Human activities accelerate changes in the freshwater cycle

The findings show that climate change, together with large-scale land and water use, is pushing the freshwater cycle further away from a stable state, as defined by the planetary boundary for freshwater change. This boundary is already considered to be transgressed, reflecting a long-term trend that jeopardises the freshwater cycle’s ability to support vital climatic and ecological Earth system processes.

Using data from 1901 to 2019, and an ensemble of global hydrological models, the researchers analysed quantitative changes in both “blue water” (in rivers, lakes and groundwater) and “green water” (in soils) around the world, distinguishing between the effects of human activities, through land and water use, and the effects through human-induced climate change on dry and wet anomalies.

“Changes in the freshwater cycle have been speeding up in recent decades, and projections indicate that this trend is likely to intensify further,” said lead author Vili Virkki from the University of Eastern Finland. “When the freshwater cycle changes faster than the environment can adapt, the risks of adverse impacts increase.”

According to the study, dry and wet anomalies now occur about twice as often as they did in the early 20th century, for both blue and green water. Regional patterns vary: While increasing dryness dominates in many tropical and subtropical regions, anomalously wet conditions have become more frequent in the northern boreal zone, reflected in floods and more prolonged, large-scale precipitation events.

In terms of attribution, climate change is the main global driver of these changes – and of the transgression of the planetary boundary for freshwater change. Wet anomalies are mainly driven by climatic factors, while direct human pressures such as land and water use intensify dry deviations in particular.

“The results clearly show that focusing only on blue water does not provide a sufficiently holistic picture of water cycle change and its potential impacts,” said co-author Sofie te Wierik from the Netherlands Environmental Assessment Agency. “Changes vary substantially between regions, over time, and depending on which component of the freshwater cycle is being examined.”

In some regions, such as parts of India and Central Asia, climate change may slightly increase seasonal water availability, but these effects are outweighed by water and land use, which contribute to drier conditions.

“Our study makes clear that returning to safe limits for the global freshwater cycle will depend on addressing climate change and land and water use as interconnected drivers of change. The findings also underscore the need to better understand how planetary boundaries interact – a task we intend to take forward,” said co-author Dieter Gerten from PIK.