Carbon budget

Emissions Gap Report 2019 published by the UN Environment Programme states in relation to carbon dioxide emission budgets (or carbon budgets): ‘For a given temperature rise limit, for example a 1.5°C or 2°C long-term limit, the corresponding carbon budget reflects the total amount of carbon emissions that can be emitted for temperatures to stay below that limit. Stated differently, a carbon budget is the area under a carbon dioxide (CO2) emission trajectory that satisfies assumptions about limits on cumulative emissions estimated to avoid a certain level of global mean surface temperature rise.’

Climate Change 2021 – The Physical Science Basis, Annex VII: Glossary, written by the Intergovernmental Panel on Climate Change (IPCC) and published by Cambridge University Press in 2023, suggests that carbon budget:

Refers to two concepts in the literature: (i) an assessment of carbon cycle sources and sinks on a global level, through the synthesis of evidence for fossil fuel and cement emissions, emissions and removals associated with land use and land-use change, ocean and natural land sources and sinks of carbon dioxide (CO2), and the resulting change in atmospheric CO2 concentration. This is referred to as the global carbon budget; (ii) the maximum amount of cumulative net global anthropogenic CO2 emissions that would result in limiting global warming to a given level with a given probability, taking into account the effect of other anthropogenic climate forcers. This is referred to as the total carbon budget when expressed starting from the pre-industrial period, and as the remaining carbon budget when expressed from a recent specified date.

Note 1: Net anthropogenic CO2 emissions are anthropogenic CO2 emissions minus anthropogenic CO2 removals. See also Carbon dioxide removal (CDR).

Note 2: The maximum amount of cumulative net global anthropogenic CO2 emissions is reached at the time that annual net anthropogenic CO2 emissions reach zero.

Note 3: The degree to which anthropogenic climate forcers other than CO2 affect the total carbon budget and remaining carbon budget depends on human choices about the extent to which these forcers are mitigated and their resulting climate effects.

Note 4: The notions of a total carbon budget and remaining carbon budget are also being applied in parts of the scientific literature and by some entities at regional, national, or sub-national levels. The distribution of global budgets across individual different entities and emitters depends strongly on considerations of equity and other value judgements.

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