But its health and future are very much in doubt.

　　Miles O'Brien has the story for our weekly segment on the Leading Edge of science and technology.

　　MILES O'BRIEN: Half the size of Texas, spanning 1,400 miles, Australia's Great Barrier Reef is the largest living structure on the planet. It is rich in beauty and diversity, but it is dying, as the ocean waters steadily warm.

　　DAVID WACHENFELD, Great Barrier Reef Marine Park Authority: It's a very confronting situation. And I hope the people of the world take this as a call to action to do more about climate change.

　　MILES O'BRIEN: Coral reef ecologist David Wachenfeld is director for reef recovery at the Great Barrier Reef Marine Park Authority. It's the second consecutive summer of extensive coral destruction, or bleaching, on the reef.

　　DAVID WACHENFELD: We are using aerial surveys and underwater surveys to try and cover that whole enormous area of the Great Barrier Reef to get a handle on the extent and severity of the event. But, certainly, this year is shaping up to be another very bad year, as was last year.

　　MILES O'BRIEN: Last year, two-thirds of the corals in the northern part of the Great Barrier Reef died, the worst die-off in history. As for this year, it is too early to tell. But the outlook is grim, as this is one big piece of an unprecedented global coral crisis.

　　C. MARK EAKIN, National Oceanic and Atmospheric Administration Coral Reef Watch: Since June of '14, we have had continuous bleaching somewhere in the world. Globally, over 70 percent of the coral reefs around the globe have been exposed to the high temperatures that cause bleaching.

　　MILES O'BRIEN: Coral reef ecologist Mark Eakin is the coordinator for the National Oceanic and Atmospheric Administration's Coral Reef Watch.

　　He relies on data from scientific satellites operated by NOAA, the Europeans, and the Japanese that measure ocean water temperature. He, along with David Wachenfeld, is co-author of a new study published in the journal "Nature" documenting the link between warm waters and dying coral in the Great Barrier Reef.

　　C. MARK EAKIN: What we did here at Coral Reef Watch was to provide the satellite data that gives the information on the areas where the high temperatures occurred.

　　So, there are charts in there showing where the bleaching was worst and where the temperatures were highest for the longest time, and the correlation between that heat stress and where the bleaching occurred was very high.

　　气候变化对大堡礁造成致命威胁

　　MILES O'BRIEN: Coral reefs are the rain forests of the sea, brimming with mind-boggling diversity that is still not fully explored.

　　JENNIFER SMITH, Scripps Institution of Oceanography, UCSD: Despite the fact that coral reefs occupy a very small footprint of the overall Earth's surface, less than 0.1 percent, an area about the size of the country of France, they're home to more species of marine organisms than any other marine ecosystem on the planet.

　　MILES O'BRIEN: Jen Smith is associate professor in marine biology at the Scripps Institution of Oceanography at the University of California, San Diego.

　　We met at Scripps' Birch Aquarium, home to some spectacular displays of reef ecosystems to nurture curious minds and, behind the scenes, an extensive nursery to nurture the coral itself.

　　Most corals are nourished through a symbiotic relationship with algae, that convert sunlight into energy, which the corals tap into.

　　JENNIFER SMITH: So, it's like having a garden growing in your stomach where 90 percent of your daily nutrition comes directly from that garden in your stomach.

　　MILES O'BRIEN: The pigments in the algae are the source of the vivid rainbow of colors in a healthy coral reef. But the algae are very temperature-sensitive. A few degrees warmer than normal, and their photosynthesis is enhanced. That may sound good, but the unfortunate byproduct is a toxin.

　　So, the coral is forced to spit out its food source, revealing its white color, thus the term bleaching. Jen Smith showed me Birch's display of an unhealthy bleached coral reef.

　　JENNIFER SMITH: As long as you still see white skeleton, that's usually an indication that the coral is still alive, because, as soon as the tissue starts dying, your seaweed will start settling on that skeleton and start growing over it.

　　So, within a matter of a few weeks or even a month during a bleaching event, you will have an idea of whether that coral is dead and getting overgrown by seaweed, or whether it's on its way to recovery.

　　MILES O'BRIEN: The sustained global bleaching event is not giving corals a chance to recover. Many coral reefs are further stressed by other types of human activity: runoff from sewage, agriculture, and overfishing.

　　But scientists say the current bleaching is happening whether those local factors are present or not. It is clear warming water is the culprit, and reducing our use of fossil fuels is the only solution.

　　Scientists say the world must adhere to obligations to do just that made by 195 nations in Paris in 2015.

　　C. MARK EAKIN: This is really the only thing that's going to deal with this global coral bleaching problem. Even if we do, though, we're going to lose a lot of coral reefs.

　　MILES O'BRIEN: But the Trump administration is filled with climate change skeptics pushing to roll back Obama-era regulations aimed at reducing greenhouse gases.

　　So, what if we do nothing to slow climate change?

　　JENNIFER SMITH: Corals reefs as we know them will not exist in the next 10, 20, 30 years. We could single-handedly be responsible for the extinction of an entire ecosystem.

　　MILES O'BRIEN: Coral reefs are more than spectacular, vivid examples of nature's beauty. The fish they harbor are also a food source for a half-billion people, and natural barriers against storms and floods.

　　But, before too long, they may only exist behind glass.

　　Miles O'Brien, the PBS NewsHour, La Jolla, California.

　　JUDY WOODRUFF: Clearly, they are a global treasure. Let's hope more attention is paid to how to save them.