Carbon budgets

What is a carbon budget?

We know that greenhouse gases warm the climate. The wealth of climate science research available also give us a good understanding of how much the climate will warm for a given level of greenhouse gas emissions. Therefore, we can work out what level of greenhouse gas emissions would cause us to reach a specific level of global warming, such as 1.5°C. This is a carbon budget.

Once we have this maximum level of emissions before a given limit is reached, we can use data on historic and current emissions to calculate how much we have already emitted. This gives us a figure for the level of greenhouse gases we can still afford to emit if we are to remain below a particular warming level. We can determine how much of the carbon budget we have already used.

Carbon budgets are not definitive; there will always be some amount of uncertainty in what level of emissions will produce a given warming level. This is because of the complex feedbacks associated with the carbon cycle, including how it changes under different levels of global warming. Therefore, you will usually see carbon budgets given with some level of probability, such as what emissions we can afford to emit and still have a 50% likelihood of remaining below 1.5°C of global warming since pre-industrial times.

Global carbon budget

The global carbon budget is assessed every year by the Global Carbon Project. They take the latest data on past carbon emissions and combine it with a detailed understanding of how the Earth will respond to rising levels of greenhouse gases to tell us how much more we can afford to emit.

Their latest analysis shows that the remaining carbon budget if we are to have a 50% likelihood of limiting warming to 1.5°C is equivalent to 380 Gigatonnes of CO₂. This is equivalent to 9 more years of emissions at 2022 levels before we will have a less than 50% likelihood of achieving the 1.5°C target set under the Paris Agreement. Alternatively, for a warming level of 2°C we could still emit 1,230 GtCO₂, equivalent to 30 more years of 2022-level emissions.

Our remaining carbon budget for various future warming levels, from 2023. Credit: Global Carbon Project

UK carbon budgets

Carbon budgets for the UK are among the reports produced by the Climate Change Committee (CCC), the UK’s independent adviser on tackling climate change. These set the pathway to a Net Zero future in which any carbon emissions are compensated for by removals. The UK has pledged to achieve this target by 2050.

The CCC’s Sixth Carbon Budget, published in December 2020, sets out the amount of greenhouse gases the UK can emit during 2033-37 and still achieve the net zero target. The recommended pathway requires a 78% reduction in UK emissions between by 2035 compared to 1990 levels. The UK government uses these reports to set legally binding carbon budgets which you can find out more about here.