Water storage in many of the world’s biggest lakes has declined sharply in the past 30 years, according to a new study, with a cumulative drop equal to the annual water consumption of the United States.
The loss of about 21.5 gigatons of water a year in natural lakes can “largely be attributed to climate warming,” a team of scientists said in a report on their research in Science for which they analyzed satellite data from 1,980 lakes and reservoirs between 1992 and 2020. When they combined the satellite images with climate data and hydrological models, they found “significant storage declines” in more than half of the bodies of water.
Combining information from different sources made it possible to determine whether the declines are related to climate factors, like increased evaporation and reduced river flows, or other impacts, including water diversions for agriculture or cities. A quarter of the world’s population lives in basins where lakes are drying up, they warned.
Vanishing lakes already have led to starvation and dislocation and increased the potential for international conflict, including in Africa, where Lake Chad is drying up, as well as in South America, where Bolivia’s Rhode Island-sized Lake Poopó, once that nation’s second-largest body of water, disappeared over the past few decades.
The study calls the southwestern United States a troubled area, confirming the challenges spurred by dwindling water supplies in the nation’s two largest reservoirs — Lake Powell and Lake Meade on the Colorado River.
The study showed lake water-storage loss prevailed across major global regions including much of interior Asia and the Middle East, northeastern Europe, as well Oceania, North America, South America and southern Africa. A total of 457 natural lakes had significant water losses of about 38 gigatons a year, 234 lakes showed gains, and 360 — about one-third of the lakes studied — showed no significant change.
Only about one-third of the total decline of water storage in drying lakes is offset by increases in other lakes, and the water bodies with rising levels mostly are in remote and sparsely populated regions like the Inner Tibetan Plateau, the Northern Great Plains in the United States and the Great Rift Valley in Africa. These storage increases were driven mainly by changes in precipitation and runoff, the study found.
Uncertainty about the Great Lakes
The research didn’t find a climate warming fingerprint affecting the Great Lakes, but that doesn’t mean it’s not there. During the 1992-2020 study period, water levels in the Great Lakes dropped steeply, then increased sharply again due to big swings in rainfall.
The analysis didn’t show a global warming signal, said lead author Fangfang Yao, who studies surface water changes at the University of Colorado Boulder’s Cooperative Institute for Research in Environmental Sciences.
“Lake Michigan-Huron shows no trend during our study period,” Yao said. “Lakes Superior and Erie — both of them show an overall increasing trend, suggesting the greater role of natural climate variability.”
The Great Lakes are being affected by other climate extremes. Ice cover has declined significantly, lake temperatures have warmed, and seasonal water cycles have changed, making some parts of the lakes more susceptible to toxic algal blooms and fish die-offs. The study warned about compounding climate impacts, said co-author Balaji Rajagopalan, associate chair of the University of Colorado’s department of civil, environmental and architectural engineering.
The federal government’s 2018 National Climate Assessment projects the levels of Lake Michigan and Lake Huron will probably fall six inches by 2100, with other Great Lakes dropping by smaller amounts, but the assessment cautions that there is still a lot of uncertainty in those projections.
Increasing lake sediment cuts storage
Nearly two-thirds of all large reservoirs covered in the study experienced significant storage declines. But reservoirs overall showed a net increase in storage due to the filling of newly created water storage lakes.
Declines in the amount of water stored in reservoirs filled before 1982 “can be largely attributed to sedimentation,” the study found. “Globally, sedimentation-induced storage loss offsets more than 80% of the increased storage from new dam construction.”
The researchers said their findings suggest that sediment build-up is the main cause of the decline in reservoirs globally.
The large-scale decline in surface water has real impacts across many regions of the world, said University of Oregon geographer and water researcher Sarah Cooley, who wasn’t involved in the study.
“Many of these local populations are dependent on lake water storage, whether for water supply, fishing and food supply, hydropower, irrigation, navigation, recreation,” Cooley said. “I think the major takeaway of this study is that drying trends in lakes are prevalent worldwide and perhaps more so than previously thought.”
