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Thursday, 16 November, 2000, 15:57 GMT
More stormy seas ahead?
Storm AP
North Atlantic waves are a metre higher than 30 years ago
By BBC News Online's Helen Briggs

Climate researchers are warning of a possible link between global warming and giant waves in the Atlantic Ocean.

They say that if the current trend towards warmer temperatures continues, roughening seas could threaten coastal areas in northern Europe.

Average winter wave heights in the north-east Atlantic have increased by about a metre (3.28 feet) over the past 30 years. Stormy conditions also persist longer.

Scientists are divided over whether these ocean changes are being driven by natural climate variation or by greenhouse gases generated by man.

But experts in Germany believe new data show human activity is a major factor.

And they warn that if the trend continues, more rough seas seem likely, threatening coastal areas and the marine industry.

Seismic shocks

Ingo Grevemeyer, of the University of Bremen, told BBC News Online: "Our data suggest a matching trend between rough seas and increased air temperatures.

"We know from computer models that temperature will increase in the near future, so if this trend remains the same, we have to face increased wave heights."

Surfer AP
Coastal surf causes tiny ground movements called microseisms
The new evidence comes from studying tiny ground movements known as microseisms associated with ocean waves and coastal surf.

Microseisms have been measured routinely around the world since the late 19th Century as a by-product of earthquake monitoring.

The records allowed the German researchers to look back to 1954, before direct measurements of wave heights began.

They say that from 1954 until the late 70s, there were about seven days of strong ocean waves per month, on average. Then, between the late 70s and the 1980s, the number doubled to 14, staying at that level ever since.

North Atlantic Oscillation

Writing in the journal Nature, Ingo Grevemeyer and colleagues said:

"The implied increase in north-east Atlantic wave height over the past 20 years parallels increased surface air temperatures and storminess in this region, suggesting a common forcing.

They added: "It seems reasonable to propose that greenhouse forcing affects the ocean's wave climate and hence coastal surf and storm surges along northern Europe's coastlines, which in turn produced the observed increase of microseisms."

But other experts are more cautious. David Woolf, from the Southampton Oceanography Centre, UK, said: "We know waves are getting higher but we don't know if this is due to global warming.

He said that a weather pattern known as the North Atlantic Oscillation, which brings wet, windy winters to Europe at its peak, and cold ones during low periods, has a strong influence on wave heights and patterns.

"It is known that the North Atlantic Oscillation Index has gone up and down for 150 years," he told BBC News Online.

"We know it moves around without the influence of greenhouse gases. It is not clear whether the recent trend is due to manmade activities or a natural variation."

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