Oceans rising faster near coasts

19:00 21 April 04

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Satellite measurements of sea levels have uncovered a bizarre effect - the sea seems to be rising faster near the coast than in mid-ocean.

Simon Holgate and Philip Woodworth of the Proudman Oceanographic Laboratory, in Bidston, UK, found this discrepancy using the Topex satellite, launched in 1992. The satellite measures sea level by bouncing microwaves off the ocean and timing the
return trip.

According to the Topex data, global average sea level rose by 2.8 millimetres a year between 1993 and 2002. This is thought to be a consequence of global warming: water in the oceans expands as it warms up, and more is added as glaciers and ice caps melt.

But during the same period, the water level within 100 kilometres of the coast rose faster, by an average of 3.7 millimetres a year.

Holgate and Woodworth wonder whether the oceans are behaving like water in a bathtub. If you splash in the bath, waves travel outwards and then run around the edges of the tub.

Boundary waves

In 1996, ocean modellers predicted that this effect could be at work in the seas. Kirk Bryan of Princeton University and his colleagues worked out what would happen when part of an ocean heats up: the water expands, creating waves that hit the coast then travel around the rim of the ocean basin for several years.

Holgate and Woodworth are the first to suggest that we can see this happening, says Jonathan Gregory, an expert in climate change at the University of Reading, UK.

This is not necessarily bad news for the many millions of people who live near the shore. The boundary waves might have been caused by some relatively sudden event, such as a particularly warm year, in which case they will subside.

But if they are driven by an ongoing process of climate change, the difference between coast and ocean could get worse, and flooding could be an even bigger problem than scientists have anticipated.

Journal reference: Geophysical Research Letters (vol 31, p L07305)

Jenny Hogan