If you had been alive 18,000 years ago, you could have walked in a straight line from Cork to Stockholm. The floor of the North Sea was land. Objects have been found from that strange, drowned world. A carefully sharpened flint scraper has been retrieved by Norwegians drilling for oil in 450 feet of water 100 miles east of Shetland. Spearheads and mammal and rhinoceros teeth have been dragged

up by trawlermen on the Dogger Bank. Sometimes in their trawls fishermen find lumps of peat from forgotten moors. It is an unsettling

fact that tens of thousands of people once knew the floor of the North Sea as well as any of us might know the Yorkshire Dales or the Sussex Downs.

When this periglacial world began to warm up about 20,000 years ago, the ice sheets melted and sea-level rose, on average, at about a centimetre a year. By 5,000BC it was some 130m (430ft) higher than it had been at glacial maximum and Britain had become an island. But

then the warming slowed. Since 2,000BC the sea level has remained extraordinarily constant, varying no more than a metre in 4,000 years. This period of sea-level stability has also seen the rise of urban and commercial civilisation. We have built our cities on a constant shore.

That long constancy has allowed us to forget that we have been living i

n a privileged world. But that privilege is now over. The physical conditions of the world are changing for the first time since humanity started to build. For thousands of years we have shaped the world.

Now, for the first time, the world is going to shape us.

It is the most subtle and unknowable of processes. The rocks of this country are, in part, still bobbing up in response to that huge ice load having been removed. An enormous boss of Scotland and northern England, stretching from Inverness to Morecambe Bay and from Edinburgh to Islay, is actually rising in relationship to the sea. But

outside that protuberant bump, the country is slowly going under, largely because, as the earth warms up, the water in the oceans is expanding. Dire predictions of a sea-level rise of 6m or even 10m have been made regularly over the years, but the evidence is contradictory. Such a vast increase will depend on the total melting of the Greenland and West Antarctic ice sheets, but it may be that a warmer atmosphere, which

can hold more moisture, will actually increase snowfall over Antarctica, thickening the ice sheet and so reducing, or at least stabilising, global

sea levels. The science remains tentative.

There may be little to worry about. There may be a lot. The

government-funded UK Climate Impacts Programme based in

Oxford has produced maps that combine the predicted rise of the land with two different estimates for sea-level rise, one for a future in which we continue to drive our 4x4s and oil-fire our central heating as if there were no tomorrow; and one for a future in which we all take a little

more care.

The low-emissions future does not look too bad. Throughout the coming century, people on the west coast of Scotland from Ardnamurchan to

the Mull of Kintyre are going to find themselves with even more beauti-

ful grassy verges between their shoreside houses and the beach. Scottish west-coast lochs are the place to invest. Elsewhere in Britain, even by 2080, the sea will have risen by no more than 20cm, and in many places less than that.

The high-emissions future is rather different. It begins to look like a

rerun of the great prehistoric drowning. South-west Scotland is still the best off, but even that is going down by 50-60cm. Other parts of the country are set to experience a rise in sea level that very nearly matches the end of the last ice age: north-east England 66cm, the rest of the east coast 77cm, the south-east 74cm, Wales the same, with the south-west experiencing the deepest immersion at 80cm.

All those Cornish fishing villages will be in severe trouble. The National Trust, which owns the tiny harbour on the west side of the Lizard at Mullion Cove, has already accepted that they cannot keep it for ever. Some repairs have been made this year, but at some time - not

specified because there is no telling "when and how the ultimate

extreme storm event or series of events" will occur - it will no longer

be viable to repair, the breakwaters will be dismantled and Mullion Harbour will be as much of a memory as the campsites on Dogger

Bank.

The assets are real enough. About 1,062,000 flats and houses, 82,000 businesses, 2.5 million people and 2m acres of agricultural land, worth about £120bn in all, are thought to be at risk from coastal flooding and coastal erosion.

If none of these things were defended from the sea, the annual average damage done to the country would be about £2bn. But the sea walls, as they stand, provide an extraordinary service, reducing the flood damage to £210m a year. Few of us recognise it, but we are already living in a fortress defended against the sea. The question is how much of that defence we are going to be able to maintain. With no improvements to the defences, the annual cost of damage by the sea is set to rise to

more than £1bn a year. Do we defend it even more expensively? Or do we abandon what we cannot maintain?

Certainly the National Trust, which owns one-tenth of the coastline of England and Wales, is going to let quite a lot of it go. The hotel and coastguard cottages at Birling Gap in East Sussex became a cause célèbre in the 90s - natural retreat from the Trust, "What about our houses?" from the residents. Ellie Robinson is assistant director of

policy and campaigns. "The coast is a canary for the rest of the country. It is where change is evident. We have all been looking at things in the short term: defend this, save that. And we have lost sight of the big systems that underpin it all. There is no doubt that to defend it all is going to be too expensive. Cities, towns will be defended. But large stretches of the coastline are going to go. The coast is a history of process and change and that is what we all now have to understand.

It is a question of inventive change in a riskier world."

Natural processes will be allowed to take their course at Birling Gap.

The retreating cliff will remain pristine white, and evidence of any

human intervention will in time be removed by the sea. At Brancaster on the north Norfolk coast, the Trust has made its youth activity centre "floodable" - electricity sockets high up the wall, tiled floors-

but in the end, says Robinson, "that will have to be abandoned too".

The current state of the Norfolk Broads, whose beauty and ecology depends on the fresh water that flows into them from inland, will

become increasingly difficult to maintain. "There will be a whole series of dynamics here," Robinson says. "Fresh water to saline, back to fresh water. It is inevitable that nat- ural processes will have to be allowed to take their course."

On the Studland peninsula in Dorset, the Trust has moved beach huts

and will move them again. Paths have been realigned at Golden Cap in Dorset and in Lancashire. At Porlock in north Somerset, the Trust has allowed the sea to breach the shingle ridge at the back of the beach

and what was once grazing is now salt marsh. In another, extraordinar-

ily expensive move, the Landmark Trust has begun to spend £900,000 transporting the prominent folly known as Clavell Tower, which has

stood for two centuries above Kimmeridge Bay in south Dorset and inspired writers including Thomas Hardy, John Fowles and PD James,

just 80 feet back from the cliff edge. It is a luxury treatment that few other buildings will be afforded.

The area that will experience the greatest difficulties and the greatest distress is bound to be the east coast. Between the two chalk promontories of Flamborough Head in the East Riding of Yorkshire and the North Foreland in Kent, the face that England presents to the sea is weak, often low, muddy and vulnerable. It is here, both in the lowlands protected by dune systems and in the high ground defended only by crumbling cliffs made of glacial boulder clay, that a taste of the future

is to be found. The east coast is what the future looks like, and it is an unsettling place. Little is really known: projected erosion on the north Norfolk coast has in places been five times what government studies thought it would be only 10 years ago; in other places, beaches have been replenished more than expected.

The greatest difficulties arise when people's attachment to their own houses and the places they love come into conflict with the new orthodoxy of managed retreat. Mike Ball, principal engineer for the East Riding of Yorkshire Council, takes me to see a street that is falling into the sea at Aldbrough in Holderness. "This is a sacrificial coast," he says. "It feeds the Humber, prevents it overdeepening, and the Lincolnshire beaches, which protect the low land behind them. And some of it goes to Holland." The soggy, boulder-clay cliffs we were looking at were visibly sliding downwards like cake drenched in rum. Above, the small bungalows and houses of Seaside Road looked hopeless and forlorn, ready to slide away to their wet and muddy fate. What about protecting them? "Look at these homes - they are not a good deal," Ball says. "The money would be better spent on the health service."

But the irony, as Ball points out, is that the same houses in the south of England, where property values even in these circumstances are higher, might well qualify for hard protection because the cost of building the protection - currently about £5m a kilometre - would be outweighed by the value of the property. "And yet," says Ball, "the trauma felt by these people is just as strong as anywhere else."

The fact is that money goes where money is. And that is the key to the greatest danger in the whole process: the creation of climate ghettoes. There are already embryonic examples. Until the early 90s, the beach at Sea Palling on the north Norfolk coast had been a horrible, eroded mass of sticky brown clay. No one wanted to spend any time there and the holiday businesses in the village had sunk to their lowest ebb. Then the Environment Agency built a series of offshore reefs at a cost of about £20m. Sand was pumped from the sea bed to create a beautiful beach within the reefs, and Sea Palling began to thrive. Charlie Roberts, proprietor of Sandy Hills Amusements just below the dunes, thinks his turnover has increased at least 500% since the reefs were put in. He has 10 staff now, where he only had two before the beach was remade, and he has just invested £100,000 in an extension of the arcade. It is a success story.

Or is it? Since the Sea Palling reefs were installed, the longshore drift of sediment has been interrupted by them and there have been major problems down the coast, where dune systems have been eroded almost completely. It looks as if Sea Palling's success has come at the expense of its neighbours.

But the tale of two villages, one in Norfolk and one in Yorkshire, may

well be symptomatic of what will happen in the years to come. Kilnsea,

in the East Riding just north of Spurn Head, is an example of how to play the system; Happisburgh, in north Norfolk, a classic example of a climate ghetto in the making.

Late last year, the Environment Agency published a document about the future of the Humber estuary. Without warning, the villagers of Kilnsea,

a small and pretty settlement, read this: "The coastal defences near Kilnsea are being threatened by erosion, and could be breached within five to 10 years, but possibly in as little as two years. There is no economic justification for realigning or replacing these defences, so

they are likely to be abandoned."

A village of poor and retired people might in these circumstances have meekly accepted their fate and shuffled off to the council housing that was being offered. But Kilnsea, which has 28 businesses and a fair share of forceful and articulate inhabitants, was not going to accept that. What riled them most was that a colony of little terns just north of the village was going to receive careful attention and money from the Environment Agency and Natural England, as the birds were to be protected under a potent European nature designation. Stuart Haywood, chairman of the village action group and an electrical technician with BP, is still angry.

"It appeared to be a fait accompli. That was it. The human side of it

was being abandoned, but our feathered friends were being accommodated."

The village activated itself, found money from a dazzling variety of sources, badgered its MP and councillors, and has managed to get

flood protection authorised for at least the next 20 years. The banks

and channels to save their houses are being dug this autumn.

Happisburgh is at the other end of the spectrum. Beach Road, a yard

or two of which is disappearing almost monthly, and which has lost 26 houses in the past 15 years, is at the poor end of quite a smart village.

It has been without any sea defence since 1991, when the groynes and revetment below the cliff were partly smashed in a storm and the rest removed. The people at the end of Beach Road have implored the authorities to spend the money to defend their houses, but Defra maintains that the cost-benefit ratio is too high: perhaps £2m for 18 rather poor houses. Exceptions can't be made. Campaigns have been waged, but there has been no movement on the part of the government.

Phyllis Tubby, a retired nurse aged 85, feels she has been deserted. Her pristine bungalow, a few yards from the cliff edge, is considered worthless. No one will buy it and it is uninsurable. She bought it 20 years ago and "had it done up. I spent an awful lot of money to make it comfortable. I had it surveyed, and the surveyor said, 'You'll be all right for 100 years.' And it wouldn't bother me then. But I do worry about it. When I go to bed at night I hope I don't wake up in the morning.' The council has offered her alternative housing but she says she would "rather die in my bed than go to a council house".

The government cannot consider compensation because, they say, to do so would distort the insurance market and, worse, create a "perverse" speculation in coastal properties. It is widely felt, though, that a financial mechanism could easily be devised by which this climate ghettoisation

of the poor and the weak could be avoided. Ellie Robinson at the National Trust thinks that "we need a fair and just way of adapting to change.

And it is not fair for the burden to fall on the people who are on the whole least able to bear it. If, for example, central government were to lend local government the money to buy out the threatened houses,

some time in advance, the local authority could get rental income from those houses as social housing and that rent would pay back the government loan. Or you could have a form of community insurance scheme, pooling the risk. There are all sorts of options. We need to

think ahead."

What seems like a strange and anomalous stretch of landscape now will soon become part of the mainstream. At Norfolk dinner parties, it is said, talk is of little except height above sea level. But no sight is more emblematic of the strange coastal future than Les and Avril Rial's property at Intack, just outside Withernsea on the east coast of Yorkshire. Two houses, one in front of the other, 200 yards apart, look nearly identical: red-brick boxes, slate roofs, doors square in the middle. But there is something slightly spooky about them. The older house, built in 1907, is right on the cliff edge. Its windows have the bleak air of abandonment. Its back door opens on to a ragged yard, the far side of which is the raw wound of the clifftop. Pipes and concrete slabs stick out in mid-air. The sheds that were once there have mostly tumbled into the sea. Inside the house, water drips on to the furniture.

Les, 59, was a teacher and in 1987 moved here from London. When he bought the farm for a knockdown £30,000 - it was a repossession - it was 70m from the edge. The council told him the cliff was receding at 1.2 to 1.5m a year. It would happily see them out. "My wife and I confidently expected to be buried at sea," he says. "I remember reading something by a native American who could not understand why white settlers would build houses that would last longer than they would." His now grown-up children, he says, will be able to make their own way in the world. As there is no compensation and no insurance available for houses that drop into the sea, they will have to.

For 10 years the cliff scarcely moved. But in 1997 disaster struck. "We lost seven metres," Rial says, like a gambler remembering a bad night

at the tables, "most of it in March." The cliff is 12m high, but in storms the house used to shake, and so much spray blew over the clifftop that

a river of salt water ran past their door to the road. They used to lie awake at night feeling the sea destroying the earth beneath them. "We thought we could market it as an earthquake experience," Rial says solemnly.

In 2000 they decided to make their own managed retreat, got planning permission for a new house, and began to abandon the old one. The

new house (still not finished and costing "in excess of £100,000") is 200m from the cliff, the same distance the old house was when it was built in 1907. At three metres a year, they will have 60 years or so. "I'd be happy with 50," Rial says. "If it lasts that long, it will have cost me £40 a week." Was he here for the frisson of living on the edge? "God, no. I like a feeling of security in my life." Two houses may, in the future, be all the security many of us will have.

· UK Climate Impacts Programme maps: http://www.ukcip.org.uk/resources/publications/documents/

124.pdf

Maps of places susceptible to flooding: http://www.hrwallingford.co.uk/downloads/projects/national%20appraisal.pdf