The Severn Estuary at dawn is a place of wild enchantment. The brown river meets the tide ripping in from the sea, sending the water dancing in strange and beautiful eddies. Out on the misty mudflats, bands of shell duck potter about purposefully in search of lugworms, and the curlews pick daintily along the shore edge, watchful as traffic wardens. The landscape here has hardly changed for 15,000 years, which was when the last Ice Age ended, and the estuary, carved by glaciers in Triassic times, was flooded. There are few richer nature spots in the whole of Britain.

It is in order to preserve Britain’s natural wonders that the Government ought to build a whacking great concrete wall across the estuary as soon as possible, to harness the extraordinary untapped power of the tides. The ambitious Severn barrage project, stretching from Cardiff to Weston-super-Mare, will cost an estimated £15 billion, last for at least 120 years and provide nearly 5 per cent of the country’s total electricity needs without pumping more CO 2 into the atmosphere.

The Severn barrage would be the largest power station in Britain, the equivalent of ten nuclear power stations; it would alter Britain’s geography more radically than any engineering project in history; and it will have profound and far-reaching consequences for the delicate ecosystem of the Severn Estuary.

But if we are serious about tackling global warming and undoing the damage already caused by inadvertent alteration of the planet’s chemical balance, then the Severn barrage, or similar technology, seems the only logical way to go. Human engineering caused the problem; only human engineering on a massive scale will begin to tackle it, and perhaps save the curlew.

The more conservative conservationists find talk of “geo-engineering” unsettling. Last week James Lovelock, author of the controversial Gaia hypothesis, upset the scientific community once again by arguing that technology should be used to pump matter from the ocean bed in order to stimulate microorganisms that would then absorb carbon from the atmosphere. To many, that sounds like man tinkering with nature once again.

Yet there is a growing understanding in both government and the environmental movement that only radical measures will solve man-made problems in nature. Take the grey squirrel. There was predictable outrage from animal lovers this week when it emerged that scientists have been asked by Defra to find ways to halt the growth of the grey squirrel population, and thus save what few red squirrels remain, by a programme of sterilisation.

A broad cull would not only save the reds from the squirrelpox virus that the greys carry, but also protect bird populations from these agile and insatiable egg thieves. The grey squirrel was introduced into Britain from North America in 1876. As an unnatural feature of our countryside, the squirrel plague demands an artificial solution: putting (Grey) Squirrel Nutkin on the Pill. Similarly the only way to save our fished-out, polluted oceans is by setting aside marine parks where fishing is banned.

In each of these examples, finding a man-made solution to a man-made problem cannot be done without sacrifices: the migrant wading birds of the Severn estuary, the grey squirrels in our parks and gardens, and the traditional fishing industry.

The barrage across the Severn will be ugly, a line of huge concrete blocks with at least 300 grinding turbines. An area of wild sea will be tamed, transformed into a sort of synthetic and controlled tidal lake. Some 11,000 acres of intertidal land, up to half the tidal range including important feeding areas, may be lost. The fish and bird life of the estuary is certain to be adversely affected.

At the same time, a barrage would stimulate the economies on both the Welsh and English coasts, offer flood protection and above all provide a reliable source of clean energy for the foreseeable future. No one (except the construction industry) wants to see a ten-mile dam of concrete snaking across this beautiful stretch of water. But the rate of carbon emissions, the mounting evidence of global warming, the Government’s own targets for renewable energy, the need to cultivate independent energy sources and, above all, the fact that the Severn’s huge tidal range makes it a natural power station all combine to suggest that a barrage is the least bad answer to a grim conundrum.

Whether the barrage proves an ecological disaster depends on a different sort of engineering. Under directives governing protected land, new wildlife habitat must be created to compensate for what will be lost. As the Sustainable Development Commission reported this week, Britain has never faced an environmental challenge on this scale before: plans for building an energy-trapping barrage already exist, but no government has ever tried to engineer a compensatory wildlife habitat equivalent to 11,000 hectares of salt marsh and mudflat.

Yet what human intervention and a changing climate have wrought, human ingenuity in adapting to climate change may also resolve. Poor quality, lowlying coastal farmland in East Anglia, Wales and elsewhere is already under threat from the rising sea and violent storms; this could be purchased, and then turned back to nature as part of the compensation package. The problems created by global warming might thus become part of the solution.

Climate change has caused a decline in certain bird species on the estuary. A barrage may further affect those populations. Failure to tackle climate change, however, could wipe them out entirely.

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