By Alex Kirby BBC News Online environment correspondent

Scientists and campaigners have joined forces to try to find out more about the world's polar bears.

The Norwegian Polar Institute (NPI) and WWF, the global environment network, have launched a website to track the bears' movements across the Arctic.

They hope to find out what effect climate change is having on the bears.

WWF thinks this may be one of the biggest challenges to the animals' wellbeing.

In a report, Vanishing Kingdom: The Melting Realm Of The Polar Bear, WWF says the Arctic is undergoing rapid warming.

Bears will be hungrier in a warmer world

The report says: "The Intergovernmental Panel on Climate Change (IPCC) has confirmed that human-induced climate change is a reality.

"It can no longer be dismissed as a theoretical, academic concept nor as a politically motivated doomsday prophecy.

"The Arctic is one of the regions on Earth where climate change will be seen early, and where the impacts are dramatic."

Forced to starve

It says traditional knowledge echoes the scientific evidence:

• Air temperatures in the Arctic have on average increased by about 5 Celsius over the last 100 years
• Arctic sea-ice extent decreased by approximately 3% per decade between 1978 and 1996
• the summer minimum thickness of Arctic sea-ice has decreased by 40% over the last 30 years.

The report says polar bears eat most of their food out on the sea-ice between late April and mid-July.

Plentiful food is critical at this time, but if it is not available the bears fast for long stretches, which can make severe demands on them, and especially on pregnant females.

Pollution hazard

WWF says the melting of the sea-ice, the platform the bears use to hunt the seals which are their primary prey, means they have less fat to help them to survive the long summer season.

Up close: Hands-on bear research

It says: "A two-week increase in the ice-free season results in an 8% weight loss for polar bears.

"This lower body weight reduces female bears' ability to lactate, leading to greater mortality among cubs."

The report says climate change is not the bears' only problem, with research in some areas showing a link between high contaminant levels and reduced immune system function.

There are thought to be about 22,000 polar bears in the wild, 60% of them in Canada.

Constant tracking

WWF says scientists now predict a 60% loss of summer sea-ice by about 2050, and its complete disappearance by 2080 during the summer months.

Some scientists do not accept the IPCC forecasts of Arctic warming, or even that climate change shows a discernible human influence at all.

They point to some of the patchiness of the ice thickness data, which is based on intermittent submarine surveys.

The two female bears being tracked by WWF and NPI are from Svalbard. Earlier this year, they were fitted with radio collars which beam their positions via satellite to the new website.

One is called Louise, after Louise Arner Boyd, the first woman to fly over the geographic North Pole, at the age of 67.

The other bear is Gro, named in honour of the former Norwegian Prime Minister Gro Harlem Brundtland, now director of the World Health Organisation.