Robin McKie, science editor, The Observer, writes:

The Arctic’s sea covering has shrunk so much that the Northwest Passage, the fabled sea route that connects Europe and Asia, has opened up for the first time since records began.

The discovery, revealed through satellite images provided by the European Space Agency (Esa), shows how bad the consequences of global warming are becoming in northerly latitudes. This summer there was a reduction of a million square kilometres in the Arctic’s ice covering compared with 2006, scientists have found.

As a result, the Northwest Passage that runs between Canada and Greenland has been freed of the ice that has previously blocked it and that, over the centuries, has frustrated dozens of expeditions that attempted to sail northwest and open up a commercial sea route between the Atlantic and the Pacific.

In addition, scientists have found that the Northeast Passage, a corresponding route that runs parallel to the north coast of Russia, may also soon become navigable - though the clearing of both passages is likely to fuel animosity between countries trying to exploit the region’s oil, fish and mineral resources, experts have warned.

According to scientists led by Leif Toudal Pedersen of the Danish National Space Centre, Arctic ice this summer dropped to around 3 million square kilometres, a decrease of 1 million square kilometres on last year’s coverage. Given that for the past 10 years Arctic ice has been disappearing at an average annual rate of only 100,000 square kilometres, this year’s reduction is ‘extreme’, said Pedersen.

‘The strong reduction in just one year raises flags that the ice [in summer] may disappear much sooner than expected and that we urgently need to understand better the processes involved,’ he added.

Pedersen and his team used 200 images - acquired earlier this month by the Advanced Synthetic Aperture Radar instrument aboard Esa’s Envisat satellite - to create a mosaic that shows the Northwest route across northern Canada is currently navigable, while the Northeast Passage along the Siberian coast remains blocked, but only partially.

Finding a Northwest Passage from the Atlantic to the Pacific was a dream of European merchants since the 16th century. Explorers including Martin Frobisher and Henry Hudson tried to find a route but failed.

Finally, in 1845, a well-equipped two-ship expedition led by Sir John Franklin attempted to find the passage. It disappeared, with all its 129 crewmen. Subsequent investigations found the bodies of Franklin’s sailors and uncovered evidence that contaminated food may have helped doom the expedition. It is also thought some crewmen may have resorted to cannibalism to try to save themselves.

The passage was finally conquered in 1906 by the Norwegian explorer Roald Amundsen, who completed the journey in a converted 47-ton herring boat called Gjoa. However, some of the waterways used by Amundsen were extremely shallow, making his route commercially impractical.

Although the discovery that the passage is now opening up dramatically, indicating it may soon be possible for shipping to take highly profitable northerly short cuts between the Europe and Asia, scientists are also very worried about the rate at which the region’s ice is melting. They fear the polar regions may have passed a crucial tipping point.

Ice reflects solar energy. But if it starts to disappear, heat is absorbed by the dark seas and rock below ice floes and glaciers. The Arctic then gets warmer and even more protective ice covering is lost - so melting accelerates dangerously.

The opening up of the sea routes is also like to increase the intensity of international disputes in the Arctic. Canada claims full rights over those parts of the Northwest Passage that pass through its territory and has announced that it will bar transit there if it wants. However, this claim is disputed by the US and the European Union.

In addition, Russia has laid claims to large tracts of the Arctic seabed, claiming these are extension of its maritime zones. Again this claim is disputed by other countries, including Canada, which recently announced it was building a fleet of naval vessels to protect its northern waters.