September 18, 2017, by Tim Radford
Sunrise on the Caspian Sea in Iran, one of five countries that share its shoreline.
Image: Amin Allen Tabrizi via Flickr
The impact of rising temperatures on the Caspian Sea is gradually reducing the world’s largest inland body of water to catastrophically low levels.
LONDON, 18 September, 2017 − While ocean levels worldwide are on the increase, one great sea is actually getting lower. As a result of climate change, the Caspian Sea is slowly evaporating.
Between 1996 and 2015, this enormous and ancient body of salt water − ringed by five nations and making a natural border between Europe and Asia − dropped at the rate of seven centimetres a year, a total of 1.5 metres.
It is now only about one metre above its previous catastrophic low of 40 years ago.
During the previous crisis, environmentalists blamed irrigation programmes and river dams that must have reduced flow into the inland sea, but the blame this time has been pinned on evaporation.
In a greenhouse world, driven by profligate human combustion of fossil fuels, the sea is drying up because the temperatures are rising.
“From our point of view as geoscientists, it’s an interesting place because it is possible to construct a sort of budget for the total amount of water that’s there,” says Clark Wilson, a geophysicist at the University of Texas’s Jackson School of Geosciences in Austin.
“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, the world’s largest inland body of water, is seen as a relic of a much more ancient ocean that washed the globe 300 million years ago. It is now a brackish pool, isolated far from the modern ocean.
It is a stretch of water bigger than Germany, a little smaller than Japan, and is home to a number of species found nowhere else. Its waters provide the spawning nursery for about 90% of the sturgeon that yield the world’s harvest of caviar.
“It’s an interesting place because it is possible
to construct a sort of budget for
the total amount of water that’s there”
Russia, Kazakhstan, Turkmenistan, Iran and Azerbaijan share its shoreline. Six rivers flow into it, but the Volga provides probably four-fifths of all the volume of freshwater that drains into the sea.
Professor Clark and his colleagues report in Geophysical Research Letters journal that they used satellite data, national records and climate models to try to calculate the flow into, and the evaporation from, the Caspian.
For the moment, such studies are academic: in a world of climate change and potentially catastrophic sea level rise, it becomes important to distinguish between the effects of long-term natural cycles and the change driven directly by human action since the Industrial Revolution.
So although climate scientists have firmly established that rising temperatures are the consequence of human-driven climate change, archaeological and geological studies have repeatedly revealed devastating ancient droughts in the Middle East, while the current drought in the eastern Mediterranean has been pronounced as the driest for the last 900 years. The latest study disentangles old natural cycles from human influence.
The Caspian Sea reached a historic low of 29 metres below sea level in 1979. After that, water levels increased by about 12 centimetres a year until 1995. But in 1996, levels began to drop again, and have continued to fall.
Evaporation in response to rising average temperatures is the best explanation. And, the researchers warn, the current long term decline “is expected to continue into the foreseeable future, under global warming scenarios”. – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.