Global warming: half of the world's lakes and reservoirs lose water

Europe 1 with AFP / Photo credits: Arnaud Le Vu / Hans Lucas / Hans Lucas via AFP 16:17 pm, May 19, 2023

Water is shrinking in more than half of the world's lakes and reservoirs, threatening a vital source of freshwater, according to a new study that largely attributes the trend to global warming and their excessive use by humans. About a quarter of the world's population lives in an area with a lake or reservoir (a body of water regulated by a dam) that is drying up, warns Thursday this study published in the prestigious journal Science.

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Lakes "allow humanity to live"

"Lakes are endangered globally, and that has broad implications," Balaji Rajagopalan, a professor at the University of Colorado Boulder and co-author of the study, told AFP. "They allow societies and humanity to live, and yet they do not receive the respect they deserve." Lakes cover about 3% of the Earth's surface, but account for 87% of the liquid fresh water on Earth. They are used for human consumption, agriculture, or electricity production.

Previous work has already looked at the decline of larger lakes individually. But this study is the first to offer a detailed view of global trends, and the causes of the observed changes, thanks to satellite observations. In total, the researchers studied 1,972 bodies of water, representing the vast majority of natural lakes (focusing on those larger than 100 km2) and reservoirs. Over the period of almost 30 years studied (from 1992 to 2020), the study concludes that they were emptied of the equivalent of all water consumption in the United States in 2015.

Declining lakes, including in wetlands

The study has an unexpected result: lakes lose water not only in drylands, but also in humid regions. "It is generally considered that when the climate warms, arid regions dry out even more, and humid regions become wetter," says Balaji Rajagopalan. But even in the latter, "the lakes are declining", which is "surprising", he stressed, giving the example of India.

To determine the causes of the observed trends, the scientists divided their analysis according to the type of body of water. For natural lakes, they attribute about half of water losses to human activities and rising temperatures, which cause increased evaporation – the latter factor being linked to climate change.

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But another important factor, the lack of rainfall, "can also be attributed to climate change in some places," Rajagopalan said. In addition, humans or herds may have to consume more water because of higher temperatures. "Clearly, the footprint of climate change is already there," says the researcher.

Lack of water can cause population displacement

As for reservoirs, the construction of new dams has increased their overall water volume, but nearly two-thirds of large reservoirs are in decline. The accumulation of sediments was the main factor reducing their storage capacity, although droughts also affect some, as in the southwestern United States.

The consequences of this decline are numerous. Lack of water can cause population displacement. Water quality can also be affected if a lake level is low. Like the oceans, lakes also store carbon, and no longer play this role when they disappear, in turn aggravating global warming.

But the researchers' message is not fatalistic: on the contrary, they seek to raise awareness. For some lakes, "it is not too late to intervene," says Balaji Rajagopalan. Policymakers are now "armed with information". An interactive map was created from the results of the study, detailing the trend observed for each lake.

A plea for more controls and follow-ups

"A global decline in water availability is worrying, but should not overshadow regional differences," Hilary Dugan, a limnology specialist at the University of Wisconsin at Madison, told AFP. "Understanding lake-by-lake variations will give officials a better understanding of the risks in their jurisdiction."

The authors of the study call for more control and monitoring, especially smaller lakes receiving less attention, and for management at the regional level, considering several lakes at the same time, on the model of river management by several jurisdictions at the same time.

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Finally, they stress the importance of preventive measures, such as transferring to another water source if the threshold of a lake reaches a critical level. "Lakes are of paramount importance. We have to take care of them, and they will take care of us," Rajagopalan said.