Carbon budget calculations for the UK


The carbon budget is how much more CO2 can be added to the atmosphere if global warming is to be kept below a particular level e.g. 1.5°C.

This web page shows a series of carbon budget calculations for the UK.

UK carbon budget chart net zero 2050The conclusions from the calculations are
The UK's current Net Zero 2050 timescale of emission cuts is grossly inadequate.
If the UK wants to keep its promises under the Paris Agreement, urgent radical reduction in energy use is necessary. The alternatives are
  • to ignore some of the CO2 emissions that that citizens are responsible for
  • to ignore the commitments in the Paris Agreement to equity between countries and take more than the UK's share of the residual global carbon budget
  • to accept a higher limit to global warming than 1.5°C.
None of these alternatives are in accord with basing policies on facts and fairness.

These calculations and conclusions are not particularly new. They are in line with the scientific consensus on carbon budgets that has been in the public domain for many years, but which is being ignored as part of society's widespread climate denial.


The carbon budget for limiting global warming to 1.5°C

Most of the CO2 released into the atmosphere from the burning of fossil fuels stays there, so the concentration of CO2 in the atmosphere has been steadily rising.

As the atmospheric CO2 concentration rises, the average global temperature rises.

From the amount of CO2 released into the atmosphere so far, and the effect that it has had, we can estimate the limit on how much more CO2 can be added to the atmosphere, if we want to keep global warming below a particular target such as 1.5°C. This is known as the CO2 budget or the carbon budget.

The scientific consensus is that global warming can be limited to 1.5°C if emissions since the start of 2020 are limited to 400 billion tonnes CO2 (this is with 67% confidence). This works out as 50 tonnes per person. This is a lifetime limit.

The UK's CO2 emissions are around 8 tonnes per person per year (depending on assumptions), so the UK has already used around 32 tonnes of this per person budget in the 4 years to the end of 2023, leaving a residual budget from the start of 2024 of 18 tonnes CO2 per person.

Some questions are:

The carbon budget charts shown below help to answer these questions. They have been generated by the calculator at carbonindependent.org/carbonbudgets.php, and they can be replicated there.


1: Constant emissions: not compliant with the carbon budget
UK carbon budget chartIf emissions continue unchanged, this residual per person carbon budget of about 18 tonnes CO2 will run out in 2 years from the start pf 2024, i.e. in 2026, as illustrated in the chart.

Clearly, CO2 emissions need to be reduced.


2: Net Zero 2050 (linear decline): not compliant with the carbon budget
UK carbon budget chart net zero 2050A pathway that is much talked about is "Net Zero 2050", where net CO2 emissions are reduced to zero in 2050. It is common for no details to be given of the pathway to net zero, and yet this is crucial since it is the cumulative emissions that matter, not the net zero date.

One version of net zero 2050 is to cut emissions steadily, by the same amount each year, which gives a sloping straight line on a chart of annual emissions. This can be termed "linear decline". This makes almost no difference to when the carbon budget runs out - it still runs out in 2026, and so the pathway would not be compliant with the residual carbon budget. It would in fact emit three times as much CO2 as the carbon budget of 50 tonnes per person.


3: Net Zero 2050 (linear decline): effect of ignoring imports
UK carbon budget chart net zero 2050The calculations shown so far have included the CO2 emissions generated in the production of all goods consumed in the UK, whether or not the goods were manufactured in the UK. These are known as "consumption emissions". An alternative is just to count emissions released in the UK whether or not the goods are consumed in the UK. These are known as "territorial emissions". This choice of accounting method is crucial. UK consumption emissions are about 50% higher than UK territorial emissions and have fallen much less over the last few decades. If imports are ignored and only territorial emissions are counted, the UK carbon budget will last until 2028. The policy does have some nonsensical consequences, e.g. firstly that an item manufactured in the UK will be treated as having a carbon footprint, whereas an identical item manufactured abroad will be treated as zero-carbon; and secondly that a country's carbon footprint can be "reduced" by the transfer of manufacturing abroad, with loss of jobs and damage to the economy.


4: Net Zero 2050 (linear decline): effect of also ignoring aviation
UK carbon budget chart net zero 2050If, in addition to ignoring emissions generated in the production of imports, emissions from aviation are ignored, then the UK's "total emissions" are reduced further, and the carbon budget will last longer, until 2030, as shown in the chart. The problem of course is that the emissions from flights by UK residents still build up in the atmosphere and increase global warming.


5: Net Zero 2050 (linear decline): effect of also ignoring equity between nations
UK carbon budget chart net zero 2050If, in addition to ignoring emissions generated in the production of imports and ignoring emissions from aviation, the commitment to equity between nations is also ignored, then the UK can take a larger share of the global carbon budget than its per-capita share in line with its current excess above the global average. The carbon budget will last longer, until 2035, as shown in the chart.


6: Net Zero 2050 (linear decline): effect of also accepting greater risk
UK carbon budget chart net zero 2050The analyses so far have required a good degree of confidence (67%) of staying within the 1.5°C limit. If, in addition to ignoring emissions generated in the production of imports, ignoring emissions from aviation, and ignoring the commitment to equity between nations, the degree of confidence of meeting the target is reduced from 67% to 50%, then the global carbon budget is higher, the UK's share is higher, and the UK carbon budget will last longer - until 2043, as shown in the chart.

These accounting choices are more or less what the UK Government and its Climate Change Committee are taking, and how they justify the net zero 2050 timescale. But not many people think that these choices accord with honesty and fairness.


7: Net Zero 2040 (linear decline): not compliant with the carbon budget
UK carbon budget chart net zero 2040A pathway that reduces emissions faster than net zero 2050 is one that cuts emissions to zero in 2040 as a linear decline. This is also inadequate - the budget still runs out in 2026, as shown in the chart. The pathway would in fact emit twice as much CO2 as the carbon budget of 50 tonnes per person.

8: Compliant Net Zero (linear decline): Net Zero 2029
UK Paris-compliant carbon budget chart linear declineAs neither a net zero 2050 pathway or a net zero 2040 pathway (with linear declines) are compatible with the carbon budget, the question arises as to how quickly cuts have to be made in order to comply with the budget.

Calculations show that for the UK, emissions need to be cut to zero in 2029 if a linear decline. This is very different to what most people believe, but it is in line with the scientific consensus - see document 33.

9: Compliant exponential decline pathway
UK Paris-compliant carbon budget chart exponential declineIt seems unrealistic to expect emissions to suddenly stop - there are many applications of fossil fuels where a sustainable substitution is difficult. So it is worth considering a pathway where emissions tail off towards zero rather than suddenly stopping. One option is the exponential decline shown ('exponential' being used in the scientific sense of constant halving time).

10: Compliant smoothed exponential decline pathway
UK Paris-compliant carbon budget chart smoothed exponential declineAlso, it seems unrealistic for emissions to suddenly start falling rapidly, and a more gradual start to cuts would be preferable. The chart shows a more 'smoothed' or 'rounded' option, that is still consistent with the carbon budget.

Options for limiting global warming to temperatures higher than 1.5°C

Option 11: Compliant smoothed exponential decline - 1.6°C
UK Paris-compliant carbon budget chart smoothed exponential decline 1.6degIf a higher limit is accepted (e.g. the 1.6°C shown in the chart), then the global carbon budget is greater, and so is the per-capita share for the UK. This means cuts can be slower. But the consequences in terms of climate related deaths, human displacement and biodiversity loss are greater. Many people consider these additional consequences to be unacceptable.

Option 12: Compliant smoothed exponential decline - 1.7°C
UK Paris-compliant carbon budget chart smoothed exponential decline 1.7degWith a still higher limit, e.g. the 1.7°C shown, the global carbon budget is still greater, and so is the per-capita share for the UK. This means cuts can be even slower. But the consequences again are even greater climate related deaths, human displacement and biodiversity loss.

Showing renewable energy

13: Showing renewable energy with linear extrapolation
UK Paris-compliant carbon budget chart smoothed exponential decline with renewable energyIn the UK, about 19% of primary energy consumption is from renewable sources.

This can be shown on a carbon budget chart via the amount of CO2 that would have been emitted if the energy had been produced from fossil fuels.

One possible scenario is that energy from renewable sources continues to increase at the same rate as it has done over recent years. This is termed a linear increase, and is shown on the chart.

14: Showing renewable energy with exponential extrapolation
UK Paris-compliant carbon budget chart smoothed exponential decline with renewable energyA second possible scenario is that energy from renewable sources continues to increase at the same annual percentage increase as it has done over recent years. This is termed an exponential increase, and is shown on the chart.

Comparison with other carbon budget calculations

The calculations and conclusions reported here are in line with the scientific consensus on the UK carbon budget e.g. as in publications from UK university departments:

These and other similar calculations have been in the public domain for years, but are being ignored as part of society's widespread climate denial.

Conclusion

It has to be concluded that in order to comply with the UK's commitments under the Paris Agreement to keep global warming below 1.5°C, urgent radical emission reductions are needed.

The alternatives are

None of these alternatives are in accord with basing policies on facts and fairness.



First published: 3 Mar 2024
Last updated: 7 Apr 2024