The densely populated urban areas of Osaka and Nagoya as well as the western Kyushu region surrounding the Ariake Sea are at risk from rising sea levels resulting from higher oceanic temperatures.

According to research released Saturday, the region could face flooding because dikes and banks may collapse in typhoon-linked high tides and seismic tidal waves.

The pace of rise in the sea level has been relatively slow but could gain momentum if global warming accelerates, according to Shinichi Iwasaki, a researcher at the National Institute for Earth Science and Disaster Prevention.

Extensive regions in western Japan have been hit hard by a series of typhoons this year.

According to Iwasaki, the sea level rises with the swelling of sea water prompted by an increase in seawater temperature. Seawater temperatures have generally fallen along the coasts of eastern Japan since around 1970 but have risen in western Japan.

Iwasaki’s research shows that the sea level fell at various coastal points in eastern Japan over a 30-year period, by as much as around 9 cm from Japan’s primary benchmark, while it rose in many parts of western Japan, by as much as 7 cm.

Eastern Japan is defined in the research as east of 137 degrees east longitude, which cuts across Honshu from Wajima in Ishikawa Prefecture in the north to Nagoya in the south.

Iwasaki checked the ground level of coastal regions in western Japan and the maximum sea level at full tide and explored what would happen if no dikes or banks were there.

The research indicates that in the western Japan region along Osaka Bay, a wide area along the Yodo River covering central Osaka and the Osaka Prefecture city of Suita, and Amagasaki, Hyogo Prefecture, to the west would be submerged.

In the Ise Bay region, most parts of Nagoya on the eastern bank of the Kiso River, as well as coastal areas in Saga, Fukuoka and Kumamoto prefectures in the Ariake inland sea region of Kyushu would also be submerged, according to the research.

All of these areas are protected by banks. But Iwasaki says, “If banks are fragile in inland regions along rivers, major damage may be inflicted even if they are away from the sea.” Inland regions along rivers that flow into the sea typically have high population concentrations.

Japan has been hit by a record-high number of typhoons this year. In August, giant waves whipped up by a typhoon surged into a river in Takamatsu, Kagawa Prefecture, overwhelming dikes and flooding more than 3,000 homes.

In Muroto, Kochi Prefecture, coastal embankments were destroyed by huge waves, as massive Typhoon Tokage whipped through western Japan last week.

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