The kind of massive coral bleaching event that afflicted 75 percent of the dominant coral species in O`ahu\u2019s Kane`ohe Bay last September is likely to become a regular occurrence within a few decades, according to modeling by scientists Jeff Maynard, Ruben van Hooidonk, Derek Manzello, and Serge Planes.<\/p>\n
Corals can usually withstand short temperature spikes, experts say, but if high temperature and bright light conditions persist for even a week or two, as they did last month, when water temperatures held steady in the mid-80s, the corals may bleach. Bleaching occurs when the toxic combination of high temperature and light cause coral to expel the zooxanthellae algae that live within it. And if stressful conditions extend over a prolonged period, the bleached corals can die, said Maynard, who spoke at this year\u2019s Hawai`i Conservation Conference in Honolulu.<\/p>\n
At the conference, Maynard, a researcher at Cornell University, provided updated projections of the impacts of climate change on coral reefs.<\/p>\n
The first \u2014 and, until recently, the only \u2014 projections were made in a 1999 paper by Australian biologist Ove Hoegh-Guldberg, who predicted that by 2050, tropical oceans would experience annual temperature anomalies several times more severe than those that occurred in the world\u2019s worst bleaching event in 1998, when 16 percent of all corals were lost, Maynard said.<\/p>\n
So what do the models show today?<\/p>\n
Under the \u201cbusiness-as-usual\u201d climate prediction in the Intergovernmental Panel on Climate Change\u2019s (IPCC) 5th assessment report, known as Representative Concentration Pathway (RCP) 8.5, Maynard and his colleagues found that by the mid-2050s, all coral reef areas \u2014 not just the tropics \u2014 are likely to experience annual events where water temperatures exceed baseline temperatures by at least six degrees Celsius for one week. This kind of event, known as a six-degree-heating week, is likely to cause severe coral bleaching, Maynard said, noting that, historically, six-degree-heating weeks have caused more than 50 percent of corals in affected areas to bleach.<\/p>\n
According to maps prepared by Maynard and his colleagues, the waters around the Main Hawaiian Islands and Northwestern Hawaiian Islands will experience annual six-degree-heating weeks between 2040 and 2050.<\/p>\n
If, somehow, greenhouse gas emissions decline drastically to a level that matches the IPCC\u2019s second-worst emission scenario (RCP 6.0), Maynard said, \u201cthe year in which six-degree-heating weeks would be exceeded annually is only pushed back two decades. It\u2019s not actually prevented.\u201d<\/p>\n
To appease critics who thought the six-degree-heating week threshold was too low, Maynard said he and his colleagues ran models to predict when eight-degree-heating weeks would begin occurring annually.<\/p>\n
In an eight-degree-heating week, intra- and inter-species differences in bleaching susceptibility matter much less and most coral species would bleach under such conditions during a summer or warm season, he said.<\/p>\n
Maynard suggested that the stricter threshold mattered little. Once a place starts to experience annual six-degree-heating weeks, eight-degree-heating weeks follow just a few years later, he said.<\/p>\n
Under the \u201cbusiness-as-usual\u201d scenario, all coral reef areas will start experiencing annual eight-degree-heating weeks between 2030 and 2060, 90 percent of them before 2055, he said.<\/p>\n
Reefs in the higher latitude will suffer the events later than those closer to the equator, he added. In Hawai`i\u2019s case, the Main Hawaiian Islands will see annual eight-degree-heating weeks shortly before the Northwestern Hawaiian Islands do, according to Maynard\u2019s maps.<\/p>\n
Even under the IPCC\u2019s two best-case scenarios for green house gas emissions \u2014 scenarios Maynard suggests are completely unrealistic \u2014 \u201c88.3 percent and 99.7 percent, respectively, of reef locations are projected to experience severe bleaching annually by 2100,\u201d Maynard, Hooidonk, Planes, and Manzello state in a paper published earlier this year in Global Change Biology.<\/p>\n
In addition to looking at when reefs will start experiencing annual coral bleaching conditions, Maynard and his co-authors have also assessed when and to what extent reefs will start suffering from the effects of ocean acidification. As oceans become more acidic, corals and other marine species have a harder time calcifying.<\/p>\n
The authors found that between now and when bleaching is expected to occur annually (around 2055), calcification is expected to decline between three and 17 percent.<\/p>\n
Maynard\u2019s conference abstract states that if carbon dioxide levels continue to rise on the current course (the RCP 8.5 scenario), as early as 2034 all reefs will have experienced at least a permanent, five percent decline in calcification. High latitude reefs will experience it about 10 years earlier than those in low latitudes, Maynard said.<\/p>\n
Maynard made it clear he was not judging which was worse for corals, bleaching or acidification. Rather, \u201cwe\u2019re looking at the interplay between declines in calcification and thermal stress induced bleaching,\u201d he said. \u201cWhat we\u2019re finding is \u2026 with current climate models and current scenarios, that there are no refugia from both threats this century.\u201d<\/p>\n
He added that the newest climate models are thought to underestimate things like El Ni\u00f1o Southern Oscillation, \u201cwhich means our projections may be too optimistic.\u201d However, he said, his projections don\u2019t consider the plausible possibility that temperature tolerance of corals will increase.<\/p>\n
Local Impacts and Solutions<\/i><\/b><\/p>\n
Hawai`i is having one of its hottest years on record and, according to state Division of Aquatic Resources administrator Frazier McGilvray, last September was one of the hottest since the 1940s. Although biologists say it\u2019s common for some corals in Hawai`i to bleach in the fall, the scale and scope of the event that occurred in late September and early October caught many off guard.<\/p>\n
\u201cThis is above normal,\u2019 said DAR\u2019s Anne Rosinski at a press conference at Kane`ohe Bay last month.<\/p>\n
Not only were most corals in the bay experiencing some level of bleaching, from paling to going completely white, observers were seeing it in Lanikai, Waimanalo, Hanauma Bay, and Waikiki on O`ahu, as well as in waters off Maui and Hawai`i island.<\/p>\n
Marine biologist Cynthia Hunter said the normal temperature range for Hawai`i waters is 77 to 78 degrees F. Temperature measurements off Lanikai indicate they had risen to the mid-80s in September and October.<\/p>\n
Hunter said that the shallower corals have suffered more than those in deeper, darker waters.<\/p>\n
Kim Hum of The Nature Conservancy of Hawai`i added that other stressors, such as sediment, pollutants, and invasive algae, need to be managed so that when the ocean does warm or become more acidic as a result of climate change, the corals don\u2019t die.<\/p>\n
Ruth Gates of the Hawai`i Institute of Marine Biology agreed.<\/p>\n
\u201cIf it\u2019s just temperature, it\u2019s more hopeful. \u2026 If it\u2019s all these things together, it\u2019s a disaster, she said.<\/p>\n
Hum pointed to TNCH\u2019s relatively pristine reserve at Palmyra Atoll, 1,000 miles south of Hawai`i, as an example of how healthy reefs are better able to rebound from stressful events than unhealthy ones. After bleaching events, the corals there recover very quickly, and when they spawn again, there are places for the larvae to settle, she said.<\/p>\n
\u201cThat\u2019s why it\u2019s so important to remove that algae,\u201d she said.<\/p>\n
For the past few years, TNCH has been using a vacuum device known as a \u201cSuper Sucker\u201d to clear invasive algae from Kane`ohe Bay. Hunter suggested that healthy marine life is also needed.<\/p>\n
\u201cAs corals are dying back, what\u2019s going to take its place? Alien algae. We need herbivores to be chewing up algae,\u201d she said.<\/p>\n
If and when those stressors are brought under control, it will still be a very long time before the reef rebounds.<\/p>\n
\u201cSome of these colonies out there are a century old. That\u2019s how long they\u2019ll take to come back,\u201d McGilvray said.<\/p>\n
— Teresa Dawson<\/i><\/p>\n
For Further Reading<\/p>\n
Environment Hawai`i reported on Ove Hoegh-Guldberg\u2019s 1999 predictions on coral decline as a result of warming oceans. See \u201cAs Temperatures Rise, Corals Fall: The Effect of Climate Change on Reefs,\u201d August 1999.<\/p>\n
All of the coral bleaching and acidification maps prepared by Maynard and his co-authors can be viewed using Google Earth. A link to file can be found at coralreefwatch.noaa.gov\/climate\/projections\/piccc_oa_and_bleaching\/index.php.<\/p>\n
Their Global Change Biology paper can be found at piccc.net\/files\/vanHooidonk_opposing_gradients.pdf<\/p>\n
Volume 25, Number 5 November 2014<\/p>\n","protected":false},"excerpt":{"rendered":"
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