Abstract
The Japan Sea is called a “miniature ocean”because it has its own thermohaline circulation, which is
similar to that of the open ocean. Therefore, it may be possible to detect oceanic responses to climate
change such as global warming in a relatively short time by monitoring physical and biogeochemical
conditions in the Japan Sea. In this paper, I introduce our finding of the past on global warming impacts
in the Japan Sea. Historical data and recent observations show a centennial-scale oxygen decrease in the
bottom water at the depth below 2000m from about 250µmol/kg in the late 1920s to about 200µmol/kg at
present. This oxygen decline implies weakening of thermohaline circulation, which corresponds to a
centennial-scale warming in the northern area of the sea where the bottom ventilation could occur. Using
the spatial distributions of chlorofluorocarbons, we found that the scale of thermohaline circulation of the
sea after 1975 have decreased by 21-30% in the deep water and 15-41% in the bottom water compared
with those before 1975. Using the vertical profiles of total carbonic acid and total alkalinity, we found that
anthropogenic CO2 concentration in the sea increase sharply over the past 20 years and ocean
acidification of the sea is more active than other marginal seas.
