The Caribbean's vibrant coral reefs could be in for another devastating year as the world's oceans experience some of their warmest surface temperatures on record, scientists warn.
The National Oceanic and Atmospheric Administration released its 2009 outlook for the world's coral reefs this week, and the results are disturbing.
The temperature patterns and heat stress that scientists are seeing, particularly in the Caribbean, are reminiscent of 2005. That year set records for coral deaths. Across the Caribbean, 25 to 95 percent of the coral colonies were affected. In the U.S. Virgin Islands, nearly 52 percent of the corals died. In Trinidad and Tobago, 73 percent of all Colpophyllia and Diploria brain coral colonies were wiped out.
The damage goes beyond the corals themselves. Reefs provide habitats and ecosystems for tens of thousands of organisms, and they support the fisheries and tourism that some 100 million people worldwide depend on for their livelihoods.
"There's a lot of similarly between what we're seeing now and what hindcasts of 2005 showed," said C. Mark Eakin, coordinator of NOAA's Coral Reef Watch. "We can't say whether it's going to be worse or how they're going to compare, but we're looking at the potential for conditions that could lead to coral bleaching."
In 2005, parts of the Caribbean were hit by a double-whammy: high surface temperatures and a lack of tropic storm activity that could have cooled the water. This year, the global ocean surface temperature reached a record 0.62 degrees Celsius above the 20th century average in July, and the warmer water is accompanied by a storm forecast for the Caribbean that promises little rain or cloud cover.
The arrival of El Nino raises further concerns, starting in the central and eastern Pacific later this year and worsening conditions for corals in the Caribbean in 2010.
There isn't much scientists can do at this point but monitor the reef ecosystems, learn from what they see and hope for the best. That's because the primary source of the problem is global warming: Human activities have pumped more carbon dioxide into the air than the oceans can handle and raised the oceans' temperatures in the process.
When carbon dioxide mixes with seawater it forms carbonic acid, lowering the pH of the water. As the pH falls, so do levels of carbonate ions, which are essential to the creation of corals' calcium carbonate skeletons.
Add heat stress from rising surface temperatures to creatures with weakened skeletons, and scientists discover mass coral bleachings.
Corals that are under stress lose their color, known as bleaching, because they expel microscopic algae from their systems. They rely on that algae for food, and without it, they slowly starve. In mild events, they can recover quickly. In more severe events, they can die.
"Even if you don't visibly see the corals bleach, they're still under a lot of stress and you tend to see aspects of infectious disease," Eakin said. "These diseases are also more active at higher water temperatures."
The worst years to date for corals were 1998, when about 16 percent of world's coral reefs died, and 2005, which held the record for global sea surface temperature until this year.
Overfishing and run-off from agriculture and sewage contribute to the health threat to corals, but the greatest underlying danger to corals is global warming. In the past 50 years, many Caribbean reefs have lost up to 80 percent of their coral cover, according a report on the 2005 bleaching by the Global Reef Monitoring Network.
"The 2005 event would be extremely rare, possibly as low as a 1-in-a-1,000-year event, without the observed warming since the Industrial Revolution," writes Simon Donner, a climate and aquatic biology expert at the University of British Columbia, in an analysis attached to the report.
"The build-up of greenhouse gases in the atmosphere has increased the probability of an event like 2005 by an order of magnitude to a less than 1-in-a-100-year event. Furthermore, the warming projected to occur over the next 20-30 years should make this once rare occurrence a biannual event."
In 2005, scientists had little warning of the coral bleaching that was about to occur. This year, they're prepared to monitor their reefs for how it effects the ecosystems. As Eakin points out, "You can't find solutions if you don't understand the problem."
They know that in nature, shade, cool water, and currents that flush out toxins can help protect some corals. There are a few experimental techniques that have been tried to create shade, such as using sprinklers to break up the water surface. Coastal areas can also reduce other stresses on corals, such as reducing agriculture run-off, Eakin says.
The only way to truly protect coral reefs, though, is to stop global warming. Along with melting glaciers, they have become the most visible canaries in the coal mine for climate change. When the International Tropical Marine Ecosystem Management Symposium released its recommendations for preserving the oceans' corals, climate change topped the list: The group urged international support for efforts to limit sea temperature increases to 2 degrees Celsius over pre-industrial levels and to keep ocean carbonate ion concentrations from falling any farther.
(Photo: Kimberlyfaye/Flickr Creative Commons)