The world watched with a sense of dread in 2018 as Cape Town, South Africa, counted down the days until the city would run out of water. The region’s surface reservoirs were going dry amid its worst drought on record, and the public countdown was a plea for help.
By drastically cutting their water use, Cape Town residents and farmers were able to push back “Day Zero” until the rain came, but the close call showed just how precarious water security can be. California also faced severe water restrictions during its recent multiyear drought. And Mexico City is now facing water restrictions after a year with little rain.
There are growing concerns that many regions of the world will face water crises like these in the coming decades as rising temperatures exacerbate drought conditions.
Understanding the risks ahead requires looking at the entire landscape of terrestrial water storage – not just the rivers, but also the water stored in soils, groundwater, snowpack, forest canopies, wetlands, lakes and reservoirs.
We study changes in the terrestrial water cycle as engineers and hydrologists. In a new study published Jan. 11, a team of colleagues from universities and institutes around the world showed for the first time how a warming climate will likely affect water availability on land from all water storage sources over the course of this century.
We found that the sum of this terrestrial water storage is on pace to decline across two-thirds of the land on the planet. The worst impacts will be in areas of the Southern Hemisphere where water scarcity is already threatening food security and leading to human migration and conflict. Globally, one in 12 people could face extreme drought related to water storage every year by the end of this century, compared to an average of about one in 33 at the end of the 20th century.
These findings have implications for water availability, not only for human needs, but also for trees, plants and the sustainability of agriculture.
Where the risks are highest
The water that keeps land healthy, crops growing and human needs met comes from a variety of sources. Mountain snow and rainfall feed streams that affect community water supplies. Soil water content directly affects plant growth. Groundwater resources are crucial for both drinking water supplies and crop productivity in irrigated regions.
While studies often focus just on river flow as an indicator of water availability and drought, our study instead provides a holistic picture of the changes in total water available on land. That allows us to capture nuances, such as the ability of forests to draw water from deep groundwater sources during years when the upper soil levels are drier.
The declines we found in land water storage are especially alarming in the Amazon River basin, Australia, southern Africa, the Mediterranean region and parts of the United States. In these regions, precipitation is…