Carbon budget

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A carbon budget is the total amount of CO2 we can emit and still have a good chance of keeping global warming within a chosen limit. It can be measured from the start of the industrial era (about 1750) as the total budget, or from a later date as the remaining budget. An emissions budget is the portion allowed in a year or period to stay within that limit. The amount of warming depends on the total cumulative emissions, not on when they happen.

Global budgets can be divided into national budgets to guide each country’s actions. Because countries differ in population, historical emissions, development, and ability to reduce pollution, scientists use fairness principles to share the budget. Common approaches include:
- common but differentiated responsibilities and respective capabilities,
- per-capita (egalitarian) allocations, and
- grandfathering (allocations based on past emissions). Grandfathering is controversial.

Estimating budgets blends science with value judgments and depends on the chosen temperature target, the required probability of staying below it, and how non-CO2 greenhouse gases are reduced. Budgets are updated as new data become available.

Mitigation actions—such as shifting to clean energy, improving efficiency, and changing transport and land use—are needed to stay within budgets. Current policies are not enough to keep warming near 1.5°C or 2°C; rapid decarbonisation and, eventually, net-zero CO2 emissions are required. Changes in consumption and behavior (demand-side solutions) could also substantially cut emissions by 2050.