Global warming is fundamentally linked to the absolute concentration of greenhouse gases in the atmosphere.  To stabilise global temperature at any level vs pre-industrial, then there is a finite amount of emissions that can be released before net emissions need to reach zero. For CO₂ emissions this can be referred to as a carbon budget.

Carbon budgets continue to be a popular approach to frame the challenge of keeping global warming to ‘acceptable’ levels. The IPCC Special Report on Global Warming of 1.5ºC (2018) is the most recent authority on ‘total’ carbon budgets – meaning the total amount of emissions that can be released and hence contribute to warming across all sectors of the economy (which can be categorised as energy sector emissions plus land use, land use change and forestry plus industrial sector emissions). Other sector-specific CO₂ estimates for a given warming outcome may also be published by various organisations, for example, the energy sector CO₂ emissions published by the IEA.

At the 2015 UNFCCC Paris COP, world governments confirmed their intention to limit global warming “well below 2°C” and pursue efforts to “limit the temperature increase to 1.5°C”.  We calculate that the remaining 1.5°C carbon budget was c.495GtCO₂ as at the beginning of 2020 (based on the carbon budgets updated by the IPCC in 2018 and emissions data from the Global Carbon Project). Based on 2019 emissions of 43.1GtCO₂ this budget can be expressed in terms of years remaining at current emissions levels – as of 2020 this equates to 11.5 years for a 50% probability of a 1.5°C warming outcome.

Carbon Tracker’s 2019 report Balancing the Budget discusses global carbon budgets further and translates these to company carbon budgets for upstream oil and gas companies based on IEA fossil fuel demand scenarios using a least-cost approach.

The nature of climate science means that the probabilistic scenarios used produce a range of budgets depending on the parameters applied to the models. In selecting / using a budget, people should consider:

• What assumptions are made for certain underlying variables? Climate sensitivity is one variable that can significantly impact the size of the carbon budget. As is the degree to which net-negative emissions are deployed.
• Does the budget cover just energy-related sources of emissions, or are other industrial, agricultural, land use change, etc. emissions covered as well?
• Is the budget for CO₂ alone, or all greenhouse gases? Some budgets are just for carbon or carbon dioxide (check the units); others cover other non-CO₂ greenhouse gases (GHGs) and are expressed in CO₂e (equivalent). An assumed high amount of non-CO₂ GHGs means less remaining budget available for CO₂ – therefore, does a CO₂ only budget assume a high or low effort to reduce other greenhouse gases?
• What is the probability of delivering the desired global warming outcome is, for example, 50%, 66%, 80%? The more likely the scenario is to achieve the specified temperature outcome, the smaller the carbon budget will be.

Our latest thinking on this concept can be found in Carbon Budgets:  Where are we now?