The cumulative quantity of carbon dioxide still emittable to hold a warming limit, which turns a temperature target into a stock constraint.
Surface warming is, to a good approximation, proportional to cumulative carbon dioxide emissions since pre-industrial times rather than to their annual level. This near-linear relationship, a physical result and not a political convention, allows a temperature limit to be converted into a finite quantity still emittable, the remaining carbon budget. The conversion has two consequences public debate often conflates. It makes time fungible with volume, since deferring effort is not neutral but simply paid for with a steeper decline afterwards, and it attaches a probability to every budget, the same temperature threshold corresponding to very different budgets depending on whether a one-in-two or a two-in-three chance of holding it is accepted. For insurance, the quantity is the missing link between a political objective and a transition risk scenario, since it states how fast carbon-intensive assets must stop producing and therefore when their residual value collapses.
Working Group I of the IPCC Sixth Assessment Report, published in August 2021, put the remaining budget consistent with limiting warming to 1.5 degrees with a fifty per cent likelihood at roughly 500 gigatonnes of carbon dioxide from 1 January 2020.
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