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Although sea levels are rising due to the climate crisis and threatening near-coastal infrastructures, higher temperatures in other areas are having exactly the opposite effect. The water levels are falling and causing massive problems.

The consequences are equally serious. But the declining water levels are receiving less attention according to Matthias Prange, Thomas Wilke of the Justus Liebig University in Gießen, and Frank P. Wesselingh of the University of Utrecht and the Naturalis Biodiversity Center Leiden (the Netherlands). (Matthias Prange, Thomas Wilke and Frank P. Wesselingh, The other side of sea level change, Communications Earth & Environment, 2020; 1 (1) DOI: 10.1038/s43247-020-00075-6)

“The Caspian Sea can be viewed as representative of many other lakes in the world. Many people are not even aware that an inland lake is dramatically shrinking due to climate change, as our models indicate,” says Matthias Prange.

The report of the Intergovernmental Panel on Climate Change (IPCC) also failed to mention lakes, and disregarded the social, political and economic consequences of global warming on the affected regions.

“This has to change. We need more studies and a better understanding of the consequences of global warming in this region.”

The goal must be to raise awareness of the consequences of climate change for inland seas and lakes so that appropriate strategies can be developed, including approaches for other large lakes and regions facing similar challenges.

Because of its size, the largest lake in the world, and because of its relatively high salinity of about one per cent, which is about one-third of the salt concentration in the oceans, the Caspian has been named a ‘Sea’. Its largest inflow is the Volga River and it has no natural connection to the ocean. The water level is determined by the proportional influences of inflow, precipitation and evaporation. Global warming is causing increased evaporation, which results in a declining water level.

The Caspian Sea is an important regional water reservoir and, despite its salt content, it is a biological and commercial center. It is bounded by Kazakhstan, Turkmenistan, Iran, Azerbaijan and Russia. Depending on the degree of global warming in the future, the water level could fall by 9 to 18 meters during this century.

“This would affect not only the biodiversity, various species, and habitats that would disappear. The economies of all the bordering countries would be impacted, including harbors, fisheries and fish farming.”

For this reason, the authors argue that in the future the Caspian Sea should be used as an example in scientific research to assess the vulnerability of certain regions to falling water levels. Because no nation can solve the resulting conflicts alone, they propose a global task force to develop and coordinate strategies.

The article suggests that “international climate funds” could offer a possibility for financing projects and adaptation measures if changes in the lake level are attributed to climate change.

Caspian Sea evaporating

In 2017, a study (J. L. Chen, T. Pekker, C. R. Wilson, B. D. Tapley, A. G. Kostianoy, J.-F. Cretaux and E. S. Safarov, Long-term Caspian Sea level change, Geophysical Research Letters, 2017; 44 (13): 6993 DOI: 10.1002/2017GL073958) found:

Water levels in the Caspian Sea dropped nearly 7 centimeters (3 inches) per year from 1996 to 2015, or nearly 1.5 meters (5 feet) total.

The current Caspian Sea level is only about 1 meter (3 feet) above the historic low level it reached in the late 1970s.

Increased evaporation over the Caspian Sea has been linked to increased surface air temperatures.

According to the data from the study, the average yearly surface temperature over the Caspian Sea rose by about 1 degree Celsius between the two timeframes studied, 1979-1995 and 1996-2015. These rising temperatures are likely a result of climate change, according to the study’s authors.

Evaporation brought about by warming temperatures appears to be the primary cause of the current drop in sea level and the decline will likely continue as the planet warms, according to the study’s authors.

“From our point of view as geoscientists, it’s an interesting place because it’s possible to construct a sort of budget for the total amount of water that’s there,” said Clark Wilson, a geophysicist with the Jackson School of Geosciences at the University of Texas at Austin, and co-author of the new study published in Geophysical Research Letters, a journal of the American Geophysical Union. “The real control that causes it to go up and down over long periods of time is really most likely the evaporation, which is almost completely dominated by temperature.”

The Caspian Sea, located between Europe and Asia, is roughly the size of Montana at 371,000 square kilometers (143,244 square miles). It has experienced substantial changes in its water level over the past several hundred years, but previous studies were unable to nail down the exact causes of the sea level changes.

The Caspian Sea contains an abundance of natural resources and diverse wildlife. The sea also contains oil and natural gas reserves, and is an important resource for fisheries in the surrounding countries.

The new study began after Wilson and Jianli Chen, the study’s lead author from the Center for Space Research at the University of Texas at Austin, along with other researchers, used the Caspian Sea to calibrate data from the twin satellites of the GRACE mission launched in 2002. By comparing measurements of the Caspian Sea from GRACE data and Earth-based measurements, the researchers helped improve the satellite data’s accuracy. In doing so, they noticed the Caspian Sea’s water levels were undergoing significant changes.

“Once we got through with [the calibration], Jianli Chen said, ‘Well, you know, this is very curious. Why is this changing so much?'” Wilson said. “That got us going on the current question, which is trying to understand what the reason is for these multimeter variations in the sea level. It’s an interesting place, and it’s been studied for a long time, but it wasn’t really clear.”

The scientists looked at the three primary influences on Caspian Sea water levels: water from rivers that drain into the sea, precipitation and evaporation.

They compiled information about water level changes observed by satellites, records of precipitation and drainage into the sea from rivers, and estimations of precipitation and evaporation from climate models.

The scientists then assembled a record of how much each of these factors contributed to observed changes in the Caspian Sea level from 1979 to 2015.

They found Caspian Sea levels increased by about 12 centimeters (5 inches) per year from 1979-1995. But in 1996, sea levels began to drop, and declined by an average of nearly 7 centimeters (3 inches) per year through 2015. From 1996-2015, the Caspian Sea level dropped nearly 1.4 meters (4.5 feet), according to sea level records used in the study.

Evaporation contributed to about half of that decline, while the combined effects of precipitation and river discharge changes contributed to the other half. According to the study, the observed evaporation rates are associated with increased surface air temperature and other climate factors such as surface humidity and wind.

The study provides the first convincing evidence that increased evaporation over the Caspian Sea is a more important driving force of Caspian sea level change than river discharge or precipitation, said Anny Cazenave, a CNES space geodesist at the Laboratoire d’Etudes en Géophysique et Océanographie Spatiales (LEGOS) at Observatoire Midi-Pyrénées in Toulouse, France, who was not involved in the study.

“An interesting finding from the study is that over the last two decades, climate-model predicted water loss … apparently cannot be balanced by water gain from discharge, and increased evaporation is a major factor leading to this imbalance,” Cazenave said. “If the temperature in the Caspian Sea region continues to increase, the evaporation rate is also expected to increase. Unless river discharge increases accordingly or precipitation in the Caspian drainage basin increases accordingly, the imbalance is likely to continue.”

The Caspian reached a historic low of 29 meters (95 feet) below mean sea level in the late 1970s before water levels increased in 1978. If current evaporation rates continue, the results suggest the sea could approach the historic low again, according to the study’s authors.

Evaporation will have the biggest impact on the northern portion of the Caspian Sea because much of the water in that area is less than 5 meters (16 feet) deep, Wilson said. If the current trend of a 7-centimeter decrease per year continues at a steady rate, it would take around 75 years for the northern part of the sea to disappear, according to the study.

The Caspian Sea supports many unique and ancient species remaining from when the sea was a part of the Tethys Ocean during the Mesozoic era, approximately 300 million years ago. Although most of these species live in the southern and middle regions of the Caspian, some use the shallow northern area as spawning grounds, including 90 percent of the world’s sturgeons. Dropping sea levels would also impact the Kara-Bogaz-Gol Bay on the eastern side of the sea, which is less than 5 meters (16 feet) deep and contains one of the world’s largest natural deposit of sea salts, according to the study’s authors.

Wilson said the next step in this research is to project future changes in Caspian Sea level using climate models. Although this study identified the trends in sea level and their causes, the scientists did not project specific estimates of how these levels might change in the future.

“If you’re going to take this to the next step, it would be to take a suite of climate models or look at some sort of ensemble predictions of future temperatures to get some idea of what those scenarios might be for the Caspian Sea,” Wilson said. “You can imagine if you had a continued decline in sea level that led to several meters of loss, you’ve considerably diminished the size of the sea.”

Lake Urmia’s decline

In 2016, another study found:

A combination of climate change and water usage are responsible for the staggering drying of Iran’s Lake Urmia, what was once the second largest salt lake in the world.

The dramatic decline of Lake Urmia, once the second-largest hypersaline lake in the world, has both direct human and climatic causes, according to the study (Somayeh Shadkam, Fulco Ludwig, Pieter van Oel, Çağla Kirmit and Pavel Kabat, Impacts of climate change and water resources development on the declining inflow into Iran’s Urmia Lake, Journal of Great Lakes Research, 2016; 42 (5): 942 DOI: 10.1016/j.jglr.2016.07.033) published in the Journal of Great Lakes Research. The study was the first to compare the relative impact of climate and water management on the water flowing into the lake.

“Saving Lake Urmia will require both national action to improve water management, and international cooperation to address the issue of climate change,” said International Institute for Applied Systems Analysis (IIASA) and Wageningen University researcher Somayeh Shadkam, who led the study.

Lake Urmia but has declined over 80% in recent decades. Previous work by Shadkam and colleagues had shown that climate change threatens the lake’s existence, using future scenarios of climate change to project water inflow into the lake. The new study aims to better understand the causes of the lake’s decline, teasing out the relative contribution of different factors such as human water usage as well as climate-related variables, using 50 years of data from 1960 to 2010.

The annual flow of water into Lake Urmia dropped by 48% over the study period. Using a model of water flow into the lake, the researchers found that 60% of this decline was caused by climate changes, such as change in precipitation and temperature, and that the remaining 40% of the decline could be attributed to water resources development, such as diverting water for irrigation that would otherwise flow into the lake.

Most previous studies have indicated the dominate role of water usage in changes in the lake surface area. But the new study clearly indicates that climate change and variability has contributed to the lake desiccation. That means that reducing water use without taking change in the climate into account might be insufficient in saving the lake.

“Water resources and climate change are inextricably interlinked,” says IIASA Director General and CEO Professor Dr. Pavel Kabat, a coauthor on the study. “This is just one area where an integrated systems viewpoint is vital for providing sound advice to policymakers trying to solve such pressing issues.”


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