WASHINGTON — Global warming took a toll on coral reefs in 2010, endangering one of the world's key ecosystems that benefit people in countless ways.
Coral reefs are habitat for almost 100,000 known marine species, including about 40 percent of all fish species. They feed millions of people, protect coasts by absorbing wave energy, and shelter creatures that could become sources of medicine for treating cancer, HIV/AIDS and other diseases.
National Oceanic and Atmospheric Administration satellite data show that 2010, the warmest on record, was hard on corals. Warmer than normal temperatures stressed tropical corals, causing them to bleach — expelling the algae that live in their tissue, giving them color and nourishment.
Some 75 percent of the world's reefs are threatened by climate change, overfishing and pollution, according to a new assessment from the World Resources Institute and other conservation organizations. The number increased dramatically from the group's last assessment in 1998.
"It will take a Herculean effort to reverse the current trajectory and leave healthy ocean ecosystems to our children and grandchildren," said Jane Lubchenco, the marine scientist who heads NOAA. "How the world rises to this challenge is a reflection of our commitment to one another and to the natural world that gives us sustenance, wisdom and a reflection of our souls."
Coral reefs cover less than a tenth of 1 percent of the oceans' acreage, but that's still about 100,000 square miles. Scientists who dive to study reefs can't cover them all, so they're turning increasingly for help from satellites.
NOAA's satellite data on ocean heat showed that bleaching is occurring in all regions and becoming more frequent. Extreme bleaching kills corals because they can't survive without the nourishment the algae provide. Less intense bleaching can weaken corals, reduce their growth and reproductive ability, and make them more vulnerable to disease.
Mark Eakin, a University of Miami-trained oceanographer who coordinates NOAA's Coral Reef Watch satellite program, said that 2010 was only the second time on record that bleaching occurred globally.
The first global bleaching, from 1997 to 1999, came when an exceedingly strong El Nino — a periodic warming of ocean surface temperatures in the tropical Pacific — was followed by an especially strong version of its opposite counterpart, La Nina. About 15 percent of the world's corals died then.
"Fast forward to 2010," Eakin said. This time, El Nino and the La Nina that followed weren't nearly as strong.
"The problem that we're seeing is, as the oceans keep warming on a year to year basis, it doesn't take as big or as unusual conditions to result in this sort of event."
The bleaching from last year in many places was the worst since 1998. In the warmest months, bleaching hit the Pacific, the Indian Ocean, Southeast Asia and the southern Caribbean.
The Florida Keys and the northern part of the Caribbean, where unprecedented bleaching occurred in 2005, were spared last year because tropical storms cooled the waters.
Coral reefs are more diverse in life forms than even rain forests. The most abundant life is in the Coral Triangle, from the Philippines down to Indonesia and across to Papua New Guinea.
"I've been diving in some places there where I see more species on any given reef than we have in all of the Caribbean," Eakin said.
In Thailand, where he visited reefs last summer, nearly all were hit by bleaching.
Andrew Baird, a scientist at the Australian Research Council's Coral Reef Studies center, said he just returned from Indonesia's Aceh province, where nearly 100 percent of corals died at most sites. He said the total loss of coral cover could range from 50 percent to 80 percent.
"This is as bad as I have ever seen," Baird said. A similar level of bleaching is likely in Thailand and India, but it isn't nearly as bad elsewhere, he said.
Baird said it's highly likely that the most recent bleaching was connected to global warming.
"It's almost certainly the heat that was primarily responsible, but also low winds and therefore higher than usual penetration of sunlight could also have been involved," he said.
Ove Hoegh-Guldberg, the director of the Global Change Institute at the University of Queensland in Australia, said it's not surprising that the hottest year on record brought widespread coral bleaching and mortality. He said that scientists have reported that mass bleaching in the Philippines and Indonesia resulted in about 30 percent to 40 percent of the corals dying.
The accumulation of carbon dioxide, the primary heat-trapping gas contributing to global warming, also is expected to damage corals by making the oceans more acidic.
"To our knowledge, ocean acidification doesn't kill corals, but what it does is it makes them grow more slowly," Eakin said.
Carbon dioxide in the atmosphere is now higher than any time in 800,000 years.
"We probably sometime around the 1980s passed the levels that are conducive to coral reef growth in terms of temperatures and CO2 in the atmosphere," Eakin said. "What we really need is to get back to where we were in the '80s."
That would mean reducing greenhouse gases to about 350 parts per million. It's 390 now and will be 560, or double preindustrial levels, by mid-century, according to the latest projections.
At that level, "we'll have major problems in reefs around the world," Eakin said.
"I don't think we're going to lose all corals, but we will probably lose species, and ecosystems will be severely degraded."
The World Resources Institute report projected that if current rates of greenhouse gas emissions continue, about half of the world's reefs will suffer enough warming to cause severe bleaching most years in the 2030s, and 95 percent of them will in the 2050s.
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