TSUKUBA, Ibaraki Prefecture--Global warming is contributing to a decrease in the oxygen level at the bottom of the Sea of Japan, which could mean dire consequences for the marine ecosystem and the fisheries industry.
“The decline is attributable to the fact that surface seawater is not cooling sufficiently in the winter due to global warming,” said a group of researchers from the National Institute for Environmental Studies (NIES) and the Japan Agency for Marine-Earth Science and Technology (JAMSTEC).
If the decline continues, the deepest portions could become even completely oxygen deprived in 100 years, it added.
The Sea of Japan exceeds 3,500 meters in depth at its deepest portions. The shape of the sea is like that of a bowl with a deep bottom.
Warm and lighter seawater, which has been carried from the south by the Tsushima Current, seldom mixes with seawater that has accumulated in the deeper portions of the Sea of Japan. Then, it flows out of the Sea of Japan through the Tsugaru Strait or Soya Strait, both located around Hokkaido.
The seawater in the deeper portions, which seldom mixes with surface seawater, is called “water unique to the Sea of Japan.”
The water unique to the Sea of Japan is mostly accumulated in portions deeper than depths of 200 meters. The water temperature ranges from zero to 1 degree.
Of the water unique to the Sea of Japan, the bottom layer of seawater, which has accumulated in sections deeper than 2,000 meters, is now suffering from a decrease in oxygen.
The NIES, based in Tsukuba, Ibaraki Prefecture, and the JAMSTEC, headquartered in Yokosuka, Kanagawa Prefecture, analyzed the changes in the bottom seawater that took place in the period from 1930 to 2012. They conducted the analysis by combining the marine observation data of the Japan Meteorological Agency with the results of their own research.
As a result of the analysis, they found that the concentration of oxygen dissolved in the bottom of the sea off Noto Peninsula in Ishikawa Prefecture declined from 7.36 milligrams per kilogram of seawater in the 1960s to 6.24 milligrams in 2012.
The decline means that the amount of oxygen dissolved in the seawater decreased by 15 percent during the past 50 years.
In the Sea of Japan, a cold seasonal wind blows from the Eurasian continent in winter, cooling the surface of the sea. Then, the cooled seawater at the surface sinks to the deeper portion as its weight becomes heavier. As a result, oxygen contained in the surface seawater is supplied to the bottom seawater.
In an extremely cold winter, in which the day’s lowest temperature in Vladivostok in the Russian Far East falls below minus 20 degrees on more than 20 days, a large amount of surface seawater sinks to the deeper portions of the Sea of Japan.
Such an extremely cold winter occurred once every two or three years during the period from 1930 to 1950. During the period from 1969 to 2012, however, such an extremely cold winter occurred only three times.
As a result, the amount of surface seawater that sank to the deepest levels decreased, leading to the decline in the amount of dissolved oxygen there.
The group of researchers said that the decrease in oxygen was caused by the fact that surface seawater is not cooled enough in winter due to the influences of global warming.
“In the future when global warming becomes more serious, the sinking of surface seawater that supplies oxygen to the bottom could stop completely in the Sea of Japan. In that case, due to oxygen consumption by bacteria and other creatures, the seawater in portions deeper than depths of 2,000 meters will fall into the condition of containing no oxygen within 100 years, according to our calculations,” said Takafumi Aramaki, a senior NIES researcher.
“The change in the ecosystem of the sea could have effects on fish and other marine creatures in the long term,” he added.
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