The
hydrological cycle as a global common good

Key takeaways

Human actions are impacting the global hydrological cycle and freshwater availability around the world.

The growth of consumption and linked changes in land use and pollution are impacting the quantity and quality of freshwater resources. Climate change, deforestation, and loss of biodiversity are mutually reinforcing drivers of shifts that are changing precipitation patterns— the source of all freshwater— and destabilising the hydrological cycle.

Current policy tends to deal with the “blue” water we can see – in rivers, lakes, and aquifers – largely overlooking “green” water – in soil, plants, and forests – that evaporates and transpires into the air, falling downwind as rain.

Green water supplies are far more interdependent than previously thought. Atmospheric moisture flows carry water from one country to another, even across continents and oceans.

We are failing to connect the feedback between land cover and rainfall as a critical component of the global hydrological cycle. Nearly half the rain that falls over land originates from the land through a process of “terrestrial moisture recycling”. Intact ecosystems and lands managed in ways that do not adversely impact their hydrological functioning are critical to securing terrestrial rainfall. A stable supply of green water in soils is also crucial for carbon sequestration.

The hydrological cycle is deeply interlinked with climate change. As global temperature rises, land and oceans respond by evaporating more freshwater, and the hydrological cycle intensifies, leading to more extreme weather events that affect billions of people.

Multiple signs are pointing to a global freshwater crisis. We have transgressed planetary boundaries for global blue and green freshwater. Regional and local scales face multiple crises in terms of water quantity and quality. Combining information on total terrestrial water storage with indicators of water shortage and physical scarcity reveal “hotspots” of particular concern.

We therefore need to: