SOMETHING TERRIBLE happened last summer beneath the startlingly blue Caribbean seas off the island of Tobago, where we have just been staying. The Buccoo coral reef, home to one of the richest marine ecologies in the world, turned a brilliant white. “It looked as if it had been bleached,” said my brother-in-law, a marine biologist. “It was a strangely beautiful sight, but in fact it was sick, so sick that we wondered whether it could recover.”

We inspected it from our glass-bottomed boat, and watched its dazzling display of exotic fish, dipping down through waving green tendrils, shivering over the strange, sponge-like surface of the coral. To our untutored eyes the reef looked pure and unspoilt. It has recovered, but the bleaching has weakened it. Like a human body infected by disease, it is in a fragile state, vulnerable to the stress of pollution and the shock of the next big hurricane.

Tobago, like many Caribbean islands, is in the front line of climate change. The bleaching of its reefs came about because the sea around its coast had warmed by three degrees centigrade above the normal, rising as high as 31 (88F), which is well above the coral’s tolerance levels. At that temperature the algal cells that provide its life-support system are expelled and may never re-grow. Like a skeleton in the desert, the whitened coral remains only as a stark warning of its own mortality. The death of the reef would signal the end of the marine life it supports, as well as the fish and the birds that feed on them.

For Tobago, that would be more than just an environmental tragedy, it would be an economic disaster. Tourism and fishing — the only indigenous industries left after the devastation of its sugar plantations by successive hurricanes — are its lifeblood; an island that advertises itself as one of the world’s great eco-tourism destinations stands to lose its prime attraction — and with it, its principal source of income.

For the people of the Caribbean, global warming, that few of us at home envisage as a serious threat for at least the next two or three generations is a danger here and now. When Sir David King, Britain’s chief scientist, says that we may have to grow used to global temperatures rising by a minimum of three degrees, he is tacitly signalling the death knell for fragile ecologies such as that of Tobago. Out in the Atlantic, the seas are warming up again, gathering themselves for the next hurricane season; the coral reef, already fragile, may not survive next time.

But Tobago is more than just a vivid example of climate change. It is also a metaphor for how we might begin to combat it. Instead of waiting for the leading nations of the world to inch their way towards global solutions, Tobago, and the environmental organisations that support it, is taking local steps to control its own pollution and to limit the land-based developments that threaten its coastal waters with erosion and sedimentation. They may not be able to stem global warming itself, but at least they can ensure that the reef is defended as far as possible.

While we were there I talked to Peter Raines, the British founder of Coral Cay Conservation, which has worked on vulnerable coral reefs around the world. He pointed to the way that vulnerable reefs in the Philippines and Fiji have been effectively protected by prompt local action. In the Philippines this happened despite, rather than because of, the attitude of the national Government.

As a contrast to the deadly apathy of most Western governments in the face of global warming, this call for local action strikes me as the healthier approach. It has already been adopted by several American cities, which have rejected the intransigent approach of the Bush Administration and have begun putting in place their own climate protection policies.

Greg Nickels, the Mayor of Seattle, who has, with 218 other city bosses, signed up to a 12-step programme to meet or beat the targets set by the Kyoto treaty, says bluntly: “If it’s not going to happen from the top down, let’s make it happen from the bottom up.” These local efforts, symbolic as they may be, stand as an example of what the world as a whole should be trying to achieve.

The fact is that we are running out of time. When James Lovelock, the great environmental guru, gives warning in his latest book The Revenge of Gaia that global “heating” — as he terms it — is accelerating, and that there is “almost no time left to act”, he is not just scaremongering. He is reflecting the views of a growing body of climatologists, who not only accept the reality of what is happening to the Earth system, but have also begun to reassess the timescale in which it is doing so. Predictions that we may be facing serious and irreversible consequences from the growth of CO2 emissions by the end of the century have been revised sharply towards something that could be a reality within the next 20 years, or even sooner. “The slow creep of environmental decay is giving way to sudden and self-perpetuating collapse”, was the way Time magazine put it in its recent special issue on global warming.

In Britain, Lovelock believes that action is needed now to protect our low-lying coasts and our sea-level cities. “It would be unwise to rely on international agreement to save civilisations from the consequence of global heating . . .” he writes. “In our small country, we have to act as if we were about to be attacked by a powerful enemy.”

That is certainly the way they see things in Tobago. If the coral turns white again this summer it will send out a warning signal not just to that beautiful island and its delightful people, but to the rest of the world as well.