Carbon budget
The cumulative amount of CO₂ compatible with a stated warming limit and probability under specified assumptions.
Example
A remaining budget estimates how much more may be emitted from a particular starting date.
The catch
Non-CO₂ warming, Earth-system responses and the chosen probability affect the result.
Related terms
Albedo, Atmospheric lifetime, Carbon dioxide, Climate change, Climate feedback, Climate model
In more depth
Carbon budget, explained.
A carbon budget exists because warming rises almost in step with the total CO₂ ever emitted, a near-linear relationship the IPCC assesses with high confidence. That makes the budget a stock to be spent. Each year of emissions draws it down, and spending past it commits the world to exceeding the temperature limit it was calculated for unless CO₂ is later removed at scale.
Cutting methane and other non-CO₂ gases enlarges the remaining CO₂ budget, which links the two strands of climate policy. Two points cause confusion. Splitting the global budget among countries or companies raises questions of climate justice that physics cannot settle. Some governments also use the phrase for legally set emission caps over fixed periods, a different instrument from the IPCC's global quantity.