Think of water, and images of vast oceans, white-capped rapids, babbling brooks, and placid lakes come to mind. Water, however, exists beyond these everyday blue bodies, and today many climate scientists are focusing their attention on another category of it entirely: green water. This is the water found in terrestrial precipitation, evaporation, and soil moisture. Stored in the soil and used by plants, green water is central to all of Earth’s ecosystems, supporting plant functions, including photosynthesis, and underpinning modern agricultural systems. According to climatologists, such as Johan Rockström, water is not just a passive player but an active driver of Earth’s climate system through multiple interconnected feedback loops. Rockström explains, “Water is the bloodstream of the biosphere. . . . If we lose the water cycle, we lose everything.” Changes to the water cycle, such as altered rainfall patterns, droughts, and reduced evapotranspiration from vegetation, can destabilize entire ecosystems. Water in soils and vegetation also has a cooling effect, regulating temperatures; when that buffering is lost, landscapes heat up faster and more intensely.

Global shifts in water endowments are reshaping our understanding of the water cycle and its intricate ties to the changing climate, and they are also impacting land use. Activities in one area can reduce evaporation and downwind precipitation elsewhere, effectively exporting water scarcity and increasing chaotic climatic patterns. Urbanization, conventional agriculture, and deforestation all contribute to decreases in green water retention, increasing runoff rather than encouraging water infiltration, and this leads to negative impacts on natural photosynthetic cycles. These shifts include changes in average and extreme rainfall patterns as well as in total water storage, encompassing green water, surface water, groundwater, and human-made reservoirs.