Carbon budget
The cumulative amount of CO2 that can still be emitted while keeping global warming below a given limit with a stated probability.
Warming is approximately proportional to cumulative CO2 emissions, the transient climate response to cumulative emissions (TCRE), estimated by IPCC AR6 at about 0.45°C per 1,000 Gt CO2 (likely range 0.27–0.63°C). This near-linear relationship makes a finite carbon budget meaningful.
IPCC AR6 WGI gave remaining budgets from the start of 2020 of 500 Gt CO2 for a 50% chance of 1.5°C and 1,150 Gt CO2 for a 67% chance of 2°C. With global CO2 emissions of roughly 40–42 Gt per year (fossil plus land use), updated assessments (Forster et al., Indicators of Global Climate Change; Global Carbon Budget 2024–2025) estimated the 1.5°C budget had fallen to about 130–200 Gt CO2 by 2025, equivalent to only 3–5 years of current emissions.
Budgets are sensitive to assumptions on non-CO2 forcing, particularly methane and aerosols, and to permafrost feedbacks. National carbon budgets are also a legal tool: the UK Climate Change Act sets five-year budgets, with the sixth (2033–2037) capping emissions at 965 Mt CO2e, about 78% below 1990.
Related terms
All terms →- Net zero
A state in which anthropogenic greenhouse gas emissions are balanced by anthropogenic removals over a specified period, so net additions to the atmosphere are zero.
- NDC (Nationally Determined Contribution)
A country’s climate action plan under the Paris Agreement, setting emissions targets and measures, updated every five years with progressively higher ambition.
- CO2e (CO2 equivalent)
A common unit that expresses emissions of different greenhouse gases as the mass of CO2 that would cause the same warming over a chosen time horizon.
- Global warming potential (GWP)
The time-integrated radiative forcing of one kilogram of a gas relative to one kilogram of CO2, over a stated period such as 20 or 100 years.