Remaining global CO2 budgets

DOI 10.5281/zenodo.17767402 published on Zenodo Status: 30/11/2025 Remaining Global CO2 Budgets Andreas Wolfsteiner www.save-the-climate.info [email protected] Abstract This document summarises the remaining global CO₂ budgets assessed in IPCC AR6 and presents updated estimates from the IGCC initiative. It explains how to interpret probabilistic budgets, clarifies the role of uncertainties and provides guidance on how policymakers can adopt precautionary interpretations of the AR6 budgets. Remaining global CO2 budgets Page 2 / 6 Contents Abstract ............................................................................................................................................. 1 List of tables ...................................................................................................................................... 2 What does the IPCC say about the remaining CO2 budgets? ............................................................ 3 Updated budgets ................................................................................................................................ 4 Interpretation of the IPCC budgets ................................................................................................... 5 References ......................................................................................................................................... 6 List of tables Table 1: Remaining budgets from 2020 onwards according to the IPCC (AR6) ............................. 3 Table 2: Updated budgets from 2025 onwards (IGCC) .................................................................... 4 Table 3: Recalculated updated budgets from 2020 onwards ............................................................ 4 Table 4: Actual emissions from 2020 onwards ................................................................................. 4 Remaining global CO2 budgets Page 3 / 6 What does the IPCC say about the remaining CO2 budgets? The goal of the Paris Agreement is to hold “the increase in the global average temperature to well below 2°C above pre-industrial levels” and pursue efforts “to limit the temperature increase to 1.5°C above pre-industrial levels”. The UN Intergovernmental Panel on Climate Change (IPCC) published the following figures in its Sixth Assessment Report (AR6) Working Group I 2021 (IPCC, 2021, cf. Tables SPM.2 and 5.8): Table 1: Remaining budgets from 2020 onwards according to the IPCC (AR6) Warming Estimated remaining carbon budgets Scenario variation Geophysical uncertainties Non-CO2 scenario variation Non-CO2 forcing and response uncertainty Historical temperature uncertainty ZEC uncertainty Recent emissions uncertainty Probabilities: 50% 67% 83% [°C] [GtCO2 from 2020 on] [GtCO2] 1.5 500 400 300 ±220 ±220 ±550 ±420 ±20 1.6 650 550 400 1.7 850 700 550 1.8 1,000 850 650 1.9 1,200 1,000 800 2.0 1,350 1,150 900 At present, the remaining CO₂ budget is being depleted at an annual rate of approximately 42 Gt. 1 A web app for calculating linear emission pathways consistent with a given CO₂ budget is available at (temporary overshooting can be taken into account): https://global-paths.climate-calculator.info. The IPCC writes about the CO2 budget (IPCC, 2021, p. 28 f., emphasis and [...] not in the original): «D.1.1 […] there is a near-linear relationship between cumulative anthropogenic CO2 emissions and the global warming they cause. Each 1000 GtCO2 of cumulative CO2 emissions is assessed to likely cause a 0.27°C to 0.63°C increase in global surface temperature with a best estimate of 0.45°C. […] This quantity is referred to as the transient climate response to cumulative CO2 emissions (TCRE). This relationship implies that reaching net zero anthropogenic CO2 emissions is a requirement to stabilize human-induced global temperature increase at any level, but that limiting global temperature increase to a specific level would imply limiting cumulative CO2 emissions to within a carbon budget.» «D.1.2 […] Remaining carbon budgets have been estimated for several global temperature limits and various levels of probability, based on the estimated value of TCRE and its uncertainty, estimates of historical warming [±550 GtCO2], variations in projected warming from non-CO2 emissions [±220 GtCO2], climate system feedbacks such as emissions from thawing permafrost [±220 GtCO2], and the global surface temperature change after global anthropogenic CO2 emissions reach net zero [ZEC].» Regarding probabilities, the IPCC notes (IPCC, 2021, p. 29, emphasis and [...] not in the original): 2 «This likelihood is based on the uncertainty in transient climate response to cumulative CO2 emissions (TCRE) and additional Earth system feedbacks and provides the probability that global warming will not exceed the temperature levels […]. Uncertainties related to historical warming (±550 GtCO2) and non-CO2 forcing and response (±220 GtCO2) are partially addressed by the assessed uncertainty in TCRE, but uncertainties in recent emissions since 2015 (±20 GtCO2) and the climate response after net zero CO2 emissions are reached (±420 GtCO2) are separate.» 1 See Table 4 for actual emissions after 2019. 2 The reddish marking of fields in Table 1 is intended to indicate that only in cases of high probability can a serious pursuit of the corresponding target be assumed, and what a significant reduction below 2 °C would entail. Remaining global CO2 budgets Page 4 / 6 Updated budgets Since 2023, the Indicators of Global Climate Change (IGCC) initiative has published an annual update on remaining CO2 budgets, which also takes into account the latest scientific findings. IGCC publishes the remaining budgets (see Table 2), taking also into account known actual emissions from 2020 onwards (cf. Forster, P.M. et al., 2025). Table 3 shows the results converted to budgets from 2020 onwards. Actual emissions were used for this purpose, according to IGCC (see Table 4). Table 2: Updated budgets from 2025 onwards (IGCC) Warming Estimated remaining carbon budgets Probabilities: 50% 67% 83% [°C] [GtCO2 from 2025 on] 1.5 130 80 30 1.6 310 240 160 1.7 490 390 290 1.8 680 550 430 1.9 860 710 560 2.0 1,050 870 690 Table 3: Recalculated updated budgets from 2020 onwards Warming Estimated remaining carbon budgets IGCC IPCC AR6 Probabilities: 50% 67% 83% [°C] [GtCO2 from 2020 on] 1.5 336 286 236 300 1.6 516 446 366 400 1.7 696 596 496 550 1.8 886 756 636 650 1.9 1,066 916 766 800 2.0 1,256 1,076 896 900 Table 4: Actual emissions from 2020 onwards 3 GtCO2 GCP IGCC EDGAR fossil LUC total fossil LULUCF total fossil LULUCF total 2020 34.4 4.5 38.9 36.0 4.4 40.4 36.2 -0.11 36.1 2021 36.0 4.7 40.7 36.5 4.3 40.8 38.2 -0.35 37.9 2022 36.7 4.8 41.5 37.1 4.1 41.2 38.5 -0.24 38.3 2023 37.3 4.7 42.0 37.8 3.6 41.4 39.1 1.30 40.4 2024 37.8 4.6 42.4 38.2 4.2 42.4 39.6 -0.05 39.6 total 205.5 206.2 192.3 2025 38.1 4.1 42.2 3 Notes: IGCC defines LULUCF here in relevance to the remaining CO2 budgets according to the IPCC (Forster, P.M. et al., 2025). EDGAR’s LULUCF estimates are not directly comparable to GCP or IGCC, as EDGAR adopts a bookkeeping-style approach with different system boundaries (EDGAR, 2025). Therefore, EDGAR total emissions should not be used for budget accounting. Process-related emissions from cement production are included in ‘fossil’. Cement carbonation sink is taken into account in GCP (GCP, 2025). The emissions for 2025 at GCP are a projection. Remaining global CO2 budgets Page 5 / 6 Interpretation of the IPCC budgets The probabilistic budgets reported in IPCC AR6 (50%, 67%, 83%) reflect uncertainty in TCRE, as well as uncertainties that are partially addressed through it — in particular those related to historical warming and non-CO₂ forcing. However, two additional sources of physical uncertainty — Zero-Emissions Commitment (ZEC) 4 and recent-emissions uncertainty 5 — are not included in these probabilistic estimates. The IPCC therefore reports ZEC-related uncertainty separately as ±420 GtCO₂, centred around a best estimate of approximately zero. This implies that the ZEC range should not be applied arithmetically to the AR6 carbon budget values. ZEC therefore widens the range of possible realized temperature outcomes without altering the central AR6 budget values. A precautionary interpretation can therefore be achieved by orienting policy towards lower temperature thresholds at higher probability levels. In contrast to these physical uncertainties, the ‘scenario variation’ term reflects scenario-dependent differences and, like ZEC and recent-emissions uncertainty, is not part of the probabilistic budgets. 6 The ±220 GtCO₂ variation reflects the spread of non-CO₂ mitigation pathways across the scenario ensembles used in AR6 that are compatible with 1.5 °C or 2 °C. Departures from the assumed nonCO₂ trajectory will therefore lead to systematically different CO₂ budgets. AR6 budgets are best-estimate values, not boundaries that incorporate all sources of Earth-system uncertainty. Example of an IPCC-consistent statement for an 83% likelihood of limiting warming to 1.8 °C For an 83% probability of limiting the increase in global mean surface temperature to 1.8 °C, the IPCC’s Sixth Assessment Report (AR6) assesses a remaining carbon budget of approximately 650 GtCO₂ from 2020 onwards (IPCC, 2021, cf. Tables SPM.2 and 5.8). When incorporating the IGCC update, the corresponding estimate amounts to 636 GtCO₂ (see Chapter “Updated budgets”). This probabilistic budget reflects uncertainties in TCRE, including those related to historical warming and non-CO₂ forcing through the assessed TCRE range. This statement also assumes a compatible non-CO₂ emissions trajectory. For ZEC, the best available estimator of zero is presumed. 4 ZEC arises from physical processes that govern the climate response after net-zero CO₂ emissions are reached, including continued ocean heat uptake, slow feedbacks (e.g. permafrost–carbon interactions), and the adjustment of nonCO₂ forcing. Because the magnitude and sign of these post-net-zero processes are uncertain, IPCC AR6 reports ZEC as a separate physical uncertainty rather than integrating it into the probabilistic remaining carbon budgets. 5 Recent-emissions uncertainty refers to uncertainties in the historical emissions data that determine how much of the remaining carbon budget has already been used. It is classified as a physical Earth-system uncertainty because it affects the diagnosed state of the climate system, not future scenario assumptions. 6 AR6 remaining CO₂ budgets are conditional on the median non-CO₂ pathway of the 1.5 °C — or 2 °C — compatible mitigation scenarios. The probabilistic (50%, 67%, 83%) budgets incorporate physical uncertainty in the climatic response to non-CO₂ forcing—partly represented through the TCRE range—but do not include uncertainty in future nonCO₂ emissions. Any deviation from the assumed non-CO₂ pathway would therefore alter the remaining CO₂ budget. Remaining global CO2 budgets Page 6 / 6 References EDGAR, 2025. European Commission, Joint Research Centre (JRC)/PBL Netherlands Environmental Assessment Agency. Emission Database for Global Atmospheric Research (EDGAR). [Online] Available at: https://edgar.jrc.ec.europa.eu/ [Accessed 09 09 2025]. Forster, P.M. et al., 2025. Indicators of Global Climate Change 2024: annual update of key indicators of the state of the climate system and human influence. [Online] Available at: https://essd.copernicus.org/articles/17/2641/2025/ GCP, 2025. [Online] Available at: https://globalcarbonbudget.org [Accessed 13 11 2025]. IPCC, 2021. Summary for Policymakers. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. [Online] Available at: https://www.ipcc.ch/report/ar6/wg1/