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FOUR years ago, a vast part of the Larsen B ice shelf broke away from the coast of the Antarctic peninsula exposing sediments that had been covered in ice for 10,000 years. Elsewhere, most of the world’s glaciers are shrinking. The extent of sea ice in the Arctic has fallen by a fifth in less than 30 years. This week we have yet more examples of ecosystems shifting to keep up with changing temperatures: the tree line in Siberia is moving north, and marine mammals are being pushed northwards as warmth-loving fish invade their feeding grounds in the northern Bering…