Book review: Indicators of Global Climate Change 2023

Book review by Simon Maxwell

Assessment Reports from the Intergovernmental Panel on Climate Change are authoritative but intermittent. The last one was in 2022-3, the next will not be till 2028-9. With the climate crisis accelerating, and 2023 having been especially warm, it is therefore exceptionally useful to have an annual update from a group of scientists who are all over the IPCC methodology. The second edition has just (5 June) been published, led once again by Professor Piers Forster from Leeds University, with a large cast of international co-authors (Figure 1). Piers Forster is the interim Chair of the UK’s independent Climate Change Committee, the body responsible for advising the Government on targets and for monitoring progress.

Figure 1

The update report is extremely dense, and there are quite a few methodological debates and choices; but the key findings are usefully summarized in a table and graphic (Table 1 and Figure 2). They cover: greenhouse gas emissions and concentrations, radioactive forcing, the earth’s energy imbalance, surface temperatures, estimates of human-induced global warming, and the remaining carbon budget.

Some key findings are:

• Greenhouse gas emissions look like they are stabilizing – at about 53 Gt per annum;

• Nevertheless, concentrations in the atmosphere are still rising, with Co2 now (in 2023) at 419 parts per million, methane at 1922 parts per billion, and N20 at 336 parts per billion;

• The decade average human-induced global mean temperature rise over pre-industrial levels is now 1.19 degrees (2014 - 2023);

• 2023 was ‘exceptionally warm’: the global surface temperature was 1.43 °C above the 1850–1900 average;

• The remaining carbon budget for 1.5 degrees has fallen from the 500 Gt estimated in AR6 to only 200 Gt, at 50% probability;

  Table 1

 Figure 2

The report repays careful study. One issue I have focused on is the remaining carbon budget for 1.5 or 2 degrees, which is falling rapidly. Table 2 has the data. At the time of AR6, based on data up to 2019, the budget for 1.5 degrees with 50% probability was 500 Gt of CO2, and for 2 degrees 1350 Gt. Those numbers are now, from 1 January 2024,  200 Gt and 1100 Gt. At 67% probability, which is arguably more appropriate than 50%, the numbers are 150 Gt and 900 Gt. CO2 emissions are currently running at about 40 Gt per annum: between the start of 2020 and the end of 2023, about 164 GtCO2 was emitted. ‘Overall’, the report concludes. ‘the 1.5 °C compatible budget is very small and shrinking fast due to continuing high global CO2 emissions.’

Table 2

There are some points to make about this.

First, the report notes that when the budget is smaller, ‘geophysical and other uncertainties . . . become larger in relative terms. This is a feature that will have to be kept in mind when communicating budgets. ‘Structural uncertainties give inherent limits to the precision with which remaining carbon budgets can be quantified.’

Second, the budget may be an overestimate. The report notes that the estimated RCB values can be higher or lower by around 200 GtCO2 depending on how deeply non-CO2 emissions are reduced. Specifically, ‘(the) estimates assume median reductions in non-CO2 emissions between 2020–2050 of CH4 (about 50 %), N2O (about 20 %) and SO2 (about 80 %) . . .  If these non-CO2 greenhouse gas emission reductions are not achieved, the RCB will be smaller.’  The report notes that methane emissions, in particular, are still rising. It adds the ominous point that

 ‘new literature not available at the time of the AR6 suggests that increases in atmospheric CH4 concentrations are also driven by methane emissions from wetland changes resulting from climate change. There is also a possible effect from CO2 fertilisation. Such carbon cycle feedbacks are not considered here as they are not a direct emission from human activity, yet they will contribute to greenhouse gas concentration rise, forcing and energy budget changes . . . They will become more important to properly account for in future years.’

Third, the conclusion to draw is not to give up because the 1.5 degree budget is close to exhausted, but rather to note that every tenth of a degree matters, and that action to limit warming needs to be accelerated. This may include carbon dioxide removal (on which see the UN Environment Emissions Gap Report for 2023), and also a discussion about whether limited overshoot during the course of the century can be tolerated. Note that new and more ambitious national plans (Nationally Determined Contributions - NDCs) are due to be submitted in 2025.

Fourth, though, it is important not to run away from reality. The RCB is kind of the canary in the coalmine.

Finally, both policy and politics need to be thought about. If 1.5 is slipping out of reach in the short term, should policy-makers be encouraged to modify their totemic attachment to the target? And, practically, should even more attention be paid to adaptation?

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Simon Maxwell is co-Chair of Climate:Change

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