Indicators of Global Climate Change: 2025 Review
Simon Maxwell
The annual update of climate change indicators led by Piers Forster is now on its third edition, and has become an essential resource. It fills the gap between major IPCC reports, with the last synthesis report (AR6) published in 2023, and the next (AR 7) not due until 2029.
The latest edition came out on 19 June. The update is a dense scientific paper with more than 60 contributors. The abstract is reproduced in Box 1. Press coverage has focused mainly on the exhaustion of the carbon budget for 1.5 degrees, on which more below. However, there are many other points of interest. For example:
Source: https://essd.copernicus.org/articles/17/2641/2025/
Emissions are still rising (Table 1). Full data for 2024 are not yet available, and different estimates make different adjudications, for example on whether to include emissions from forest fires. Not included is the impact of warming on methane emissions from wetlands. The authors say these so-called ‘indirect’ emissions will need to be accounted for properly in future years.
Table 1
Global GHG Emissions by Source and Decade
Source: https://essd.copernicus.org/articles/17/2641/2025/
With the ‘tap’ still opening wider, concentrations of greenhouse gases in the atmosphere are unsurprisingly also rising. The concentration of CO2 reached 423 ppm in 2024, up from about 370 ppm in 2000. (Confidence limits omitted in this and subsequent paragraphs)
Global warming has a linear relationship with the total cumulative amount of CO2. Thus, temperatures are also up, by 1.24 degrees for the decade 2015-24 over 1850-1900. The authors add that ‘The decade 2015–2024 was 0.31 °C warmer than the previous decade (2005–2014) . . . amplified somewhat by the exceptionally warm years in 2023 and 2024 . . .’. The mean surface temperature in 2024 was 1.52 °C higher than the mean, last year thus becoming ‘the first calendar year since pre-industrial times more likely than not exceeding 1.5 °C’.
Warming is faster over the land than the sea. The report says that ‘land temperatures increased by 1.79 °C from 1850–1900 to 2015–2024 and ocean temperatures by 1.02 °C over the same period, implying that most land areas have already experienced more than 1.5 °C of warming from the 1850–1900 period’.
The indicators for drivers of extreme weather events are also moving in the wrong direction. They include maximum temperature and rainfall level and variability.
Sea level rose by 23 cm between 1901 and 1924, and the rate of increase is accelerating.
The remining carbon budget (RCB) is of particular interest: see my previous work on personal carbon budgets at different ages (here, here and here) and also a recent piece on strategic priorities for climate research and action in 2025. In the latter piece, I said that the budget for 1.5 would run out on 27 June 2027. I said that ‘there are complications to do with multi-decadal warming averages, the scope for limited overshoot, the pace of future carbon dioxide removal, and the time frame for calculating global warming potential (GWP), but the picture is grim.’
The three editions of the report provide evidence of a rapidly diminishing budget. For example, taking a 67% probability of holding warming to 2 degrees as an example, the budget from 1 January 2023 was 950 Gt; by 1 January 2024, it had fallen to 900 Gt; and by 1 January 2025 it had fallen to 870 Gt. The latest estimates are given in Table 2. The fall is not surprising as emissions have not stopped, but the small size of the budget now available to hold warming to 1.5 degrees is notable. The Financial Times reported that the Earth was set to exhaust the carbon budget for 1.5 degree warming in 3 years; the Guardian cut this to 2.
Table 2
Remaining carbon budget (RCB) from 1 January 2025
Source: https://essd.copernicus.org/articles/17/2641/2025/
Some elucidation is needed, however. There have been some significant adjustment of the numbers to do with aerosols and earth feed-back effects, but an important issue has to do with non-CO2 emissions, which are assumed to fall, but are not in fact doing so, as Table 1 showed. The report says that ‘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 GHG emission reductions are not achieved, the RCB for all temperature targets would be smaller than the values reported here’. It then concludes that
‘The values in (the table) are all greater than zero, implying that we have not yet emitted the amount of CO2 that would commit us to these levels of warming. However, including . . . uncertainty, non-CO2 emission and forcing uncertainty, and underrepresented Earth system feedbacks results in negative RCB estimates for limiting warming to low temperature limits with high likelihood. A negative RCB for a specific temperature limit would mean that the world is already committed to this amount of warming and that net negative emissions would therefore be required to return to the temperature limit after a period of overshoot.’
More information is provided in the paper by Rogelj and Lamboll which underpins the analysis. They say that
‘Assuming global CH4 emissions do not decline but instead are kept constant at 2020 levels would reduce the RCB by 431, 370, and 280 GtCO2 warming equivalent for RCBs compatible with 1.5 °C, 1.7 °C, and 2 °C, respectively. In other words, choosing not to reduce CH4 emissions and correctly adjusting for this decision in RCB estimates would cause 1.5 °C-compatible RCBs to be exhausted as of today, in effect putting the 1.5 °C ambition of the Paris Agreement out of reach.’
On this basis the press coverage is not so much alarmist as unduly cautious with regard to 1.5. On the other hand, there is still time to act to avoid 2 degree warming, and possibly lower thresholds also.
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Simon Maxwell is Co-Chair of Climate:Change
Perspective pieces are the responsibility of the authors, and do not commit Climate:Change in any way.