What Is a Carbon Budget?

A carbon budget is the cumulative quantity of carbon dioxide that can be emitted globally while keeping warming below a specified temperature threshold with a stated probability. It rests on a physical result established in the climate science literature: global mean surface temperature rises roughly in proportion to total cumulative CO2 emissions, largely independent of the path by which those emissions occur. That near-linear relationship, expressed as the transient climate response to cumulative emissions, converts an abstract temperature goal into a finite quantity of carbon measured in gigatonnes.

The term also carries a second, related meaning in environmental economics and corporate practice, where a carbon budget is an allocated emissions allowance assigned to a country, sector, company, or facility for a compliance period. Both senses share the same logic: emissions are treated as a depletable stock rather than an annual flow, so a tonne emitted today permanently reduces what remains available later. This framing is what makes cumulative accounting central to climate policy design.

The Remaining Carbon Budget

The remaining carbon budget is the portion of the total budget still unspent from a given start date. Estimates are derived from five components: the observed warming to date, the transient climate response to cumulative emissions, the zero emissions commitment, the warming contribution of non-CO2 gases, and an adjustment for unrepresented Earth system feedbacks such as permafrost thaw. Each carries its own uncertainty, and a peer-reviewed assessment published in Nature Climate Change on the size and uncertainty of remaining carbon budgets found the budget for a 50 percent chance of limiting warming to 1.5 degrees Celsius had fallen to roughly 250 gigatonnes of CO2 as of early 2023, equivalent to about six years of emissions at then-current rates. Budgets shrink for two reasons at once: ongoing emissions draw them down, and revised estimates of aerosol cooling and non-CO2 forcing tighten the arithmetic.

Physical Basis and Uncertainty

Carbon budget estimates are probabilistic statements, not thresholds. A budget quoted for a 67 percent likelihood is substantially smaller than one quoted for a 50 percent likelihood of the same temperature limit, and quoting a single number without its probability obscures that difference. The Intergovernmental Panel on Climate Change Sixth Assessment Report sets out the underlying framework and reports budgets across a range of temperature limits and likelihood levels. Annual updates of emissions, land and ocean sinks, and atmospheric growth come from the Global Carbon Budget assessment produced by the Global Carbon Project, which reconciles fossil and land use emissions against measured atmospheric accumulation and closes the remaining discrepancy as a budget imbalance term.

Allocation and Policy Use

Translating a global budget into national or corporate targets requires an allocation rule, and no rule is scientifically determined. Equal per capita shares, grandfathering from historical emissions, ability to pay, and responsibility for cumulative historical emissions all produce sharply different allocations from the same global total. Sectoral pathways, science-based target frameworks, and net zero commitments are all downstream of some allocation choice. Budgets also interact with carbon dioxide removal: because the temperature relationship is driven by net cumulative emissions, verified removals extend a budget, though the permanence and measurement of those removals determine whether the extension is real.

Applications

Carbon budgets have applications in a range of fields, including:

• National climate policy and nationally determined contributions under the Paris Agreement
• Corporate science-based emissions targets and net zero transition plans
• Integrated assessment modeling of mitigation pathways and technology deployment
• Energy system planning, including estimates of stranded fossil fuel assets
• Climate risk analysis in financial regulation and portfolio alignment testing