<p>Projections of global terrestrial evapotranspiration (ET) are plagued by sizeable uncertainties. Here, we uncover bivariate emergent constraint relationships between projected global ET trends (2015–2100) and historical vapour pressure deficit (VPD) trends (1980–2014) under the low emission scenario of SSP126 when water supply is sufficient, and with historical precipitation trends under the high emission scenarios of SSP370 and SSP585 when water availability is limited, across 28 CMIP6 models. Combining multiple observational datasets into a Hierarchical Emergent Constraint framework, we find the raw CMIP6 models overestimate future annual ET growth rates. The original projections of 0.233 ± 0.107 mm year⁻¹ (SSP126), 0.360 ± 0.244 mm year−¹ (SSP370) and 0.506 ± 0.365 mm year⁻¹ (SSP585) are adjusted downwards to 0.193 ± 0.074 mm year⁻¹, 0.272 ± 0.184 mm year⁻¹ and 0.391 ± 0.299 mm year⁻¹. The revised projection uncertainties are reduced by 18.1–31.1%. These findings highlight the value of incorporating observational constraints to improve the reliability of ET projections, which are critical for understanding the future global water cycle.</p>