Visitors strolling on the beach at Borth, in mid Wales, are often puzzled by what appear to be tree stumps in the sand near the low water mark. They are indeed tree stumps, the remains of a forest that grew here before being submerged by the sea thousands of years ago. Local legend speaks of the Cantre'r Gwaelod, the drowned land of "sixteen noble cities" lying beneath the waves of Cardigan Bay. Similar curiosities can be found elsewhere around our coast, attesting by myth and hard evidence that sea levels today are higher than in previous centuries.
About 10,000 years ago the British Isles emerged from the last ice age, which at its peak saw much of northern Europe and North America covered with ice. Glaciers extended as far south as the Thames. The melting of the glaciers raised sea levels inundating the forest of Borth and cutting us off from the European mainland.
At present sea levels are rising by around 2 mm a year. Should we be worried? Yes, because global warming, caused largely by our emission of greenhouse gases, is accelerating this rise. As the Earth gets warmer the glaciers and ice sheets, most crucially those in Greenland and Antarctica, melt faster and add more water to the oceans. The water itself warms and expands, raising the level still further. The Intergovernmental Panel on Climate Change, a UN body set up to provide impartial scientific advice, forecasts that sea levels could rise by up to 88 cm by 2100. But this is not the whole story. Northern Britain, relieved of the weight of the ice following the end of the last Ice Age, is gently rising compared with the south-east. That means that estuaries and plains on the south and east coasts of England will be the most vulnerable to flooding.
Most sea flooding occurs in storm surges, where high tides coincide with storms at sea. It was just such an event that caused the devastating floods of 1953 in which 307 people were killed along the east coast of England and many more in Belgium and the Netherlands. In 1990 a surge breached the sea wall at Towyn, in North Wales, flooding four square miles and 2800 houses. We can expect many more such events in the coming decades. Large parts of London are already below high water level and are protected only by the embankments and the Thames flood barrier.
Global warming can affect the coast in other ways. Warmer temperatures mean that more water is evaporated into the air which in turn means more stormy weather. Every summer you will find Open University students visiting the Sussex hamlet of Birling Gap, in the South Downs, where the chalk cliffs are being rapidly eroded, threatening a row of houses. Further east, at Beachy Head, a massive cliff fall in 1999 saw hundreds of thousands of tonnes of chalk fall into the sea. It is not rising seas that are doing this, but the pounding of the waves, driven by storms. Again, such erosion will become more common as the world warms.
What can we do to save our coastline? The traditional approach is to strengthen sea defences, but most experts now believe this is a battle that cannot be won. Even if the amount of greenhouse gases in the atmosphere levelled off tomorrow, sea levels would continue to rise - slowly - for centuries. And there is evidence that defences on one part of the coast can actually increase erosion elsewhere. In the long run it may be better to accept that we cannot hold back the sea. Even the Dutch, those legendary masters of coastal defence, are now planning to work with nature rather than against her.
The National Trust, which owns 1130 kilometres of our coastline, now accepts that it cannot protect all of its sites. Some will be allowed to erode away, like Formby Sands in Lancashire which is disappearing at the rate of three to four metres a year, while others will be left to flood. The trust is letting the sea come in and create new saltmarshes at Porlock in Somerset and near Newtown harbour on the Isle of Wight.
If global warming is not arrested then the consequences are dire. A team at University College London recently produced maps showing the effects of large sea level rises on the shape of the British Isles. A seven-metre rise, equivalent to the complete melting of the Greenland ice sheet, would inundate extensive swathes of the Fens and the Humber basin. Other areas under water include the Somerset levels and the coastal plains of Essex, Kent, Sussex and Lancashire, though no part of the British coast would remain untouched. Melting of the Antarctic ice sheets would have even more catastrophic consequences. But such outcomes presume that global warming is allowed to run away unchecked for centuries and we're not going to let that happen. Are we?