2.1 Focus on Paris Agreement relevant scenarios from mitigation decision-maker viewpoint

The elements outlined in the Paris Agreement, more specifically in relation to the long-term temperature goal expressed in Article 2.1 and Article 4.1, should be fully explored (Schleussner et al., 2022). Under the UNFCCC, the second periodic review of the long-term global goal under the Convention, and of overall progress towards achieving it, concluded its work in 2022. Furthermore, the global stocktake provides a checkpoint on whether aggregate emission levels by parties to the agreement are consistent with the long-term goals of the Convention (UNFCCC, 2022a). The Conference of the Parties under the UNFCCC explicitly “acknowledges that limiting the global average temperature increase to 1.5 °C above pre-industrial levels with no or limited overshoot would avoid increasingly severe climate change impacts” (UNFCCC, 2022b), building on the conclusions of the IPCC “Special Report on Global Warming of 1.5 °C” (Hoegh-Guldberg et al., 2018), as well as the IPCC AR6 WG I and WG II reports (IPCC, 2022b, 2021). They also concluded that while “information and knowledge have improved significantly since the first periodic review (2013–2015)” (UNFCCC, 2022b, paragraph 7), “there continue to be important information and knowledge gaps” in relation to its scope, including on “the long-term global goal and scenarios towards achieving it in the light of the ultimate objective of the Convention”. The scientific community is explicitly encouraged to address those gaps. We are of the view that any scenario design process needs to be cognisant of this explicitly expressed call by governments for more scientific information on Paris Agreement compatible scenarios.

Against a backdrop of “the emission world avoided” context provided by high-end emission scenarios, it is paramount that decision makers also understand the implications of stronger mitigation efforts in terms of climate benefits and avoided impacts. Whether we follow a scenario that delays mitigation efforts by 10, 20 or 30 years and reaches net-zero CO₂ emissions by 2050 or 2060 or 2070 makes trillion-dollar differences in terms of directing government incentives and private capital (Riahi et al., 2022; van der Wijst et al., 2023), but also in terms of adaptation costs, limits to adaptation, irreversible loss, as well as economic and non-economic costs of anticipated losses and damages (IPCC, 2022b). While natural variability in any single year influences global-mean temperatures by ±0.25 °C (Box 4.1 in IPCC AR6 WGI; i.e. Lee et al., 2021), climate extremes (Seneviratne et al., 2021) and impacts that reflect long-term, cumulative climate changes (e.g. glacier melt or sea level rise) can be substantially different between a scenario peaking at 1.6 or 1.8 °C in the middle of the century (Mengel et al., 2018; Pfleiderer et al., 2018). Only immediate action will slow anthropogenic warming in the near term (McKenna et al., 2021), a crucial element to enable sustainable development (Schleussner et al., 2021).

2.2 Comprehensive range to inform risk management, adaptation needs, as well as loss and damage assessments

Output from ESMs based on the framing pathways should explore a comprehensive range of plausible warming futures. Plausible very-high-end or worst-case outcomes are a key foundation for risk management and adaptation decision-making. Also, those high-end outcomes can be indicative of the high-end tail of the distributions, including high-end climate sensitivity, that are helpful in examining limits to adaptation that may need to be addressed. Information about high-end global warming outcomes is of particular importance on adaptation- and loss-and-damage-relevant timescales, i.e. up to 2050. It is important to be cognisant of the fact that on adaptation-relevant timescales until mid-century, climate uncertainties dominate scenario uncertainty (Lehner et al., 2020; Lee et al., 2021). As such, even a very-high-emission pathway would not allow for a full appraisal of high-end outcomes on those timescales. Rather, an assessment of high-risk outcomes needs to be based on assessing higher risk percentiles of any given pathway.

In principle, there are two options for examining the high-end warming futures for these purposes: use a high-end pathway, such as “the emission world avoided” scenario proposed here (e.g. SSP5-8.5 or SSP3-7.0), and then – by examining impacts at different global warming levels (GWLs) – map climate characteristics onto the high percentiles of expected tails of the distribution of lower-emission pathways, such as the “current policy” pathways. Alternatively, use the higher-end warming tails of the ESMs that were run for those “current policy” pathways, acknowledging that a lack of high ensemble sizes and a lack of a complete representation of uncertainties might hinder a full examination of the tails of the projected distribution.

In contrast to the high end of the scenario space, the plausible and very low end of the scenario space is a prerequisite to deriving minimal adaptation needs and identifying unavoidable loss and damage. Understanding the very low end of the scenario space is also key to understanding the consequences of delayed action, i.e. what we are leaving behind or taking off the table. Thus, a useful framing pathway is one in which immediate action had started in 2015 and that has at least a 50 % probability of decadal average temperatures remaining below 1.5 °C.

2.3 Non-exhaustive list of policy-relevant questions

We postulate a non-exhaustive list of policy-relevant questions. The degree to which the framing pathway design can address those questions or not will influence to what degree the framing climate pathway dataset can inform the forthcoming seventh assessment cycle of IPCC (AR7) and the wider scientific literature based on CMIP7 to be policy relevant.

• a.

  What are the potential climate outcomes of current climate policy targets? The climate ambition reflected in the current set of nationally determined contributions (NDCs) is insufficient to meet the Paris Agreement temperature goal (Riahi et al., 2022; Meinshausen et al., 2022). However, beyond the near term, different interpretations of the stringency and credibility of expressed net-zero targets provide for a broad range of different emission trajectories and subsequent warming outcomes ranging from projected warming between 2.5 and 3 °C by 2100 (and continuation thereafter) to a pathway in which median peak warming is limited to less than 2 °C (Rogelj et al., 2023). Exploring this range is key to inform the implementation and refinement of established targets.

• b.

  What mitigation is required to limit warming to around 1.5 °C and what climate impacts can still be avoided? The design criteria would be strong mitigation consistent with the Paris Agreement and national targets (such as net-zero CO₂ by 2050). Only by exploring such a lower-emission future in the ESMs can science support integrated and informed decision-making to pursue synergies of mitigation and adaptation actions consistent with the Paris Agreement. It might be that such a pathway with net-zero CO₂ emissions around the middle of the century, limited cumulative emissions until then and net-zero GHG emissions in the second half of the century (as Article 4.1 of the Paris Agreement aims to achieve) is at the very low end of future emissions that some integrated assessment models can produce. ESMs themselves would also help inform confidence in the lower warming bound, providing a process-driven representation of the climate outcome of a maximally strong decarbonisation and land use strategy proposed in the scenario. Such low-end scenarios are paramount to estimate what the remaining carbon budget is and what climate impacts might still be avoidable, as well as to inform questions of climate justice. (The low-end emission pathways are assumed here to be loosely defined by the maximal speed of the transition towards a net-zero future (and beyond) from a technological and resourcing point of view – which will differ in each IAM model's implementation of the latest cost, technology and deployment information. This lower bound is subject to uncertainties and definitional choices.)

• c.

  Within the range implied by the Paris Agreement temperature goal, what are the different feasible mitigation strategies? The extent to which a temporary overshoot above global warming of 1.5 °C, always constrained by holding warming to “well below” 2 °C, is regarded to be in compliance with the Paris Agreement differs among different policy stakeholders (e.g. Mace, 2016). While some interpretations have been suggested (Schleussner et al., 2022) the exact definition of “well below” 2 °C has also not been established in the policy domain. Not pre-empting those decisions requires a set of scenarios with different median peak warming broadly within the range of temperature levels referred to in Article 2.1 of the Paris Agreement (i.e. between 1.5 °C or below and less than 2 °C). This might include distinguishing peak-and-decline pathways from stabilisation pathways at 1.5 °C, or at levels above 1.5 °C but less than 2 °C.

• d.

  What are the interlinkages of Paris Agreement compatible climate action and a broader sustainability agenda? Climate and environmental policy aim for a broader set of sustainability objectives beyond emission outcomes alone. Many of those interlinkages with sustainable development goals (SDGs) are explored in the framework of different SSPs, but insofar as they relate to different land use futures will also be of direct relevance for climate outcomes. Global sustainable land futures will be decisively different from a continuation of unequal trends, both in terms of GHG emission trajectories and also for land-cover change, water and fertiliser use, biodiversity, etc. (Humpenöder et al., 2022). Exploring such sustainable scenarios is directly relevant not just for informing climate policy but also in other fora such as the discourse on biodiversity.

• e.

  What are the consequences of delaying mitigation? In the Glasgow Climate Pact, countries recognised the need for accelerated action in this critical decade (UNFCCC, 2021). However, a yawning gap remains between 2030 emission levels implied by current nationally determined contributions under the Paris Agreement (as of 2023) and 1.5 °C pathways (IPCC, 2022a). To inform the ambition expressed in the Glasgow Climate Pact, robust scientific information on the consequences of delaying stringent climate action is highly policy relevant. Without both an immediate and a delayed scenario that aim to reach, for example, 1.5 °C by the end of the century, policymakers will not be able to obtain relevant information on the consequences of delay from dedicated impact studies that are dependent on ESM simulations. These ESM simulations will be fundamentally important to enable, for example, the impacts and adaptation limits assessment of IPCC AR7.

• f.

  Non-CO₂: what are the effects of non-CO₂ mitigation? Addressing non-CO₂ GHGs, in particular methane, has risen in prominence in policy circles. For example, the Global Methane Pledge, supported by more than 150 countries, specifically focused on stringent methane emission reductions by 2030. Distinguishing between different emission strategies with focus on shorter- or longer-lived GHGs, and/or air pollution (namely aerosols, ozone and their precursors) reduction, is policy relevant. Within CMIP5, RCP-driven simulations, non-CO₂ emissions and, in particular, the SO₂ emissions were rather closely aligned across the low- to high-end scenarios. The SSPs somewhat corrected for this with consideration of various air pollution controls derived from the overarching SSP narrative and, as a result, show a wider variation across both non-CO₂ GHGs (and their precursors) and aerosols (see e.g. Cross-Chapter Box 1.4 and Fig. 2 in IPCC, 2021; see also Gidden et al., 2019). However, the sustainable pathways included both strong climate change mitigation and air pollution control preventing disentanglement of the co-benefits of climate change mitigation policies from actions focused on air pollution (Szopa et al., 2021). It would be useful to investigate the implications of different gas-to-gas emission strategies, including regionally varying mitigation policies to capture geographical patterns of short-lived climate forcer emissions, both in the framework scenarios as well as in dedicated sensitivity scenarios in additional MIPs (Persad et al., 2022, 2023). Generally, examining the flexibility governments have to combine mitigation strategies for CO₂ and non-CO₂ GHGs, as well as air pollutants, in terms of climate outcome consequences, is of interest – aiming for a more encompassing reflection of non-CO₂ atmospheric interactions and uncertainties. Assessing the co-benefits of climate-driven policies on air pollution can be important for decision-making. These latter aims also rely on the emerging capability of some ESMs for non-CO₂ greenhouse gases to be emissions driven (Folberth et al., 2022).

• g.

  Land-based carbon dioxide removal: what are the relative risks and the effectiveness of different negative-emission strategies? RCP-based designs do not allow significant uncertainties around land-based carbon dioxide removal to be assessed, because emission fluxes are calculated in the context of IAMs using semi-empirical land carbon accounting that does not represent climate impacts on the strength of the land and ocean carbon sink, and on nature-based solutions overall (IPCC, 2022b). Complex carbon process representations used in land surface components of ESMs are increasingly able to capture dynamics associated with carbon fertilisation effects, plant demographic responses, heat stress, drought response and fire risk, all of which could potentially radically alter the capacity for land-based negative-emission fluxes. Defining scenarios in terms of fossil emissions and land use patterns or decisions allows these processes to be represented directly.

• h.

  Overshoot: to what extent are climate change and the impacts of climate change reversible, and on what timescales? Significant differences in mitigation requirements in the second half of the century between different classes of overshoot scenarios have been identified in the latest IPCC assessment (for example, as reflected in IPCC WG3's C1 and C2 categories (Riahi et al., 2022)). Information on the climate system consequences and related impacts of overshoot, however, is not comprehensively available (Asaadi et al., 2024; Santana-Falcón et al., 2023). Exploring the differences between different policy-relevant overshoot scenarios with ESMs is critical to address this gap. This would also enable policy-relevant information regarding the ways in which envisaging declining future temperatures might influence policy choices today, while temperatures are still rising. Any co-benefits and more ancillary impacts of future negative-emission options (beyond land-based carbon dioxide removal) require an enhanced focus on multiple options to limit warming to 1.5 °C in the long term.

• i.

  Providing a backdrop of “the world that could have been”. To calculate benefits of past and future climate action and inaction, it is useful to have hypothetical backdrop scenarios, both at the high and the low end. The “world that could have been” at the high end could be a medium/high-emission scenario, such as SSP3-7.0 from the CMIP6 cycle – depicting a future that unfolds in the absence of climate policies and in the absence of economic shifts that were largely initiated by climate policies (like “learning-by-doing” renewable technology cost reductions). At the low end, a “world that could have been” can provide a hypothetical backdrop of what would have been still possible if countries had acted fast enough to avoid 1.5 °C warming at all times from 2015 onwards (when the 1.5 °C goal was first adopted within the UNFCCC context in the Paris Agreement). Such scenarios will be vital to inform an emerging policy discussion around loss and damage under the Paris Agreement, as it provides a reference point of the loss and damage that could have still been avoided.

• j.

  What could a worst-case-outcome world look like? Due to different types of uncertainties (biogeochemical but also societal and geopolitical), it is important also to understand a low-likelihood but high warming outcome (Lee et al., 2021). A more elaborate way to explore the tails of the warming distributions of the scenarios seems warranted (Kemp et al., 2022).

• k.

  What are the climate effects of different regional emissions? For shorter-lived air pollutants, in particular aerosols, as well as for land use changes, regional climate effects can markedly differ for geographically variable forcings, and emissions from various regions will have different regional and global implications (Seneviratne et al., 2018b; Persad et al., 2022). In addition, some short-lived forcers have physically distinct mechanisms of interaction with the global climate, such as directly absorbing sunlight or altering clouds, perturbing precipitation and large-scale circulation, and have effects distinct from those of well-mixed GHGs (Tang et al., 2018; Liu et al., 2018; Sillmann et al., 2019; Persad et al., 2023).

• l.

  What is the time dependence of climate change impacts? How does the climate evolve even under near-stable global temperatures? Net-zero CO₂ emissions are expected to result in slowly changing global temperatures (King et al., 2021; MacDougall et al., 2020), but understanding of regional climate and long-term biogeophysical impacts of net-zero CO₂ emissions remains limited. In the literature, there is currently a large dependence of global-warming-level-based projections on sampling from fast-warming scenarios such as SSP5-8.5 (e.g. AR6 Interactive Atlas). Exploring how regional climates and different aspects of global climate evolve at a stable Paris Agreement-aligned global warming level (e.g. Sigmond et al., 2020; Mengel et al., 2018; Seneviratne et al., 2018a) would improve understanding of the effects of climate stabilisation and inform policy- and decision-making.

• m.

  What are the long-term implications beyond 2100? Scenarios supplied by IAMs have focused on a time horizon of 2100, which has been consistent throughout the evolution of scenarios from SRES to RCPs to SSPs. In the early 2000s this was considered appropriate, but 2100 is now less than one human lifespan into the future and within the life cycle of a lot of new infrastructure (Lyon et al., 2022). Several Earth system consequences, including sea level rise (Mengel et al., 2018), ice sheet loss and carbon cycle dynamics (Koven et al., 2022), as well as impacts on the natural system (Santana-Falcón et al., 2023), will continue beyond 2100, even in strong mitigation scenarios. While RCPs and SSPs included extended scenarios for running ESMs beyond 2100 (Meinshausen et al., 2011, 2020), they were not based on detailed scenario modelling provided by the IAM community and, particularly in CMIP6, had low take-up among modelling groups (Lee et al., 2021). Irreversibility is also coupled with the question of overshoot within this century and beyond (Frölicher and Joos, 2010).

5.1 Framing pathway storylines consistent with recent IEA and NGFS scenarios

We note that the set of categories identified here – which are to be represented by specific framing pathways – bear some similarities to scenarios that have been identified by other initiatives, not necessarily related to the IPCC assessment, that are of key policy relevance, such as the scenario set investigated by the International Energy Agency (IEA) (IEA, 2022) and the Network for Greening the Financial Sector (NGFS) (Richters et al., 2022). In particular, those key pathway sets examine the worlds under current policies, current climate targets on the higher side and emission levels consistent with the ultimate goal of the Paris Agreement on the lower side. To us, this illustrates a convergence in views of what policy-relevant perspectives are on pathways that the climate science community may want to consider. The initial June 2023 workshop discussions for the forthcoming ScenarioMIP protocol, at which this paper's proposal was presented, also picks up some elements of this convergence of views (Fig. 1 in van Vuuren et al., 2023). We hope that the science and policy objectives, as well as the presented pathway categories and timing considerations, outlined in this paper can further inform the deliberations under ScenarioMIP, as well as other MIPs, under the CMIP umbrella.

5.2 High granularity of lower pathway categories

We have identified three climate pathway categories between 1.5 °C and “below 2 °C”. One of the pathway categories (IA2015), a lower priority one, represents a low “the world that could have been” pathway, i.e. it investigates a hypothetical world in which global emissions would have diverged from historical emissions in 2015 to stay below 1.5 °C. The other two categories, IAPD and DAPD (see Table 1 above), resemble the lowest two AR6 WGIII categories C1 and C2 that in the central outcome are expected to return global-mean temperatures to below 1.5 °C by the end of the century again. One argument against a high granularity of ESM simulations is often that they are too close to each other to detect climate differences. This presumption is in direct contrast with a high level message from the IPCC AR6 that “every bit of warming matters” and requires further reflection. Firstly, differences between small increments of warming can indeed be detected (e.g. Pfleiderer et al., 2018) including for long-term sea level rise where differences in peak warming of 0.2 °C may amount to a 40 cm difference in 2300 sea level rise commitment or more (Mengel et al., 2018), and also in the near-term emergence of climate extremes on the country level when considering large ensembles (Beusch et al., 2022a). While for a single ESM framework large ensembles of simulations are required to statistically investigate differences in “close by” scenarios or weak single-forcing differences (Shiogama et al., 2023; Smith et al., 2022), the multi-model nature of CMIP could allow us to investigate the multi-model average differences across close-by scenarios, without the need for each modelling centre to generate very large ensembles. Furthermore, while pathways might differ by less than 0.2 °C in terms of global-mean temperature, differences in regional emissions across those scenarios imply stronger differences in regional climate outcomes (e.g. Persad et al., 2023). In addition, the category characteristics outlined above would imply very different land use futures. These differences would translate to different climate futures in particular at the regional level and, given the differences in the representation of land–climate feedbacks across ESMs, also a range of different global climate outcomes that are of key importance to inform the policy discourse on land-based mitigation.

In addition to differences in peak warming, very different overshoot outcomes would be implied under an IAPD- in comparison with a DAPD-type future. A systematic exploration of those differences, and the robustness of different mitigation strategies when considering climate impacts, is critically important given the profound differences between such pathways in the mitigation space. Whether we advance mitigation efforts to achieve a low overshoot instead of a higher one is a question of high societal relevance. To meet the high policy interest around future temperature outcomes (that can be argued to be) within the long-term Paris Agreement goals, the AR6 WGIII report (IPCC, 2022a), for example, placed strong emphasis on so-called illustrative mitigation pathway (IMP) scenarios that are largely in the lower 1.5 °C with no or low overshoot category. Similarly, we argue that it is time to enable WG II to provide the corresponding impact assessment via adequate ESM simulations that provide the needed geophysical input for ecosystem, land use and biogeophysical impact models, as well as associated impact studies. Those types of models that directly feed off ESM output have emerging importance in the quest to arrive at a finer-grained picture of future impacts and their difference between different pathways.

Lastly, sometimes the argument is made that emerging pattern scaling or regional emulator approaches could fill in and extrapolate ESM results. While that might be increasingly possible in the future (especially if proposed ESM experiments provide good training datasets), the current set of regional emulators is not yet able to fill that niche. That is particularly true for peak-and-decline (overshoot) scenarios that have the potential to exhibit hysteresis in terms of large-scale and regional warming and precipitation patterns (Pfleiderer et al., 2023). For example, approaches that rely on global-mean temperatures as one of their input parameters are not yet adequately capable of distinguishing warming, stable and cooling worlds, before, at or after a peak in global-mean temperatures (see point 5.6 below).

An additional pathway that could be similar to the delayed action DAPD one could be one that limits peak warming to below 2 °C with a likely (66 %) chance and also avoids strongly negative emissions. It would miss the 1.5 °C warming level by 2100 and arguably not be in line with the Paris Agreement's “pursuing efforts for 1.5 °C” element, and most probably also not with achieving net-zero greenhouse gases, but could present a third plausible pathway within the 1.5 to below 2 °C range (similarly to “C3” category pathways investigated in IPCC AR6 WGIII). Given the overall resource constraints, we consider the pair of IAPD and DAPD pathways to provide more relevant scientific and policy-relevant insights compared with either an IAPD–C3 or a DAPD–C3 combination, yet we acknowledge the advantage that a triple IAPD, DAPD and C3 investigation might bring. The reason is that only DAPD would explore a strong overshoot and net negative emission behaviour, and without IAPD it would be impossible to adequately illustrate the lower region of plausible futures.

In summary, we suggest that future framing pathways separately explore pathways from an immediate-action, sustainable-future category (IAPD) and a delayed-action, high-overshoot, high-negative-emission category (DAPD). These categories could arguably also be seen as exploring different futures within the Paris Agreement temperature goal range. Having at least two scenarios will prevent a singular defacto definition by the scientific community of what the Paris Agreement goal means exactly (which could be regarded as policy prescriptive). Exploring pathway variants that yield different pathways within the Paris Agreement temperature goal range obviously belongs to one of the most highly policy-relevant questions (IPCC, 2023) and should be a more than worthwhile investment of resources.

5.3 High warming pathway

Our deliberations still include a high warming pathway category, i.e. “the emission world avoided”, as those high warming pathways are widely used in the community and serve the scientific purpose of understanding climate change under large forcings. In addition, the more idealised 1 % CO₂ and abrupt forcing runs adequately assist the scientific quest to better understand Earth system characteristics in a high forcing/high warming world. Several scientific applications, i.e. related to emulator calibration and global warming level assessments, would also continue to rely on such high forcing pathways, and we do not argue that such high forcing outcomes should not be modelled in the next generation of ESM framing pathways. Our proposal, however, no longer includes a high warming pathway category that could be mistaken for a “business-as-usual” scenario and we argue that it would be beneficial to separate high forcing pathways for scientific purposes from the more policy-oriented framing pathway categories.

The high-end “emission world avoided” pathway at the upper end could also serve a strong communication purpose. Frequently, the success of the Montreal Protocol in limiting the emissions of ozone-depleting substances is showcased by comparing current emissions to “the world avoided” scenarios (Velders et al., 2007). Having a similar comparison point or range in climate science would be a useful indicator of where we might have been if we had failed to put climate action on the political agenda. In contrast, at the other end, exploring the low-end emissions “world that might have been” is also a reminder of what we could have achieved if not for political and economic forces that inhibited swift global-scale emission reductions over the past decades (Supran et al., 2023).

5.4 Separate consideration of socioeconomic pathways

It is worth noting that this proposed set of scenarios does not prescribe specific socioeconomic futures but can accommodate different narratives of socioeconomic development as reflected in the original shared socioeconomic pathway (SSP) framework, as well as different perspectives on burden sharing, equity and fairness that have been identified as key elements of scenario development arising from the Bangkok discussions. The one methodological challenge will be to derive characteristic land use and aerosol precursor emission patterns that are representative of some future socioeconomic evolutions, with potential variations then studied in additional sensitivity MIPs (Sect. 5.5).

5.5 Framing pathways to be complemented by sensitivity MIPs

A limited set of climate “framing pathways” should be complemented by a much broader set of explorations in different MIPs under the new round of CMIP. Those sensitivity cases should explore land use patterns, different aerosol assumptions (both spatially and different time series evolutions), methane and other ozone precursor reductions, global warming levels, etc. However, in our view the climate “framing pathways” presented here would provide a good basis for interlinkages with different MIP explorations on those key topics and key research questions as outlined above. We also note other, potentially policy-relevant emerging approaches for scenario design beyond emission- or concentration-driven simulations that explore adaptive, ESM-specific approaches to match different warming level outcomes, thus filling a critical gap in explicitly exploring warming level dependent impacts in the ESM space (Terhaar et al., 2022).

5.6 Regionally explicit emulators are still in the early stages of development

Separate from regional downscaling that benefits from a long tradition and uses ESM output as a starting point, the question has emerged as to what degree regionally explicit emulators can replace ESM simulations. As mentioned in Sect. 5.2, the use of emerging tools, such as spatially explicit emulators in combination with well-established reduced-complexity climate models (e.g. Beusch et al., 2020, 2022b; Tebaldi et al., 2022), can provide for very promising applications to explore a range of different futures in rapid fashion. However, such approaches, often based on the idea of pattern scaling of global-mean temperature, can only be used for emulation of ESM output, and without adequate ESM training pathways, they will not be able to capture key features of peak-and-decline or stabilisation pathways, for which the central assumption of scaling with global-mean temperatures does not hold anymore in the same way (Pfleiderer et al., 2023). Furthermore, none of those emulators are yet able to provide the full richness of variables, as well as the spatial, temporal and cross-variable correlations that ESMs are able to provide and which are key for a range of specific impact models such as those linked in the ISIMIP project (Frieler et al., 2023; Warszawski et al., 2014). Thus, while emulators will be able to play an increasingly important role, they do not provide sufficient capabilities to address the science and policy questions identified above. ESMs therefore remain the key tool to synthesise, diagnose and analyse our best available climate system science under various driving forces. This might change at some point, but it would be a large risk to bet that emulators will be sufficiently mature to fill our key scenario gaps by the time of AR7.

5.7 Linking projections of GHG and ozone-depleting substances, as well as new gases

To date, projections of the effects of atmospheric accumulation of GHGs, ozone-depleting substances (ODSs) and ODS replacements (hydrofluorocarbons (HFCs)) have occurred primarily through two communities: the IPCC for GHGs and the WMO/UNEP Ozone Depletion Assessments (e.g. World Meteorological Organization, 2022; Velders and Daniel, 2014) for ODSs and HFCs. While there has been increasing connectivity and collaboration between these two communities, projections of GHG as well as emissions of ODSs and their replacements have often been generated through substantially independent frameworks resulting in the underlying emission “storylines” often not being consistent. For example, in a given ozone-depletion assessment, the baseline ODS scenario from the previous assessment has been commonly used as the baseline for the 3-D model calculations. This can lead to inconsistencies when IPCC assesses and uses a new set of ODS emission scenarios to quantify the direct and indirect radiative forcings of ODSs and their replacements, with these scenarios often being distinctly different from any WMO/UNEP ODS scenarios. Furthermore, even the framework scenarios assessed by the IPCC scenarios, whether to attain an overall radiative forcing by some year or to follow a prescribed global development path, is not consistent with the approach taken by WMO/UNEP assessments. Given this and given that ESMs now increasingly include an interactive stratospheric chemistry scheme that explicitly couples the chemistry–climate effects of atmospheric GHG and ODS loading, a set of self-consistent GHG and ODS emission scenarios would be another opportunity to link these two research communities and provide a single set of simulations that can serve the needs of both communities, as well as benefit their intended policymaker stakeholders. As industrial processes, energy generation and transport activities transition to GHG and ODS alternatives, it is essential that the next generation of REP climate pathways include the new compounds emitted from the associated activities (e.g. the effects of fugitive hydrogen emissions on atmospheric chemistry, the stratosphere and climate from a hydrogen-based economy (e.g. Tromp et al., 2003)).

5.8 Timely provision of updated pathway information to inform the global stocktake

Countries striving to implement their climate plans, including mitigation and adaptation objectives as well as responses to loss and damage, will require significant investments in the envisioned economic and societal transformation (IPCC, 2022a). To inform policy and investment decisions, timely provision of scientific analysis and assessments is key. In response to this demand, agencies such as the IEA or the NGFS provide annual updates of emission pathways. Such a rigid timeline is impractical for community pathway developments focused on ESMs. However, we would argue that a reflection on the timeline for pathway design in the light of stakeholder needs, and in particular political processes under the Paris Agreement, is very much in order.

Figure 3A stylised timeline for scientific information and IPCC to inform the second global stocktake. Key dates are derived based on the global stocktake cycle and its modalities, including a technical phase for scientific input. The stylised timeline for IPCC products allows the establishment of a schedule for the design and execution of a new generation of ESM simulations aimed at informing the relevant IPCC products. Note that cut-off dates are indicated relative to the earliest IPCC product approval timing and would move accordingly backwards for a later approval date.

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The second global stocktake (GST) under the Paris Agreement will be conducted in 2027/2028 (Fig. 3). Informed by its outcomes, parties to the agreement will be invited to develop their NDCs for 2040. A 2040 horizon is close to expressed net-zero CO₂ and net-zero GHG targets for several major emitters, and very deep emission reductions, as well as building up carbon dioxide removal capacities, will be required to achieve them (Edenhofer et al., 2023). Several of the policy and science questions identified above are directly relevant to informing the development of NDCs on this timeline. Striving to provide major scientific community inputs in time for the second GST will be essential. Thus, an enhanced timeline of some CMIP7 outputs (or also potential continued CMIP6 activities to fill critical gaps in the existing scenario space) is almost certainly required (CMIP Panel, 2024). It would be unfortunate if the best available science at the time of the second GST in 2027/2028 would still need to predominantly rely on a set of ESM scenarios dating back to 2015 (the current CMIP6 SSP–RCP framework).

Reflections on scientific inputs into the second GST need to be cognisant of the timeline of the IPCC 7th Assessment Report (AR7) cycle, as the IPCC will be a key source of input to the global stocktake process. Even before the AR7 had started with the election of its new Bureau in July 2023, the discussions at the Bangkok meeting suggested the need to consider producing relevant outputs by 2028 (IPCC, 2023). As part of the COP28 decisions on the outcome of the first global stocktake and the IPCC AR6, the UNFCCC invited the IPCC to “consider how best to align its work with the second and subsequent global stocktakes” and “to provide relevant and timely information for the next global stocktake” (1/CMA.5, paragraph 184 in UNFCCC, 2023b), and also invited the IPCC to “continue providing relevant information to Parties on the scientific, technical and socioeconomic aspects of climate change and to take into account, in determining its future products and assessment cycles, work under the Convention and the Paris Agreement” (see 20/CMA.5 paragraph iv in UNFCCC, 2023a). At its 60th session in January 2024, the IPCC considered the above invitations and decided that the AR7 work programme will include the three Working Group Assessment reports, a Synthesis Report, the previously agreed “Special Report on Climate Change and Cities” and two TFI Methodology reports on short-lived climate forcers and carbon dioxide removal technologies, carbon capture utilisation and storage. Since no additional dedicated special report was agreed, the three IPCC Working Group Assessment reports will provide the only avenue to adequately inform the second GST. Figure 3 depicts the time frame of when this series of IPCC products will need to be produced given past experience and the hard deadline presented by the second GST. From that range, we can work backwards with potential literature cut-off dates for any such product. Based on this, we argue that aiming for providing ESM data by mid to end 2025, and thereby making forcing data available by late 2024, would be required. This tight timeline highlights the need for early preparation and reflection by the scientific community well ahead of the finalisation of the IPCC schedule. Furthermore, the challenges experienced in AR6 with “hot models” might caution against making simulations available too close to the deadline (Hausfather et al., 2022).

Missing the opportunity for input into the second GST will not only just forego the chance to inform one of the key processes for scientific input under the UNFCCC, but, and arguably even more importantly, also the time window where countries can still take on board new climate science insights in the preparation of their NDCs with a 2040 target year. Furthermore, some link between the IPCC timelines and the international climate change deliberations between countries seems pertinent to fulfil the IPCC's mandate to be policy relevant, as well as necessary to maintain and build the IPCC's standing in the international discourse.