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Mar. 10, 2015 - Updated 04:16 UTC

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Coral Reefs on the Edge

Sep. 8, 2016

Global warming is causing oceans to heat up, threatening marine ecosystems. And researchers say the high sea temperatures are damaging corals in southwestern Japan, especially in Okinawa.

The region is home to 400 of the 700 coral species that exist. Experts say water temperatures this summer have been unusually warm, raising concern about the survival of the reefs.

The National Institute for Environmental Studies in Japan conducted research around islands in Okinawa prefecture and southern Kyushu. Researchers found widespread evidence of bleaching -- the process in which high temperatures cause corals to expel the algae that live in their tissues, providing their color.

Photos taken in July, and then again in August, show that much of the corals have become bleached. The water around Okinawa has been about 2 degrees warmer than usual this year. Coral bleaching tends to happen in 2 to 4 weeks under such conditions.

"This year, bleaching is occurring all over the world, just like in 1998. The water temperatures are also similar to the levels recorded back then," says Hiroya Yamano of the National Institute for Environmental Studies.

This summer, Okinawa has had less typhoons than usual. He suspects that may have caused ocean temperatures to stay high, because strong winds from tropical storms usually help to mix colder water from the depths of the ocean with warmer water on the surface.

And that's what's causing the widespread coral bleaching this year.


NHK World's science correspondent Soichiro Kurose joins anchors Aki Shibuya and Sho Beppu in the studio.

Shibuya: What impact does coral bleaching have on the marine environment?

Kurose: Coral reefs serve as "cradles of lives" for a wide range of species. Researchers at Japan's National Institute for Environmental Studies say they are home to 90,000 species, even though they cover only one-tenth of a percent of the total seabed. So the death of corals means the loss of habitats for many sea creatures. Shrimps, crabs and fish eat organic matter produced by corals and some creatures use coral reefs as hiding places. So coral bleaching could lead to the destruction of ecosystems.

Beppu: How much has the bleaching affected marine ecosystems in Okinawa?

Kurose: Local fishermen say they are feeling the impact. I spoke to a man who catches fish that live amongst reefs. He said catches this summer have only been around half the usual amount, and concern is spreading among fellow fishermen. It's not easy for corals to recover. Back in 1998, the high temperatures caused widespread bleaching of Okinawa's corals. Only some are believed to have bounced back. There are fears it may have caused permanent damage to the area's ecosystems.

Beppu: What does the coral bleaching in Okinawa mean globally?

Kurose: A large number of coral species exist in Okinawa. If the corals in the region die, it would be a serious loss globally. Yamano, the expert we spoke to, says the bleaching shows global warming is progressing. He says seawater has been warming up across the world, causing greater stress to corals. He warns that major damage will occur if ocean temperatures remain high for nearly a month, as they did in Okinawa this year.

Shibuya: Is there a way to prevent bleaching?

Kurose: Researchers see Okinawa's coral bleaching as one consequence of global climate change. That's an issue that needs to be dealt with globally, not just by Okinawa. So all we can do is focus more than ever on the issue of global warming, and be aware of what is happening in the oceans.