Remaining Carbon Budget

A remaining carbon budget is the maximum additional amount of net human-caused carbon dioxide that may be emitted from a specified date while limiting warming to a chosen level. It is stated for a defined probability of meeting that limit and accounts for the warming effects of non-CO2 climate forcers.

Scientific basis

The budget approach rests on the approximately linear relationship between cumulative anthropogenic CO2 emissions and the rise in global surface temperature. A total carbon budget covers emissions over the entire relevant period; the remaining budget is obtained after accounting for historical emissions up to the chosen starting date.

• The warming limit must be specified, such as 1.5°C or 2°C relative to a stated baseline.
• A higher probability of staying below the limit generally implies a smaller remaining budget.

What determines its size

A remaining budget is not a single immutable number. Its estimate depends on scientific assumptions and methodological choices.

• It depends on the warming already observed and the amount of past cumulative CO2 emissions.
• Assumptions about future methane, nitrous oxide, aerosols and other non-CO2 influences affect the available CO2 budget.
• Uncertainty in climate response, Earth-system feedbacks and temperature measurement changes the estimate.
• Delaying emission reductions consumes the budget and requires faster subsequent reductions to reach net-zero CO2 emissions.

Use and interpretation

The concept translates a temperature goal into a finite cumulative emissions constraint, helping assess whether emission pathways are consistent with that goal.

• It concerns global cumulative emissions, not merely emissions in a particular year.
• It is not automatically a national emissions entitlement; dividing it among countries raises questions of equity, historical responsibility and development needs.
• Temporary overshoot followed by carbon dioxide removal involves additional uncertainties and risks, so it is not equivalent to avoiding the limit.