Yes, the Water's Warm . . . Too Warm

The reef looked small from the boat, but once I plunged into the Caribbean I found myself gazing at what looked like a vast and intricate community, which was oddly dead and alive at the same time. In one section of Glover's Reef, which surrounds an atoll 55 miles off the coast of Belize, delicate spiny corals jutted out fiercely, surrounded by vibrant green and silver fish; in other places, only ghostly white corals remained, a testament to how warmer water temperatures had wreaked ha-voc deep beneath the sea.

When people think of habitats collapsing from rising global temperatures, they tend to think of frigid climes where polar bears have been frolicking on snow and ice for centuries. Think again. Coral reefs rank as one of the ecosystems most vulnerable to climate change: Recent scientific studies suggest global warming has already destroyed 20 percent of the world's reefs, and an additional 50 percent are in danger of disappearing.

Two aspects of global warming -- hotter and more acidic oceans -- threaten corals the most. In seven tropical regions where most coral reefs occur, waters have warmed between 1.3 and 3 degrees in the past century. Though that might not seem like much, a seawater temperature rise of 1.8 to 3.6 degrees above the summer maximum can trigger bleaching on many reefs. When bleaching occurs, the single-celled algae that live in symbiosis with a coral are expelled, and if the coral does not take up other algae within a certain period of time, it dies. Scientists call this bleaching because the living tissue of a coral is white; if the algae leaves, the coral loses its color because it no longer has a living creature inside. At that point the white limestone skeleton shows through.

At the same time that seawater temperatures are on the rise, absorption of human-generated carbon emissions has altered the oceans' pH level, making it more difficult for reefs and other marine organisms to construct their calcium skeletons.

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