Weakening of the abyssal circulation (conveyor belt) in the Japan Sea during the 20th century is deduced from the decreasing trend of bottom dissolved oxygen (O2). This trend indicates a shortened O2 supply, caused by too inactive conveyor belt to compensate for biological O2 consumption at the bottom of the sea. Recent climatic changes during the winter season in the northern Japan Sea may play a significant role in prohibiting the formation of surface seawater dense enough to sink to the bottom. It is predicted that the Japan Sea bottom water will become anoxic within a few hundred years, as it was in the last glacial maximum. ¿ 1999 American Geophysical Union