The present study was conducted to identify the key meteorological variables that have a significant effect on reference evapotranspiration (ET0) by using principal component analysis (PCA) and sensitivity method in a irrigated semi-arid region of North-West India (Karnal). Further, the climate change effect on ET0 was studied with the projected increase in temperature ranging from 1 to 5°C and CO2 concentration between 330 to 660 ppm. The investigation of PCA indicated that temperature and sunshine hours had greater linearity with ET0 with eigen values of 3.81, 3.70, and 3.18 in annual, summer, and winter periods, respectively. While, sensitivity analysis revealed that ET0 had significant variability with temperature, followed by sunshine hours, relative humidity, and wind speed. It was also found that changes in ET0 by increased temperature of 1 and 5°C is neglected the effect of 430 ppm and 660 ppm CO2 concentrations, respectively Hence, elevated temperature and CO2 concentration had opposite effects on ET0 and the combined effect of elevated CO2 and temperature contradicts the individual effect of each other on ET0.