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Coral reefs, such as the a reef-building favites abdita seen here at low tide, are vulnerable to bleaching and possible extinction.

Warmer waters, more acidic oceans and stronger storms are taking their combined toll on coral reefs. "Coral reefs may prove to be the first ecological victims of unchecked global warming," says Environmental Defense scientist Rod Fujita.

Loss of coral reefs would translate into huge economic losses in coastal regions dependent on reefs—they provide about $375 billion each year in food and tourism income. (U.S. Commission on Ocean Policy)

Severe damage to reefs is also an ecological catastrophe. Coral reefs are sometimes called "rain forests of the ocean" because they are home to a rich diversity of marine life such as reef fish, turtles, sharks, lobsters, anemones and sponges.

Warmer water linked to "bleaching," death

Corals get both their food and their spectacular color from tiny algae called zooxanthellae that live in them. Corals are very sensitive to temperature and thrive within a narrow range of heat and cold. An increase of just 1.8 degrees Fahrenheit above the typical maximum summer temperature can cause corals to expel their algae, or "bleach." After prolonged bleaching, they often die.

A massive bleaching of corals occurred during one of the warmest 12-month periods on record, in 1997 and 1998. About 16 percent of the world's reefs suffered severe damage, and thousand-year-old corals perished. Continued increases in ocean temperature could make mass bleachings an annual event. Environmental Defense scientist Doug Rader says that "within a century, very large portions of coral reefs could be gone."

Double damage: Oceans getting more acidic

Coral reefs face another threat related to global warming: carbon dioxide (CO₂) pollution. Carbon dioxide is the main heat-trapping gas that causes global warming, but that's not the only damage it does. A report by the U.K.'s Royal Society found that the increased levels of CO₂ in the ocean are making it more acidic.

When CO₂ dissolves in ocean waters, it produces carbonic acid, which corrodes the limestone structures of coral reefs and seashells. In acidic water, "there is a greater tendency for seashells to dissolve, like putting them in vinegar, but not quite as dramatic," says Environmental Defense climate scientist Dr. James Wang.

"The world's seas are naturally alkaline," adds Fujita, "and many of these marine creatures that have been around for eons will not survive in an acid sea."

As waters become more acidic, coral reefs and other marine ecosystems could suffer. The Royal Society's panel of scientists report that acidification will hurt tropical and subtropical reefs the most, but cold-water corals are also in danger. Since acidification is "irreversible in our lifetimes," the authors say, "the only practical step is to reduce emissions of carbon dioxide as quickly as possible to minimize large-scale, long-term harm to the world's oceans and marine ecosystems."

Guard reefs from stress, and they are resilient

Rader has spent years diving in the Caribbean and seen first-hand the decimation to reefs. Disease has nearly destroyed elkhorn and staghorn reefs throughout the region, he says. "Add to that more frequent bleaching events and an abundance of CO₂ in the water," says Radar. "It seems hard to believe that it is happening—and happening on our watch."

But Fujita offers a sliver of hope. "Corals are sensitive but also very resilient—if conditions are right. If we can reduce some of the other direct stresses from human activities on coral reefs, like pollution from diffuse sources, that may also enable reefs to cope better with threats like climate change."

Creating more protected areas for coral reefs may help them better withstand the rigors of too-warm water and be less vulnerable to extinction. Kelp forests seem to be able to cope with warmer water better in marine reserves, explains Fujita. But even so, cautions Fujita, "the number of corals that can adapt to or withstand such dramatic, rapid changes may be just a tiny fraction."

Sources

Fujita, Rod. Heal the Ocean: Solutions for Saving our Seas. Gabriola Island, B.C., Canada: New Society Publishers, 2003. 74-76, 78, 85.

The Heat Is On: A White Paper on Climate Action. Environmental Defense. 2004. www.environmentaldefense.org/documents/3777_TheHeatIsOn.pdf.

The National Assessment Synthesis Team, U.S. Global Change Research Program. Climate Change Impacts on the United States: The Potential consequences of Climate Variability and Change. U.S. Global Change Research Program, Washington D.C. 2000. www.usgcrp.gov/usgcrp/Library/nationalassessment/1IntroA.pdf.

Raven, John (Chair), The Royal Society. Ocean acidification due to increasing atmospheric carbon dioxide. June 2005. www.royalsoc.ac.uk/displaypagedoc.asp?id=13539.

U.S. Commission on Ocean Policy.  An Ocean Blueprint for the 21st Century: Final Report, Washington, D.C. Chapter 21: Preserving Coral Reefs and other Coral Reef Communities. 2004. www.oceancommission.gov/documents/full_color_rpt/21_chapter21.pdf.

Wilkinson, Clive (editor). Australian Institute of Marine Science. Status of the World's Coral Reefs: 2004. www.aims.gov.au/pages/research/coral-bleaching/scr2004/index.html.