numbers, not adjectives — D. J. C. MacKay

Are Current UK Greenhouse Gas Emission Limits Fit For Purpose?

by Carl Edward Rasmussen, 2023-10-26

In the UK, the Climate Change Act 2008 requires legally binding limits to be set on greenhouse gas emissions. Carbon budgets are set for 5 year intervals at least 12 years in advance. Advice on the budgets is provided by the independent Climate Change Committee and adopted by government, see the recent budgets.

Such a longstanding and transparent system is rare in the international context and should be lauded. But are the limits themselves fit for purpose? To answer that, let's first clarify the purpose. There are two: firstly to insure that the UK lives up to the Paris Agreement and secondly to provide intermediate stepping stones to make sure the process in on track to accomplishing Net Zero by 2050. Below I assess these separately.

UK Emission Limits and the Paris Agreement

The Paris Agreement obliges signatories to "pursue efforts to limit global warming to 1.5°C" (above the preindustrial value). Let's start by figuring out what that means in terms of greenhouse gases. Carbon dioxide or CO₂ is the dominant greenhouse gas (appart from water vapour, which we can't directly control). The CO₂ concentration in the atmosphere is currently increasing by around 2.5 ppm per year, predominantly driven by fossil fuel use. Empirically, over recent history there has been very tight correlation between atmospheric CO₂ concentration and temperature; an increase of 10 ppm CO₂ is associated with 0.1°C temperature rise; combining these two facts shows that temperature is currently increasing by 0.1°C every 4 years. In the same plot we can also see that the current CO₂ concentration of 422 ppm corresponds to a 1.2°C temperature rise (compared with preindustrial times). Combining these facts, we conclude that we will reach 1.5°C (a further rise of 0.3°C) in 12 years, if current emissions are maintained. This estimate is almost certainly too optimistic, because of inertia in the climate system, we would expect the temperature to keep increasing for a while, even if the CO₂ level stabilised; but since it's difficult to know exactly how much inertia there is, let's ignore this for the sake of keeping this argument simple.

What are the consequences of this 12 years business as usual limit? Globally current fossil fuel combustion causes the release of 40 Giga tons of CO₂ per year. We are around 8 billion people, so that is 5 tons of CO₂ per person per year. Therefore the total budget left consistent with 1.5°C temperature rise is 60 tons per person. Of course it doesn't matter whether this budget is used over 12 years or a longer period, it's only the total amount that matters.

And what are the consequences for UK emissions? The legislated carbon budgets for the next 15 years are 1950 + 1725 + 965 = 4640 million tons of CO₂ (equivalents). Since we are about 67 million people in the UK, that is 69 tons per person. Note that this value exceeds the global budget of 60 tons per person.

The above argument shows that the limits set are not consistent with our obligations in the Paris Agreement. We are not "pursuing efforts to limit global warming to 1.5°C", on the contrary our legislation limits are inconsistent with this goal. So, I think the UK must do one of three things:

1. strengthen the limits to make them consistent with our Paris Agreement obligations, or
2. explain to the international community that while the UK still commits to the Paris Agreement it thinks that UK should contribute less than in proportion to its population and expects other countries to contribute more, or
3. withdraw from the Paris Agreement.

The inescapable conclusion is that the legislated carbon budgets are inadequate wrt UK's Paris Agreement obligations, and pretending otherwise is disingenuous and dangerous.

UK Emission Limits and Net Zero

Greenhouse gas emission targets for the far future, such as 2030 or 2050 cannot stand on their own. If they do, we risk arriving at those dates without having achieved the targets, and being out of time to correct our actions. Instead, future targets must be supplemented by frequent binding checkpoints along the way. The UK has been brilliant, and almost unique in the world, in legislating yearly emission limits starting in 2008.

The purpose of the yearly UK government targets for greenhouse gas emissions is to ensure that the UK is on track to reduce our impact on climate change. The 2050 goal is to reach Net Zero. One worry is that action now gets substituted with promises for the future. Such a substitution is undesirable because 1) it may turn out impossible to achieve at the time (but will then be too late to do anything about it) and 2) it commits people in the future to act, rather than taking action ourselves.

So, the emission limits are brilliant in principle, but are the actual values appropriate? Are the current limits set by the government fit for purpose? Do they make it plausible that we're on track to the 2050 Net Zero goal? Below I've plotted these limits together with the actual emissions for 2018-2022 for context, expressed in tons of CO2 equivalent per person per year.

In the plot above, I've taken the 5 year budgets from the Climate Change Act, normalised them by dividing by the UK population size, and plotted the individual yearly limit such that the year to year reduction is 6% (and that the 5 year total coincides with the legislated limit). For the third budget 2018-2022 I've plotted the actual emissions. For the 5th budget 2028-2032, I'm assuming that the NDC 2030 commitment isn't supposed to be a one off isolated limit so I've extended to 2031 and 2032 and split the remainder of the budget 2 ways for 2028 and 2029. I've superimposed a hypothetical insufficient curve starting from the actual average 2018-2022, with a constant yearly rate of reduction of 6%. If we followed that line, we would reach 1 ton per person per year in 2050, and not Net Zero.

Are the limits set on target? The answer is no. Every one of the 15 targets set are above a curve of constant relative reduction of 6% per year, a curve which is insufficient to get close to Net Zero by 2050. The 4th budget 2023-2027 exceeds the insufficient 6% curve by more than 20%. The limits for 2028-2029 are so large, that they're practically vacuous. The CCC claim in their 6th Carbon Budget Report (chapter 10, section 3) that "It is our assessment that the Fourth [2023-2027] Carbon Budget is on track...". I think those limits are not on track, they're inadequate. The first two years of the 5th budget are nearly 100% in excess of the insufficient 6% curve. The CCC's opinion of the 5th budget is that it is not adequate. However, they say "It is for the Government to decide whether the currently legislated budgets are amended to bring them in line with the revised 2050 target or the 2030 NDC, but the Committee does not consider it to be necessary". I think it is highly necessary to amend those limits. The whole point of having limits is to keep the process on track, so how can inadequate limits be deemed adequate? Finally the 6th budget limits are also inadequate in meeting the insufficient 6% curve (but I acknowledge the extra complication that this includes international aviation and shipping, which for the purposes of all the other limits have been excluded).

One may perhaps criticise the use of even relative reduction of 6% for comparison. But I think even relative reduction is in fact quite a conservative strategy (at least up until very close to reaching Net Zero), as some reductions are much easier to achieve that others. Therefore, the reductions will initially focus on the "low hanging fruit", and will become progressively harder over time.

Sadly, while the principle is brilliant, the limits set are mostly not fit for purpose. The current limits are not on target even for an insufficient 6% reduction per year. These targets must be updated and strengthened, to make sure we are on track to our goals, and not merely promising that someone in the future will sort it out.