A personal lifetime carbon budget

(There has been an important update on the Remaining Carbon Budget in this paper by Piers Forster et al. It reduces the RCB significantly, and thus the numbers used in this piece. See the note at the end for more detail).

This is a companion piece to one on personal carbon footprinting.  It introduces the idea of a personal lifetime carbon budget. Taking the carbon budget as in 2023, it suggests a global average lifetime budget of 100 tons CO2e/capita for 1.5 degrees or 200 tons/capita for 2 degrees. That is a lot higher than implied by current footprints: on current emissions, the average global citizen could run out of budget at 16. If the global budget were reallocated to favour current low emitters, the higher emitters (in rich and poor countries) would run out even faster.

There is no great surprise in these numbers, but having a personal budget, combined with personal footprinting does focus the mind on the choices we make – and on the system changes needed to support them.

Making the global carbon budget concrete

You know about the global carbon budget for 1.5 or 2 degrees. How about your personal carbon budget? Let’s approach this in four steps, and try to make it concrete.

• First, the global budget and the timeline to net zero;
• Second, the number of people who have to share the budget;
• Third, translating this into the lifetime carbon budget for an average person in the world; and
• Fourth, the ethical dilemma of distribution.

The bottom line is that your personal budget is much smaller than you think – and for most of you reading this, much smaller even than that.

Step 1: The global budget and the timeline

We know from the IPCC that the relationship is more or less linear between warming and CO2 concentrations in the atmosphere. As summarised by the UN Environment Emissions Gap Report 2022

‘The carbon budget refers to the maximum amount of cumulative net global anthropogenic CO2 emissions that would result in limiting global warming to a given level with a given chance, taking into account the effect of other anthropogenic climate forcers. The IPCC estimates that the remaining carbon budget from the beginning of 2020 for limiting warming to a maximum of 1.5°C is approximately 400 GtCO2 and 1,150 GtCO2 for 2°C (both with a 67 per cent chance).’

CO2 emissions from 2020-2022 have been running at about 38 Gt a year (a bit less in the peak COVID years), so the CO2 budget from January 2023

• For 1.5 degrees is approximately 286 Gt of CO2; and
• For 2 degrees is approximately 1036 Gt of CO2.

Note that these numbers are for CO2, not all greenhouse gases (GHGs). The budget depends a lot on how fast the emissions fall of non-CO2 gases, especially methane.  The Emissions Gap Report had a useful box on this in 2022, focusing specifically on methane and other short-lived pollutants. Together these added about 40% to emissions (15 Gt on top of the 38 Gt). The EGR said

‘In conjunction with CO2 emission reductions, rapid reductions in emissions from methane and other short-lived climate pollutants are critical to lower peak warming . . .. Global average methane concentrations in the atmosphere have increased by 162 per cent compared with pre-industrial levels. This increase is largely driven by anthropogenic sources, mainly enteric fermentation of livestock and manure, rice cultivation, waste, and fossil fuel exploration. Methane has a significantly higher global warming potential than CO2 (80 and 83 times higher over 20 years for biogenic and fossil methane, respectively), but a much shorter atmospheric lifetime (about 12 years). . . .

Estimates of the remaining global carbon budget for 1.5°C assume that methane is strongly reduced by at least 30 per cent, 40 per cent and 50 per cent relative to 2020 levels in 2030, 2040 and 2050 respectively. Every ca 100 Mt shortfall in methane reductions compared with these benchmarks diminishes the already very small cumulative remaining carbon budget by around 450 GtCO2. Reducing emissions from methane is therefore an essential part of Paris-compatible mitigation strategies.’

As it happens, methane is increasing, not decreasing, from direct and indirect anthropogenic sources. The latter are particularly alarming as climate change itself may be impacting on the natural environment, for example on the emission of methane from expanding wetlands as rainfall rises in sensitive areas. That would reduce the CO2 budget even further.

How many years can the budget stretch? That depends on when we need to achieve net zero. The EGR, again, this time the 2021 Emissions Gap Report, is a good source on this. A ‘back-of-the-envelope’ estimate is mid-century for CO2 and about 20 years later for all GHGs. Specifically, 2050/2070 to limit warming to 1.5 degrees, and 2070/2090 for 2 degrees. These dates are based on the least cost pathways, the assumptions and limitations of which need to be understood (a box on this from EGR 2022 is appended). The costs of extreme weather impacts are largely excluded, which would reduce the timeframe; but on the other hand, co-benefits of things like cleaner air are also excluded, which would stretch the timeframe.

Anyway, let’s make this simple:

• For 1.5 degrees, a budget of 286 Gt of CO2 between now and 2050.
• For 2 degrees, a budget of 1036 Gt of CO2 between now and 2070.

Step 2: Sharing out the budget

Up to a thousand billion tons of CO2 sounds like a lot – but there are a lot of people wanting a share. Remember, the population of the world reached 8 bn in November 2022 , and is increasing by about 70 million people per year.

How many ‘life years’ will there be between now and 2050 or 2070? This is basically the world population from now until then, so

• From 2023-2050: 251 billion
• From 2023-2070: 454 billion

From here, the arithmetic is easy. Each person in the world, on average, can emit: for 1.5 degrees: 286/251 = 1.1 tons of CO2 per annum and for 2 degrees:  1036/454 = 2.3 tons of CO2 per annum.

I don’t know how easy it is to work out carbon footprints for all GHGs, rather than just CO2, but in the UK at least, CO2 accounts for about 80% of household emissions, so we could add 25% to the numbers above, giving

• 1.4 GHG tons per person per annum for 1.5 degrees; and
• and 2.9 tons for 2 degrees.

Just to be clear, though, that these are approximations. 

Step 3: a lifetime carbon budget

It is helpful to have an annual target, but what happens if you overshoot one year? Then, of course, you are borrowing from future years. And vice-versa: undershooting in one year, gives room for manoeuvre in subsequent years. In this model, once you have a budget, you can spend it whenever you like.

At present, global life expectancy is about 73, and expected to rise to 77 by 2050. A child born in 2023 on average therefore has a lifetime carbon budget of (73x1.4=) 102 GHG tons for 1.5 degrees, and (73x2.9=) 211 GHG tons for 2 degrees.

Let’s simplify:

• The average lifetime budget for 1.5 degrees is 100 tons of GHGs;
• The average lifetime budget for 2 degrees is 200 tons of GHGs.

Does that sound generous? Well, the average global per capita emissions are currently about 6.3 tons, of course more in rich countries and a lot more for rich people, and less for poor countries, much less for poor people.

Let’s stick to averages. On current numbers, the average global citizen will ‘run out’ of CO2e by the age of 16 for 1.5 degrees, and by the age of 32 for 2 degrees. Yes, you can spend your budget whenever you like, but if you are profligate now, the pain will come later.

All this puts me in mind of the story of the Sleeping Beauty. The new born baby is given a precious gift by each fairy, but one decrees a curse, that on her 16^th birthday, she will prick her finger on the spindle of a spinning wheel and fall asleep. Our new born baby receives a certificate entitling her or him to 100 (maybe 200) tons of greenhouse gas emissions. Is that the precious gift, or the spindle of the spinning wheel? How, one might ask, to avoid the curse?

Step 4: the ethical dilemma of distribution

We know that emissions are highly unequal, between countries and between individuals. Lucas Chancel et al sum it up in the Climate Inequality Report 2023: country-level emissions range from 1.6 tons CO2e/capita in sub-Saharan Africa to 20.8 tons/capita in North America (Figure 1); and from 1.4 tons/capita for the poorest 50% to 28.7 for the richest 10% and 101 tons/capita for the richest 1%. Importantly, they conclude that ‘carbon inequalities within countries now appear to be greater than carbon inequalities between countries.’

Figure 1

Source: https://wid.world/document/climate-inequality-report-2023/

Figure 2

Source; https://wid.world/document/climate-inequality-report-2023/

The Doughnut Economic Action Lab at Leeds has more data on whether countries live within planetary boundaries, and whether they meet the needs of the people. Some countries need to reduce their environmental footprint, others may well need to increase it. Kate Raworth has described these trajectories as ‘Reduce’ and ‘Rise’.

We could try to keep this simple. The average global lifetime budget is 100 tons/capita for 1.5 degrees and 200 tons/capita for 2 degrees. 85% of the world’s population lives in low and middle income countries, and the share is growing. That means reallocation from rich to poor has to be on a very large scale to make a difference. If the rich country budget per person was reduced from 100 tons to 50 tons, that would only add 9 tons or so to the budget of those living in low and middle income countries – before taking account of population change.

The ethics of this are also complicated. There is a large literature. Carbon Independent have an entry-level brief. Williges et al provide a more detailed and scientific exploration of different allocation rules. A summary is in Table 1, listing different criteria.

Table 1

Source: https://www.sciencedirect.com/science/article/pii/S095937802200019X

So what’s the answer? I don’t think I have one. I guess less than 100/200 tons in rich countries and more than that in poor ones. But much less in rich countries will be hard, and a bit more in poor countries will not make much difference. Anyway, a lifetime budget of 100 tons is already a big ask for those of us in rich countries. The average person in the UK has a carbon footprint of 10 tons p.a. (some say as much as 13 tons if international flights and shipping are included), so on current consumption, the lifetime budget would run out before the age of ten.

Conclusion

There is no great surprise in these numbers. We know the global carbon budget is shrinking fast, and that we are running out of time. It is a shock, though, to translate the global numbers into personal budgets and to be forced to examine the choices and trade-offs. Taken together with personal carbon footprinting, the use of a personal budget might help make decision-making more transparent and more strategic. It would also focus even greater attention on the system changes that are needed to enable and support personal action.

Image: © <a href='https://www.123rf.com/profile_copterphotographer'>copterphotographer</a>, <a href='https://www.123rf.com/free-images/'>123RF Free Images</a>

__________________

Appendix

Putting cost estimates from least-cost emissions scenarios in context

Least-cost scenarios are constructed to achieve global emission reductions at the lowest cost possible. However, estimates of mitigation costs vary extensively and depend critically on the reference and mitigation scenario assumptions and data parameterization chosen (Köberle et al. 2021; Riahi et al. 2022). If a reference scenario in which global and local economies are at their efficiency frontier is assumed, climate policies will inevitably entail macroeconomic costs. However, the literature, including the latest IPCC assessment, illustrates that this is a stylized and unrealistic assumption (Riahi et al. 2022). An economy at its efficiency frontier implies no fossil fuel subsidies, no taxation that distorts the allocation of labour, no “misallocation or under-utilization of production factors such as involuntary unemployment”, and no “imperfect information or non-rational behaviours” (Riahi et al. 2022). Each of these economic imperfections are common across real-world economies at all levels of development. However, the models that produce least-cost pathways rarely represent all these aspects and hence disregard them in their model estimates of mitigation costs. This results in mitigation cost estimates that are biased high (Köberle et al. 2021; Riahi et al. 2022).

Studies that model a reference economy below the efficiency frontier find that a low-carbon transformation can result in economic stimulus and increase economic growth, conditional on green investments not replacing investment in other parts of the economy (Pollitt and Mercure 2018; Mercure et al. 2019; Riahi et al. 2022). Because of regional differences in governance, development and societal and technological context, mitigation cost estimates differ between countries. For example, under the idealized assumption that emission reductions are achieved through a globally uniform carbon price, countries with carbon-intensive economies or fossil fuel exporting countries would have relatively higher macroeconomic costs as their economies require a deeper transformation (Stern, Pezzey and Lambie 2012; Tavoni et al. 2015; Böhringer et al. 2021). For a detailed discussion see Riahi et al. (2022).

In addition, mitigation cost estimates of least-cost pathways disregard the economic benefits that accrue through avoided damages and societal co-benefits of a low-carbon transition, such as improved public health because of improved air quality (Köberle et al. 2021; Riahi et al. 2022). Even when considering least-cost mitigation scenarios where costs are biased high, these benefits likely outstrip the modelled costs (see figure 4.1) (Riahi et al. 2022). For example, one study found that health co-benefits outweigh the policy cost of achieving fair national contributions to limit warming to 1.5°C in China and India, with the modelled costs compensated by health co-benefits (Markandya et al. 2018).

In conclusion, typically an ideal, perfectly working economy is assumed when mitigation costs are quantified, while the economic co-benefits and avoided damages are unaccounted for. As a result, modelled mitigation cost estimates typically only provide limited real-world insights about the net burden to economies or society. In all cases, these modelled costs occur in a world of continued economic development and growth.

Source: Emissions Gap Report 2022

Update (June 2023)

As noted at the top of this piece, there has been an important update on the Remaining Carbon Budget (RCB) in this paper by Piers Forster et al. It reduces the RCB significantly, and thus the numbers used here.

The new data are in this table:

At 67% probability, the RCB for 1.5 degrees has fallen from 400Gt to 150Gt, and for 2 degrees from 1150Gt to 950 Gt.

Using the same assumptions as in the main text, the lifetime budget has fallen

•  100 tons of GHGs (for 1.5 degrees); and
•  200 tons of GHGs for 2 degrees.

to a lifetime budget of only

55 Gt for 1.5 degrees; and 

190 Gt for 2 degrees

This means the average child born today runs out before the age of 9 for 1.5 degrees, rather than 16; and  at 30 rather than 32 for 2 degrees.

i guess there will be further updates in years to come. if emissions do not fall quickly, the time scales will shrink further.