Updated scenarios show 1.5°C overshoot is unavoidable but limitable

Updated scenarios show 1.5°C overshoot is unavoidable but limitable | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Updated scenarios show 1.5°C overshoot is unavoidable but limitable Laila Gohar, Camilla Mathison, Benjamin Sanderson, Marit Sandstad, and 9 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8407383/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract A lack of meaningful action in reducing greenhouse gas emissions has fuelled debate on whether global warming can still be limited to 1.5°C. In 2022, the Working Group III contribution to the Sixth Assessment Report found 97 scenarios that could limit global warming to 1.5°C above pre-industrial levels with no or limited overshoot, while 9 kept median warming projections under 1.5°C throughout the 21st century. These pathways relied on emissions reductions by 2025 that have not materialized. We show that updating the emissions and climate model calibrations to 2023, provides additional constraints on the projected warming outcomes. The window to keep warming below 1.5°C with a greater than 50% likelihood has closed. We use several approaches to show that the objective of limiting warming to 1.5°C needs to be pursued from above after a temporary overshoot. Immediate, rapid deep reductions in CO 2 emissions and non-CO 2 forcers, can still limit peak warming to around 1.7°C, and returning below 1.5°C before 2100 is attainable. Keeping peak warming as low as possible and pursuing to reverse it below 1.5°C thereafter will contain the adverse consequences of temperature overshoot and limit the scale of negative emissions required. Earth and environmental sciences/Climate sciences/Climate change Earth and environmental sciences/Climate sciences Earth and environmental sciences/Climate sciences/Climate change/Climate-change mitigation Figures Figure 1 Figure 2 Figure 3 Figure 4 Main The Paris Agreement long-term temperature goal 1 , 2 (LTTG) includes an ambition to pursue efforts to limit warming to 1.5°C relative to preindustrial levels. However, there is recent debate 3 – 10 about whether global warming can still be held to 1.5°C and how this reflects on whether the Paris Agreement’s 1.5°C ambition remains within reach. While there has not yet been a definitive conclusion that keeping global warming to 1.5°C is impossible 8 , recent emissions drivers and climate trends make it increasingly difficult to achieve 7 , 11 – 13 . To inform political discussions under the United Nations Framework Convention on Climate Change, the 2018 Intergovernmental Panel on Climate Change (IPCC) Special Report on 1.5°C (SR1.5) made a distinction between scenarios that remained under 1.5°C throughout the 21st century at the 50th percentile (“no overshoot”), and those that included a temporary overshoot of approximately 0.1°C at the 50th percentile (“low overshoot”, see Extended Data Table 3) 14 . However, due to the narrow literature base for “no overshoot” scenarios, insights for policymakers were reported for a combined “no or limited overshoot” scenario category, combining the two sets 15 . More recently, in the 2022 IPCC Sixth Assessment Report (AR6) Working Group III (WG3) contribution, this practice was continued with the scenario category with the lowest warming (C1) containing scenarios with both no and low overshoot of 1.5°C 16 . AR6 WG3 projected the global warming outcome for 1202 emissions scenarios derived from Integrated Assessment Models (IAMs). These scenarios span a broad range of potential climate futures, from futures with deep greenhouse gas emissions reductions to futures where greenhouse gas emissions strongly increase because of further fossil fuel expansion. 97 of the available scenarios were categorised as C1 scenarios, which is defined in the IPCC AR6 WG3 report (IPCC, 2022) as limiting peak warming to 1.5°C with at least a 33% likelihood and end-of-century warming below 1.5°C with at least 50% likelihood, loosely translating to a median peak warming of no more than 1.6°C. A further 133 scenarios are categorised as C2 scenarios. These C2 scenarios are allowed to result in a higher overshoot (defined formally as not meeting the 33% likelihood threshold for remaining under 1.5°C, reaching median peak warming of up to approximately 1.8°C) but with an end-of-century goal still focused on 1.5°C (see Extended Data Table 3 17 ). Scenarios typically follow historical emissions up to a given year (2015 in AR6 WG3), after which their modelled future starts. With time passing, the branching point of some scenario vintages, including those in AR6 WG3, might already lie well in the past. Particularly if recent real-world trends in energy and emissions are very different from the trends modelled in the scenarios, some aspects of scenarios might not be achievable anymore. As IPCC assessment reports are produced at intervals of 5‒8 years, this is also true for some of the findings in the most recent IPCC AR6 WG3 assessment. Scenarios were derived from literature published between 2016‒2021, branched off from historical emissions that ended in 2015. Despite this early branching point, emissions of CO 2 and GHGs in C1 scenarios are nevertheless expected to be consistent with historical trends until 2020, after which they project rapid emissions reductions. This is because a vetting process as part of the AR6 WG3 assessment pipeline filtered out scenarios in which the emissions of major GHGs and energy generation capacities were not consistent with historical best estimates until 2019 18,19 . This vetted set of scenarios was ultimately assessed using constrained climate emulators from the 2021 AR6 Working Group I (WG1) report 18 that were calibrated with a final observational year of 2019 16,19,20 . In this analysis, we update the AR6 WG3 scenarios using the latest historical emissions and integrate recent climate trends into our modelling framework (see Methods) to reassess the temperature outcomes for the 1.5°C ambition. Using a similar updating concept, Zhong et al., (2025) 21 highlight the recent divergence of China’s emissions from the AR6 published pathways, where they include latest emissions and reharmonize the emissions to SSP2. They develop new scenarios based on SSP2 (initially following SSP2-4.5) called SSP2-com and SSP2-com+, focussing on a 2°C global warming level; these are part of a larger framework that aligns with China Net Zero emissions pathways. Here, we present four arguments which illustrate that complying with the AR6 C1 definition is not a possibility anymore, implying a commitment to at least a temporary warming in excess of 1.5°C. We show that it is still possible to limit warming to 1.7 or 1.8°C with strong immediate mitigation action. However, this will require focused adaptation as well as mitigation. Returning to 1.5°C will require a strengthened resolve for continued rapid deep emissions cuts to limit the peak warming as low as possible, while a return to lower temperatures thereafter will require negative emissions. Ultimately, a higher mitigation ambition to limit peak warming will reduce the negative emissions burden for returning to 1.5°C, which we argue should continue to be the anchor point for mitigation ambition. Emissions above AR6 1.5°C scenario range The updated scenarios show that the most recent trends and scientific updates now indicate markedly higher risks of 1.5°C overshoot over the 21st century. It is evident that greenhouse gas emissions in 2024 are above the emissions range of the 1.5°C-compatible C1 scenarios and diverge strongly over the most recent years (Fig. 1 ). Global total CO 2 emissions have increased from 2015 to 2024 by an estimated 4% 22 (solid black lines), whereas the C1 (C2 not shown) scenarios show a change in CO 2 emissions ranging from ‒39 to + 1% (-26% to 12%) over the same timeframe (minimum-maximum range, brown plumes, Fig. 1 a). Similarly, total CO 2 -equivalent emissions of all greenhouse gases (GHG, aggregated using 100-year Global Warming Potentials from IPCC AR6 WG1 18 ) show an estimated increase of 4% (Fig. 1 b) for the 2014–2024 period while C1 (C2) scenarios show a ‒36 to ‒2% (‒22 to + 6%) change. Figure 1 also highlights our approach to updating scenarios in light of more recent historical emissions (blue curves). Scenarios are updated to a 2023 starting date with historical emission overriding the scenario-modelled emissions. To ensure a smooth transition from the recent historical emissions to IAM projections the scenarios’ emissions trajectories are harmonized 22 , and infilled 23 to ensure a complete set of GHG and short-lived climate forcer (SLCF) emissions are provided as input required for the climate assessment (see Methods). Harmonization is necessary to transition from the updated historical emissions (black lines, Fig. 1 ) to the emissions pathways of the underlying scenarios which have since diverged from history. Therefore, we consider a modified scenario set relative to AR6 WG3, which we refer to as 2023-harmonized scenarios. This approach allows the study to capture the general characteristics of emission scenarios in context of the most recent real-world trends. To understand the underlying systems transformations, additional modelling experiments would be required..The updating of scenarios to a more recent historical harmonization year means that the net-zero year for CO 2 and GHG emissions changes (bars, Fig. 1 a). Ultimately, the divergence of real-world emissions trends and ambitious mitigation scenarios in the recent past does not imply that the long-term temperature outcomes of these scenarios cannot be achieved any more, but that the near-term peak warming and overshoot is necessarily higher than it would have been following the pathways that diverged from history in 2015. Remaining carbon budget overspent Next we analyse the cumulative emissions implied by the 2023-harmonized scenarios and compare them to assessed estimates of remaining carbon budgets (RCBs) to net zero that would hold global warming under 1.5°C with 50% likelihood 24 . The IPCC AR6 WG1 assessment reported a 500 GtCO 2 RCB value from the start of 2020 onwards for limiting warming to 1.5°C with 50% probability 25 . Most recent updates that consider emissions since 2020 and updates in attributable warming revise the 1.5°C RCB downwards 12 to 130 GtCO 2 from the start of 2025. These central estimates can be larger or smaller by at least 220 GtCO 2 25 , depending on how successful or unsuccessful the world is in reducing non-CO 2 emissions, meaning that the most up-to-date range of RCBs compatible with limiting warming to 1.5°C with 50% likelihood lies between ‒90 and + 350 GtCO 2 starting from 2025. In 2023-harmonized C1 scenarios, the lowest cumulative CO 2 emissions between the start of 2025 and net zero CO 2 is 177 GtCO 2 , implying that the central estimate of the 50% likelihood 1.5°C RCB will be exceeded even with maximally ambitious emissions reductions. While theoretically falling within the uncertainty range of 1.5°C budgets if non-CO 2 mitigation is maximized, the cumulative emission value of 177 GtCO 2 exceeds the central estimate of the remaining carbon budget. Limited progress on non-CO 2 mitigation also makes that option less likely 13 . By applying reharmonization to updated historical emissions annually for each year from 2014 to 2023 (see Methods), we find that 2024 was the first year in which no re-harmonized scenario remained within the central estimate of a 50%-likelihood 1.5°C RCB (Fig. 2 ). This implies that, bar exceptionally strong reductions in non-CO 2 emissions, all updated scenarios would have a greater than 50% probability of at least temporarily exceeding 1.5°C. The RCB analysis is not unlike the “infrastructure commitment” argument that existing fossil fuel generating capacity will emit more over their remaining lifetimes than the central 1.5°C RCB for a 50% likelihood permits 27 . However, the RCB line of argument is stronger than the infrastructure commitment, as high ambition IAM scenarios have the flexibility to retire infrastructure before the originally planned end of their lifetime 28 , and can bring carbon dioxide removal online before positive emissions from fossil infrastructure are phased out. Central climate projections exceed 1.5°C The assessment of global warming outcomes of scenarios uses well-calibrated and flexible simple climate models 18 , 29 , 30 . The AR6 WG3 climate assessment was calibrated to the climate system evidence from WG1 18,31,32 , which extended to a final year of 2020 or earlier. Since 2020, increasingly more observations are available to constrain projections. Here we use climate system observations up until 2023 from the CMIP7 historical emissions dataset 33 to constrain updated climate projections from the reharmonized C1 scenarios (see Methods). This update shows an increased risk of exceeding 1.5°C over the course of the century. Using the MAGICCv7.5.3 climate emulator, AR6 WG3 determined that 97 scenarios available in the IPCC AR6 scenario database were in the C1 category (Fig. 3 a). Using a different climate emulator, FaIR v2.2.2 34,35 , with a similar calibration 30 to AR6 WG3 (Methods) and the same emissions scenarios, we find similar results; 107 scenarios fall in the C1 category, of which 14 do not overshoot 1.5°C in the ensemble median (Fig. 3 b). The AR6 WG3 assessment included solar and volcanic forcings in their scenario classification. However, we argue here that including natural forcings to assess future warming projections is not in the spirit of determining progress towards the Paris Agreement long-term temperature goal 36 . Firstly, the natural 11-year solar cycle is clearly visible in future temperature projections, creating oscillations around an anthropogenically-forced smooth transition in climate projections (Extended Data Fig. 1 ). This would allow scenarios that were close to the threshold to potentially exceed or remain under a 1.5°C threshold based on whether their peak warming from mitigation efforts happened to coincide with a hypothetical solar minimum or maximum. Secondly, the method of implementing volcanic forcing post-2014 in CMIP6 Earth System Models 37 , where the historical (1850‒2014) mean volcanic forcing is defined as the zero reference point, also impacts the assessment of future warming. The volcanically quiescent years in the end of the historical time period have a small positive forcing relative to this historical zero-mean. In other words, as the end of the historical period had few volcanoes, there is an artificial offset in effective radiative forcing in the AR6 WG3 simulations of around ‒0.15 W m − 2 between 2015 and 2025 (Extended Data Fig. 1 ), meaning future projections are artificially slightly cooler than they otherwise would be. Owing to this, if natural forcings are removed from historical and future projections following the AR6 WG3 calibration and 2014 harmonization, fewer scenarios achieve C1 (75) and 1.5°C-no-overshoot (14) categorizations (Fig. 3 c) when assessed with FaIR. Other studies 38 , 39 have shown that including volcanoes in future projections increases the likelihood of remaining under 1.5°C, though in a similar vein to betting on the solar cycle, the serendipitous timing of a cooling volcanic eruption should not be used to claim credit for achieving mitigation goals. When natural forcing terms are excluded and observational data up to 2023 is used to constrain FaIR, we find that even for emissions scenarios starting in 2015, no scenario avoids overshoot of 1.5°C, only 6 have a low overshoot of less than 0.1°C, and there are 20 scenarios that less than 0.3°C overshoot (Fig. 3 d). Finally, when updating the climate constraints together with a 2024 harmonization of historical emissions, the median global warming projections of all of the available scenarios in this analysis exceed 1.5°C but there are still 24 scenarios that peak median warming between 1.6 and 1.8°C (Fig. 3 e). Applying the reharmonization to historical emissions with an end point each year from 2014 to 2024, and using the 2023 calibration of FaIR (as in Figs. 3 d and 3 e), we can track the changes in the number of 2023-harmonized scenarios that exceed 1.5°C (Extended Data Fig. 2 ). Our analysis shows that while there is a marked decline in achievability of 1.5°C with low- or no-overshoot, it is still possible that peak warming could remain under 1.7°C or 1.8°C with post-2020 emission trends. Our analysis also shows that as of 2024, no updated AR6 WG3 scenario limits peak warming to 1.6°C with 50% probability, and that while the window for 1.7°C is rapidly closing options to stay below it still remain. The counterintuitive result of an increase in scenarios limiting warming to specific thresholds between 2015 and 2020 (Extended Data Fig. 2 ) is a result of the emissions vetting process, in which historical and scenario emissions were approximately in line between 2015 and 2020. From 2020 onwards, every year delay in deep emissions cuts makes it more difficult to limit warming to defined thresholds. The warming response to abrupt zeroing of all emissions The scenarios assessed in AR6 WG3 represent a useful ensemble of opportunity, but the physical challenge of limiting warming can be framed independently of their trajectories. Another perspective on the physical challenge of limiting warming is to explore the climate system response to a hypothetical abrupt zeroing of all anthropogenic emissions 3 . Although extremely stylized and unrealistic, such exercises offer insight into how the Earth system responds to changes in forcing from the zeroing of emissions. Earlier studies showed how an abrupt zeroing of all emissions can lead to a warming peak within a decade, followed by slow cooling as GHG concentrations decline, and that peak warming is limited to the lowest level when emissions are abruptly zeroed compared to slower transitions towards zero emission 3 . We find that a positive committed peak warming to zeroing all emissions is likely, since in the near term the balance and timescale of reductions in short-lived aerosol forcing (net negative) acts faster than the slower decline in greenhouse gas concentrations (net positive) 40 . However, it is possible that some other pathway to net zero for all anthropogenic emissions could result in lower peak temperature or even no further warming, so this ‘immediate phase out of all emissions’ sensitivity experiment does not necessarily represent an absolute lower bound for committed warming rather a conservative estimate which attempts to quantify the sensitivity of the system. Note that this concept differs from the more common definition of the “zero emissions commitment” (ZEC), which considers zeroing CO 2 emissions in isolation 41 . A recent ZEC assessment highlights the considerable uncertainty surrounding ZEC due to missing processes in ESMs 42 . In addition, recent model experiments to quantify ZEC have focused on CO 2 because no pathways exist that show a complete elimination of CH 4 , N 2 O or aerosol emissions. A wide range of processes that could affect ZEC, for example, zeroing emissions in CO 2 is expected to lead to a drop in atmospheric CO 2 as well as a decline in ocean heat uptake have been identified 42 ; these will act in the opposite direction in terms of their effect on temperatures, but there are many other processes not yet represented in the ESMs that could also affect this energy balance 42 . The ZECMIP 41 experiments show that the ZEC in ESMs is affected by the cumulative emissions giving a range from − 0.36 to + 0.29°C for 1000PgC and − 0.40 to + 0.52°C for 2000 PgC. The picture at regional scales after the zeroing of emissions is varied, with the patterns of change far more complex and little consistency between different models. The sign of temperature change (warming or cooling) also varies by model and climatic zone 42 . Using the 2023 calibration and harmonization of FaIR, an abrupt zeroing of emissions from 1 January 2024 results in additional peak warming of 0.24°C in the ensemble median and 0.16°C at the 33rd percentile. Using a definition of human-induced warming over the 2015‒2024 period of 1.22°C as the baseline as in IGCC 2024, this results in a peak commitment that exceeds 1.5°C in the median but only narrowly remains under 1.5°C at the 33rd percentile (Fig. 4 ). In other words, this hypothetical zero-emissions scenario would be classed as a low-overshoot scenario in the C1 category using the AR6 WG3 definition. Any internally consistent mitigation pathway that reaches zero emissions slower than instantaneously, which includes the scenarios with realistic socio-technological transitions, would reach higher warming levels and therefore be committed to a larger overshoot regardless of definition. Discussion Using a variety of methods, we show that the likelihood of limiting peak warming below 1.5°C is strongly decreasing when AR6-generation scenarios are updated to be consistent with recent emissions and climate history. With this decrease in likelihood for limiting warming to 1.5°C comes a corresponding increase in the risk of temporary or permanent overshoot. Already, the literature is starting to shift focus to 2°C 21 . We show in this analysis that in keeping the focus on the long-term temperature target of 1.5°C, peak warming of well below 2°C is still in reach provided we keep firmly to strong and immediate mitigation of emissions. We show that delays in mitigation action have reduced our ability to limit warming to 1.5°C, but further delays will reduce our ability to limit warming to 1.6, 1.7 or 1.8°C. Deep sustained emissions cuts are needed to reduce the negative emissions burden, avoid additional and committed warming and limit the size and duration of overshooting 1.5°C. The Paris Agreement 1.5°C goal of pursuing efforts to limit warming below 1.5°C remains central and critical 10 . It is imperative that countries maintain and ratchet up their ambition to set Nationally Determined Contributions under the Paris Agreement that are aligned with this goal. Insufficient emission reductions between 2015 and 2024 have compounded the difficulty of limiting warming to 1.5°C. Indeed, the likelihood of limiting peak warming to 1.5°C is strongly reduced and limiting warming to 1.5°C in the long term now requires a period of globally net negative emissions to reverse the global temperature increase 43 , 41 . This new reality raises issues of equity around the responsibility for carbon dioxide removal and warming drawdown 44 . Limiting peak warming to as low a level as possible is still critical for avoiding escalating climate impacts 45 and triggering tipping elements in the Earth system 46 . The number of scenarios that can return global warming to 1.5°C by the end of the century after a maximum medium warming between 1.6°C and 1.8°C (such as those in the C2 and C3 AR6 WG3 scenarios, which also limit warming to 2°C with 66% or greater likelihood) has not markedly declined compared to 2015 (Extended Data Fig. 3 ). There are other non-AR6 based scenarios, such as the Network for Greening the Financial Sector (NGFS) that produce updated IAM scenarios on an annual basis that account for technological developments but still use historical emissions up to 2014 for harmonisation and climate assessments based on 2019 information 47 . We carry out the same process to the AR6 scenarios, reharmonizing these scenarios to 2023, and we show these NGFS scenarios in the broader context of the AR6 scenarios in the Extended data Fig. 4 showing the distribution of peak warming against 2030 emissions. These 2023-harmonized NGFS and AR6 scenarios are still within the distribution suggesting that scenario development in terms of mitigation has not moved on significantly and therefore the reharmonized 2023 NGFS and AR6 scenarios are still relevant. It is likely that AR7 will assess scenarios that take into account the latest understanding, knowledge/assumptions on costs, learning rates and concentration constraints 48 . Our analysis shows that the door is not closed on temperatures below 2°C, and settling for 2°C 21 as a long term global warming level is unwarranted and unnecessary. Every additional tenth of a degree of warming increases risks of both high impact globally important tipping points 49 and more local impacts with potentially high adaptation costs. Our findings have implications for future scenario development. They suggest that while the window to avoid overshoot of 1.5°C has closed, pathways that limit the peak warming to 1.6 or 1.7°C are still available with global cooperation on rapid deep emissions reductions. A focus on limiting 1.5°C exceedance in magnitude and duration remains essential to limit future climate harms. Methods IPCC AR6 WG3 emissions scenarios We start with the 1202 emissions scenarios from IAMs that received a climate categorization in the IPCC AR6 WG3 16,50 . These scenarios are the subset of the 3131 submitted scenarios, submitted through an open call to the community, that passed basic vetting 19 , 20 . The vetting checks were (1) scenarios contained at least emissions of CO 2 from fossil fuels and industrial processes (FFI), CO 2 from agriculture, forestry and other land use (AFOLU), CH 4 , and N 2 O from 2015 to 2100; (2) the emissions of total CO 2 , CO 2 FFI and CH 4 were close to inventory best estimates in 2019; and (3) energy sector variables for primary energy, carbon capture and storage, nuclear, and solar and wind were within reported values from industry bodies such as the International Energy Agency for 2019 or 2020. Firstly, we re-run the 1202 scenarios in IPCC AR6 WG3 which branch from historical emissions in 2015 and were harmonized and infilled (described below) as part of the IPCC AR6 WG3 process. The historical emissions from 1750‒2014 were the global annual total emissions dataset prepared for the Reduced Complexity Model Intercomparison Project (RCMIP) 29 , 32 , covering 43 GHGs and seven SLCFs. For CO 2 FFI and the SLCFs (BC, OC, SO 2 , NH 3 , NOx, non-methane volatile organic compounds (NMVOCs) and CO), the historical emissions were the same as supplied to CMIP6 Earth System Models (ESMs). As ESMs in CMIP6 were not designed to (and generally not capable of) running with emissions of greenhouse gases other than CO 2 , emissions datasets for non-CO 2 GHGs (and CO 2 AFOLU, which in carbon-cycle resolving models is a diagnosed flux) were not prepared for CMIP6 and were derived from a number of sources including PRIMAP-Hist 51 , the WMO Scientific Assessment of Ozone Depletion 52 , Community Emissions Data System (CEDS) 53 , Global Carbon Budget 22 , biomass burning emissions prepared for CMIP 54 , and inversion of historically observed concentrations 29 . Updated historical emissions As part of the CMIP7 process 55 , a new historical emissions dataset for 1750‒2023 has been prepared 56 , covering the same 43 GHGs and 7 SLCFs. Fossil and industrial emissions of CO 2 , CH 4 and N 2 O and the seven SLCFs are taken from the Community Emissions Data System (CEDS) and are complete for 1750 to 2023 using the v2025_03_18 of CEDS 57 , except for CH 4 and N 2 O which are complete from 1970 and backward extensions are performed to 1750 using the third-party emissions inventories from PRIMAP-Hist (PRIMAP-Hist-TP) 51 . Biomass burning emissions for the SLCFs, CH 4 and N 2 O are used from an update to the BB4CMIP dataset prepared for CMIP7. Noting substantial year-to-year variability in biomass burning emissions that has shown to be problematic for ESMs 58 , a five-year smoothing filter is applied to biomass burning emissions. This also avoids spurious harmonization artefacts by initialising future projections from years in which biomass burning emissions were unusually high and likely a result of climate influences rather than anthropogenic activity, resulting in stronger than expected aerosol forcing 26 . AFOLU emissions for CO 2 are taken from the Global Carbon Budget (1850‒2023), with the period 1750‒1850 filled in using an estimate of 30 GtC cumulative emissions over this period 22 that is modelled using an exponential ramp up from 1750 to 1850. Following previous precedents, we do not explicitly consider CO 2 biomass burning emissions which are considered to be part of the steady-state carbon cycle over decadal timeframes 12 . Minor GHG emissions are from a number of sources. These are Adam et al. 59 for HFC-23, Velders et al. 60 for HFC-125, HFC-134a, HFC-143a, HFC-152a, HFC-227ea, HFC-236fa, HFC-245fa, HFC-32, HFC-365mfc, HFC-4310mee, WMO Scientific Assessment of Ozone Depletion 2022 (WMO2022) 52 for CF 4 , C 2 F 6 , SF 6 , CCl 4 , CFC-11, CFC-12, CFC-113, CFC-114, CFC-115, CH3CCl3, HCFC-141b, HCFC-142b, HCFC-22, Halon-1202, Halon-1211, Halon-1301 and Halon-2402, and the inversion of observed concentration time series prepared for CMIP7 for C 3 F 8 , C 4 F 10 , C 5 F 12 , C 6 F 14 , C 7 F 16 , C 8 F 18 , CH 2 CCl 2 , CH 3 Br, CH 3 Cl, CHCl 3 , NF 3 , SO 2 F 2 and c-C 4 F 8 . Data is complete in the CMIP7 historical dataset until 2023 for all emissions species and to 2024 for GHGs in the Velders et al. and WMO2022 datasets. For other species, we perform extrapolations to estimate emissions for 2024. CO 2 FFI emissions in 2024 are 0.8% higher than 2023 using preliminary estimates from the Global Carbon Budget (GCB) 22 , which we adopt by increasing the 2023 emissions by 0.8% for 2024. CO 2 AFOLU emissions are estimated to be 4.2 GtCO 2 in GCB, and this value is also used directly. For other species, which includes CH 4 , N 2 O and SCLFs, an extrapolation of emissions is performed using a continuation of the 2022 to 2023 trend into 2024. Harmonization Although vetted to ensure they are within range, the raw emissions projections from the 1202 IAM scenarios are model outputs and do not correspond exactly to historical emissions in 2014. Therefore, in AR6 WG3, a harmonization 19 , 61 process was undertaken to ensure a smooth transition from historical best estimate emissions in 2014 to the IAM-derived emissions projections for the 21st century. The harmonization process adjusts the raw IAM emissions time series up or down to align them with the historical best estimate dataset in 2014. This also provides a common branching-off point for all IAM emissions scenarios. For this study we also harmonize the 1202 IAM scenarios to the updated and extended CMIP7 historical emissions ending in 2023 using the aneris package, v0.3.1. Aneris was developed to aid in the scenario assessment in the AR6 WG3 61 . Aneris allows emissions pathways (primarily from IAM scenarios) to be harmonized or align with historical emissions of greenhouse gases and short lived climate forcers. The software package consists of established methods of harmonization including ratio based methods and offset based methods, The software is designed so that it maintains the sign of the original emissions and avoids distortions. The harmonization method we use is the same for each species as it was in the AR6 WG3 process: “reduce_ratio_2150_cov” for C 2 F 6 , C 6 F 14 , CF 4 , CO, CO 2 AFOLU, OC and VOC, “reduce_ratio_2080” for total CO 2 and CO 2 FFI, “constant_ratio” for HFC-125, HFC-134a, HFC-143a, HFC-227ea, HFC-23, HFC-32, HFC-4310mee and SF 6 , and CH 4 , N,O, NH 3 , NOx and SO 2 harmonized according to the decision tree in Gidden et al. 61 . The “reduce_ratio” method starts by scaling model emissions to align with historical values, then gradually reduces this adjustment until it disappears by the specified convergence year. The suffix “_cov” indicates that a convergence function controls how quickly the adjustment fades over time. If this suffix is absent, the reduction follows a simpler pattern, such as a fixed or linear decrease. While the 2023 harmonization is the focus of this study, we also harmonize the IAM scenarios to the updated CMIP7 historical emissions for each year from 2014 to 2023 for analysis in Extended Data Figs. 2 and 3 , giving 10 sets of future emissions projections with different starting years. Infilling Most IAM scenarios do not provide a complete set of the 43 GHG and 7 SLCF emissions species required to perform a full climate assessment. Therefore, an infilling 23 process is undertaken, where emissions of missing species are estimated based on patterns of how these species change over time with a lead species (in this case CO 2 FFI) in a subset of the IAM scenarios that do include the required minor emissions species. The logic of infilling minor species this way is that many substances are co-emitted, or share similar sectoral origins, to fossil fuel and industrial CO 2 emissions and therefore are likely to change in step 62 . Including a temporal dimension to the relationship allows legislative developments (e.g. around air pollution control) and efficiency improvements to be incorporated in the infilling relationship. We use two infilling databases: one from AR6 WG3 63 which includes SLCFs, CH 4 , N 2 O, the majority of hydrofluorocarbons (HFCs) and some fluorinated (F-)gases for multiple scenarios, and RCMIP 29 which includes all required gases including the less commonly emitted HFCs and PFCs as well as the Montreal gases for eight SSP-RCP combinations. For the updated historical emissions from CMIP7, we first harmonized the infilling databases to historical emissions ending each year from 2014 to 2023. This is necessary to ensure a consistent baseline for infilling. Thus, we create an additional 11 versions of the infilling databases that we use for each of the 2014‒2023 harmonization years with the CMIP7 historical emissions. Infilling is performed using the silicone 23 package v1.3.0, using the same decision methodology as in the AR6 WG3. We use the Quantile Rolling Windows method to infill missing SLCFs, CO 2 AFOLU, CH 4 and N 2 O based on their co-evolution with CO 2 FFI in the reharmonized AR6 WG3 infilling dataset. HFCs and F-gases with broad representation across AR6 scenarios (HFC-125, HFC-134a, HFC-143a, HFC-227ea, HFC-23, HFC-32, HFC-4310mee, C 2 F 6 , C 6 F 14 , CF 4 , SF 6 ) are also infilled using this dataset using the scenario with the root-mean-squared closest difference. All of the GHGs are infilled to the closest scenario of the CMIP6 SSP trajectories. Infilling is only performed for missing species and does not override harmonized emissions in scenarios where they exist. Harmonization and infilling implicitly assumes that the technological and socioeconomic assumptions underpinning the original IAM scenarios are still valid, since all updates are performed in emissions space. Efforts such as the Network for Greening the Financial Sector (NGFS) and the Scenario Compass Initiative produce updated IAM scenarios on an annual basis that account for technological developments, but draw from a smaller literature 47 . We show that the latest NGFS scenarios (Phase V) do not assume more stringent mitigation to 2030 than the AR6 WG3 scenarios (Extended Data Fig. 4 ), so are not likely to remain under 1.5°C with a 2023 harmonization and climate assessment. Climate categorization We follow the same categorization for scenarios that was used in the AR6 WG3 16,64 . The 1202 scenarios that received a climate categorization were labelled from C1 to C8 depending on their 21st century global mean surface temperature (GMST) projections as assessed probabilistically in the MAGICCv7.5.3 reduced-complexity climate model. C1 is formally defined as peak warming remaining under 1.5°C with at least a 33% probability, and 2100 temperature anomaly below 1.5°C with 50% or greater probability. To a good first approximation this is roughly equivalent to the space of scenarios which limit the 50th percentile of peak warming to below 1.6°C (Extended Data Fig. 2 ). We subset the C1 category additionally by analysing scenarios that remain under 1.5°C with greater than 50% probability (“no overshoot”) in Fig. 3 , which was a separate category in the IPCC Special Report on 1.5°C 14 . While the focus of this paper is on the C1 definition, we provide an analysis of the evolution of scenarios over time using different harmonization years in all categories (with the 2023 calibration of FaIR) in Extended Data Fig. 3 . The number of scenarios in the C2 (1.5°C high-overshoot; defined as scenarios that are below 1.5°C in 2100 at the 50th percentile but exceed peak warming of 1.5°C at the 33rd percentile) and C3 (likely below 2°C; defined as 67% or higher probability of remaining below 2°C throughout the 21st century) categorizations increased from 2014 to 2020 before declining after 2020. Nevertheless, there are more scenarios that are “well-below” 2°C (C1‒C3 combined) in 2023 than there were in 2014, showing that the overarching aim of the Paris Agreement is still very much alive. Climate model and calibrations The primary analysis is conducted with v2.2.2 of the Finite-amplitude Impulse Response (FaIR) model 30 , 35 with calibration versions 1.5.0 (for the 2023 update to historical emissions and climate constraints) 65 and 1.5.1 (for the AR6 WG3 historical emissions and AR6 WG1 constraint set) 30 . FaIR is a reduced-complexity climate model that takes emissions of anthropogenic-source GHGs and SLCFs and natural forcings from solar activity and volcanic eruptions as exogenous time series. FaIR generates greenhouse gas concentrations and radiative forcings from the main categories of anthropogenic and natural influences on the climate system 31 , including GHGs, land use change, ozone and aerosols. Global mean surface temperature and ocean heat content change is calculated by driving a three-layer energy balance model with the calculated forcings. FaIR is calibrated using existing CMIP6 models and IPCC AR6 assessments 30 . The model ensemble is constrained using ten observed and IPCC-assessed metrics (Extended Data Table 1) to produce a probabilistic posterior ensemble of 841 members. A two-step constraining process first filters out ensemble members that are not within 0.19°C of the historical GMST measured using a root-mean-squared difference (RMSD), and second performs a distribution fitting to eight assessed or observed climate metrics. A ninth secondary constraint, ensuring that the airborne fraction of CO 2 is higher at successive doublings in a 1pctCO2 run, is imposed to prevent negative carbon-climate feedbacks that are present in some calibrations resulting in questionably low CO 2 concentrations in high emissions scenarios. When calibrating FaIR, we include natural forcings from solar variability and volcanic eruptions, and internal climate variability, in the determination of matching to the historical record. For our recommended assessment using anthropogenic warming only (Figs. 2 c-e, Fig. 4 ), we use the parameter sets calibrated with all forcings, but run simulations with anthropogenic forcings only. We produce an ensemble that is close to the AR6 assessed constraints on historical climate observations, labelled calibration v1.5.1. The fair-1.6.2 used in AR6 WG3 produced substantially cooler projections than fair-2.2.2 used in this study when calibrated on the AR6 WG1 constraints; this calibration produces similar outcomes to MAGICCv7.5.3 in terms of number of scenarios falling into the C1 category from the 2014 harmonization (Figs. 3 a, 3 b). One adjustment is made to the target constraint ranges from AR6 WG1 for aerosol forcing, noting that all emulators found it difficult to reproduce the assessed strength of aerosol effective radiative forcing in AR6 WG1 31 , we use the posterior distribution from fair-1.6.2 in AR6 WG1 in place of the target assessment. Baseline periods for warming assessments For determining climate categorisations such as 1.5°C alignment we follow the AR6 WG3 method of anchoring the recent warming period to a defined temperature level above 1850‒1900. This characterises compliance with 1.5°C in terms of remaining allowable warming headroom rather than accounting for all historical observational uncertainty since 1850 to influence the results, and is a similar methodology to the remaining carbon budget assessments 11 . The AR6 WG3 uses a best estimate of 0.85°C for 1995‒2014 relative to 1850‒1900 as the warming baseline, where this value includes anthropogenic and natural factors. For our recommendation of using the anthropogenic component of warming only to determine climate projections, estimates of the anthropogenic-only warming baselines are also necessary. For generating annual baselines for each emissions harmonization from 2014 to 2024, we use the mean of the last 10 years of anthropogenic warming as calculated using the Global Warming Index (GWI) 66 . These values are shown in Extended Data Table 2. A 2014‒2023 GWI of 1.194°C is used for the 2023 harmonization (Figs. 3 d, 3 e). For the committed warming analysis in Fig. 4 , we draw upon three available methods for assessing the anthropogenic warming contribution for the ten-year period 2014‒2023 that are assessed in IGCC2024, namely the GWI, the Kriging for Climate Change 67 method and the Regularised Optimal Fingerprint method 68 . The combination of methods gives a best estimate anthropogenic warming of 1.194°C for 2014‒2023 relative to 1850‒1900. Peak warming commitment to zero emissions For the analysis in Fig. 4 , following Dvorak et al. (2022) 3 , we set emissions of all species back to their 1750 levels rather than zero, since this is the baseline for effective radiative forcing calculations and some of the forcing relationships, particularly for aerosols, are highly non-linear. Declarations Acknowledgements CSmith was supported by the Horizon Europe research and innovation programs under grant agreement no. 101081661 (WorldTrans) and 101081369 (SPARCCLE), and acknowledges travel support from the Research Council of Norway under the TRIFECTA project (grant agreement number 334811) which also supported the work of BMS and MS. LKG and CM were supported by the Hadley Centre Climate Programme funded by DSIT. CM was supported by OptimESM (grant agreement No 101081193) and TipESM (grant agreement No 101137673) both funded by the European Union under the European Union's Horizon Framework research and innovation programme and by UK Research and Innovation (UKRI) under the UK government’s Horizon Europe funding Guarantee. WT acknowledges funding from the European Research Council (ERC) under the European Union's Horizon Framework research and innovation programme (grant agreement No 101076909; ERC Consolidator Grant ‘LACRIMA’). 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Sci Rep 7 Ribes A, Qasmi S, Gillett NP (2021) Making climate projections conditional on historical observations. Sci Adv 7:eabc0671 Gillett NP (2021) Constraining human contributions to observed warming since the pre-industrial period. Nat Clim Change 11:207–212 Additional Declarations There is NO Competing Interest. Supplementary Files ExtendedDataUpdatedscenariosshow1.5XXCovershootunavoidable191225.docx Updated IPCC-assessed emissions scenarios cannot limit warming to 1.5°C: Supplementary material Cite Share Download PDF Status: Under Review Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8407383","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":571618638,"identity":"25523485-b9a6-4b27-b759-08548abee28e","order_by":0,"name":"Laila 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12:34:11","extension":"png","order_by":11,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":18238,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8407383/v1/c8ae103f0fb57197372ff506.png"},{"id":100588877,"identity":"04c05eb7-fd88-4538-9313-e34ab9cda554","added_by":"auto","created_at":"2026-01-19 12:33:45","extension":"xml","order_by":12,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":135221,"visible":true,"origin":"","legend":"","description":"","filename":"NCOMMS251032850structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8407383/v1/bab9dbcc66845869aacff992.xml"},{"id":100588821,"identity":"19d7fea4-1c64-47a7-b7c4-d3add8362180","added_by":"auto","created_at":"2026-01-19 12:32:23","extension":"html","order_by":13,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":146600,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8407383/v1/03bbc3eb2959983adf56a1e0.html"},{"id":100588840,"identity":"f9e9eea1-22fc-4e0a-b144-0b529b91138e","added_by":"auto","created_at":"2026-01-19 12:32:38","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":125501,"visible":true,"origin":"","legend":"\u003cp\u003e(a) CO\u003csub\u003e2\u003c/sub\u003e and (b) all greenhouse gas emissions (expressed in CO\u003csub\u003e2\u003c/sub\u003e-equivalence with 100-year Global Warming Potentials) from the 97(133) C1(C2) scenarios drawn from AR6 WG3. Dashed black lines show the original AR6 WG3 historical emissions to 2014 (black); solid black lines show the updated historical emissions to 2023 (Methods), including revisions made to pre-2015 historical emissions since publication of AR6. Brown(Purple) lines and plumes show median and minimum-to-maximum ranges of emissions in the original AR6 WG3 C1(C2) scenarios from 2015. Blue and green lines and plumes show the median and minimum-to-maximum ranges of 2023-harmonized emissions scenarios from the C1 and C2 category respectively. Circular markers correspond to AR6 (brown circle) and 2023-harmonized emissions (blue circle) in 2015. In panel a, lines and boxes above the plot show 5‒95% and interquartile range of net zero year respectively, with the white line showing the median.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-8407383/v1/b6fa26e6df291e47f4c6a921.png"},{"id":100588862,"identity":"8df0d866-6571-4f34-b5c6-f07f08152302","added_by":"auto","created_at":"2026-01-19 12:33:06","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":36852,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eEvolution of the lowest cumulative CO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e emissions in the most ambitious AR6 WG3 scenarios\u003c/strong\u003e between 1 January in the year following the emissions harmonization and net zero (red line). The shaded area below the red line shows the space of RCB levels for which no scenario exists. Black crosses show literature assessments\u003csup\u003e11,25,12,26\u003c/sup\u003e of the RCB to limit warming to 1.5°C (with 50% likelihood) assuming central estimates of comparable non-CO\u003csub\u003e2\u003c/sub\u003e emission reductions, with the \u003cem\u003ex\u003c/em\u003e-axis intercept being the date they apply to. The vertical bars show the uncertainty range around the estimates, The observational estimates are: in 2020 (Canadell et al., 2021), 2023 (Lamboll et al., 2023), 2024 (Forster et al., 2024) and 2025 (Lamboll et al. (2023) with recent emissions - upper cross - and Forster et al. (2025) - lower cross\u0026nbsp; For the 2024 update (RCB from 1 January 2025), both an update of Lamboll et al. (2023)\u003csup\u003e11\u003c/sup\u003e taking into account CO\u003csub\u003e2\u003c/sub\u003e emissions in 2023 and 2024, and an updated assessment from Forster et al. (2025)\u003csup\u003e12\u003c/sup\u003e that is similar to the Lamboll et al. (2023) method and also accounts for updates in recent historical warming, show a central RCB that is below the lowest updated scenario. The paler shaded area denotes the space post the emissions harmonization year.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8407383/v1/dc0dd1823e1a51c7913fea42.png"},{"id":100588867,"identity":"97cea45e-dd69-4f4a-8cbf-b9e9c007c8ba","added_by":"auto","created_at":"2026-01-19 12:33:20","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":513239,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDemonstration of evolving emissions scenario projections from AR6 to the present analysis. \u003c/strong\u003e(a) AR6 WG3 projections; (b) AR6 WG3 projections using an equivalent AR6 calibration of fair-2.2.2; (c) defining climate categorization with respect to anthropogenic warming only; (d) updating fair-2.2.2 calibration to climate constraints and recent anthropogenic warming baseline from 2023 historical emissions update\u003csup\u003e33\u003c/sup\u003e ; (e) 2023 harmonization of WG3 emissions scenarios based on recently updated historical emissions. In each panel, ensemble median global mean surface temperature projections are shown for scenarios that satisfy a 1.5°C with low (light blue) or no (dark blue) overshoot criteria. Black lines in each case show the best-estimate historical global mean surface temperature anomaly from the AR6 WG1 (dashed) or the IGCC2024 (solid). The grey bands show the baseline periods used to assess future warming (see Methods).\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8407383/v1/684c1a13cfa93ae258406266.png"},{"id":100588813,"identity":"d6edccf7-808e-48dc-8010-6b1b13e411bb","added_by":"auto","created_at":"2026-01-19 12:32:08","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":57699,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eWarming response to a hypothetical cessation of all anthropogenic emissions at the start of 2024\u003c/strong\u003e using a 2015‒2024 anthropogenic warming baseline.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-8407383/v1/94facc53273db8a018291992.png"},{"id":100589356,"identity":"8ecd522b-1d9d-402e-ae44-dc30d3fd78e6","added_by":"auto","created_at":"2026-01-19 12:38:03","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1169079,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8407383/v1/f0a29f90-62e8-4333-ae89-37919fefcddd.pdf"},{"id":100588861,"identity":"d97d0e5b-a60d-484b-b00e-6c0afdfda6e1","added_by":"auto","created_at":"2026-01-19 12:33:03","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":2245773,"visible":true,"origin":"","legend":"Updated IPCC-assessed emissions scenarios cannot limit warming to 1.5\u0026#x00B0;C: Supplementary material","description":"","filename":"ExtendedDataUpdatedscenariosshow1.5XXCovershootunavoidable191225.docx","url":"https://assets-eu.researchsquare.com/files/rs-8407383/v1/16070593f0d5c4cb4546a994.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Updated scenarios show 1.5°C overshoot is unavoidable but limitable","fulltext":[{"header":"Main","content":"\u003cp\u003eThe Paris Agreement long-term temperature goal\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e (LTTG) includes an ambition to pursue efforts to limit warming to 1.5\u0026deg;C relative to preindustrial levels. However, there is recent debate\u003csup\u003e\u003cspan additionalcitationids=\"CR4 CR5 CR6 CR7 CR8 CR9\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e about whether global warming can still be held to 1.5\u0026deg;C and how this reflects on whether the Paris Agreement\u0026rsquo;s 1.5\u0026deg;C ambition remains within reach. While there has not yet been a definitive conclusion that keeping global warming to 1.5\u0026deg;C is impossible\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e, recent emissions drivers and climate trends make it increasingly difficult to achieve\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan additionalcitationids=\"CR12\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eTo inform political discussions under the United Nations Framework Convention on Climate Change, the 2018 Intergovernmental Panel on Climate Change (IPCC) Special Report on 1.5\u0026deg;C (SR1.5) made a distinction between scenarios that remained under 1.5\u0026deg;C throughout the 21st century at the 50th percentile (\u0026ldquo;no overshoot\u0026rdquo;), and those that included a temporary overshoot of approximately 0.1\u0026deg;C at the 50th percentile (\u0026ldquo;low overshoot\u0026rdquo;, see Extended Data Table\u0026nbsp;3)\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. However, due to the narrow literature base for \u0026ldquo;no overshoot\u0026rdquo; scenarios, insights for policymakers were reported for a combined \u0026ldquo;no or limited overshoot\u0026rdquo; scenario category, combining the two sets\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. More recently, in the 2022 IPCC Sixth Assessment Report (AR6) Working Group III (WG3) contribution, this practice was continued with the scenario category with the lowest warming (C1) containing scenarios with both no and low overshoot of 1.5\u0026deg;C\u003csup\u003e16\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eAR6 WG3 projected the global warming outcome for 1202 emissions scenarios derived from Integrated Assessment Models (IAMs). These scenarios span a broad range of potential climate futures, from futures with deep greenhouse gas emissions reductions to futures where greenhouse gas emissions strongly increase because of further fossil fuel expansion. 97 of the available scenarios were categorised as C1 scenarios, which is defined in the IPCC AR6 WG3 report (IPCC, 2022) as limiting peak warming to 1.5\u0026deg;C with at least a 33% likelihood and end-of-century warming below 1.5\u0026deg;C with at least 50% likelihood, loosely translating to a median peak warming of no more than 1.6\u0026deg;C. A further 133 scenarios are categorised as C2 scenarios. These C2 scenarios are allowed to result in a higher overshoot (defined formally as not meeting the 33% likelihood threshold for remaining under 1.5\u0026deg;C, reaching median peak warming of up to approximately 1.8\u0026deg;C) but with an end-of-century goal still focused on 1.5\u0026deg;C (see Extended Data Table\u0026nbsp;3\u003csup\u003e17\u003c/sup\u003e). Scenarios typically follow historical emissions up to a given year (2015 in AR6 WG3), after which their modelled future starts. With time passing, the branching point of some scenario vintages, including those in AR6 WG3, might already lie well in the past. Particularly if recent real-world trends in energy and emissions are very different from the trends modelled in the scenarios, some aspects of scenarios might not be achievable anymore. As IPCC assessment reports are produced at intervals of 5‒8 years, this is also true for some of the findings in the most recent IPCC AR6 WG3 assessment. Scenarios were derived from literature published between 2016‒2021, branched off from historical emissions that ended in 2015. Despite this early branching point, emissions of CO\u003csub\u003e2\u003c/sub\u003e and GHGs in C1 scenarios are nevertheless expected to be consistent with historical trends until 2020, after which they project rapid emissions reductions. This is because a vetting process as part of the AR6 WG3 assessment pipeline filtered out scenarios in which the emissions of major GHGs and energy generation capacities were not consistent with historical best estimates until 2019\u003csup\u003e18,19\u003c/sup\u003e. This vetted set of scenarios was ultimately assessed using constrained climate emulators from the 2021 AR6 Working Group I (WG1) report\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e that were calibrated with a final observational year of 2019\u003csup\u003e16,19,20\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eIn this analysis, we update the AR6 WG3 scenarios using the latest historical emissions and integrate recent climate trends into our modelling framework (see Methods) to reassess the temperature outcomes for the 1.5\u0026deg;C ambition. Using a similar updating concept, Zhong et al., (2025)\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e highlight the recent divergence of China\u0026rsquo;s emissions from the AR6 published pathways, where they include latest emissions and reharmonize the emissions to SSP2. They develop new scenarios based on SSP2 (initially following SSP2-4.5) called SSP2-com and SSP2-com+, focussing on a 2\u0026deg;C global warming level; these are part of a larger framework that aligns with China Net Zero emissions pathways.\u003c/p\u003e \u003cp\u003eHere, we present four arguments which illustrate that complying with the AR6 C1 definition is not a possibility anymore, implying a commitment to at least a temporary warming in excess of 1.5\u0026deg;C. We show that it is still possible to limit warming to 1.7 or 1.8\u0026deg;C with strong immediate mitigation action. However, this will require focused adaptation as well as mitigation. Returning to 1.5\u0026deg;C will require a strengthened resolve for continued rapid deep emissions cuts to limit the peak warming as low as possible, while a return to lower temperatures thereafter will require negative emissions. Ultimately, a higher mitigation ambition to limit peak warming will reduce the negative emissions burden for returning to 1.5\u0026deg;C, which we argue should continue to be the anchor point for mitigation ambition.\u003c/p\u003e \u003cp\u003eEmissions above AR6 1.5\u0026deg;C scenario range\u003c/p\u003e \u003cp\u003eThe updated scenarios show that the most recent trends and scientific updates now indicate markedly higher risks of 1.5\u0026deg;C overshoot over the 21st century. It is evident that greenhouse gas emissions in 2024 are above the emissions range of the 1.5\u0026deg;C-compatible C1 scenarios and diverge strongly over the most recent years (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Global total CO\u003csub\u003e2\u003c/sub\u003e emissions have increased from 2015 to 2024 by an estimated 4%\u003csup\u003e22\u003c/sup\u003e (solid black lines), whereas the C1 (C2 not shown) scenarios show a change in CO\u003csub\u003e2\u003c/sub\u003e emissions ranging from ‒39 to +\u0026thinsp;1% (-26% to 12%) over the same timeframe (minimum-maximum range, brown plumes, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). Similarly, total CO\u003csub\u003e2\u003c/sub\u003e-equivalent emissions of all greenhouse gases (GHG, aggregated using 100-year Global Warming Potentials from IPCC AR6 WG1\u003csup\u003e18\u003c/sup\u003e) show an estimated increase of 4% (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb) for the 2014\u0026ndash;2024 period while C1 (C2) scenarios show a ‒36 to ‒2% (‒22 to +\u0026thinsp;6%) change.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e also highlights our approach to updating scenarios in light of more recent historical emissions (blue curves). Scenarios are updated to a 2023 starting date with historical emission overriding the scenario-modelled emissions. To ensure a smooth transition from the recent historical emissions to IAM projections the scenarios\u0026rsquo; emissions trajectories are harmonized\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e, and infilled\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e to ensure a complete set of GHG and short-lived climate forcer (SLCF) emissions are provided as input required for the climate assessment (see Methods). Harmonization is necessary to transition from the updated historical emissions (black lines, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) to the emissions pathways of the underlying scenarios which have since diverged from history. Therefore, we consider a modified scenario set relative to AR6 WG3, which we refer to as 2023-harmonized scenarios. This approach allows the study to capture the general characteristics of emission scenarios in context of the most recent real-world trends. To understand the underlying systems transformations, additional modelling experiments would be required..The updating of scenarios to a more recent historical harmonization year means that the net-zero year for CO\u003csub\u003e2\u003c/sub\u003e and GHG emissions changes (bars, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea).\u003c/p\u003e \u003cp\u003eUltimately, the divergence of real-world emissions trends and ambitious mitigation scenarios in the recent past does not imply that the long-term temperature outcomes of these scenarios cannot be achieved any more, but that the near-term peak warming and overshoot is necessarily higher than it would have been following the pathways that diverged from history in 2015.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eRemaining carbon budget overspent\u003c/p\u003e \u003cp\u003eNext we analyse the cumulative emissions implied by the 2023-harmonized scenarios and compare them to assessed estimates of remaining carbon budgets (RCBs) to net zero that would hold global warming under 1.5\u0026deg;C with 50% likelihood\u003csup\u003e24\u003c/sup\u003e. The IPCC AR6 WG1 assessment reported a 500 GtCO\u003csub\u003e2\u003c/sub\u003e RCB value from the start of 2020 onwards for limiting warming to 1.5\u0026deg;C with 50% probability\u003csup\u003e25\u003c/sup\u003e. Most recent updates that consider emissions since 2020 and updates in attributable warming revise the 1.5\u0026deg;C RCB downwards\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e to 130 GtCO\u003csub\u003e2\u003c/sub\u003e from the start of 2025. These central estimates can be larger or smaller by at least 220 GtCO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e25\u003c/sup\u003e, depending on how successful or unsuccessful the world is in reducing non-CO\u003csub\u003e2\u003c/sub\u003e emissions, meaning that the most up-to-date range of RCBs compatible with limiting warming to 1.5\u0026deg;C with 50% likelihood lies between ‒90 and +\u0026thinsp;350 GtCO\u003csub\u003e2\u003c/sub\u003e starting from 2025.\u003c/p\u003e \u003cp\u003eIn 2023-harmonized C1 scenarios, the lowest cumulative CO\u003csub\u003e2\u003c/sub\u003e emissions between the start of 2025 and net zero CO\u003csub\u003e2\u003c/sub\u003e is 177 GtCO\u003csub\u003e2\u003c/sub\u003e, implying that the central estimate of the 50% likelihood 1.5\u0026deg;C RCB will be exceeded even with maximally ambitious emissions reductions. While theoretically falling within the uncertainty range of 1.5\u0026deg;C budgets if non-CO\u003csub\u003e2\u003c/sub\u003e mitigation is maximized, the cumulative emission value of 177 GtCO\u003csub\u003e2\u003c/sub\u003e exceeds the central estimate of the remaining carbon budget. Limited progress on non-CO\u003csub\u003e2\u003c/sub\u003e mitigation also makes that option less likely\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. By applying reharmonization to updated historical emissions annually for each year from 2014 to 2023 (see Methods), we find that 2024 was the first year in which no re-harmonized scenario remained within the central estimate of a 50%-likelihood 1.5\u0026deg;C RCB (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). This implies that, bar exceptionally strong reductions in non-CO\u003csub\u003e2\u003c/sub\u003e emissions, all updated scenarios would have a greater than 50% probability of at least temporarily exceeding 1.5\u0026deg;C.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe RCB analysis is not unlike the \u0026ldquo;infrastructure commitment\u0026rdquo; argument that existing fossil fuel generating capacity will emit more over their remaining lifetimes than the central 1.5\u0026deg;C RCB for a 50% likelihood permits\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e. However, the RCB line of argument is stronger than the infrastructure commitment, as high ambition IAM scenarios have the flexibility to retire infrastructure before the originally planned end of their lifetime\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e, and can bring carbon dioxide removal online before positive emissions from fossil infrastructure are phased out.\u003c/p\u003e \u003cp\u003eCentral climate projections exceed 1.5\u0026deg;C\u003c/p\u003e \u003cp\u003eThe assessment of global warming outcomes of scenarios uses well-calibrated and flexible simple climate models\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e,\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e. The AR6 WG3 climate assessment was calibrated to the climate system evidence from WG1\u003csup\u003e18,31,32\u003c/sup\u003e, which extended to a final year of 2020 or earlier. Since 2020, increasingly more observations are available to constrain projections. Here we use climate system observations up until 2023 from the CMIP7 historical emissions dataset\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e to constrain updated climate projections from the reharmonized C1 scenarios (see Methods). This update shows an increased risk of exceeding 1.5\u0026deg;C over the course of the century.\u003c/p\u003e \u003cp\u003eUsing the MAGICCv7.5.3 climate emulator, AR6 WG3 determined that 97 scenarios available in the IPCC AR6 scenario database were in the C1 category (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). Using a different climate emulator, FaIR v2.2.2\u003csup\u003e34,35\u003c/sup\u003e, with a similar calibration\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e to AR6 WG3 (Methods) and the same emissions scenarios, we find similar results; 107 scenarios fall in the C1 category, of which 14 do not overshoot 1.5\u0026deg;C in the ensemble median (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). The AR6 WG3 assessment included solar and volcanic forcings in their scenario classification. However, we argue here that including natural forcings to assess future warming projections is not in the spirit of determining progress towards the Paris Agreement long-term temperature goal\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eFirstly, the natural 11-year solar cycle is clearly visible in future temperature projections, creating oscillations around an anthropogenically-forced smooth transition in climate projections (Extended Data Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). This would allow scenarios that were close to the threshold to potentially exceed or remain under a 1.5\u0026deg;C threshold based on whether their peak warming from mitigation efforts happened to coincide with a hypothetical solar minimum or maximum.\u003c/p\u003e \u003cp\u003eSecondly, the method of implementing volcanic forcing post-2014 in CMIP6 Earth System Models\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e, where the historical (1850‒2014) mean volcanic forcing is defined as the zero reference point, also impacts the assessment of future warming. The volcanically quiescent years in the end of the historical time period have a small positive forcing relative to this historical zero-mean. In other words, as the end of the historical period had few volcanoes, there is an artificial offset in effective radiative forcing in the AR6 WG3 simulations of around ‒0.15 W m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e between 2015 and 2025 (Extended Data Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), meaning future projections are artificially slightly cooler than they otherwise would be. Owing to this, if natural forcings are removed from historical and future projections following the AR6 WG3 calibration and 2014 harmonization, fewer scenarios achieve C1 (75) and 1.5\u0026deg;C-no-overshoot (14) categorizations (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec) when assessed with FaIR. Other studies\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e,\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e have shown that including volcanoes in future projections increases the likelihood of remaining under 1.5\u0026deg;C, though in a similar vein to betting on the solar cycle, the serendipitous timing of a cooling volcanic eruption should not be used to claim credit for achieving mitigation goals.\u003c/p\u003e \u003cp\u003eWhen natural forcing terms are excluded and observational data up to 2023 is used to constrain FaIR, we find that even for emissions scenarios starting in 2015, no scenario avoids overshoot of 1.5\u0026deg;C, only 6 have a low overshoot of less than 0.1\u0026deg;C, and there are 20 scenarios that less than 0.3\u0026deg;C overshoot (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed). Finally, when updating the climate constraints together with a 2024 harmonization of historical emissions, the median global warming projections of all of the available scenarios in this analysis exceed 1.5\u0026deg;C but there are still 24 scenarios that peak median warming between 1.6 and 1.8\u0026deg;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eApplying the reharmonization to historical emissions with an end point each year from 2014 to 2024, and using the 2023 calibration of FaIR (as in Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed and \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee), we can track the changes in the number of 2023-harmonized scenarios that exceed 1.5\u0026deg;C (Extended Data Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Our analysis shows that while there is a marked decline in achievability of 1.5\u0026deg;C with low- or no-overshoot, it is still possible that peak warming could remain under 1.7\u0026deg;C or 1.8\u0026deg;C with post-2020 emission trends. Our analysis also shows that as of 2024, no updated AR6 WG3 scenario limits peak warming to 1.6\u0026deg;C with 50% probability, and that while the window for 1.7\u0026deg;C is rapidly closing options to stay below it still remain. The counterintuitive result of an increase in scenarios limiting warming to specific thresholds between 2015 and 2020 (Extended Data Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) is a result of the emissions vetting process, in which historical and scenario emissions were approximately in line between 2015 and 2020. From 2020 onwards, every year delay in deep emissions cuts makes it more difficult to limit warming to defined thresholds.\u003c/p\u003e \u003cp\u003eThe warming response to abrupt zeroing of all emissions\u003c/p\u003e \u003cp\u003eThe scenarios assessed in AR6 WG3 represent a useful ensemble of opportunity, but the physical challenge of limiting warming can be framed independently of their trajectories. Another perspective on the physical challenge of limiting warming is to explore the climate system response to a hypothetical abrupt zeroing of all anthropogenic emissions\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. Although extremely stylized and unrealistic, such exercises offer insight into how the Earth system responds to changes in forcing from the zeroing of emissions. Earlier studies showed how an abrupt zeroing of all emissions can lead to a warming peak within a decade, followed by slow cooling as GHG concentrations decline, and that peak warming is limited to the lowest level when emissions are abruptly zeroed compared to slower transitions towards zero emission\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. We find that a positive committed peak warming to zeroing all emissions is likely, since in the near term the balance and timescale of reductions in short-lived aerosol forcing (net negative) acts faster than the slower decline in greenhouse gas concentrations (net positive)\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e. However, it is possible that some other pathway to net zero for all anthropogenic emissions could result in lower peak temperature or even no further warming, so this \u0026lsquo;immediate phase out of all emissions\u0026rsquo; sensitivity experiment does not necessarily represent an absolute lower bound for committed warming rather a conservative estimate which attempts to quantify the sensitivity of the system.\u003c/p\u003e \u003cp\u003eNote that this concept differs from the more common definition of the \u0026ldquo;zero emissions commitment\u0026rdquo; (ZEC), which considers zeroing CO\u003csub\u003e2\u003c/sub\u003e emissions in isolation\u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e. A recent ZEC assessment highlights the considerable uncertainty surrounding ZEC due to missing processes in ESMs\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e. In addition, recent model experiments to quantify ZEC have focused on CO\u003csub\u003e2\u003c/sub\u003e because no pathways exist that show a complete elimination of CH\u003csub\u003e4\u003c/sub\u003e, N\u003csub\u003e2\u003c/sub\u003eO or aerosol emissions. A wide range of processes that could affect ZEC, for example, zeroing emissions in CO\u003csub\u003e2\u003c/sub\u003e is expected to lead to a drop in atmospheric CO\u003csub\u003e2\u003c/sub\u003e as well as a decline in ocean heat uptake have been identified\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e; these will act in the opposite direction in terms of their effect on temperatures, but there are many other processes not yet represented in the ESMs that could also affect this energy balance\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e. The ZECMIP\u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e experiments show that the ZEC in ESMs is affected by the cumulative emissions giving a range from \u0026minus;\u0026thinsp;0.36 to +\u0026thinsp;0.29\u0026deg;C for 1000PgC and \u0026minus;\u0026thinsp;0.40 to +\u0026thinsp;0.52\u0026deg;C for 2000 PgC. The picture at regional scales after the zeroing of emissions is varied, with the patterns of change far more complex and little consistency between different models. The sign of temperature change (warming or cooling) also varies by model and climatic zone\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eUsing the 2023 calibration and harmonization of FaIR, an abrupt zeroing of emissions from 1 January 2024 results in additional peak warming of 0.24\u0026deg;C in the ensemble median and 0.16\u0026deg;C at the 33rd percentile. Using a definition of human-induced warming over the 2015‒2024 period of 1.22\u0026deg;C as the baseline as in IGCC 2024, this results in a peak commitment that exceeds 1.5\u0026deg;C in the median but only narrowly remains under 1.5\u0026deg;C at the 33rd percentile (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). In other words, this hypothetical zero-emissions scenario would be classed as a low-overshoot scenario in the C1 category using the AR6 WG3 definition. Any internally consistent mitigation pathway that reaches zero emissions slower than instantaneously, which includes the scenarios with realistic socio-technological transitions, would reach higher warming levels and therefore be committed to a larger overshoot regardless of definition.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eUsing a variety of methods, we show that the likelihood of limiting peak warming below 1.5\u0026deg;C is strongly decreasing when AR6-generation scenarios are updated to be consistent with recent emissions and climate history. With this decrease in likelihood for limiting warming to 1.5\u0026deg;C comes a corresponding increase in the risk of temporary or permanent overshoot. Already, the literature is starting to shift focus to 2\u0026deg;C\u003csup\u003e21\u003c/sup\u003e. We show in this analysis that in keeping the focus on the long-term temperature target of 1.5\u0026deg;C, peak warming of well below 2\u0026deg;C is still in reach provided we keep firmly to strong and immediate mitigation of emissions. We show that delays in mitigation action have reduced our ability to limit warming to 1.5\u0026deg;C, but further delays will reduce our ability to limit warming to 1.6, 1.7 or 1.8\u0026deg;C. Deep sustained emissions cuts are needed to reduce the negative emissions burden, avoid additional and committed warming and limit the size and duration of overshooting 1.5\u0026deg;C.\u003c/p\u003e \u003cp\u003eThe Paris Agreement 1.5\u0026deg;C goal of pursuing efforts to limit warming below 1.5\u0026deg;C remains central and critical\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e. It is imperative that countries maintain and ratchet up their ambition to set Nationally Determined Contributions under the Paris Agreement that are aligned with this goal. Insufficient emission reductions between 2015 and 2024 have compounded the difficulty of limiting warming to 1.5\u0026deg;C. Indeed, the likelihood of limiting peak warming to 1.5\u0026deg;C is strongly reduced and limiting warming to 1.5\u0026deg;C in the long term now requires a period of globally net negative emissions to reverse the global temperature increase\u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e,\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e. This new reality raises issues of equity around the responsibility for carbon dioxide removal and warming drawdown\u003csup\u003e\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e. Limiting peak warming to as low a level as possible is still critical for avoiding escalating climate impacts\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e and triggering tipping elements in the Earth system\u003csup\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e. The number of scenarios that can return global warming to 1.5\u0026deg;C by the end of the century after a maximum medium warming between 1.6\u0026deg;C and 1.8\u0026deg;C (such as those in the C2 and C3 AR6 WG3 scenarios, which also limit warming to 2\u0026deg;C with 66% or greater likelihood) has not markedly declined compared to 2015 (Extended Data Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). There are other non-AR6 based scenarios, such as the Network for Greening the Financial Sector (NGFS) that produce updated IAM scenarios on an annual basis that account for technological developments but still use historical emissions up to 2014 for harmonisation and climate assessments based on 2019 information\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e. We carry out the same process to the AR6 scenarios, reharmonizing these scenarios to 2023, and we show these NGFS scenarios in the broader context of the AR6 scenarios in the Extended data Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e showing the distribution of peak warming against 2030 emissions. These 2023-harmonized NGFS and AR6 scenarios are still within the distribution suggesting that scenario development in terms of mitigation has not moved on significantly and therefore the reharmonized 2023 NGFS and AR6 scenarios are still relevant. It is likely that AR7 will assess scenarios that take into account the latest understanding, knowledge/assumptions on costs, learning rates and concentration constraints\u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e. Our analysis shows that the door is not closed on temperatures below 2\u0026deg;C, and settling for 2\u0026deg;C\u003csup\u003e21\u003c/sup\u003e as a long term global warming level is unwarranted and unnecessary. Every additional tenth of a degree of warming increases risks of both high impact globally important tipping points\u003csup\u003e\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e and more local impacts with potentially high adaptation costs.\u003c/p\u003e \u003cp\u003eOur findings have implications for future scenario development. They suggest that while the window to avoid overshoot of 1.5\u0026deg;C has closed, pathways that limit the peak warming to 1.6 or 1.7\u0026deg;C are still available with global cooperation on rapid deep emissions reductions. A focus on limiting 1.5\u0026deg;C exceedance in magnitude and duration remains essential to limit future climate harms.\u003c/p\u003e "},{"header":"Methods","content":" \u003cp\u003eIPCC AR6 WG3 emissions scenarios\u003c/p\u003e \u003cp\u003eWe start with the 1202 emissions scenarios from IAMs that received a climate categorization in the IPCC AR6 WG3\u003csup\u003e16,50\u003c/sup\u003e. These scenarios are the subset of the 3131 submitted scenarios, submitted through an open call to the community, that passed basic vetting\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. The vetting checks were (1) scenarios contained at least emissions of CO\u003csub\u003e2\u003c/sub\u003e from fossil fuels and industrial processes (FFI), CO\u003csub\u003e2\u003c/sub\u003e from agriculture, forestry and other land use (AFOLU), CH\u003csub\u003e4\u003c/sub\u003e, and N\u003csub\u003e2\u003c/sub\u003eO from 2015 to 2100; (2) the emissions of total CO\u003csub\u003e2\u003c/sub\u003e, CO\u003csub\u003e2\u003c/sub\u003e FFI and CH\u003csub\u003e4\u003c/sub\u003e were close to inventory best estimates in 2019; and (3) energy sector variables for primary energy, carbon capture and storage, nuclear, and solar and wind were within reported values from industry bodies such as the International Energy Agency for 2019 or 2020.\u003c/p\u003e \u003cp\u003eFirstly, we re-run the 1202 scenarios in IPCC AR6 WG3 which branch from historical emissions in 2015 and were harmonized and infilled (described below) as part of the IPCC AR6 WG3 process. The historical emissions from 1750‒2014 were the global annual total emissions dataset prepared for the Reduced Complexity Model Intercomparison Project (RCMIP)\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e,\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e, covering 43 GHGs and seven SLCFs. For CO\u003csub\u003e2\u003c/sub\u003e FFI and the SLCFs (BC, OC, SO\u003csub\u003e2\u003c/sub\u003e, NH\u003csub\u003e3\u003c/sub\u003e, NOx, non-methane volatile organic compounds (NMVOCs) and CO), the historical emissions were the same as supplied to CMIP6 Earth System Models (ESMs). As ESMs in CMIP6 were not designed to (and generally not capable of) running with emissions of greenhouse gases other than CO\u003csub\u003e2\u003c/sub\u003e, emissions datasets for non-CO\u003csub\u003e2\u003c/sub\u003e GHGs (and CO\u003csub\u003e2\u003c/sub\u003e AFOLU, which in carbon-cycle resolving models is a diagnosed flux) were not prepared for CMIP6 and were derived from a number of sources including PRIMAP-Hist\u003csup\u003e\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e, the WMO Scientific Assessment of Ozone Depletion\u003csup\u003e\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e, Community Emissions Data System (CEDS)\u003csup\u003e\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e\u003c/sup\u003e, Global Carbon Budget\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e, biomass burning emissions prepared for CMIP\u003csup\u003e\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e, and inversion of historically observed concentrations\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eUpdated historical emissions\u003c/p\u003e \u003cp\u003eAs part of the CMIP7 process\u003csup\u003e\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e\u003c/sup\u003e, a new historical emissions dataset for 1750‒2023 has been prepared\u003csup\u003e\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e\u003c/sup\u003e, covering the same 43 GHGs and 7 SLCFs.\u003c/p\u003e \u003cp\u003eFossil and industrial emissions of CO\u003csub\u003e2\u003c/sub\u003e, CH\u003csub\u003e4\u003c/sub\u003e and N\u003csub\u003e2\u003c/sub\u003eO and the seven SLCFs are taken from the Community Emissions Data System (CEDS) and are complete for 1750 to 2023 using the v2025_03_18 of CEDS\u003csup\u003e\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e\u003c/sup\u003e, except for CH\u003csub\u003e4\u003c/sub\u003e and N\u003csub\u003e2\u003c/sub\u003eO which are complete from 1970 and backward extensions are performed to 1750 using the third-party emissions inventories from PRIMAP-Hist (PRIMAP-Hist-TP)\u003csup\u003e\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e. Biomass burning emissions for the SLCFs, CH\u003csub\u003e4\u003c/sub\u003e and N\u003csub\u003e2\u003c/sub\u003eO are used from an update to the BB4CMIP dataset prepared for CMIP7. Noting substantial year-to-year variability in biomass burning emissions that has shown to be problematic for ESMs\u003csup\u003e\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e\u003c/sup\u003e, a five-year smoothing filter is applied to biomass burning emissions. This also avoids spurious harmonization artefacts by initialising future projections from years in which biomass burning emissions were unusually high and likely a result of climate influences rather than anthropogenic activity, resulting in stronger than expected aerosol forcing\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. AFOLU emissions for CO\u003csub\u003e2\u003c/sub\u003e are taken from the Global Carbon Budget (1850‒2023), with the period 1750‒1850 filled in using an estimate of 30 GtC cumulative emissions over this period\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e that is modelled using an exponential ramp up from 1750 to 1850. Following previous precedents, we do not explicitly consider CO\u003csub\u003e2\u003c/sub\u003e biomass burning emissions which are considered to be part of the steady-state carbon cycle over decadal timeframes\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eMinor GHG emissions are from a number of sources. These are Adam et al.\u003csup\u003e59\u003c/sup\u003e for HFC-23, Velders et al.\u003csup\u003e60\u003c/sup\u003e for HFC-125, HFC-134a, HFC-143a, HFC-152a, HFC-227ea, HFC-236fa, HFC-245fa, HFC-32, HFC-365mfc, HFC-4310mee, WMO Scientific Assessment of Ozone Depletion 2022 (WMO2022)\u003csup\u003e\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e for CF\u003csub\u003e4\u003c/sub\u003e, C\u003csub\u003e2\u003c/sub\u003eF\u003csub\u003e6\u003c/sub\u003e, SF\u003csub\u003e6\u003c/sub\u003e, CCl\u003csub\u003e4\u003c/sub\u003e, CFC-11, CFC-12, CFC-113, CFC-114, CFC-115, CH3CCl3, HCFC-141b, HCFC-142b, HCFC-22, Halon-1202, Halon-1211, Halon-1301 and Halon-2402, and the inversion of observed concentration time series prepared for CMIP7 for C\u003csub\u003e3\u003c/sub\u003eF\u003csub\u003e8\u003c/sub\u003e, C\u003csub\u003e4\u003c/sub\u003eF\u003csub\u003e10\u003c/sub\u003e, C\u003csub\u003e5\u003c/sub\u003eF\u003csub\u003e12\u003c/sub\u003e, C\u003csub\u003e6\u003c/sub\u003eF\u003csub\u003e14\u003c/sub\u003e, C\u003csub\u003e7\u003c/sub\u003eF\u003csub\u003e16\u003c/sub\u003e, C\u003csub\u003e8\u003c/sub\u003eF\u003csub\u003e18\u003c/sub\u003e, CH\u003csub\u003e2\u003c/sub\u003eCCl\u003csub\u003e2\u003c/sub\u003e, CH\u003csub\u003e3\u003c/sub\u003eBr, CH\u003csub\u003e3\u003c/sub\u003eCl, CHCl\u003csub\u003e3\u003c/sub\u003e, NF\u003csub\u003e3\u003c/sub\u003e, SO\u003csub\u003e2\u003c/sub\u003eF\u003csub\u003e2\u003c/sub\u003e and c-C\u003csub\u003e4\u003c/sub\u003eF\u003csub\u003e8\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003eData is complete in the CMIP7 historical dataset until 2023 for all emissions species and to 2024 for GHGs in the Velders et al. and WMO2022 datasets. For other species, we perform extrapolations to estimate emissions for 2024. CO\u003csub\u003e2\u003c/sub\u003e FFI emissions in 2024 are 0.8% higher than 2023 using preliminary estimates from the Global Carbon Budget (GCB)\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e, which we adopt by increasing the 2023 emissions by 0.8% for 2024. CO\u003csub\u003e2\u003c/sub\u003e AFOLU emissions are estimated to be 4.2 GtCO\u003csub\u003e2\u003c/sub\u003e in GCB, and this value is also used directly. For other species, which includes CH\u003csub\u003e4\u003c/sub\u003e, N\u003csub\u003e2\u003c/sub\u003eO and SCLFs, an extrapolation of emissions is performed using a continuation of the 2022 to 2023 trend into 2024.\u003c/p\u003e \u003cp\u003eHarmonization\u003c/p\u003e \u003cp\u003eAlthough vetted to ensure they are within range, the raw emissions projections from the 1202 IAM scenarios are model outputs and do not correspond exactly to historical emissions in 2014. Therefore, in AR6 WG3, a harmonization\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e\u003c/sup\u003e process was undertaken to ensure a smooth transition from historical best estimate emissions in 2014 to the IAM-derived emissions projections for the 21st century. The harmonization process adjusts the raw IAM emissions time series up or down to align them with the historical best estimate dataset in 2014. This also provides a common branching-off point for all IAM emissions scenarios.\u003c/p\u003e \u003cp\u003eFor this study we also harmonize the 1202 IAM scenarios to the updated and extended CMIP7 historical emissions ending in 2023 using the \u003cem\u003eaneris\u003c/em\u003e package, v0.3.1. Aneris was developed to aid in the scenario assessment in the AR6 WG3\u003csup\u003e61\u003c/sup\u003e. Aneris allows emissions pathways (primarily from IAM scenarios) to be harmonized or align with historical emissions of greenhouse gases and short lived climate forcers. The software package consists of established methods of harmonization including ratio based methods and offset based methods, The software is designed so that it maintains the sign of the original emissions and avoids distortions. The harmonization method we use is the same for each species as it was in the AR6 WG3 process: \u0026ldquo;reduce_ratio_2150_cov\u0026rdquo; for C\u003csub\u003e2\u003c/sub\u003eF\u003csub\u003e6\u003c/sub\u003e, C\u003csub\u003e6\u003c/sub\u003eF\u003csub\u003e14\u003c/sub\u003e, CF\u003csub\u003e4\u003c/sub\u003e, CO, CO\u003csub\u003e2\u003c/sub\u003e AFOLU, OC and VOC, \u0026ldquo;reduce_ratio_2080\u0026rdquo; for total CO\u003csub\u003e2\u003c/sub\u003e and CO\u003csub\u003e2\u003c/sub\u003e FFI, \u0026ldquo;constant_ratio\u0026rdquo; for HFC-125, HFC-134a, HFC-143a, HFC-227ea, HFC-23, HFC-32, HFC-4310mee and SF\u003csub\u003e6\u003c/sub\u003e, and CH\u003csub\u003e4\u003c/sub\u003e, N,O, NH\u003csub\u003e3\u003c/sub\u003e, NOx and SO\u003csub\u003e2\u003c/sub\u003e harmonized according to the decision tree in Gidden et al.\u003csup\u003e61\u003c/sup\u003e. The \u0026ldquo;reduce_ratio\u0026rdquo; method starts by scaling model emissions to align with historical values, then gradually reduces this adjustment until it disappears by the specified convergence year. The suffix \u0026ldquo;_cov\u0026rdquo; indicates that a convergence function controls how quickly the adjustment fades over time. If this suffix is absent, the reduction follows a simpler pattern, such as a fixed or linear decrease. While the 2023 harmonization is the focus of this study, we also harmonize the IAM scenarios to the updated CMIP7 historical emissions for each year from 2014 to 2023 for analysis in Extended Data Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, giving 10 sets of future emissions projections with different starting years.\u003c/p\u003e \u003cp\u003eInfilling\u003c/p\u003e \u003cp\u003eMost IAM scenarios do not provide a complete set of the 43 GHG and 7 SLCF emissions species required to perform a full climate assessment. Therefore, an infilling\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e process is undertaken, where emissions of missing species are estimated based on patterns of how these species change over time with a lead species (in this case CO\u003csub\u003e2\u003c/sub\u003e FFI) in a subset of the IAM scenarios that do include the required minor emissions species. The logic of infilling minor species this way is that many substances are co-emitted, or share similar sectoral origins, to fossil fuel and industrial CO\u003csub\u003e2\u003c/sub\u003e emissions and therefore are likely to change in step\u003csup\u003e\u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e\u003c/sup\u003e. Including a temporal dimension to the relationship allows legislative developments (e.g. around air pollution control) and efficiency improvements to be incorporated in the infilling relationship.\u003c/p\u003e \u003cp\u003eWe use two infilling databases: one from AR6 WG3\u003csup\u003e63\u003c/sup\u003e which includes SLCFs, CH\u003csub\u003e4\u003c/sub\u003e, N\u003csub\u003e2\u003c/sub\u003eO, the majority of hydrofluorocarbons (HFCs) and some fluorinated (F-)gases for multiple scenarios, and RCMIP\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e which includes all required gases including the less commonly emitted HFCs and PFCs as well as the Montreal gases for eight SSP-RCP combinations. For the updated historical emissions from CMIP7, we first harmonized the infilling databases to historical emissions ending each year from 2014 to 2023. This is necessary to ensure a consistent baseline for infilling. Thus, we create an additional 11 versions of the infilling databases that we use for each of the 2014‒2023 harmonization years with the CMIP7 historical emissions.\u003c/p\u003e \u003cp\u003eInfilling is performed using the silicone\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e package v1.3.0, using the same decision methodology as in the AR6 WG3. We use the Quantile Rolling Windows method to infill missing SLCFs, CO\u003csub\u003e2\u003c/sub\u003e AFOLU, CH\u003csub\u003e4\u003c/sub\u003e and N\u003csub\u003e2\u003c/sub\u003eO based on their co-evolution with CO\u003csub\u003e2\u003c/sub\u003e FFI in the reharmonized AR6 WG3 infilling dataset. HFCs and F-gases with broad representation across AR6 scenarios (HFC-125, HFC-134a, HFC-143a, HFC-227ea, HFC-23, HFC-32, HFC-4310mee, C\u003csub\u003e2\u003c/sub\u003eF\u003csub\u003e6\u003c/sub\u003e, C\u003csub\u003e6\u003c/sub\u003eF\u003csub\u003e14\u003c/sub\u003e, CF\u003csub\u003e4\u003c/sub\u003e, SF\u003csub\u003e6\u003c/sub\u003e) are also infilled using this dataset using the scenario with the root-mean-squared closest difference. All of the GHGs are infilled to the closest scenario of the CMIP6 SSP trajectories. Infilling is only performed for missing species and does not override harmonized emissions in scenarios where they exist.\u003c/p\u003e \u003cp\u003eHarmonization and infilling implicitly assumes that the technological and socioeconomic assumptions underpinning the original IAM scenarios are still valid, since all updates are performed in emissions space. Efforts such as the Network for Greening the Financial Sector (NGFS) and the Scenario Compass Initiative produce updated IAM scenarios on an annual basis that account for technological developments, but draw from a smaller literature\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e. We show that the latest NGFS scenarios (Phase V) do not assume more stringent mitigation to 2030 than the AR6 WG3 scenarios (Extended Data Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), so are not likely to remain under 1.5\u0026deg;C with a 2023 harmonization and climate assessment.\u003c/p\u003e \u003cp\u003eClimate categorization\u003c/p\u003e \u003cp\u003eWe follow the same categorization for scenarios that was used in the AR6 WG3\u003csup\u003e16,64\u003c/sup\u003e. The 1202 scenarios that received a climate categorization were labelled from C1 to C8 depending on their 21st century global mean surface temperature (GMST) projections as assessed probabilistically in the MAGICCv7.5.3 reduced-complexity climate model. C1 is formally defined as peak warming remaining under 1.5\u0026deg;C with at least a 33% probability, and 2100 temperature anomaly below 1.5\u0026deg;C with 50% or greater probability. To a good first approximation this is roughly equivalent to the space of scenarios which limit the 50th percentile of peak warming to below 1.6\u0026deg;C (Extended Data Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). We subset the C1 category additionally by analysing scenarios that remain under 1.5\u0026deg;C with greater than 50% probability (\u0026ldquo;no overshoot\u0026rdquo;) in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, which was a separate category in the IPCC Special Report on 1.5\u0026deg;C\u003csup\u003e14\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eWhile the focus of this paper is on the C1 definition, we provide an analysis of the evolution of scenarios over time using different harmonization years in all categories (with the 2023 calibration of FaIR) in Extended Data Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The number of scenarios in the C2 (1.5\u0026deg;C high-overshoot; defined as scenarios that are below 1.5\u0026deg;C in 2100 at the 50th percentile but exceed peak warming of 1.5\u0026deg;C at the 33rd percentile) and C3 (likely below 2\u0026deg;C; defined as 67% or higher probability of remaining below 2\u0026deg;C throughout the 21st century) categorizations increased from 2014 to 2020 before declining after 2020. Nevertheless, there are more scenarios that are \u0026ldquo;well-below\u0026rdquo; 2\u0026deg;C (C1‒C3 combined) in 2023 than there were in 2014, showing that the overarching aim of the Paris Agreement is still very much alive.\u003c/p\u003e \u003cp\u003eClimate model and calibrations\u003c/p\u003e \u003cp\u003eThe primary analysis is conducted with v2.2.2 of the Finite-amplitude Impulse Response (FaIR) model\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e,\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e with calibration versions 1.5.0 (for the 2023 update to historical emissions and climate constraints)\u003csup\u003e\u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e65\u003c/span\u003e\u003c/sup\u003e and 1.5.1 (for the AR6 WG3 historical emissions and AR6 WG1 constraint set)\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e. FaIR is a reduced-complexity climate model that takes emissions of anthropogenic-source GHGs and SLCFs and natural forcings from solar activity and volcanic eruptions as exogenous time series. FaIR generates greenhouse gas concentrations and radiative forcings from the main categories of anthropogenic and natural influences on the climate system\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e, including GHGs, land use change, ozone and aerosols. Global mean surface temperature and ocean heat content change is calculated by driving a three-layer energy balance model with the calculated forcings.\u003c/p\u003e \u003cp\u003eFaIR is calibrated using existing CMIP6 models and IPCC AR6 assessments\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e. The model ensemble is constrained using ten observed and IPCC-assessed metrics (Extended Data Table\u0026nbsp;1) to produce a probabilistic posterior ensemble of 841 members. A two-step constraining process first filters out ensemble members that are not within 0.19\u0026deg;C of the historical GMST measured using a root-mean-squared difference (RMSD), and second performs a distribution fitting to eight assessed or observed climate metrics. A ninth secondary constraint, ensuring that the airborne fraction of CO\u003csub\u003e2\u003c/sub\u003e is higher at successive doublings in a \u003cem\u003e1pctCO2\u003c/em\u003e run, is imposed to prevent negative carbon-climate feedbacks that are present in some calibrations resulting in questionably low CO\u003csub\u003e2\u003c/sub\u003e concentrations in high emissions scenarios. When calibrating FaIR, we include natural forcings from solar variability and volcanic eruptions, and internal climate variability, in the determination of matching to the historical record. For our recommended assessment using anthropogenic warming only (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec-e, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), we use the parameter sets calibrated with all forcings, but run simulations with anthropogenic forcings only.\u003c/p\u003e \u003cp\u003eWe produce an ensemble that is close to the AR6 assessed constraints on historical climate observations, labelled calibration v1.5.1. The fair-1.6.2 used in AR6 WG3 produced substantially cooler projections than fair-2.2.2 used in this study when calibrated on the AR6 WG1 constraints; this calibration produces similar outcomes to MAGICCv7.5.3 in terms of number of scenarios falling into the C1 category from the 2014 harmonization (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea, \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). One adjustment is made to the target constraint ranges from AR6 WG1 for aerosol forcing, noting that all emulators found it difficult to reproduce the assessed strength of aerosol effective radiative forcing in AR6 WG1\u003csup\u003e31\u003c/sup\u003e, we use the posterior distribution from fair-1.6.2 in AR6 WG1 in place of the target assessment.\u003c/p\u003e \u003cp\u003eBaseline periods for warming assessments\u003c/p\u003e \u003cp\u003eFor determining climate categorisations such as 1.5\u0026deg;C alignment we follow the AR6 WG3 method of anchoring the recent warming period to a defined temperature level above 1850‒1900. This characterises compliance with 1.5\u0026deg;C in terms of remaining allowable warming headroom rather than accounting for all historical observational uncertainty since 1850 to influence the results, and is a similar methodology to the remaining carbon budget assessments\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe AR6 WG3 uses a best estimate of 0.85\u0026deg;C for 1995‒2014 relative to 1850‒1900 as the warming baseline, where this value includes anthropogenic and natural factors. For our recommendation of using the anthropogenic component of warming only to determine climate projections, estimates of the anthropogenic-only warming baselines are also necessary. For generating annual baselines for each emissions harmonization from 2014 to 2024, we use the mean of the last 10 years of anthropogenic warming as calculated using the Global Warming Index (GWI)\u003csup\u003e\u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e66\u003c/span\u003e\u003c/sup\u003e. These values are shown in Extended Data Table\u0026nbsp;2. A 2014‒2023 GWI of 1.194\u0026deg;C is used for the 2023 harmonization (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed, \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee).\u003c/p\u003e \u003cp\u003eFor the committed warming analysis in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, we draw upon three available methods for assessing the anthropogenic warming contribution for the ten-year period 2014‒2023 that are assessed in IGCC2024, namely the GWI, the Kriging for Climate Change\u003csup\u003e\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e67\u003c/span\u003e\u003c/sup\u003e method and the Regularised Optimal Fingerprint method\u003csup\u003e\u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e68\u003c/span\u003e\u003c/sup\u003e. The combination of methods gives a best estimate anthropogenic warming of 1.194\u0026deg;C for 2014‒2023 relative to 1850‒1900.\u003c/p\u003e \u003cp\u003ePeak warming commitment to zero emissions\u003c/p\u003e \u003cp\u003eFor the analysis in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, following Dvorak et al. (2022)\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e, we set emissions of all species back to their 1750 levels rather than zero, since this is the baseline for effective radiative forcing calculations and some of the forcing relationships, particularly for aerosols, are highly non-linear.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eCSmith was supported by the Horizon Europe research and innovation programs under grant agreement no. 101081661 (WorldTrans) and 101081369 (SPARCCLE), and acknowledges travel support from the Research Council of Norway under the TRIFECTA project (grant agreement number 334811) which also supported the work of BMS and MS. LKG and CM were supported by the Hadley Centre Climate Programme funded by DSIT. CM was supported by OptimESM (grant agreement No 101081193) and TipESM (grant agreement No 101137673) both funded by the European Union under the European Union's Horizon Framework research and innovation programme and by UK Research and Innovation (UKRI) under the UK government\u0026rsquo;s Horizon Europe funding Guarantee. WT acknowledges funding from the European Research Council (ERC) under the European Union's Horizon Framework research and innovation programme (grant agreement No 101076909; ERC Consolidator Grant \u0026lsquo;LACRIMA\u0026rsquo;). JR was supported by the Horizon Europe research and innovation programs under grant agreement no. 101003536 (ESM2025) and 101081369 (SPARCCLE)\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eSchleussner C-F (2016) Science and policy characteristics of the Paris Agreement temperature goal. 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Nat Clim Change 11:207\u0026ndash;212\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-8407383/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8407383/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eA lack of meaningful action in reducing greenhouse gas emissions has fuelled debate on whether global warming can still be limited to 1.5\u0026deg;C. In 2022, the Working Group III contribution to the Sixth Assessment Report found 97 scenarios that could limit global warming to 1.5\u0026deg;C above pre-industrial levels with no or limited overshoot, while 9 kept median warming projections under 1.5\u0026deg;C throughout the 21st century. These pathways relied on emissions reductions by 2025 that have not materialized. We show that updating the emissions and climate model calibrations to 2023, provides additional constraints on the projected warming outcomes. The window to keep warming below 1.5\u0026deg;C with a greater than 50% likelihood has closed. We use several approaches to show that the objective of limiting warming to 1.5\u0026deg;C needs to be pursued from above after a temporary overshoot. Immediate, rapid deep reductions in CO\u003csub\u003e2\u003c/sub\u003e emissions and non-CO\u003csub\u003e2\u003c/sub\u003e forcers, can still limit peak warming to around 1.7\u0026deg;C, and returning below 1.5\u0026deg;C before 2100 is attainable. Keeping peak warming as low as possible and pursuing to reverse it below 1.5\u0026deg;C thereafter will contain the adverse consequences of temperature overshoot and limit the scale of negative emissions required.\u003c/p\u003e","manuscriptTitle":"Updated scenarios show 1.5°C overshoot is unavoidable but limitable","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-19 12:21:13","doi":"10.21203/rs.3.rs-8407383/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"ef420acf-8e51-4090-bcc9-4a4f8bbd9ac2","owner":[],"postedDate":"January 19th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":60830763,"name":"Earth and environmental sciences/Climate sciences/Climate change"},{"id":60830764,"name":"Earth and environmental sciences/Climate sciences"},{"id":60830765,"name":"Earth and environmental sciences/Climate sciences/Climate change/Climate-change mitigation"}],"tags":[],"updatedAt":"2026-01-19T12:21:13+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-19 12:21:13","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8407383","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8407383","identity":"rs-8407383","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}