Carbon budget

A carbon budget translates a temperature goal — such as the Paris Agreement's 1.5°C or "well below 2°C" targets — into a finite tonnage of carbon dioxide that humanity can release before that threshold is likely breached. The concept rests on the near-linear relationship between cumulative CO₂ emissions and peak global mean surface temperature, a finding formalised in the IPCC's Fifth Assessment Report (2013–14) and refined in subsequent cycles.

The Intergovernmental Panel on Climate Change (IPCC) publishes headline estimates. In its Sixth Assessment Report (AR6, Working Group I, 2021), the remaining budget from the start of 2020 for a 50% chance of limiting warming to 1.5°C was estimated at roughly 500 GtCO₂, and around 1,150 GtCO₂ for a 67% chance of staying below 2°C. The Global Carbon Project issues annual updates; by its 2023 assessment, the 1.5°C budget had shrunk to approximately 275 GtCO₂ — equivalent to under seven years of emissions at then-current rates of about 40 GtCO₂/year.

Budgets can be:

• Global or national — national shares are contested, with allocation methods including grandfathering, per-capita equity, and historical responsibility.
• Gross or net — net budgets assume future negative-emissions technologies (BECCS, direct air capture) to offset overshoot.
• CO₂-only or multi-gas — methane and N₂O are usually treated separately because of their different atmospheric lifetimes.

Carbon budgets underpin Nationally Determined Contributions (NDCs) under the Paris Agreement, the UK's statutory five-year budgets set under the Climate Change Act 2008, and the EU's "Fit for 55" trajectory. They also feature in litigation: in Neubauer v. Germany (2021), the German Federal Constitutional Court ruled parts of the Federal Climate Protection Act unconstitutional partly because they shifted budget burdens onto future generations. Critics note that budgets carry large uncertainty ranges and depend on assumptions about non-CO₂ forcers, climate sensitivity, and permafrost feedbacks.