The notion that air temperature causes evapotranspiration (ET) from land has held much weight in the understanding of the impacts of climate change on hydrology. However, even the correlation between air temperature and ET is largely limited to variations associated with the seasonal cycle, and application of the idea of air temperature causing ET, such as methods based on the work of Thornthwaite, can result in overestimates of the quantity of increase in ET associated with climate change. While higher temperatures during summer relative to winter are indicative of increased net radiative input at the surface (a requirement for increased ET), higher temperatures under increased greenhouse gas conditions are instead associated mainly with insulation, i.e. higher temperatures required to balance the same radiative input, which has less tendency to promote increased ET. Examples will be presented of simulations in the Great Lakes basin and observations from other regions.