The environmental campaign group Greenpeace says it has found evidence that walruses and other Arctic species are being affected by climate change.

It says some of the species affected are barometers of the changes taking place, and are a clear warning that more change is inevitable.

The crew of a Greenpeace vessel, the Arctic Sunrise, which recently sailed along the edge of the Arctic pack ice between Alaska and the Chukotka region of Russia, found that the edge of the ice was much further north than usual.

Greenpeace campaigners say the edge of the ice is normally only a few tens of miles north of Point Barrow in Alaska.

But this year they found it was at least 150 nautical miles north of the settlement, retreating towards the North Pole.

Walruses need thick ice

Walruses use the pack ice for resting and breeding. An adult male weighs getting on for two tons, and a female about half of that.

The thinner ice cannot support weights like these, so the animals' habitat is shrinking.

The walruses dive for food to the seabed, where they feed on molluscs and other invertebrates.

But with the ice so far further north than usual, the edge of the pack is now over much deeper water than the walruses are used to.

As the ice edge retreats beyond the continental shelf waters of the Chukchi Sea, Greenpeace says, the water could be too deep for the walruses to dive for food.

Breeding affected

Dr Brendan Kelly, of the School of Fisheries and Ocean Sciences at the University of Alaska, who was on the Arctic Sunrise, says the ratio of walrus cows to calves is much lower than he would like to see.

"If the trend continues," says Dr Kelly, "we will definitely see a decline in the population. That may very well be due to the retreat of the ice".

The Greenpeace expedition also says it found problems with some Arctic seabirds, especially the black guillemot. It says the birds' numbers were quite healthy during the 1970s and 80s, but that they have fallen dramatically since then.

The guillemots nest on shore but, like the walruses, they depend entirely on food found at the edge of the pack ice.

Dr George Divoky, of the Institute of Arctic Biology at the University of Alaska, says black guillemot are "like the proverbial canary in the coal mine".

They and similar species "are an excellent indicator of climate change, because the effects of warming on these habitats are direct and immediate".