Global warming will not only cause sea levels to rise but lakes to shrink, a team of Dutch scientists warned this week. As a case in point, they predict a catastrophic drop in water levels in the Caspian Sea – presently the largest lake in the world – by the century’s end.
“Many people are not even aware that an inland lake is dramatically shrinking due to climate change, as our models indicate,” stated co-author Matthias Prange of the MARUM - Center for Marine Environmental Sciences at the University of Bremen, writing in Communications Earth and Environment. Which means countries are starting to think how to adapt to rising seas and creeping loss of low-lying coastland, but they’re not considering a decline in lake water levels.
This also applies to so-called seas such as the Sea of Galilee (a freshwater lake), the Caspian Sea and other inland water bodies. It bears qualifying that water level in the Sea of Galilee is presently very high thanks to heavy rainfall in recent winters, but the long-term trend projected for the region implies that relief will be transient.
Globally, in some areas precipitation is expected to decline with climate change. By and large, North Africa, the Mediterranean basin and the Middle East, including Israel, are among the regions expected to suffer aridification.
So is the area of the Caspian Sea, though the region feeding its main source, the Volga River, might actually get more rain. “However, again, the effect of increasing Caspian lake evaporation will clearly exceed this effect, leading to the projected lake level decline,” Prange tells Haaretz.
Sounds worrying. But the problem of lake shrinkage is being overlooked to the degree that it isn’t even mentioned in the latest report of the Intergovernmental Panel on Climate Change, the researchers say.
Yet much like climate change, which scientists have been warning about for some 50 years, the impact on lakes is no surprise. Mounting surface temperatures naturally intensify evaporation, explain Prange, Thomas Wilke and Frank Wesselingh, of the University of Utrecht, in the new paper.
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Lakes with no outlet, known as endorheic lakes, are particularly vulnerable to climatic change, the researchers say. The Caspian Sea is exactly that. It’s the largest lake in the world but is in an endorheic basin, as is Israel’s Dead Sea. (The Sea of Galilee isn’t.) In the case of endorheic lakes, water level is a function of rainfall, the inflow of water from rivers and streams, and evaporation from the lake surface.
Israel’s Dead Sea, a hypersaline lake, also lies in an endorheic basin, with no outlet. Water arriving stays until it evaporates, which is why the lake is so salty – minerals arriving with the water never leave (unless mined). The Dead Sea’s condition, however, is divorced from climate change in the sense that historically its main source is the Jordan River. In modern times, only a small fraction of the river water reaches the lake. Most of the river water is diverted for agriculture and other uses. One upshot of the Dead Sea’s falling water level has been the collapse of parts of the shoreline on a massive scale. Some call it ecological disaster, but leading geologists with Israel’s Geological Survey suggest it’s not calamity but “change” and is, if anything, an opportunity to create a “Sinkhole Park” for tourists.
As for the Caspian Sea, how much of its shrinkage is due to water diversion? It’s “important to disentangle the effects of human water withdrawals, river damming and diversions versus climate change, which can be quite challenging, depending on the lake and its setting,” Prange explains to Haaretz. “In the case of the Caspian Sea, however, the situation is very clear. The model simulations show that due to the sheer size of the Caspian Sea, the climate-driven increase in lake evaporation will be orders of magnitude larger than any other (man-made) effects.”
The Caspian Sea is salty, with about 1 percent salinity compared with the Dead Sea’s 34 percent. Surface salinity impedes evaporation – a paper published in 2018 notes that the evaporation rate from the surface where freshwater enters the Dead Sea was as much as three times higher than from the salty surface.
So the salinity of the Caspian Sea is relatively less of an impediment to evaporation, and the new paper projects a stunning drop of 9 to 18 meters (almost 30 to 60 feet) by 2100. These estimates are based on the medium to high emissions scenarios (RCP4.5 and RCP8.5).
Asked why they didn’t check the low scenario (RCP2.6), Prange explains they haven’t simulated it yet. But while on the topic, he adds: “A recent study published [in PNAS] has shown that higher emissions scenarios (in particular RCP8.5) better match real fossil fuel emissions under current and stated policies.
He also points out that if the Caspian Sea level drops by “only” 9 meters, since the vast majority of the Caspian shelf is only at a depth of 5 meters, most of this shelf would also fall dry in a more optimistic scenario, with all the negative consequences outlined in the new paper.
Their projected Caspian Sea decline by the century’s end is double the rate projected until now, the paper says. If indeed the water level falls by 9 to 18 meters, the northern Caspian shelf, the Turkmen shelf in the southeast, and land along the coasts in the middle and south of the lake, will break through the surface, they write. The Kara-Bogaz-Gol Bay will be no more.
All in all, they project the Caspian Sea surface shrinking by around 23 to 34 percent by 2100. Among the devastating effects will be the impact for the Caspian seal, which is already endangered.
What can be done? “Global mitigation measures such as greenhouse gas emissions reductions will likely take effect too late,” the team writes, noting that the lake level is already declining by 6 to 7 centimeters a year. That leaves adapting to the situation, best done by regional planning and collaboration. However, “Different communities and littoral countries might perceive that they are less affected by Caspian Sea level decline than others. Such a bias could hamper coordinated adaptation actions,” they write.
It bears adding that like so many of the world’s water bodies, the Caspian Sea is already experiencing stress from overexploitation, pollution and invasive species. The desiccation of its shallow-water habitats will decimate their marine fauna and flora, among other things depriving the seals of key sources of food.
On a global scale, contraction of lakes will have devastating environmental, economic and geopolitical ramifications, the authors warn. At the Caspian Sea, just for one example: present-day ports will become ruins perching on dry land. The nations around the Caspian will likely squabble about its diminishing resources and, the authors note, about the area’s “coveted oil and gas reserves.”
And there you have it. There may be good reason why they didn’t factor in a scenario wherein greenhouse gas emissions come under control.