What is a carbon budget?
A carbon budget is the total amount of greenhouse gases that can be emitted over a specific time period to limit global temperature rise to a target level, such as 1.5°C above pre-industrial levels.
In 2018, the Intergovernmental Panel on Climate Change (IPCC) released its Special Report on Global Warming of 1.5°C (SR15). This report provided updated emission reduction pathways required to limit warming to 1.5°C and outlined several key findings for policy makers:
Cumulative emissions matter - Limiting global warming requires limiting total cumulative global human-induced emissions since the pre-industrial period, that is, staying within a total carbon budget. As of 2018, the global carbon budget was being depleted at a rate of approximately 42 Gt CO2 per year.
Speed of action increases success - Faster carbon reductions result in a higher probability of limiting global heating to 1.5°C. To have a 66% chance of staying below 1.5°C, the report estimated the remaining global carbon budget of 420 Gt CO2 (or approximately 10 years of 2017 emissions). To have a 50% chance of staying below 1.5°C, the report estimates approximately 14 years of current emissions.
Net-zero timing is critical - Emissions globally will need to be reduced to net-zero by 2050 but if the date of reaching net-zero emissions is brought forward to 2040 the chance of limiting warming to 1.5 ̊C is considerably higher.
How is your carbon budget calculated?
If the findings of the Intergovernmental Panel on Climate Change (IPCC) SR15 Special Report are applied to cities or council areas, this effectively means establishing a carbon budget of approximately 10 years of 2017 baseline emissions and working towards reducing these emissions as quickly as possible to stay within this carbon budget.
We have determined each council's estimated community emissions at 2017 and multiplied this value by 10 (i.e. 10 years) to calculate each Council's community carbon budget. Because 2017 emissions are not available for each Council using the same methodology as the current inventory, 2017 emissions for each Council are determined by taking the 2023/24 inventory (total emissions) and back-projecting this to 2017 using changes to the national inventory as a proxy:
Council 2017 Emissions =Council 2023/24 emissions x (1 + % reduction in national inventory between 2017 and 2023/24)
Based on the above, the carbon budget for each Council is calculated as follows:
Council Carbon Budget = Council 2017 Emissions x 10
What does the remaining carbon budget mean in Community Decarbonisation?
The remaining carbon budget reported in Community Decarbonisation equates to how much of your council’s carbon budget remains at 2050 based on subtracting annual emissions from your overall carbon budget (including emissions already emitted between 2017 and 2023/24).
Why carbon budgets matter
A carbon budget allows councils to understand not only a pathway to net-zero emissions but also the speed at which emission reductions are required to contribute to meeting our global warming threshold targets. Understanding your remaining carbon budget provides a clear framework for planning and testing different intervention scenarios across sectors and timeframes. This enables councils to compare their current pathway against intermediate or aggressive policy options and see how the timing and intensity of action impacts their ability to stay within their carbon budget.