Coral reefs — a key element in ocean ecosystems that provide not only coastline protection but billions of dollars in benefits from tourism, as well as ingredients used in cutting-edge medicines — are increasingly threatened from the effects of global warming and other hazards, according to a new U.S. government report.
The report estimates that nearly half of the coral reefs in areas from the Caribbean to the Pacific "are not in good condition and are continuing steadily on a long-term decline."
"It's a pretty alarming situation," said Jeannette Waddell, the report's co-editor and a marine biologist with the National Oceanic and Atmospheric Administration's National Ocean Service.
"Coral reefs around the world are confronted by the same types of threats. In some places it is worse. In some places, it's slightly better. But we're finding that even remote reefs are showing signs of decline," she told ABC News.
The NOAA report looked at the health of coral reefs in 15 areas under the jurisdiction of the United States and a group of countries called the Pacific Freely Associated States, which include Palau, the Marshall Islands and Micronesia.
A major threat facing corals is climate change, the report says, which affects coral reefs in multiple ways.
First, warmer ocean temperatures cause corals to expel the colorful living algae in their tissues, leaving them with a "bleached" white look.
"It really stresses out the coral and makes them more susceptible to things like disease," Waddell said.
A major bleaching and disease event in 2005 devastated coral reefs across the Caribbean. In the U.S. Virgin Islands and Puerto Rico, scientists say an average of 50 percent of the coral was lost. Some areas lost 90 percent of their coral.
Another problem for corals is that human-induced climate change is altering the chemistry of the oceans, making them more acidic. It happens as fossil fuels are burned, releasing carbon dioxide into the atmosphere. Much of that carbon dioxide is absorbed by the ocean, which becomes more corrosive.
"If the ocean continues to acidify, it's possible that it would preclude corals from growing, because they won't be able to draw the nutrients and elements out of the water that they need to create the structures that they produce as coral colonies," Waddell said. "It's also possible that ocean acidification may become so extreme that it may begin to dissolve the corals that already exist, which would spell disaster for costal communities."
A 1997 report in the science journal Nature estimated that the resources and economic benefits derived from coral reefs are worth $375 billion a year.
"Coral reefs only cover about one percent of the world's surface, but they are a very diverse and important environment or ecosystem," said Mark Monaco, a marine biologist with NOAA's National Centers for Coastal Ocean Science.
"They provide us fisheries, they provide us culture from the cultural resources, they provide us pharmaceuticals, and they provide us protection from storm events," he told ABC News.
In areas that have been hit by severe tsunamis, experts point out that damage is usually less severe in places with intact coral reefs just offshore.
Scientists who study the medical benefits of coral reefs say there are about 20 compounds in clinical trials derived from the corals themselves or the many organisms that depend on them.