A slightly more complicated graph following up on the previous one showing how much carbon is left to burn, showing that even that one is unreasonably optimistic.
The curve plots a reasonable estimate of the (Bayesian) probability, given available knowledge, of staying within 2 degrees C above the preindustrial global mean surface temperature. Normally, we base our estimates on the 50% line; to have a 50/50 shot of staying under 2 C, we have used up a bit over half of our available emissions.
As David Spratt explains, we don’t fly in an airplane with a 1 in 100,000 chance of falling out of the sky. (We have government regulations for that!) But the usual carbon budget is based on a 50% chance of staying within 2 C of warming. If we take a more, ahem, conservative approach, and stick to only a 10% chance of failure, there is no carbon budget left.
I think there are things that mitigate Spratt’s position. But we shouldn’t forget that in the limit of having perfect information about the system, there’s a something on the order of a 10% chance that we may have already passed the 2 C mark by any reasonable definition.