The carbon clock suggests that the cumulative total of long-lived greenhouse gases in the atmosphere so far is 3.64tn metric tonnes, carbon dioxide equivalent. It is rising by 2bn tonnes a month. To have a good chance of stopping at the all-important temperature barrier, we need to produce, across the remainder of human history, not much more than a quarter of the total accumulation so far. In other words, no more than 500 months (42 years) of current production. The clock must stop at 4.6tn. There’s our challenge in stark numbers. Sobering to have it spelt out.