WHILE Tony Blair was delivering his first speech in three-and-a-half years on the environment on Tuesday, an academic from Durham University was in Marseilles talking about seven years' research into rising sea levels.

For the first time, Dr Ben Horton and a team of researchers have shown how a large area of land can be flooded, causing catastrophe to its human inhabitants, when the globe warms up.

Once upon a time, Britain was part of mainland Europe. By taking plugs of sediment out of the North Sea floor and analysing the contents, Durham's Sea Level Research Unit has discovered a vast land mass that spread from just south of the River Tees to Denmark.

"Ten thousand years ago, the UK wasn't an island," says Dr Horton. "We were a headland sticking out of western Europe. A tidal flat like Holy Island off the Northumbrian coast stretched all the way from the North-East coast to Denmark.

"It was an area equal to the whole of England and Wales, about 100,000 square miles, and it disappeared with a sea level rise of about 20 metres.

"Global warming, with a melting of the polar ice caps, could give a similar rise which would be devastating."

We can only imagine how devastating it must have been to our ancestors 10,000 years ago. The wooded plain would have been attractive to the hunter-gatherers of the Stone Age. They would have hunted in its forests, collected plants from its land and fished in its seas.

"The only evidence we had of them was from trawlers dredging up flints and tools and human bones which showed there had been a human community there, and it looks like the type of environment which was very flat and fertile - particularly around the lagoon that formed 8,000 years ago which I can imagine being very attractive."

The lagoon - bordered to the north by Dogger Bank which was then a 140-mile long island rather than just a vague reference on the shipping forecast - was created as sea levels rose and flooded the North Sea plain.

As it took about 2,500 years for Britain to breakaway from Europe, the flooding would have been a gradual process. The research shows that there were storm surges once every 25 years with waves up to 15ft high breaking on the land. Although the sea would have receded after each storm, it never dropped to its previous level. Another piece of Britain's Atlantis would have been lost.

The people of the region might well have got so fed up of their coastal areas being flooded once a generation that they retreated to permanently dry land. However, it is believed that some of the largest of the tidal surges would have inundated such large areas of land that the human population would have been caught napping. As many as 3,000 would have been killed in each surge.

There is evidence of a catastrophic flood that hit the east coast of North England and Scotland about 7,000 years ago, caused by a tsunami. A tsunami is a giant tidal wave usually caused by either a volcano or an earthquake under the sea. There has recently been scientific speculation that one day a giant cliff fall off the Canary Islands will cause a tsunami to cross the Atlantic Ocean and devastate New York.

The same thing happened in the North Sea in Stone Age times. "We have indications that a tsunami hit the North-East coast 7,000 years ago," says Dr Horton. "It was caused by a landslide below the sea's surface off Norway. Deposits from it have been found along the coast of Scotland and Northumbria, and it would have had a devastating effect on this environment because you only need a small rise in sea level to devastate these flat areas."

The tsunami was one of the final straws for the lost land mass. It helped open the English Channel into the North Sea, and submerge the Dogger Bank. Britain was now an island.

This had a dramatic affect on the humans who had been driven back to the dry lands of England. "The human species had been able to develop freely from Scotland to the Mediterranean, and to exchange technologies," says Dr Horton.

Britain was now cut-off. As a result, when the rest of Europe developed new forms of arrow heads, Britain lagged 2,000 years behind. The introduction of agriculture in Britain was delayed by 1,000 years. The effects on British culture and British genes - unable to mate with Continentals, we became an island race - have yet to be fully understood.

All of this rise in sea level, and the repercussions for the humans of the North-East, were caused by nature. The Earth is on a 100,000-year cycle from one Ice Age through a warmer period and then back to another Ice Age.

"So 10,000 years isn't that long ago," reminds Dr Horton. In fact, the North-East has yet to recover from the last Ice Age 100,000 years ago. "During that glaciation, there were ice caps over northern Scotland and northern England that squashed the land down. When they melted the Earth started to rebound back. Northumberland is rising by about 0.5mm a year and the centre of the uplift is in central Scotland." This is why Ben Nevis is getting taller, whereas the south east is dropping slightly.

It was towards the end of the last warming period that, 10,000 years ago, the North Sea plain was flooded and Britain became an island.

"We had a warming from the deglaciation and that caused a relatively small but continuous rise in sea level," says Dr Horton. "With global warming we also have a rise in sea level and so from this research we know what might happen in the future."

The research is part of the Land Ocean Interaction Study commissioned by the National Environmental Research Council. It has cost many millions of pounds spread across several universities, and has lasted seven years. Dr Horton is currently in Marseilles presenting the team's findings to a European Union climate conference, although perhaps the most revealing part of the study is about to begin.

"The next stage is to see what a two degrees Centigrade rise in temperature does to tidal highs and ranges," he says. Using the new knowledge of the past, they will be able to discover what effect global warming will have on future humans.

However, while the floods in south England this month show the problems global warming is already causing, no one is sure what long-term effect it will have on the Earth's climate. We are now beginning to approach the next Ice Age, but no one knows by how much global warming will delay it.

"The Earth is on this 100,000 year cycle, and in that time scale we are nearing the end of an interglacial so we will be moving to a colder climate," says Dr Horton. "Sea levels will drop as water freezes and Britain will become part of the European land mass again."

But will we have survived global warming to take advantage of the new European single continent?