Secondly because, as two recent papers in Nature show, cumulative emissions are the most important measure of whether or not we’re winning. One of them suggests that only 1500-1800bn tonnes of carbon dioxide (or 400-500bn tonnes of carbon) stand between current temperatures and two degrees of global warming. The other gives us a 25% chance of exceeding two degrees if we produce 1000bn tonnes of CO2.
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.
The New York carbon counter will be updated online at know-the-number.com.