The data are a long-term (1980-present), daily reanalysis of reference evapotranspiration, covering the globe at a spatial resolution of 0.625° Longitude x 0.5° Latitude. Reference evapotranspiration is a measure of evaporative demand, or the "thirst of the atmosphere"--basically how much moisture from the surface could evaporate into overpassing air, assuming (i) that enough water is available to evaporate and (ii) the surface is covered with a specific reference crop that completely shades the ground (some other conditions also apply). For this dataset, reference evapotranspiration is derived from the daily implementation of the Penman-Monteith reference evapotranspiration equation (Monteith, 1965) as codified in the FAO-56 report (FAO, 1998), which is the international standard for reference evapotranspiration estimation. The equation is driven by data from the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2; Gelaro et al., 2017). The MERRA-2 input data are at a spatial resolution of 0.5 deg lat x 0.625 deg long; the reference evapotranspiration output data are at the same spatial resolution. There are two outputs, one for each reference crop: ETos is the reference evapotranspiration from a short crop (0.12-m grass); ETrs is the reference evapotranspiration from a long-crop (0.5-m alfalfa).