Minimize disruptions to the timing and quantity of the water cycle

The water cycle underpins life

The water cycle — also known as the hydrologic cycle — underpins life on Earth. In this cycle, a fixed quantity of water moves through and is stored across the atmosphere and ecosystems: water falls from the sky via precipitation, after which it may run off the surface into blue water ecosystems like rivers, streams and lakes or be absorbed by plants as green water. Some of these blue and green water sources will later return to the atmosphere via evaporation or transpiration. Blue water that does not evaporate may also trickle further below the Earth's surface into the groundwater zone. There, water may remain for long periods of time or eventually discharge back into rivers, lakes and springs, continuing the cycle.

This cycle of movement and storage of water — also known as freshwater flows and stocks — provides drinking water, food, energy, sanitation and jobs, supports ecosystems and the rich species biodiversity that each are home to, and plays a critical role in climate regulation by absorbing heat and cooling the Earth’s surface. Yet today the quantity and timing of the world’s freshwater flows and stocks are being fundamentally altered, with devastating knock-on effects for people, nature and climate, including reduced water availability and increased water scarcity in many regions.
 

Climate change, land-use change and overextraction are disrupting the water cycle

While these disruptions are driven by a variety of forces, climate change is likely causing the most far-reaching effects. Global temperature rise increases evaporation rates, shrinking both surface water and groundwater stocks; shifts precipitation patterns, making some areas more likely to experience heavier rainfall and others likely to face prolonged periods of dryness; and intensifies extreme weather events including storms, floods and droughts.

These changes have already led to alarming continental drying patterns, surface water and groundwater depletion, alterations to rivers’ environmental flows, reductions in soil moisture that underpins all food production, and increasing water stress and flooding faced by communities. And, just as climate-driven alterations to the quantity and timing of freshwater flows and stocks are increasing risks for people, they are also harming ecosystems and the species that depend on these ecosystems' health and intactness. Monitored freshwater species populations have declined by 85% since 1970, with growing evidence climate change is taking an increasing toll on freshwater biodiversity.

At the same time, other drivers are also disrupting the world’s freshwater flows and stocks. For instance, land-use changes like deforestation driven by agriculture and urban development can result in reduced soil moisture, increased runoff and diminished groundwater recharge, leading to greater flood and drought risks while weakening local and regional climate regulation caused by altered evapotranspiration. Because forests and other terrestrial ecosystems play a critical role in regulating the water cycle, conversion of these ecosystems to other land uses is particularly detrimental.

Meanwhile, overabstraction of water for agriculture, industry and domestic use can lower river flows and deplete aquifers, increasing the likelihood of drought and disrupting seasonal water availability and degrading habitats for fish and other freshwater species. And, as the population grows, demand for food, infrastructure, goods and services that fundamentally alter the quantity and timing of freshwater flows and stocks is also rising. The infrastructure built to meet this demand — from hydropower dams to data centers — can simultaneously intensify water scarcity, amplify flood risk and accelerate ecosystem degradation.
 

Protecting the water cycle is paramount

Targets aligned with the latest scientific research and international frameworks like the U.N. Sustainable Development Goals call for dramatically reducing disruptions to blue and green water stocks and flows to safeguard communities and ecosystems. Yet almost all measurable outcome indicators monitored in this shift that track how the world is faring on these critical objectives are heading in the wrong direction.

To reverse course, the world must immediately double down on key actions, including scaling up regulatory policy to alleviate pressures from increasing demand for freshwater resources, investing in water efficiency technologies and strategies that enable agricultural, industrial and domestic users to do more with fewer water withdrawals, and accounting for and adapting to inherent water quantity and timing variability in future planning and management efforts.

Perhaps most important, these measures must be complemented by simultaneous efforts to immediately cut emissions across the global power, buildings, industry, transport, forests and land, and food and agriculture sectors, as well as to scale up carbon dioxide removal and climate finance. Indeed, minimizing disruptions to the quantity and timing of freshwater flows and stocks for both people and nature will largely hinge on the world’s collective ability to mitigate climate change.