There have been some extraordinary events in the Arctic recently. Last week two German merchant ships sailed through the North East Passage around the Arctic coast of Russia, normally a treacherous route full of ice floes. But this summer was so warm that huge areas of sea ice melted — the third-lowest area of Arctic sea ice on record.

Summer temperatures rose so high that Ellesmere Island in the far north of Canada reached a record high of 20.9C (69.6F) in July. For the past two years large numbers of Arctic sea geese have no longer migrated to Mexico over winter, preferring to stay in Alaska in the warmer seas. But polar bears are running out of ice to hunt for seals and as a result they are attacking human beings in increasing numbers.

Are these signs of global warming? The Arctic has had bouts of large ice melts long before global warming began. In 1815 whaling ships reported an unprecedented break-up of sea ice off the east coast of Greenland. The president of the Royal Society, London, wrote to the Admiralty: “New sources of warmth have been opened and give us leave to hope that the Arctic Seas may at this time be more accessible than they have been for centuries past.” But the ice returned after a few years. The difference in recent times is that the Arctic has been warming for about 30 years, and looks set to continue.