A bleached colony of Acropora coral off American Samoa in 2015.

NOAA

Almost all of the 29 coral reefs on U.N. World Heritage list damaged by bleaching

There was good news and bad news for the world's coral reefs last week. The good news, announced 19 June, is that the global coral bleaching event that started in 2015 appears to be over, according to the U.S. National Oceanic and Atmospheric Administration (NOAA) in Silver Spring, Maryland. The bad news, released 23 June, is that the 3 successive years of bleaching conditions damaged all but three of the 29 reefs that are or are contained within United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage sites. And the prognosis is grim: Without dramatic reductions in greenhouse gas emissions, all these reefs "will cease to host functioning coral reef ecosystems by the end of the century," predicts the report from UNESCO’s World Heritage Center in Paris.

Bleaching occurs when overly warm water leads corals to expel symbiotic algae called zooxanthellae. Without the colorful algae, which use photosynthesis to produce nutrients for themselves and their hosts, the corals turn white, or bleach. If the waters cool soon enough, algae return; if bleaching persists, the corals die. Reefs are ecosystems that support more than a million marine species. And an estimated half billion people around the world rely on reefs for livelihoods from fishing and tourism.

NOAA's Coral Reef Watch uses satellite observations of sea surface temperatures and modeling to monitor and forecast when water temperatures rise enough to cause bleaching. In the most recent case, waters in the Atlantic, Pacific, and Indian ocean basins began rising in mid-2014 and bleaching started in 2015. The 3-year duration of this latest event is unprecedented; previous bouts of global bleaching came and went within a year.

To get some idea of what climate change will likely mean for the reefs, the World Heritage Centre asked coral experts at NOAA and elsewhere to produce what they claim is a first of its kind study "that scientifically quantifies the scale of the issue, makes a prediction of where the future lies, and indicates effects up to the level of individual sites," says Fanny Douvere, marine program coordinator at the center. "This has never been done before in a World Heritage context," she adds.

The team reviewed published reports of on-site observations and NOAA data. Aside from the three reefs spared, 21 of 29 suffered severe and/or repeated exposure to water hot enough to cause bleaching. The scientists found that even remote, pristine reefs that have suffered minimal human degradation, including Papahānamokuākea in Hawaii and the Seychelles' Aldabra Atoll, were heavily bleached. "Coral mortality during the third global bleaching event has been among the worst ever observed," the report states.

Reefs can recover from bleaching, but it takes 15 to 25 years. Yet 13 of the 29 World Heritage listed reefs were exposed to bleaching more than twice per decade between 1985 and 2013, that is, even before the latest bleaching event which killed significant numbers of corals on Australia's Great Barrier Reef 2 years in a row. The team projects that intervals between bleaching will get shorter and shorter if carbon dioxide (CO2) emissions continue to rise. The report predicts that by the end of the century, all 29 World Heritage site reefs would be essentially destroyed "under a business-as-usual emissions scenario."

But even if CO2 emissions are curbed, reefs face challenges from climate change. The Paris Agreement sets the goal of holding the increase in the global average mean temperature to well below 2°C above preindustrial levels but calls for efforts to limit that increase to 1.5°C. The report states that any increase greater than 1.5°C will likely cause "severe degradation of the great majority of coral reefs." Still, limiting the rise in atmospheric temperatures will at least give reefs some time to adapt.

The report goes to the World Heritage Committee at a meeting in Kraków, Poland, which starts 2 July. The World Heritage Centre and the International Union for Conservation of Nature have prepared draft decisions for the committee to adopt.

But the draft decision on the coral reef report "is very disappointing," says Jon Day, a report contributing author and a resource manager formerly with Australia's Great Barrier Reef Marine Park Authority. Although the draft decision expresses "its utmost concern" regarding the impacts of climate change on the World Heritage reefs, any action is delayed by a call "to further study the current and potential impacts of climate change" for possible action when the committee meets in 2018. Day thinks there is enough substance in the current report to set a course of action now. "This really is a fob-off, rather than addressing what is clearly an enormous and growing issue," Day says.

Other report co-authors are more patient. "We are thrilled" that the World Heritage Committee is now considering climate change in conservation policy decisions, report lead authors Scott Heron and Mark Eakin, both coral reef scientists at NOAA, wrote in a joint email to ScienceInsider. Though they agree with Day that a stronger stance now would have been welcome; "this year’s action paves the way for significant change in 2018," they wrote.