Increase of fractional vegetation coverage enhancing rise of water use efficiency over globe from 1982 to 2021

Ecosystem water use efficiency (WUE), defined as the amount of carbon sequestration per unit of water consumed, is significantly influenced by various factors, including elevated atmospheric CO₂, changing climate, vegetation growth, nitrogen deposition, and so on. However, the response of the water and carbon coupled cycles to global change and the underlying comprehensive driving mechanisms of multiple factors remain unclear. Based on global 475 situ eddy covariance fluxes sites data released by FLUXNET, AmeriFLUX, ICOS, OzFlux and ChinaFLUX, and integrated with multiple satellite remote sensing products, we upscaled global ecosystem WUE from 1982 to 2021 by using an ensemble of 27 machine learning models. Importantly, our findings suggest, largely attributed to the increase in fractional vegetation coverage (FVC), that the rapid increase in global WUE after 2012, while CO₂ fertilization effect is not significant. Notably, FVC enhances gross primary productivity (GPP) but also limits evapotranspiration (ET) to some extent. This asynchronous effect doesn’t lead to a proportional increase in water consumption costs as carbon sinks rise. This study systematically elucidates the mechanisms through which WUE responds to climate change, thereby providing a more accurate prediction of the future water-carbon coupling cycle.