the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
A perspective on the next generation of Earth system model scenarios: towards representative emission pathways (REPs)
Malte Meinshausen
Carl-Friedrich Schleussner
Kathleen Beyer
Greg Bodeker
Olivier Boucher
Josep G. Canadell
John S. Daniel
Aïda Diongue-Niang
Fatimah Driouech
Erich Fischer
Piers Forster
Michael Grose
Gerrit Hansen
Zeke Hausfather
Tatiana Ilyina
Jarmo S. Kikstra
Joyce Kimutai
Andrew King
June-Yi Lee
Chris Lennard
Tabea Lissner
Alexander Nauels
Glen P. Peters
Anna Pirani
Gian-Kasper Plattner
Hans Pörtner
Joeri Rogelj
Maisa Rojas
Joyashree Roy
Bjørn H. Samset
Benjamin M. Sanderson
Roland Séférian
Sonia Seneviratne
Christopher J. Smith
Sophie Szopa
Adelle Thomas
Diana Urge-Vorsatz
Guus J. M. Velders
Tokuta Yokohata
Tilo Ziehn
Zebedee Nicholls
Abstract. In every IPCC Assessment cycle, a multitude of scenarios are assessed, with different scope and emphasis throughout the various Working Group and Special Reports and their respective chapters. Within the reports, the ambition is to integrate knowledge on possible climate futures across the Working Groups and scientific research domains based on a small set of ‘framing pathways’, such as the so-called RCP pathways from the Fifth IPCC Assessment report (AR5) and the SSP-RCP scenarios in the Sixth Assessment Report (AR6). This perspective, initiated by discussions at the IPCC Bangkok workshop in April 2023 on the “Use of Scenarios in AR6 and Subsequent Assessments”, is intended to serve as one of the community contributions to highlight needs for the next generation of framing pathways that is being advanced under the CMIP umbrella for use in the IPCC AR7. Here we suggest a number of policy research objectives that such a set of framing pathways should ideally fulfil, including mitigation needs for meeting the Paris Agreement objectives, the risks associated with carbon removal strategies, the consequences of delay in enacting that mitigation, guidance for adaptation needs, loss and damage, and for achieving mitigation in the wider context of Societal Development goals. Based on this context we suggest that the next generation of climate scenarios for Earth System Models should evolve towards ‘Representative Emission Pathways’ (REPs) and suggest key categories for such pathways. These ‘framing pathways’ should address the most critical mitigation policy and adaptation needs over the next 5–10 years. In our view the most important categories are those relevant in the context of the Paris Agreement long-term goal, specifically an immediate action (low overshoot) 1.5 °C pathway, and a delayed action (high overshoot) 1.5 °C pathway. Two other key categories are a pathway category approximately in line with current (as expressed by 2023) near- and long-term policy objectives, and a higher emissions category that is approximately in line with “current policies” (as expressed by 2023). We also argue for the scientific and policy relevance in exploring two ‘worlds that could have been’. One of these categories has high emission trajectories well above what is implied by current policies, and the other has very low emission trajectories that assume that global mitigation action in line with limiting warming to 1.5 °C without overshoot had begun in 2015. Finally, we note that timely provision of new scientific information on pathways is critical to inform the development and implementation of climate policy. For the second Global Stocktake under the Paris Agreement in 2028, and to inform subsequent development of Nationally Determined Contributions (NDCs) up to 2040, scientific inputs are required well before 2028. These needs should be carefully considered in the development timeline of community modelling activities including those under CMIP7.
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Malte Meinshausen et al.
Status: open (until 01 Nov 2023)
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CC1: 'A value-of-information lens is needed', Robert Kopp, 12 Sep 2023
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I thank the authors for this manuscript but am concerned the proposal does not fully grapple with the costs of identifying a scenario as a high-priority scenario. Running high-priority scenarios through dozens of Earth system models represents a major commitment of scientists' time – time that cannot be spent otherwise advancing the scientific frontier – and should not be treated as cost-free. There should be good reason to think that the ESM scenarios are sufficiently distinct that the differences among them will be meaningful and will truly provide insights that can only be gleaned from an ESM.
In this regard, I am particularly concerned about the overlap between the DAPD, IAPD, and IA2015 scenarios. I am skeptical that ESMs will tell us enough here that cannot be gleaned by taking a warming-level lens to climate model emulator output, and would strongly urge that one or more of the better-resourced climate modeling groups demonstrate the value of such scenarios before they become a global recommendation of CMIP7.
The warming-level framing (that a 2°C world is climatologically a 2°C world regardless of whether it happens in 2060 or 2100) has significant limitations. From a human perspective, this is certainly not the case – the impacts of 2°C to a world with 2060 socioeconomics is quite different from a world with 2200 socioeconomics. But that’s irrelevant to the Earth system model.
It is also true from a physical perspective that the climate continues changing after temperature stabilizes, as the oceans continue to warm. So a world that stabilizes at 2 C in 2070 looks physically different from that same world in 2150. Likewise, a world that reaches 1.5°C in 2150 after high overshoot will not be identical to one that stabilizes at 1.5°C with low or no overshoot. This is important to investigate – but I don’t think the threshold has been met for saying it’s a high priority for every modeling team participating in CMIP7 to run multiple emissions scenarios that indirectly probe this. In the absence of clear evidence, the payoff is not convincing to me.
I am also concerned by the deprioritization of the very high emissions (TEWA) scenario. While indeed this scenario should not be portrayed as a reference scenario, stress-testing Earth system models is important for understanding the behavior of the Earth system and characterizing deep uncertainty. There is likely to be greater scientific insight gleaned from one very high emissions scenarios than three very low emissions scenarios, and from a scientific perspective, it is quite useful to stress-test every model with such a scenario.
I recognize that my comments here point to some tension between a WG1 perspective and a WG2/3 perspective. This should be acknowledged. It is indeed important to produce the physical projections needed for impact and mitigation analysis. But I am unconvinced that standardized scenarios run with full-complexity Earth system models, which have substantial opportunity-costs, are always the right approach, and I would like to see a critical value-of-information lens brought to the scenario discussion before such an approach is adopted.
Citation: https://doi.org/10.5194/gmd-2023-176-CC1 -
CC2: 'Reply on CC1', Zebedee R. Nicholls, 12 Sep 2023
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Thanks for a very nice comment. Some excellent points. I particularly like the idea of "that one or more of the better-resourced climate modelling groups demonstrate the value of such scenarios before they become a global recommendation of CMIP7". I hope this idea can be fed to the CMIP7/ScenarioMIP steering committees, who then have the fun job of working out if this is feasible on the timelines required or not :) Without that information, it is indeed an interesting yet tricky debate to have, with pros, cons and unknowns.
Citation: https://doi.org/10.5194/gmd-2023-176-CC2 -
CC3: 'Reply on CC1', Malte Meinshausen, 13 Sep 2023
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Thanks. A worthwhile consideration we thought about in the author team....
Two points:
a) to some degree it is a question of balance and focus. Do we want to focus on extreme scenarios (e.g. take SSP5-8.5) to advance scientific insights w.r.t the climate system behaviour under high warming or do we want to focus on scenarios that have a closer link to being of relevance for societies (and all scenarios cater to both aspects, just to different degrees of course). The trillion-dollar decisions in this and the next decade are made against the backdrop of 1.5C versus 1.7C versus 2C and if we do not populate that space with scientific WGI & WGII studies on relative impacts, then IPCC AR7 can again inform only in relatively general statements ("every bit of warming matters") rather than drilling down into the specifics. See below the relevant text excerpt from our study.
"Against a backdrop of ‘the emission world avoided’ context provided by high-end emission scenarios, it is paramount for decision makers to 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 315 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 and economic and non-economic costs of anticipated losses and damages (Pörtner et al., 2022). 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 320 level rise) can be substantially different between a scenario peaking at 1.6°C or 1.8°C in the middle of the century (Mengel et al., 2018; Pfleiderer et al., 2018). "
b) While maybe not from the ESM output per se, but further down the line in the cause-effect WGI-WGII model chain, there could be ample new science on the impacts of, e.g., slight versus higher overshoot of 1.5C. At the moment, such (often more WGII focussed) science is simply not possible given that the first piece of the chain (the ESM output) is not available. Thus, am not sure that we would solve the consideration of where to place resources with simply "one ESM model test run", given that the chain of impact models would need to be employed.Citation: https://doi.org/10.5194/gmd-2023-176-CC3 -
CC4: 'Reply on CC3', Robert Kopp, 13 Sep 2023
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I agree on the value of understanding the difference of impacts in different 1.5-2 C scenarios. My question is whether more (expensive) ESM scenarios will provide you that information. Much of the difference in impacts will have to do with the state of the human system when different warming levels occur — not something you necessarily need new ESM simulations to address. Yes, emulation has limitations, but before spending millions of dollars of scientist time on new simulation, it seems reasonable to do some experiments to see whether those limitations are large relative to internal variability and structural uncertainty.
In the quote from the paper, you highlight the difference between 1.6 and 1.8 C in terms of sea level rise. This I think points to a limit of the “run everything through ESM” approaches. ESMs of course neither generally include all sea level processes nor sample sea level uncertainty adequately (compare the differences in medians in Mengel et al 2018 to the overall range). Further, putting aside ice sheet instability temperature thresholds that ESM scenarios are not optimized for probing, global mean sea level change is closely related to integrated temperature — so yes, an extended period over 1.5 C adds up, but not in a way that an ESM is needed to quantify.
I think the paper perhaps too readily dismisses the utility of smart emulation methods (eg Tebaldi et al 2022’s STITCHES methodology, https://esd.copernicus.org/articles/13/1557/2022/). It might be a better use of researchers’ time to have a smaller number of standardized ESM scenarios combined with standardized emulated scenarios that could also be used by impact models. The comparison between the two approaches should at least be made on quantitative grounds rather than intuition.
Citation: https://doi.org/10.5194/gmd-2023-176-CC4
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CC4: 'Reply on CC3', Robert Kopp, 13 Sep 2023
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CC2: 'Reply on CC1', Zebedee R. Nicholls, 12 Sep 2023
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CC5: 'Comment on gmd-2023-176', Alexandre Magnan, 19 Sep 2023
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Dear colleagues,
First of all, many thanks to the authors for these thoughts on how to improve the scenario approach in the AR7. That is a critical question that the scientific community should seriously reflect on over the coming years, and that relates to how to capture the various components of climate action together (mitigation + adaptation) and project them into the future… and in a way that speaks in the end to decision-making at large. The scientific and methodological challenges are enormous, and so that is extremely valuable to have papers that give a try, such as this one.
I concur with the authors that we need a pathways approach that shows the range of potential futures, beyond only the ones that one could judge today as “realistic”. As scientists, our role is probably to speak to policy today, yes, but also lay foundations for thinking beyond present day-driven path dependencies and allow for imagining potential radical shifts in one way or another.
The proposal to classify scenarios into NFA, DASMT, DAPD, IAPD + TEWA and IA2015 is interesting, but being not myself a climate modeler, I will let experts to comment on the technical aspects and the overall relevance of such a framing.
On the multi-“Representative x Pathways” (RxP) approach, it makes sense in principle as we need, along with emission/warming projections, insights on what other aspects of climate action, especially adaptation-related, could look like in the future. And the authors are right to advocate for not stopping at 2100, though challenging from a social science perspective (but still, the role of the scientific community is to try envisaging the very long-term).
My two general comments rather refer to the bottom part of Figure 1. Maybe the authors will see them as being a bit out of the scope of this Discussion paper, sorry for that, but that’s an attempt on my side to expand on the ideas suggested by the authors. These are just food for thought; I don’t have neither turnkey solutions nor lessons to give; so please take the below as complementary reflections and not as pure (negative) criticism :-)
- Figure 1 seems to suggest that there is a deterministic path from RTPs to RSPs and then RACs. That doesn’t not align with what the adaptation science says and that views adaptation has being driven by more than warming-related and economic-related drivers. I know drawing figures means being reductionist in a way, so forgive this comment if your intention was not to advocate for a “deterministic view”.
- One question at one point will be: how to combine the various pathway scenarios into a kind of synthetic view of climate action options for the future? That is, what could be the common language/metric across the various RxP, or more specifically, the language/metric to be used to move from Representative Socioeconomic Pathways to Representative Adaptation Pathways, and then from these latter to Representative Impact/Risk Pathways ? In other words : how to connect information on Hazards (influenced by REPs, but maybe not only) to information on Exposure and Vulnerability? Answering this question could be decisive from a cross-Working Group perspective and in terms of laying foundations for the AR7 Synthesis Report. The question itself seems trivial from a theoretical perspective (the Risk propeller diagram shows connections !), but in practice the answer is far from being an easy one… Using quantified indicators for designing adaptation pathways that are susceptible to land on clear information on Exposure and Vulnerability, would be a way to come up with numbers and create bridges with the outcomes of REPs (with a meeting point around Risk/Hazards, as suggested in Figure 1); but the history of developing quantitative indicators for adaptation proved to be unsatisfying. Especially because that often means reducing adaptation aspects to parameters that can be quantified and informed with existing databases, which usually ends up with selecting GDP-related dimensions (e.g. income, proportion of population/activities at risk) and leaving aside more qualitative ones (e.g., risk perception, power relations). There is therefore, at least to me, a danger with moving towards a quantitative indicator-based approach, for example through IAMs, what the direct arrow from RSPs to RAPs suggests under the lines. Other methods rather use expert judgments to assess adaptation progress/gaps in a more integrative way (i.e. not only GDP- and warming-driven) and offer more relevant ways to think the future of adaptation, but then how to connect them to RSPs on the one hand, and to RIPs on the other one? Again, I don’t have any clear answer to this, but such a question should also drive the way scenarios are developed in view of feeding the AR7.
- The scale issue : while it makes perfect sense to develop REPs at the global level (+ possibly a regional perspective), developing RIPs and RAPs requires to further consider sub-national context-specificities (i.e. intra-national variability). So that hoping for globally relevant “climate change scenarios” (i.e. at the crossroads of multiple RxPs in Figure 1) could reveal problematic — as Shared Socioeconomic Pathways revealed to be (a point that has been raised during the IPCC AR6 Scenario workshop in April 2023). On the other hand, we cannot expect from the IPCC to consider a wide range of sub-national context-specific RxPs… which would anyway also raise the “aggregation” issue that I mention above. Here again, no solution on my side, but that’s a debate that needs to happen between the IPCC and the policy community: who are the decision-makers that the IPCC primarily wants to talk to ? And what is the level of information and scenario outcomes these policy-makers truly need? My understanding from the recent years is: global-level information is not enough, but local-level information is too much detailed; so where to put the cursor for the IPCC? I guess this question is at the heart of the design of multi-perspective scenarios/pathways in the perspective of the AR7.
Citation: https://doi.org/10.5194/gmd-2023-176-CC5 -
CC6: 'Reply on CC5', Malte Meinshausen, 20 Sep 2023
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Thank you Alexandre for these very pertinent questions and the food for thought. I can think of some sentences that would point to (rather than solve) these key issues for the manuscript (as e.g. inserting "various methods, including qualitative expert judgment" or similar into the green part of the figure 1). However, ...
.... maybe your comments provide the incentive for some bright minds to chime in, especially from the WGII and WGIII domains. If not for the manuscript, it would be great to start thinking about the options to address these two all-important issues. For one, how to link the various RxPs up (without it ending up be a large hodgepodge that impedes synthesis at the end) and secondly, how to integrate the richness and diversity of the national perspective (both on the adaptation and mitigation side).
Looking forward to the further discussion. Hopefully some here, maybe a bit elsewhere.Citation: https://doi.org/10.5194/gmd-2023-176-CC6
Malte Meinshausen et al.
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