On Campbell Island in the Southern Ocean, some 400 miles south of New Zealand, is a single Sitka spruce. More than 170 miles from any other tree, it is often credited as the “world’s loneliest tree”. Planted in the early 20th century by Lord Ranfurly, governor of New Zealand, the tree’s wood has recorded the radiocarbon produced by above ground atomic bomb tests – and its annual layers show a peak in 1965, just after the tests were banned. The tree therefore gives us a potential marker for the start of the Anthropocene.