"It's a bit strange to think that the trips I have been doing may not be possible in 10-20 years," he told reporters after attending a climate seminar in the Norwegian parliament. "But it may well happen."

That would end just over a century of trips across the ice -- American Robert Peary was the first to claim to reach the North Pole in 1909.

"Over time I have seen the changes myself," said Ousland, aged 45, who has been to the North Pole several times.

On a first trip in 1990 the ice was about three metres thick around the North Pole. "Now it is 30 percent thinner," he said.

There were also far more and wider gaps in the ice with open water, requiring risky swims in a special survival suit while tugging provisions and other gear along in a floating sledge.

The Arctic ice shrank in September 2007 to the smallest on record, eclipsing a 2005 low, according to US satellite data.

It is now expanding again as winter approaches but many climate scientists say that the ice could vanish in summer well before the end of the century because of a build-up of greenhouse gases, mainly from burning fossil fuels.

HELICOPTER TO ICE

And the summer ice now starts several hundred kilometres (miles) further north than a century ago. Few expeditions can now begin from Russia's Cape Arkticheskiy, as Ousland did in 1990, because a helicopter ride is needed to reach firm ice.

The receding ice is also revealing new islands.

Ousland and a colleague this year, retracing a 1896 trip by Norwegian polar hero Fridtjof Nansen, found that an island called Northbrook Island in the Russian Arctic was in fact two -- melting ice had exposed a channel between them.

They took a photograph of walruses swimming between the two islands. Polar bears in the region looked thin, forced to eat nesting seabirds rather than seals, which live on the ice.

Ousland said that even a trip he made with South African Mike Horn staring in January 2006 -- the first winter trek to the North Pole -- revealed gaps in the ice.