The Arctic ice cap has shrunk by an area twice the size of France's land mass over the last two years, the Paris-based National Centre for Scientific Research (CNRS) said Wednesday.

"The year 2008 promises to be a critical year on every level," said Jean-Claude Gascard, the body's research director and coordinator of European scientific mission Damocles, which is monitoring the effects of climate change across the Arctic.

September 2007 measurements show ice covering 4.13 million square kilometres (1.6 million square miles), down from 5.3 million square kilometres in 2005.

"Melting could result in the loss of another million in one (2008) summer," he added at a press conference.

"Summer 2007 was marked by a major retreat in the ice-cap, one we were not anticipating," Gascard said. "The rate of decline is also two or three times faster than (observed) beforehand."

International models used to predict retreating ice have some "catching-up" to do, he said.

Over the last 20 years, 40 percent of the ice-cap has melted with the average thickness halved from three to 1.5 metres.

Year-round ice coverage has reduced, with summer melting also lasting longer, the centre reported.

The Damocles' exploration vessel Tara has been able to cross the 5,000-kilometre Arctic Ocean in just over 16 months -- less than half the time taken by a late 19th century Norwegian explorer.

Gascard said the ship had been able to travel at "twice the pace expected by organisers, and three times the speed models suggested".

Disruption to the thermal layers of atmosphere stacked over Earth's far north was cited as the principal cause by Swedish researchers earlier this month, in a study published in the journal Nature.

The Tara team recorded a temperature of 10 degrees Celsius (50 degrees Fahrenheit) at altitudes between 500 and 1,000 metres.

"The reduction in the intensity of cold (temperatures) during winter over these last 20 years corresponds to an accumulation (rise) of 1,000 degrees Celsius," Gascard said.

The team highlighted the role of ocean currents, namely in the northern Pacific, behind warming of waters.

Gascard's research colleague, Gerard Ancellet, also spoke of recently-formed Arctic mist, pollution clouds which "trap" Earth's naturally-emitted infrared rays thereby raising temperatures.

"Internal" Arctic pollution is the source, Ancellet said, highlighting Russian and northern Scandinavian gas and oil exploitation.

Carbon dioxide emissions among the major north American, European and south-east Asian economies was not the only other factor, he added.

Shipping traffic with additional nitrogen oxide emissions is a growing complication, given he estimated that 25 percent of the increase in future maritime transport "will be confined to the Arctic zone".

In summer 2007, the Northwest Passage, historically an ice-jammed potential shortcut between Europe and Asia, was "fully navigable" for the first time since monitoring began in 1978, according to the European Space Agency.

It lasted five weeks, according to Canada's environment ministry, with 100 vessels getting through.