Almost all Arctic glaciers have receded. The area of Arctic land covered by snow in early summer has shrunk by almost a fifth since 1966. But it is the Arctic Ocean that is most changed. In the 1970s, 80s and 90s the minimum extent of polar pack ice fell by around 8% per decade. Then, in 2007, the sea ice crashed, melting to a summer minimum of 4.3m sq km (1.7m square miles), close to half the average for the 1960s and 24% below the previous minimum, set in 2005. This left the north-west passage, a sea lane through Canada’s 36,000-island Arctic Archipelago, ice-free for the first time in memory.