The National Marine Fisheries Service, part of the US government, has this month ruled that two species of coral . . . elkhorn and staghorn . . . must officially be registered as threatened under the country's Endangered Species Act, partly because they are imperilled by rising sea temperatures.

They are they 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%-98%. The fisheries service concluded that "elevation of the sea surface temperature" was partly to blame and added that there were "no regulatory mechanisms in place to control or prevent" it.

The ruling accepts that the corals die when the sea temperature exceeds 290C. Corals are really colonies of billions of tiny animals called polyps, which lay down limestone skeletons that build the reef. They, in turn, depend on tiny algae called zooxanthellae, which harness the power of the sun through photosynthesis to feed the polyps.

Over a million of the algae throng each square centimetre of coral, and give it its colour. When the water temperature rises the polyps expel them into their stomachs, and then spit them out through their mouths. The coral then becomes transparent and eventually dies. This white death, virtually unknown 25 years ago, is happening increasingly around the globe as global warming takes hold.

Under US law, the government is legally bound to take measures to diminish the threat by cutting back the pollution that causes it. Environmental groups are preparing to take it to court if it continues to refuse to do so.