Remaining global CO2 budgets

www.save-the-climate.info A. Wolfsteiner Version: 27/11/2025 DOI 10.5281/zenodo.16754510 Download this paper: http://ipcc-co2-budgets.climate-calculator.info 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 20021 (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 1000 850 650 1.9 1200 1000 800 2.0 1350 1150 900 A remaining CO2 budget is currently being used up with annual emissions of around 42 Gt. 1 Here is a web app for calculating linear global emission paths that comply with a given CO2 budget (temporary overshooting can be taken into account): http://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): «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 Actual CO2 emissions in 2024 were estimated at 42.4 Gt (GCP, 2025). The projection for 2025 is 42.2 Gt. See also footnote 2. Remaining global CO2 budgets Page 2 / 3 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. 2 Table 2: Updated budgets from 2025 onwards 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 Probabilities: 50% 67% 83% [°C] [GtCO2 from 2020 on] 1.5 336 286 236 1.6 516 446 366 1.7 696 596 496 1.8 886 756 636 1.9 1.066 916 766 2.0 1.256 1.076 896 2 Notes on the following table: IGCC defines LULUCF here in relevance to the remaining CO2 budgets according to the IPCC. Process-related emissions from cement production are included in ‘fossil’. Cement carbonation sink is taken into account in GCP. GtCO2 GCP (GCP, 2025) IGCC (Forster, P.M. et al., 2025) fossil LUC total fossil LULUCF total 2020 34.4 4.5 38.9 36.0 4.4 40.4 2021 36.0 4.7 40.7 36.5 4.3 40.8 2022 36.7 4.8 41.5 37.1 4.1 41.2 2023 37.3 4.7 42.0 37.8 3.6 41.4 2024 37.8 4.6 42.4 38.2 4.2 42.4 total 205.5 206.2 Remaining global CO2 budgets Page 3 / 3 References 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/