A carbon budget is a concept used in climate policy to help set emissions reduction targets in a fair and effective way. It looks at "the maximum amount of cumulative net global anthropogenic carbon dioxide () emissions that would result in limiting global warming to a given level". When expressed relative to the pre-industrial period it is referred to as the total carbon budget, and when expressed from a recent specified date it is referred to as the remaining carbon budget.
A carbon budget consistent with keeping warming below a specified limit is also referred to as an emissions budget, an emissions quota, or allowable emissions. An emissions budget may also be associated with objectives for other related climate variables, such as radiative forcing or sea level rise.
Total or remaining carbon budgets are calculated by combining estimates of various contributing factors, including scientific evidence and value judgments or choices.
Global carbon budgets can be further divided into national emissions budgets, so that countries can set specific climate mitigation goals. Emissions budgets are relevant to climate change mitigation because they indicate a finite amount of carbon dioxide that can be emitted over time, before resulting in dangerous levels of global warming. Change in global temperature is independent from the geographic location of these emissions, and is largely independent of the timing of these emissions.
Carbon budgets are applicable to the global level. To translate these global carbon budgets to the country level, a set of value judgments have to be made on how to distribute the total and remaining carbon budget. This involves the consideration of aspects of equity and fairness between countries as well as other methodological choices. There are many differences between nations, including but not limited to population, level of industrialisation, national emissions histories, and mitigation capabilities. For this reason, scientists have made attempts to allocate global carbon budgets among countries using methods that follow various principles of equity.
Definition
The IPCC Sixth Assessment Reports defines carbon budget as the following two concepts:
"An assessment of carbon cycle sources and sinks on a global level, through the synthesis of evidence for fossil fuel and cement emissions, emissions and removals associated with land use and land-use change, ocean and natural land sources and sinks of carbon dioxide (CO2), and the resulting change in atmospheric CO2 concentration. This is referred to as the global carbon budget."; or "The maximum amount of cumulative net global anthropogenic CO2 emissions that would result in limiting global warming to a given level with a given probability, taking into account the effect of other anthropogenic climate forcers. This is referred to as the total carbon budget when expressed starting from the pre-industrial period, and as the remaining carbon budget when expressed from a recent specified date." Global carbon budgets can be further divided into national emissions budgets, so that countries can set specific climate mitigation goals.
An emissions budget may be distinguished from an emissions target, as an emissions target may be internationally or nationally set in accordance with objectives other than a specific global temperature and are commonly applied to the annual emissions in a single year as well.
Estimations
Recent and currently remaining carbon budget
Several organisations provide annual updates to the remaining carbon budget, including the Global Carbon Project, the Mercator Research Institute on Global Commons and Climate Change (MCC) and the CONSTRAIN project. In March 2022, before formal publication of the 'Global Carbon Budget 2021' preprint, scientists reported, based on Carbon Monitor (CM) data, that after COVID-19-pandemic-caused record-level declines in 2020, global emissions rebounded sharply by 4.8% in 2021, indicating that at the current trajectory, the carbon budget for a ⅔ likelihood for limiting warming to 1.5 °C would be used up within 9.5 years.
In April 2022, the now reviewed and officially published The Global Carbon Budget 2021 concluded that fossil emissions rebounded from pandemic levels by around +4.8% relative to 2020 emissions – returning to 2019 levels.