search engines: Serendipity On the rebound

IN THE 18th century, fishermen on the island of Lovgrund, a few miles off the Swedish coast, noticed that things were not as they used to be. The village elders remembered how seals used to climb upon a particular rock in the harbour, where they could easily be harpooned or shot. But the seals could no longer clamber on to the rock, because the sea level seemed to have fallen and the rock was too high out of the water.

In 1731 Anders Celsius, inventor of the Celsius temperature scale, decided to investigate and rowed out to the island. He marked sea level on the rock, rowed back and waited. Sure enough, as the years passed, the sea level gradually and consistently fell below Celsius's mark.

When I visited the rock, while filming part of the BBC's Earth Story series, I could see that the mark was several feet above sea level. The apparent fall in sea level is not unique to Lovgrund. There are Viking harbours that are now stranded far inland and it is possible to find pebbly beaches halfway up a hillside.

When scientists first tried to make sense of what was happening, some argued that there was a hole in the bottom of the ocean. Others believed that this was simply the result of the Great Flood subsiding. In fact, the sea is not falling; rather, it is the land that is rising. Celsius was probably the first to suggest this, although at the time he had no idea what was forcing the land upwards.

The correct explanation concerns the last Ice Age, when a massive ice-sheet covered Scandinavia. Its sheer weight squeezed out the rock below the crust towards more southerly regions. Hence Scandinavia was depressed and places like England and France were pushed up by the incoming rock. It is just like a waterbed - lying on one side of the bed will cause a depression, and the water is displaced, resulting in uplift on the other side. Although rock is rock- hard, over geological time it can flow like water.

When the ice melted, roughly 10,000 years ago, much of Scandinavia was pushed down and submerged. Then, over the millennia, the rock gradually flowed back from England and France, effectively re-inflating countries like Sweden. The phenomenon is known as glacial rebound.

Nowadays, scientists are showing a renewed interest in glacial rebound. There are sea-level records dating back centuries, made by tide gauges in ports around the world. However, because the vast majority of these gauges are located in places where glacial rebound has caused the land either to rise or fall, the sea-level measurements are distorted. For scientists interested in ascertaining the extent of sea-level change caused by global warming, it is vital to ascertain the extent and take into account the effects of glacial rebound.