As a major pathway for ecosystems to lose water, evapotranspiration (ET) is an essential process that affects the water cycle and land-atmosphere feedback in the earth system. Elevated CO₂ significantly affects ecosystems, by increasing photosynthesis and/or reducing stomatal conductance, which affects both ET and carbon assimilation. These direct and indirect effects have been quantified by model simulations, however are rarely constrained with ecosystem level direct, and long-term observations. Here we analyzed the high-frequency eddy covariance (EC) based observations at 80 sites from Fluxnet and AmeriFlux that have more than 10 years of observations to quantify the effects of CO₂ on ET, and water use efficiency (WUE). We investigated the drivers of trends and interannual variation of WUE at all sites. Results indicate that among all sites, the majority of sites did not have a significant trend of ET over time; at the same time, 25 sites showed a significant increase in the ET over time and 11 sites have a significant ET decrease over time. As for the WUE, we found a significant increase trend in WUE at 26 sites. We also found that while net radiation, gross primary production, and soil water content contributed most for the interannual variation of ET for most sites, the elevated CO₂ also played a significant role in regulating the long-term trends of ET at most sites.