More than half of the world’s major lakes and reservoirs have lost significant volume since 1992, with only a quarter gaining water. That’s the result of a large data analysis by a team led by Fangfang Yao of the University of Colorado Boulder in Science publishhas no light. The researchers cite more evaporation due to rising temperatures, changes in precipitation patterns and changes in water use as the most important reasons.
Although natural and artificial lakes cover only three percent of the earth’s surface, they store 87 percent of the liquid surface fresh water. They are therefore important not only as ecosystems, but also for agriculture and drinking water supply. So far, however, there has been no global inventory of the development of the amount of water in the countless small and large lakes worldwide. Yao’s team has now created a data set of the water level development of the almost 2000 largest lakes in the world from satellite measurements of the extent and water level of the lakes, combined with climate and hydrological models and published in an interactive map.
Two bodies of water are also represented on this that are – wholly or partly – in Germany: Lake Constance, which, according to the researchers’ data, has lost almost six million tons of water annually over the period under consideration, that would be over the entire 536 square kilometer lake distributes around one centimeter of water level per year. The researchers cite climate change as the main reason. And the Müritz, which according to the data has gained an average of almost 200,000 tons of water annually since 1992, no reason is given here.
The Caspian Sea alone loses 19 billion tons of water annually
According to the data, the losses in the shrinking lakes worldwide can only compensate for about a third of the increasing amounts of water in the growing ones. Overall, the bottom line for the measured lakes is an annual decline of almost 22 gigatonnes of water, which corresponds to about seven times the volume of Lake Starnberg.
The majority of this can be traced back to a few gigantic lakes: the Caspian Sea alone, the largest – albeit salty – lake on earth, is shrinking by around 19 gigatonnes every year. Mainly due to less rainfall and inflow, researchers say, but other studies had attributed the losses to increased evaporation. If you took this lake out of the equation, the global water balance would look much better.
The researchers emphasize, however, that the dominance of human influences remains clear: even if one disregards the Caspian Sea, around a quarter of the water losses in the other shrinking lakes are due to more evaporation due to rising temperatures, a quarter to overexploitation and a quarter to less precipitation and inflow, which is also often associated with climate change. After all, the researchers estimate that every fourth person on earth lives in the catchment area of a shrinking lake.
The data also suggest that a popular summary of the impact of climate change on water management may be oversimplifying: “Wet gets wetter, dry gets drier,” it’s often said. Which is not entirely wrong either, because regions that are already humid today will tend to receive even more precipitation in the future, while dry regions will receive even less. However, if even many lakes in humid regions – such as Lake Constance between wet southern Germany, the even wetter Austria and soaking wet Switzerland – are losing water overall, this shows that the principle has its limits. “We shouldn’t expect that more available water in wet areas will compensate for water losses in dry ones,” says University of Oregon geographer Sarah Cooley in an accompanying article in Science.