The East (Japan) Sea has been in a warming trend during the last more than 40 years: a 0.1--0.5 degree of warming in the upper 1000 meters. The warming is accompanied by the deepening of the oxygen minimum layer by more than 1000 meters. The analysis of chemical tracers such as dissolved oxygen and CFCs clearly implies that the warming is associated with changes in deep water structures in the area, resulted from a replacement of the past bottom-water formation with an intermediate water formation in recent time. This shift has a remarkable resemblance to that anticipated to the ocean conveyor-belt system in coming century associated with recent global warming. In considering a rapid turn-over time of time scale less than 100 years, the East Sea may serve as a natural laboratory for global changes in the future. ¿ 2001 American Geophysical Union