With their rainbow hues and the splendid variety of species they cater to, coral reefs are often called the rainforests of the seas. They might also be described as the ocean's architects--a consortium of tiny polyps hurling up high-rises of calcium-carbonate, which they mine from the seawater. Covering just 1 percent of the Earth's surface, they may host 25 percent of all marine life, which makes the reefs true underwater metropolises.
Now the sea's city builders are under siege. The ocean bottom might be the last place you'd think to be troubled by a climate out of kilter. But recently scientists have found that reefs are taking the brunt of the warming atmosphere. Some 30 percent of coral reefs are already irreparably damaged, thanks mainly to rising sea-surface temperatures, which hampers the coral's ability to gather nutrients. Another 30 percent may be poised for collapse. The reefs of the Seychelles never recovered from a 1998 heat surge in the Indian Ocean, another study says. Ten species of fish are either extinct or on the brink. Overall, the variety of marine life among the reefs has fallen by half. As reefs go, so go the oceans.
A coral is one of nature's odder couples: a plant inside an animal. The animal, a tiny polyp, shelters a tinier plant (an algae called zooxanthellae), which returns the favor by sharing the energy it produces through photosynthesis with its host. The skein of algae also lends the reefs their dazzling colors. The energy produced by this duo goes back into reef building, a neat bit of aquatic chemistry that mixes carbonic acid with calcium from seawater to form calcium-carbonate, the chalky stuff of seashells. But when temperatures spike, the partnership falters. The animal expels the algae, and the coral goes bone white and barren.
The bad news doesn't end there. Many experts now think creeping acidity, caused by carbon build-up in the atmosphere, is even more menacing to the oceans than the rise in temperatures. Normally seawater handily converts atmospheric carbon dioxide to carbonic acid, and buffers it with calcium to make the concrete of coral reefs. Too much carbon dioxide, however, leads to excess acid, which eats away the coral shells. The oceans are now 30 percent more acidic than in preindustrial times. "It's like going in for a routine checkup and seeing your blood pressure has gone wacko," says Thomas E. Lovejoy, head of the Washington based H. John Heinz Center for Science, Economics and the Environment. Lovejoy calls acid oceans the "single most profound environmental problem."
The irony is telling. For years, many climate scientists reckoned that the oceans were part of the solution, sopping up excess heat and carbon. Now even the forgiving oceans need saving.
