2023-05-18 18:08:32
(Washington) Water is shrinking in more than half of the world’s lakes and reservoirs, threatening a vital source of fresh water, according to a new study that largely attributes the trend to global warming and overuse 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) which is drying up, this study published in the prestigious journal Science.
“Lakes are in danger globally, and this has broad implications,” said Balaji Rajagopalan, a professor at the University of Colorado Boulder, and co-author of the study. “They allow societies and humanity to live, and yet they don’t get the respect they deserve. »
Lakes cover regarding 3% of the earth’s surface, but account for 87% of liquid fresh water on Earth. They are used for human consumption, agriculture, or even the production of electricity.
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 over 100 km2) and reservoirs.
Over the nearly 30-year period studied (from 1992 to 2020), the study concludes that they drained the equivalent of all the water consumed by the United States in 2015.
Rising temperatures
The study has an unexpected result: lakes lose water not only in arid areas, but also in humid regions.
“It is generally considered that when the climate warms up, the arid regions dry out even more, and the humid regions become wetter”, explains Balaji Rajagopalan. But even in the latter, “the lakes are in decline”, which is “surprising”, he underlined, giving the example of India.
To determine the causes of the observed trends, the scientists segmented their analysis according to the type of body of water.
For natural lakes, they attribute regarding half of water loss to human activities and rising temperatures, which cause increased evaporation – the latter factor being linked to climate change.
But another important factor, namely the lack of rainfall, “can also be attributed to climate change in some places,” notes Balaji Rajagopalan. In addition, humans or herds may need to consume more water due to higher temperatures.
“Clearly, the footprint of climate change is already there,” says the researcher.
With regard to reservoirs, the construction of new dams has made it possible to increase their overall volume of water, but nearly two-thirds of the large reservoirs are in decline.
Sediment accumulation was the main factor reducing their storage capacity, although droughts also affect some, such as in the southwestern United States.
” Not too late ”
The consequences of this decline are numerous. Lack of water can cause population displacements. Water quality can also be affected if a lake’s level is low.
Like the oceans, the 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’s not too late to intervene”, emphasizes Balaji Rajagopalan. Decision makers are now “armed with information”.
An interactive map was created from the results of the study, detailing the observed trend for each lake.
“A global decline in water availability is worrisome, but should not overshadow regional differences,” said Hilary Dugan, a limnologist at the University of Wisconsin-Madison. “Understanding lake-by-lake variations will give officials a better understanding of the risks in their jurisdiction. »
The authors of the study plead for more controls and monitoring, in particular of the smallest lakes benefiting from less attention, and for management at the regional level, by considering several lakes at the same time, on the model of the management of rivers by several jurisdictions at the same time.
Finally, they emphasize the importance of preventive measures, such as switching to another water source if the threshold of a lake reaches a critical level.
“The lakes are of paramount importance. We must take care of them, and they will take care of us”, insists Balaji Rajagopalan.
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