warming trends caused by global warming, atmospheric evaporative demands of study area could rise drastically which would be a challenging situation for future agricultural water management.