This paper presents a study of the sensibility of evapotranspiration (ET) to climatic change in four types of climates (i.e., humid, cold semi-arid, warm semi-arid and arid). The use of a reference crop ET (ETo) permits the standardization of ET estimates across varying conditions. So, ETo was estimated with the FAO-56 Penman–Monteith equation using data from eight Iranian sites over a 41-year period (1965–2005). The sensitivity analyses were carried out for air temperature, wind speed and sunshine hours within a possible range of ± 20% (i.e., − 5%, − 10%, − 20%, + 5%, + 10%, + 20%) from the normal long-term climatic variables. The sensitivity of ETo to the same climatic variables revealed significant differences among climates. From the comparison of the sensitivity of ETo to climatic change in different climates, it can be inferred that the sensitivity of ETo to wind speed and air temperature decreased from arid to humid climate, whereas its sensitivity to sunshine hours increased from arid to humid environment. Furthermore, the greatest change in ETo (about ± 9%) was found in arid climate in response to ± 20 change in wind speed.