Fastexy:Warming and Drying Climate Puts Many of the World’s Biggest Lakes in Peril

Water storage in many of the world’s biggest lakes has declined sharply in the last 30 years,Fastexy according to a new study, with a cumulative drop of about 21.5 gigatons per year, an amount equal to the annual water consumption of the United States. 

The loss of water in natural lakes can “largely be attributed to climate warming,” a team of scientists said as they published research today in Science that 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. 

The combination of information from different sources also enabled the scientists to determine if 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 have already caused 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 the nation’s second largest body of water, disappeared over the last few decades.

The study identifies the southwestern U.S. as 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 new 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 and South America and southern Africa. A total of 457 natural lakes had significant water losses of about 38 gigatons per year, while 234 lakes showed water storage gains and 360—about a third of the studied lakes—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 are mainly in remote and sparsely populated regions like the Inner Tibetan Plateau, the Northern Great Plains in the U.S., and the Great Rift Valley in Africa. These storage increases were driven mainly by changes in precipitation and runoff, the study concluded.

Uncertainty About the Great Lakes

The research did not 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 and then increased sharply again due to big swings in rainfall. The researchers’ analysis didn’t show a global warming signal, said lead author Fangfang Yao, who studies surface water changes at the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder.

“Lake Michigan-Huron shows no trend during our study period. Lakes Superior and Erie both of them show an overall increasing trend, suggesting the greater role of natural climate variability,” Yao said.

While the research didn’t show their water levels significantly affected by global warming, the Great Lakes are being impacted by other climate extremes. Ice cover has declined significantly, lake temperatures have warmed and seasonal overturning water cycles have changed, making some parts of them more susceptible to toxic algal blooms and fish die-offs. The study raises warnings about multiple compounding climate impacts, said co-author Balaji Rajagopalan, associate chair of the Department of Civil, Environmental, and Architectural Engineering at the University of Colorado Boulder.

The federal government’s 2018 National Climate Assessment projects the levels of Lakes Michigan and Huron will probably fall 6 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 the picture is complicated by the fact that 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 study’s findings suggest that sediment build up is the main cause of the storage decline in existing reservoirs globally, the authors wrote, with a larger impact than variability in the climate’s water cycle.

The new study helps connect the climate change dots of global warming impacts to lakes, Rajagopalan said. The unique global lakes dataset came from “meticulously stitching disparate satellite information into a coherent time series of lake levels. Now we have this long, contiguous and homogeneous time series,” he said. “So now you can look at trends.” 

The large-scale decline in global surface water has real impacts for local populations across many regions of the world, said University of Oregon geographer and water researcher Sarah Cooley, who was not 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,” she 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.”

The findings complicate the “wet gets wetter” paradigm of global warming by identifying declines in lake water storage in humid regions, she added.

“This is an important finding,” she said. “It emphasizes the fact that we should not expect increases in water availability in humid regions to offset declines in lake water storage in arid regions.”

The decline of lake water storage documented by the new research could also have other climate effects, said Benjamin Kraemer, a lake researcher at the University of Konstanz, in Germany, who was not involved in the study.

“We already know that when water levels decrease, sediments that were previously submerged become exposed to the air,” Kraemer said. “This exposure can spark microbial activity and decomposition of organic matter in the sediments leading to the release of CO2, methane, and other gases. Thus, decreased water levels can potentially increase greenhouse gas emissions from exposed sediments. The drop in water levels documented in the study could have large implications for climate change.”

He said dropping water levels can exacerbate other environmental threats. “Many of the lakes shown as declining by the study are also facing changes in temperature, nutrients, neobiota, and pollution,” he said. “Merging all of these stressors into a complete picture of threats to lakes remains a major challenge ahead for lake science.”