Global warming is killing coral reefs, the Bush administration has formally admitted. And the admission means that, under US law, it will finally be obliged to take action to reduce the pollution that causes climate change.

The US National Marine Fisheries Service (NMFS) has this month ruled that two species of coral - elkhorn and staghorn - must officially be registered as threatened under the US Endangered Species Act, partly because they are imperilled by rising sea temperatures. They are the first species ever to be listed as a result of global warming.

The two corals are the main reef-building species in the Caribbean and the Gulf of Florida, but they have declined by 80 to 98 per cent throughout the region. The NMFS - part of the National Oceanic and Atmospheric Administration - concluded that "elevation of the sea surface temperature" was partly to blame. The ruling accepts that the corals die when the sea temperature exceeds 29C, because a curious relationship between tiny algae and the coral breaks down.

Corals are really colonies of billions of tiny animals called polyps. They lay down limestone skeletons that build the reef. But the tiny reef-builders in turn depend on algae called zooxanthellae. Not only do they give coral its colour, they harness the power of the sun through photosynthesis to feed the polyps.

More than a million of the algae throng each square centimetre of coral. But when the sea temperature rises, the polyps expel them into their stomachs, and then spit them out through their mouths. The coral becomes transparent, and eventually dies. This white death - virtually unknown 25 years ago - is now happening increasingly around the world as global warming takes hold.

But under the Endangered Species Act, the US government is legally bound to take measures to diminish the threat, by cutting back the pollution that causes climate change. And environmental groups are preparing to take it to court if it continues to refuse to do so.