Carbon budget
The cumulative amount of carbon dioxide that can still be emitted without breaching a warming limit: the temperature target becomes a stock constraint.
IPCC Sixth Assessment Report, Working Group I, August 2021: to hold warming to 1.5 degrees with a fifty per cent likelihood, roughly 500 gigatonnes of carbon dioxide remained from January 1, 2020. The number rests on physics. Surface warming tracks, to a good approximation, cumulative CO2 emissions since pre-industrial times rather than the annual rate, which turns a temperature cap into a finite stock. Public debate tends to blur the two consequences. Time becomes fungible with volume, so any deferred effort is paid for with a steeper decline later. And every budget carries a probability: the same threshold yields very different budgets depending on whether you accept even odds or two-in-three. For insurance this is the link between a policy target and a transition risk scenario, because it says how fast carbon-intensive assets have to stop producing, and therefore when their residual value falls off a cliff.
In August 2021, Working Group I of the IPCC Sixth Assessment Report put the remaining budget at roughly 500 gigatonnes of carbon dioxide from January 1, 2020. That is what was left to limit warming to 1.5 degrees with a fifty percent likelihood.
carbon budget, remaining carbon budget
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