Photo/IllutrationBrown seaweed covers coral killed by bleaching in the Sekiseishoko area of Okinawa Prefecture in this photo taken on Dec. 21. (Provided by the Environment Ministry)

Coral bleaching has killed 70.1 percent of the nation’s largest coral reef as of the end of 2016, up from 56.7 percent just a few months earlier, the Environment Ministry said.

Warmer seawater temperatures last summer are believed to have caused coral bleaching to spread to 90 percent of the Sekiseishoko coral reef in Okinawa Prefecture.

The ministry report, released on Jan. 10, was based on a study conducted in November and December on conditions at 35 points in the Sekiseishoko coral reef, a popular diving area covering about 400 square kilometers between Ishigakijima and Iriomotejima islands.

The previous figure of 56.7 percent was based on a survey conducted in September and October.

Coral bleaching occurs when warmer water temperatures cause coral to expel algae living symbiotically within. The coral turns white and can die.

In the Sekiseishoko area, widespread coral bleaching occurred in 1998 and 2007. Experts warn that the phenomenon could occur more frequently if global warming progresses.