Japan has reported an alarming rate of death in its largest coral reef.

The environment ministry says that 70 percent of the coral at Sekiseishoko, off Okinawa, is dead.

91.4 percent of the coral in the largest reef in the Northern Hemisphere has also been bleached due to warm temperatures — meaning they're vulnerable and very likely to die, as well.

Photo taken Sept. 12, 2016 from a drone shows Japan's largest coral reef, Sekiseishoko, between Ishigaki and Iriomote islands in the southernmost prefecture of Okinawa. Image: Kyodo/AP

The Japan Times cited a ministry official saying that accelerated coral bleaching was taking place in 35 points at the reef, located between Ishigaki and Iriomote islands in Okinawa.

A screenshot from Google Maps where the Sekiseishoko coral reef is located. Image: Ng Yi Shu/Mashable

The report at Sekiseishoko comes after the ministry found in November last year, that most of the reef was bleached due to high water temperature that lasted till early September.

Ministry officials also told NHK News that water temperatures have been decreasing since autumn, but it is still unclear whether it will recover.

What is coral bleaching?

Coral bleaching occurs when coral loses the algae that lives within it, due to increased water temperatures and pollution. The algae gives coral its colour, and supplies nutrients; without it, the coral loses a major supply of food, and its skeleton is exposed, exacerbating the bleaching further.

Bleached corals can recover if they are not exposed to extra stress, and some reefs are more resilient than others.

The die-off is another alarming sign of global warming, after temperatures in the North Pole reached melting point on Christmas, and an iceberg the size of Delaware is posed to break off Antarctica.

This report comes just months after the mass bleaching crisis in the Great Barrier Reef, which saw over 67 percent of corals in one area dead.