Scientists in the Arctic are reporting a rare mass migration of thousands of walrus from the ice floes to dry land along Alaska's coast.

Researchers from the US Geological Survey (USGS), who have been tracking walrus movements using satellite radio tags, say 10,000 to 20,000 of the animals, mainly mothers and calves, are now congregating in tightly packed herds on the Alaskan side of the Chukchi Sea, in the first such exodus of its kind.

"It's something that we have never seen before in this area," said Geoff York, of the WWF's global Arctic programme. "As the ice decreases, the walrus are abandoning it earlier and earlier. They are having to swim ashore, or to linger on less suitable drift ice for long periods of time."

The flight of the walrus, first reported by the Alaska Dispatch, has reinforced warnings from scientists that the lumbering animal may be headed for extinction because of climate change.

Arctic sea ice dropped to its third lowest level in recorded history this month. The USGS study noted that the entire Chukchi shelf could be completely ice-free during August, September and October by the end of the century.

The USGS, which has been tracking the walrus since June, put the chances of extinction or serious population decline among walrus at 40% by 2095 because of the rapid and widespread loss of summer sea ice due to warming temperatures.

Walrus are not the only animals facing depleted numbers or extinction because of climate change. The Arctic is warming at twice the rest of the world on average, and its seas are growing increasingly acidic because of increased concentrations of carbon dioxide. A new report today warned that 17 species - from tiny plankton to the weighty narwhal - were threatened by the disappearing sea ice and rising seas.

In addition to polar bear – whose plight is widely recognised – and walrus, the report from the Centre for Biological Diversity said several species of whale and seal, and land-based animals such as caribou and fox were facing declines.

Meanwhile, local residents in Point Lay told the Dispatch the numbers of walruses coming ashore could be much higher than government estimates.

The rare onshore mass sightings have raised fears of a grisly repeat of last summer when some 130 of the beasts, mainly calves, were trampled to death as the herd foraged for food.

"Walrus mums and calves need the sea ice to rest," said Shaye Wold of the Centre for Biological Diversity. "When the sea ice disappears they are forced to come to shore and their calves are extremely vulnerable to being trampled in a stampede as you can imagine with 10,000 to 20,000 walruses on shore trying to forage in a limited area."

Because of their huge girth, walrus are relatively clumsy in water. During their annual migration, they rely on large floating patches of ice as resting stops or mobile fishing platforms as they make their voyage cross the shallow frigid waters between Alaska and Russia.

In 2007 and again last year "thousands of walruses hauled out along the coast of north-western Alaska and tens of thousands of walruses hauled out along the coast of northern Chukotka when ice disappeared," the USGS report said. "These events led to the trampling and death of hundreds of walruses in Alaska and thousands in Russia," the report said.

Increased concentrations of carbon dioxide in the ocean was also depleting the walruses' food supply, making the waters too corrosive for the clams and other shellfish that are their staple.

The USGS study went on to note that disappearing summer sea ice, due to climate change, was likely to spur an increase in shipping traffic through the Chukchi and Bering seas, putting the walruses in further jeopardy.

In a statement, Rebecca Noblin, the director of the Centre for Biological Diversity, argued that the forecast failed to take stock of the risks of oil spills in the pristine Arctic environment, because of the increase in shipping, or the potential for damage if the area is opened up for offshore oil and gas drilling.

"Unless we dramatically reduce our greenhouse emissions, the walrus is on a trajectory toward extinction," Noblin said.

The centre launched a law suit two years ago to place the Pacific walrus on the list of endangered species. The federal government must rule by January 2011 whether to grant endangered species protection to the Pacific walrus, which would put restrictions on offshore oil drilling and shipping routes through the Arctic.