Economist: 350 is a popular number among people concerned about climate change. That is because 350 parts per million (ppm) of carbon-dioxide in the atmosphere is the level that, in the words of James Hansen, a prominent climate scientist, is needed “to preserve a planet similar to the one on which civilization developed and to which life on Earth is adapted”. Today the ratio is nudging 400ppm, the highest since the Pliocene, 4m years ago, when Canada was a tropical jungle. This prompted some, including the…

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