Climate change: More than half of the world's largest lakes are losing water

Zoom Image

Largest lake in the U.S. state of California: Lake Salton also has a negative water balance

Photo: NASA

Water supplies in lakes are dwindling. This is the conclusion reached by an international research team published in the journal "Science". More than half of the world's largest lakes lose large amounts of fluid, often due to human activities. Understanding the causes in more detail should help to better protect the important ecosystems and preserve them as water reservoirs.

Although lakes make up only about three percent of the world's land area, they contain more than 85 percent of the Earth's liquid fresh water. The reservoirs supply households with water, irrigate fields and maintain ecosystems. It has long been known that the water is dwindling in many places, but the details and exact background have so far been little documented.

Now, researchers have combined data from Earth observation satellites with hydrological and climate models to identify and estimate changes in water volume in 1972 lakes and reservoirs worldwide (an interactive map can be found here). The data refer to the period from 1992 to 2020.

According to the study, 53 percent of the water reservoirs examined have lost significant water in the past 28 years. Water loss is "in the most important regions of the world," the team writes, mentioning western Central Asia, the Middle East, the West Indies, eastern China, northern and eastern Europe, Oceania, the adjacent United States, northern Canada, southern Africa, and most of South America.

In contrast, in just under a quarter of the lakes and reservoirs examined, the amount of water increased significantly. This was especially true for underpopulated regions such as the Inner Tibetan Plateau and the northern Great Pains in North America.

22 gigatonnes less water in lakes – every year

"This is the first comprehensive analysis of trends and drivers of fluctuations in the amount of water stored in lakes based on a set of satellite data and models," said first author Fangfang Yao in a statement. He has led the work as a visiting scientist at the Cooperative Institute for Research In Environmental Sciences (Cires), a project of the American space agency Nasa and the University of Colorado Boulder. He now works at the University of Virginia.

According to the study, the total water loss of the world's large lakes and reservoirs was around 22 gigatonnes or about 602 cubic kilometres of water – annually. This corresponds to the water consumption in the USA for the entire year 2015 or 17 times the volume of Lake Mead, the largest reservoir in the United States, writes the expert team. The global volume loss is thus about 40 percent greater than the usual fluctuations in the amount of water in the lakes and reservoirs examined within one year.

According to the team of authors, both lakes and reservoirs in dry and humid areas of the world are affected. Lakes in humid, tropical regions and in the Arctic could also lose more water than previously thought. The group estimates that about a quarter of the world's population lives in the catchment area of an increasingly dry lake, which corresponds to about two billion people.

However, the extent of water loss differs significantly. About 80 percent can be traced back to the largest 26 bodies of water. The Caspian Sea alone, the largest lake in the world, contributes 49 percent to the net water loss in lakes and reservoirs, according to the study.

According to the statement, Yao's team was motivated to work by the drying Aral Sea between Kazakhstan and Uzbekistan. The body of water was once almost as large as Bavaria, making it the fourth largest inland lake in the world. In the meantime, there are only fragments.

Irrigation of entire regions

The scientists attribute a significant part of the water loss in natural lakes to climate change and the associated higher evaporation and human water consumption. Both factors dominated the global decline in natural lake volume in about 100 large lakes, Yao said. "Many of these effects of humans and climate change on water loss in lakes were previously unknown."

In reservoirs, on the other hand, sedimentation is often responsible for declining water volumes. When water from rivers is dammed up in reservoirs, sand, silt and stones carried along in the streams accumulate. In this way, the water basins gradually become smaller. In older reservoirs that were filled before 1992, sedimentation had a greater impact on the filling quantity than, for example, droughts or years of heavy rainfall, the team reports.

Commenting on the study, geography professor Sarah Cooley of the University of Oregon points to the great importance of lakes for ecosystems as well as water supply and irrigation of entire regions. The potential consequences of drying up are important both locally and globally. The effects of climate change and sedimentation must be taken into account more in lake management plans, Yao's group demands.

JME