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Dead-end pathways: Conceptualizing, assessing, avoiding [1]

['Daniel Rosenbloom', 'School Of Public Policy', 'Administration', 'Carleton University', 'Ottawa', 'Ontario', 'James Meadowcroft', 'Jochen Markard', 'Department Of Management', 'Technology']

Date: 2025-09 

Abstract Despite rising climate urgency, decision-makers continue to support emission reduction options that appear promising on the face of it but hinder progress in practice. Whether through more efficient gasoline engines or waste heat recovery from fossil fuel combustion, many proposed solutions encourage partial emissions reductions without adequate consideration of whether they can build toward net zero systems of the future. As a result, it is essential that policy decisions are interrogated in terms of their alignment with net zero pathways (or lack thereof) and that decision-makers are both informed about and held to account for the compatibility of near-term choices with long-run system change. This study conceptualizes particularly problematic directions as ‘dead-end pathways’ and outlines a framework for identifying and avoiding them. The framework assesses pathways in relation to three dimensions: depth (how close they can come to virtually eliminating emissions in a stipulated system context), breadth (how widely they can be applied across the specified system), and timeliness (how rapidly they can be deployed). The study then applies this framework to three brief case studies drawn from road transportation, each of which fail on one of these dimensions.

Citation: Rosenbloom D, Meadowcroft J, Markard J, Landry J-S, Kabbara M (2025) Dead-end pathways: Conceptualizing, assessing, avoiding. PLOS Clim 4(8): e0000693. https://doi.org/10.1371/journal.pclm.0000693 Editor: Laurence L. Delina, The Hong Kong University of Science and Technology, HONG KONG Received: January 20, 2025; Accepted: July 25, 2025; Published: August 20, 2025 Copyright: © 2025 Rosenbloom et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: All data are in the paper. Funding: DR acknowledges the support of the Ivey Foundation (https://www.ivey.org/). JM acknowledges funding from the Swiss Federal Office of Energy’s ‘‘SWEET’’ programme as part of the PATHFNDR project under Grant Number SI/502259. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.

1. Introduction It has been roughly a decade since the Paris agreement crystallized global ambition around reaching net zero emissions by mid-century, whereby anthropogenic greenhouse gas (GHG) emissions are balanced globally by anthropogenic GHG removals. During this time, governments have enshrined this commitment in legislation, developed enhanced climate plans, and designed and implemented more robust climate policies [1]. Even though climate policy retrenchment and obstruction have recently deepened in multiple jurisdictions [2,3], global investments in decarbonization efforts are estimated to have more than doubled between 2020 and 2024 [4]. As a consequence, the cumulative capacity of renewable energy technologies reached over 4400 gigawatts in 2024 [5] and nearly 60 million battery electric vehicles are now being driven on the world’s roadways [6]. Despite these promising signs, however, the pace and scale of action remains out of step with achieving net zero by mid-century [7]. We contend that at least part of the reason for the slow pace of change relates to an overriding emphasis in climate policy on the sequential adoption of least-cost or operationally convenient abatement options (e.g., installation of higher efficiency gas furnaces or blending biofuels with gasoline) without sufficient consideration as to whether and how these elements could build toward genuine pathways to net zero by mid-century [8–11]. While in any given context there may be many ways to reduce emissions, not all contribute to sequences of change that can ultimately lead to net zero. Failing to assess individual policy decisions in light of an understanding of net zero pathways can waste societal resources, delay investment in more consequent solutions, and ultimately undermine efforts to achieve climate objectives. This paper contributes to addressing this issue by introducing a practical yet accessible framework to weed out emission reduction options that are unlikely to contribute meaningfully to building net zero pathways. The framework is the product of a transdisciplinary research collaboration among policy practitioners, civil society leaders, and scholars concerned that government decisions to support mitigation technologies and projects are often still made on an ad-hoc basis, and that billions of dollars of public and private funds continue to be allocated to emission abatement initiatives which cannot substantively contribute to achieving net zero by mid-century. The action research that generated the decision framework presented here started from an intuitive understanding of ‘dead-end pathways’ as sequences of choices that would be unable to achieve the desired (net zero) goal. This initial, intuitive concept was then extended and grounded in theory through a thematic review that bridges relevant literatures from multiple perspectives, including on net zero planning [12], carbon lock-in and stranded assets [13,14], path dependency [15], technology assessment [16], and transitions [17]. Building on this, iterative engagement with real-world policy decision contexts in Canada over a three-year period enabled the elaboration and refinement of a structured assessment approach to help with the identification of such dead-end pathways. The paper is organized as follows. It begins with an overview of the research approach and then draws on the thematic literature review to articulate a theory-informed definition of a dead-end pathway in the net zero context. Next, it outlines a decision support framework to identify such pathways, highlighting assessment of the depth, breath, and timeliness of the proposed sequence of change. The framework is applied to three case studies from the road transport sector centered on investment in (a) compressed natural gas (CNG) for long-haul trucking in Canada; (b) ethanol for personal road transport in the United States; and (c) e-fuels to decarbonize personal road transport in Germany. The paper then considers some potential objections to the framework and explores future research needs. We close out the discussion by reinforcing the significance of identifying and avoiding dead-end pathways.

2. Research approach This study adopts a transdisciplinary action research method [18], which involves: (a) a focus on responding to real-world challenges, (b) embedding of the research in practical settings, and (c) coproduction of knowledge with practitioners. Given the urgency of addressing climate change and other pervasive sustainability challenges, this methodology has become increasingly common within the field of sustainability transitions [18–20] and there have been growing calls for its uptake within broader climate research [21–24]. This method embraces the role of the researcher as an active participant in processes of transformative change. It is particularly concerned with research questions taking the form of “how to” – in this instance, how to identify and, ultimately, avoid dead-end pathways. Our study embodies the above approach in the following ways. First, the research team is intentionally transdisciplinary, drawing together policy practitioners working on climate policy in Canada, civil society leaders building out transition pathways in practice as part of the Transition Accelerator in Canada, and scholars working to advance sustainability transitions in policy and planning. Second, the research team brings together a diversity of perspectives from political science, policy studies, engineering, and climate science. Third, the research was carried out in an applied climate policy and planning setting over the span of July 2021 to October 2024. In this fashion, the research and its findings emerge from the real-world challenges facing Canadian policy practitioners wrestling with the climate file and are likely to hold lessons for other national climate policy settings. The research proceeded in the following manner. From July 2021 to December 2021, the early conceptualization of dead-end pathways and a preliminary framework were co-developed in the context of collaborations with the staff of the Transition Accelerator (see Fig 1). This involved multiple meetings and a half-day workshop. From January 2021 to June 2024, the framework was further co-developed and refined through a thematic literature review that bridges complementary perspectives (described below) as well as practical collaboration with policy practitioners working on federal climate policy. This iterative co-development process involved regular engagement with practitioners through presentations, meetings, and trial runs. From July 2024 to October 2024, the transdisciplinary research team worked to reflect feedback received throughout this process. PPT PowerPoint slide

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TIFF original image Download: Fig 1. Simple visualization of a dead-end pathway. Adapted from Layzell & Beaumier [25] and Meadowcroft et al. [26]. https://doi.org/10.1371/journal.pclm.0000693.g001 The thematic literature review allowed us to refine our understanding of dead-end pathways and anchor it in relevant strands of research. In doing so, this review bridges literatures from various perspectives covering net zero planning [12], lock-in and stranded assets [13], path dependency [15], technology assessment [16], and transitions [17,27,28]. Particularly salient written sources were identified based on: (a) iterative searches of the Scopus database of academic literature, using a set of relevant keywords (e.g., “net zero” AND “lock-in”); (b) reference snowballing from these texts; and (c) author knowledge. An initial review of abstracts was used to determine salience followed by an in-depth review of selected papers. Given the objectives of our study (i.e., to develop a practical framework to identify and avoid dead-end pathways), we opted for a targeted literature review. This meant that we emphasized existing review articles [e.g., 27–29], studies engaging with the climate dimensions of particular research strands (e.g., carbon lock-in rather than generic lock-in), and those pieces with critical insights on mechanisms that might generate dead-ends [16]. Articles were then analyzed in an interpretive review tradition [30] to identify patterns across the selected bodies of work, which we summarize in the following section.

3. Thematic literature review: conceptualizing dead-end pathways This review brings together multiple strands of scholarship to conceptualize dead-end pathways, helping to elucidate the issues associated with pursuing proposed solutions that are unable to achieve long-term climate objectives. Emerging from research on climate planning, scholarship on net zero planning often adopts an expansive temporal and geographic perspective on human-climate interactions [9,12,31]. Typically informed by scenario-based modelling [27], this research emphasizes the time-limited task of bending emissions trajectories to align with long-run global temperature goals that avert the worst climate impacts. This scholarship highlights the importance of driving emissions as close to zero as possible, with carbon dioxide removal (CDR) addressing limited residual emissions and potential overshoot [12]. This literature suggests that embarking upon emission trajectories that cannot ultimately meet emission and temperature targets delays the adoption of genuine net zero trajectories and defers more disruptive adjustments until later [32]. Work on net zero planning is well exemplified by the contributions of Working Group III to the assessment reports of the Intergovernmental Panel on Climate Change (IPCC), which define wide corridors for action grounded in the best available science and remain neutral with respect to specific policy choices [27,33]. While still offering an important backdrop to inform policy, decision-makers may therefore find it challenging to translate these general results to their specific policy options. With roots in economics, political science, and innovation studies, the literature on lock-in and stranded assets examines the multi-decadal material implications of investments [13,14,34]. This research highlights the enduring carbon liabilities that such investments can introduce in the context of longer-term (e.g., mid-century) climate commitments. Reinforcing the role of sunk costs and infrastructural inertia, this work reveals how decisions that may appear economically attractive in the short-term can materially entrench carbon-intensive development trajectories that are costly to unwind [14,35]. In particular, it points to the challenge of dismantling problematic pathways that have been partially built out given long-lived capital stock (e.g., technologies, production facilities, and infrastructure networks) and accompanying socio-institutional supports (e.g., professional standards, regulatory environments, and user practices). Thus, course corrections entail wasting scarce resources and time not only on physical assets [36] but also on accumulated experience [13,14]. Although helpful in drawing attention to investments that leave emissions unabated, this work does not sufficiently distinguish between investments that entrench undesirable trajectories and those that might serve as stepping-stones toward net zero systems. Closely related to research on lock-in, political perspectives on path dependency investigate the way in which institutional and actor dynamics underpin certain trajectories of societal development [11,15,37,38]. This work emphasizes how policy choices can create self-reinforcing feedback processes that constrain future choices, narrowing the envelope of possibilities in each round of decision-making and, over time, making deviation from the established trajectory increasingly difficult. As Pierson [39] notes “once on a particular path, political actors will generally have powerful incentives to stay on it”. In the climate context, this dynamic can manifest as vested interests seeking to sustain or advance unworkable pathways that are misaligned with long-term climate goals [40,41]. Interests may be tied to technological choices (e.g., equipment manufacturers, feedstock producers, technical experts), certain types of policy instruments (e.g., industrial carbon pricing frameworks) that obscure compliance costs [42–44], and proposals for interim solutions that delay more profound change [45]. Recognizing the deleterious effects of path dependency, research also shows that path dependent processes can help drive innovation (e.g., through positive learning effects) and be deliberately leveraged to, for instance, build constituencies supportive of climate policies [see 36,46,47]. While this duality makes path dependency a critical process to understand and leverage, it also makes it an insufficient indicator for decision-makers seeking to determine whether a specific choice leads along a desired or problematic pathway. Scholarship on technology assessment has a long history in practical decision-making settings, having been institutionalized in the 1970s through the work of the United States Office of Technology Assessment and spreading to other jurisdictions over the ensuing decades [48]. In the decarbonization space, technology assessment examines the impacts of emerging and established technologies with respect to climate goals [16,49]. It underscores that it is no trivial task to identify problematic technological approaches in open-ended processes involving complex systems [50] and competing societal priorities [51]. A technology may at first appear promising but ultimately be unable to realize this potential due to one or more limiting factors [16]. Indeed, Hillman and Sandén [49] highlight how a technology can be “constrained in some dimensions, prohibiting it from solving the problem in a more fundamental way in the longer term”. This suggests that technology assessment for net zero cannot be reduced to simply identifying those innovations capable of delivering the greatest, least-cost, and/or most proximate emissions reductions. Nor can early judgements be entirely definitive as innovations and the associated systems within which they are embedded change over time. In applied decision-making contexts, technology assessment can sometimes be applied narrowly to focus on a single technology or individual dimensions (e.g., conversion efficiency), underemphasizing embeddedness within and interaction with a broader system [28,29] – calling for greater appreciation of how technologies contribute to pathways for system change [52,53]. Emerging from a rich set of interdisciplinary perspectives (evolutionary economics, complex systems theory, and innovation studies, among others), transition scholarship focuses on the evolution of socio-technical systems responsible for societal functions such as personal mobility or indoor heating [19,54]. Within this strand of work, transitions are understood as shifts between current and future system configurations that allow for more sustainable provision of these functions [52,55]. Take, for example, moving from an indoor heating system based around natural gas furnaces and distribution networks to one oriented around electric heat pumps. These processes not only involve changes in technologies and prices but also interrelated adjustments in infrastructures, social practices, institutions, and ideas [56]. Pathways, in this view, can be understood as relating to (a) the current configuration of the system of interest, (b) the envisioned end state of the transition, and (c) the intervening steps that can bridge current and future system configurations [27,52,55,57]. Therefore, specific technological, investment, or regulatory steps can encourage or inhibit movement along such a transition pathway. Although transition perspectives offer a useful lens on pathways, their application to decision-making can sometimes point to too many assessment dimensions (as transitions are multi-causal processes) without more focused application [26,58] or the benefit of complementary approaches such as those discussed above [28,29]. Taken together, the reviewed literature sheds light on processes that can lead to the entrenchment of measures that cannot ultimately deliver on climate goals. In doing so, the review bridges these perspectives to help elucidate critical elements which can be used to enhance our understanding of dead-end pathways (see Table 1). But before we move to present our conceptualization of dead-end pathways, we must first briefly engage with established understandings of net zero pathways. PPT PowerPoint slide

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TIFF original image Download: Table 1. Summary of literature review results. https://doi.org/10.1371/journal.pclm.0000693.t001 Drawing on transition scholarship, we understand net zero pathways as sequences of change across multiple social and technological dimensions which lead from current (GHG-emitting) systems to future net zero configurations [26,27]. Such pathways involve the progressive development, deployment, and/or adjustment of various system elements: technologies, infrastructure, business models, policies, consumer attitudes, and so on (see Fig 2). Typically, they include ‘anchor technologies’ that enable core system functions (e.g., indoor heat or food) to be met in a decarbonized fashion, supported by complementary technologies, infrastructures, and adjacent systems [59–61]. For example, decarbonizing personal road transport requires advances in electric propulsion and battery technologies (anchor technologies), progress around charging infrastructure (supporting technologies), and upgrades to the electricity grid (adjacent system). PPT PowerPoint slide

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TIFF original image Download: Fig 2. Elements of a net zero compatible transition pathway. https://doi.org/10.1371/journal.pclm.0000693.g002 Bridging the strands of work covered in our review, a dead-end pathway, in contrast, (a) shows initial promise, but (b) ultimately lacks the capacity to address the underlying problem in a fundamental way due to specific constraints, and so (c) its continued pursuit channels societal resources in directions that inhibit system change and/or must be dismantled later on. We apply this definition to pathways directed at the socially determined goal of net zero and emphasize the debilitating impacts of continued investment in efforts that cannot deliver on this goal. In what follows, we operationalize this definition in terms of three critical dimensions: depth, breadth, and timeliness.

4. Results of the action research process: A dead-end pathway framework Having defined net zero pathways and their dead-end counterparts, it is now possible to outline the assessment framework that emerged from the action research process. The framework involves three steps. The first step is to clearly define the pathway that is to be examined. This includes identifying the anchor technologies underpinning a proposed pathway and how they are intended to deliver net zero in a particular system. It requires an understanding of the current system configuration, an outline of the future net zero system configuration, and the sequence of steps that could realistically bridge the two. A clear definition of the system under examination and how a proposed pathway is intended to lead to a net zero end state are essential for the assessment exercise. This implies that near-term emission reduction options and bridging solutions (intermediate steps) are primarily evaluated not for what they can deliver in the short to medium term (GHG emissions avoided at a specific cost), but rather on whether they constitute a genuine advance that leads toward a net zero compatible end state. The second step of the assessment framework focuses on determining the ability of the postulated pathway to deliver net zero for the defined system context by assessing its performance on three diagnostic tests relating to (a) depth, (b) breadth, and (c) timeliness. Failure to deliver on one or more of these tests, particularly in the presence of alternatives that offer superior performance, indicates the proposed option may constitute a dead-end. The final step draws together the analysis across the three diagnostic tests and considers whether, notwithstanding a failure to prove viable for the proposed system, the pathway could pass the diagnostic tests in a more limited system context. A visual representation of this assessment process is presented in Fig 3. PPT PowerPoint slide

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TIFF original image Download: Fig 3. Overview of the assessment framework. See Fig 5 for details on the components of each assessment step. https://doi.org/10.1371/journal.pclm.0000693.g003 Before we examine the three dimensions (depth, breadth, and timeliness) that form the heart of this assessment approach, it is necessary to clarify a few additional points that emerged during the development and application of the framework as part of the action research process. First, while the framework emphasizes technology, this is not to deny the relevance of social factors in processes of system change or the significance of pathways that foreground transformation of societal and behavioral norms. We agree that climate policy modelling has often neglected the mitigation potential associated with behavioral and demand-side adjustments [62]. Nevertheless, we justify the emphasis here on technologically anchored pathways for three reasons: (a) today these options lie at the heart of international climate mitigation efforts; (b) governments and investors are currently allocating billions of dollars to fund research and development, incentivize technology adoption, build out infrastructure, and establish manufacturing facilities for competing technological options; and (c) even those pathways that appear at first glance to center on individual behavior or cultural norms often rely on complementary technical processes (e.g., a shift away from conventional meat consumption may appear to be a matter of individual choice or cultural norms, but it is closely intertwined with technological developments in plant-based protein and even potentially cultured meat alternatives, supply chain restructuring, and changes in food processing and agricultural practices). Indeed, innovation and transition studies reinforce the co-evolutionary nature of technological and social change [63–65]. Second, the evaluation of transition pathways is most easily performed once detailed analysis, research, debate, and practical experiments with novel technologies in a particular system are well underway. In the very early stages of innovation, uncertainties about the future viability of alternatives (e.g., technical potential, costs, contribution to sustainability challenges, and unwanted side-effects) may be so great as to render grounded judgement difficult. But where existing analyses have already been conducted, from technology assessment [16] to prospective life cycle assessment [66,67], there are opportunities to inform and complement the identification of dead-end pathways. Third, assessments of this kind are always made at a particular point in time and potentially subject to revision. For example, a pathway that looked promising a decade ago may appear more dubious today. This is why the proposed framework focuses on the likelihood that a given pathway will turn out to be a dead-end, rather than attempting to offer a final and definitive binary (yes/no) evaluation. In this vein, the framework employs a distinction between pathways that are ‘very likely’ to represent a dead-end and others that are merely ‘likely’ to constitute such dead-ends. Finally, it is important to note that a pathway that is not categorized as a dead-end under this framework does not imply that it is necessarily a good one. It simply means that this pathway cannot at this point be ruled out, and that its relative strengths and weaknesses should be compared with other potential pathways when making investment decisions, designing policy supports, and so on. 4.1. Overview of the diagnostic tests The framework operationalizes dead-end pathways as those that can be identified today as being unlikely to deliver on one or more of depth, breadth, and timeliness. What this means is that if a pathway is underpinned by an anchor technology (or set of interlinked anchor technologies) that cannot (a) get close to zero GHG emissions (depth), (b) be applied across a relevant system context (breadth), or (c) diffuse until far into the future while other options are already viable today (timeliness), it probably constitutes a dead-end. These diagnostic tests are conducted sequentially – only pathways that can deliver on depth of decarbonization are then assessed for their potential breadth of application, while only those that pass both the first two tests are evaluated for timeliness. We designate pathways that fail on depth or breadth as very likely dead-end pathways, whereas those that fail only on timeliness are designated likely dead-end pathways. This distinction turns on the greater uncertainties associated with the pace of international climate mitigation efforts (and ability to achieve 2050–2060 net zero targets) and the possibility that some of these likely dead-end pathways may ultimately prove useful at some later date. Idealized representations of the implications of succeeding or failing on the abovementioned dimensions are presented in Fig 4. The following subsections outline the pertinence and application of the diagnostic tests relating to each of these dimensions. PPT PowerPoint slide

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TIFF original image Download: Fig 4. Illustrative pathways. https://doi.org/10.1371/journal.pclm.0000693.g004 4.2. Depth: Contributing to a net zero world Net zero entails two generic strategies, emissions reduction and CDR. Since CDR approaches remain uncertain (e.g., in terms of their viability at scale) and come with substantial land and energy demands [12,68], science-informed net zero policy emphasizes getting as close to zero (absolute) emissions as possible, leaving CDR for contexts where deep emissions reductions are technologically infeasible or prohibitively expensive [12]. From this, an assessment of depth concerns the extent to which a proposed pathway enables progress toward the desired system end state, essentially eliminating emissions from the specified system context. If a pathway is anchored in technologies with inherently limited abatement potential – such that it cannot plausibly lead to a future configuration that essentially eliminates emissions – it is very likely to represent a dead-end [25]. Importantly, the point is not that the anchor technology can deliver zero or near zero emissions tomorrow but rather that it constitutes a step on a pathway that can in principle (when fully developed, combined with additional measures, and contributions from adjacent systems, etc.) virtually eliminate emissions. Thus, for example, electric vehicles in the context of the road-based personal transport system would pass this test, even though present automobile and battery production and electricity generation are far from being carbon-neutral. While this diagnostic test underscores essentially eliminating emissions, there are system contexts (e.g., some heavy industry) that necessitate relaxing this condition to allow for deep emission reductions. Given varied system conditions, there can be no absolute rule as to what this entails. However, options that can secure 85-90% reductions have now been identified for many systems, even in contexts such as steel or cement that were considered ‘difficult to decarbonize’ as little as a decade ago [69,70]. Except in rare cases where viable alternatives have not yet been identified, technologies which cannot at least secure such deep emissions reductions are prime candidates for dead-end pathways. This still leaves the question of how to handle intermediate solutions (say that provide 30% or 60% abatement) that are intended as bridging options [71]. As noted earlier, a positive judgement rests on whether the partial emission reduction results from a pathway that can ultimately lead to the virtual elimination of emissions (e.g., as the anchor technology is perfected, combined with complementary solutions that enable deep emissions reductions, and/or accompanied by progress in adjacent fields). But if the reductions are secured through investments that must later be overwritten to make decisive progress towards net zero [13], then there is a high chance it represents a dead-end pathway. As there are scarce resources and a finite timeframe within which to make progress, the framework places weight on stepping-stones towards an authentic emissions elimination solution rather than the implementation of stopgap technologies that distract from the build out of net zero solutions [72]. 4.3. Breadth: Securing progress across a defined domain Since the goal is functional net zero systems, the framework places emphasis on proposed solutions that are scalable. That is, breadth entails that solutions can scale across the system context for which they are intended. Factors that could limit the applicable scale of pathway anchor technologies could in principle include varied functional needs across a system; requirements for physical space (e.g., land, pore space), raw materials, or labor; economic costs (projected, in many instances); public attitudes and existing laws; and the impacts of other environmental considerations [16,53]. In practice, many potential limiting factors can be overcome on timescales of a decade or more – for example, by accessing new mineral deposits, material substitution, labor force development, cost reductions with mass deployment, application of pollution control technologies, and so on. Consequently, two issues deserve particular attention. The first concerns the availability of bioresources and the land on which they are produced. In a net zero economy, there will be multiple competing demands for land to produce food, fiber, energy, and chemical feedstocks, as well as for carbon storage and other ecosystem services [73,74]. Proposed uses for biological resources in a net zero world already exceed by several times actual productive potential and, notwithstanding the possibility of new sources of biomass (ocean resources, algal farms in deserts, etc.), pathways that depend on technologies reliant on large-scale harvesting of these resources deserve scrutiny [75,76]. The second issue relates to technological approaches that display negative rather than positive learning rates. In some contexts, the limiting factors cited above, often in conjunction with social or political considerations, can mean that costs rise (rather than fall) as the scale of deployment increases. In large part, conventional nuclear electricity generation in Western Europe and North America has been subject to these processes over the past four decades [77,78]. Sometimes these barriers can be overcome with further technological adjustment or a shift in societal factors, which is, for example, the hope of proponents of small modular reactors. But, where present, negative learning rates and diseconomies with increased deployment invite careful evaluation as to whether the proposed anchor technology may in fact underpin a dead-end pathway. Of course, anchor technologies that can essentially eliminate emissions but are not available for general application across a system might still find a more limited niche. Large functional systems may contain many specialized applications. While gasoline and diesel-fueled internal combustion engines overwhelmingly dominated road transport over the past century, distinct niches existed in specific countries at various points in time for alternatives (e.g., electric milk floats in the UK in the 1950s and 1960s or ethanol fueled automobiles in Brazil in the 1980s and 1990s). Variegated niches relying on different technologies are likely to exist in a decarbonized world, so technologies which anchor dead-end pathways for a whole system may still be viable in specialized applications (e.g., biodiesel to power remote communities rather than all communities). But from the perspective of rationally allocating societal investment, it is critical to know the scale potential of the proposed pathway. 4.4. Timeliness: Meeting the urgency of the climate challenge Climate change is already producing serious impacts [79], and the associated mid-century net zero target indicate that time is of the essence [80]. An assessment of timeliness, therefore, means that a pathway and its underlying anchor technologies must be capable of being rolled out in a timeframe consistent with responding to the problem. In the face of pathways which can deliver net zero at scale and that are already being deployed, anchor technologies that face multiple developmental challenges and remain decades from large-scale deployment may represent dead-ends – not in the sense that the proposed technologies could never work, but in the practical sense that they cannot deliver in time. This is particularly true when one is being asked to endorse a long bridging phase where the system remains dependent on unmitigated fossil energy (even as other options are increasingly available), while waiting for the underdeveloped anchor technology to mature. This does not mean that societies should write off continuing research and development on more exotic or uncertain technological options. It is far from evident that developed countries will meet the mid-century target, so lagging technologies may still be needed. Ultimately, they may have better cost profiles or other attributes than technologies that can do the lifting until 2050 or 2060. Even if net zero targets are met in the next three or four decades, the struggle to maintain decarbonized societal development, and perhaps to go ‘net negative’, will continue in the second half of the century and beyond [81]. Nevertheless, the hope that emerging technologies may one day pan out should not dilute current efforts to accelerate implementation of net zero pathways that already perform well on depth, breadth, and timeliness. 4.5. Putting the elements together The logic of the assessment framework, which draws together the above diagnostic tests, is detailed in Fig 5. A pathway that fails on the depth dimension is classed as either a ‘very likely dead-end’ or ‘inconclusive’ depending on the presence or absence of viable alternatives. If it passes on depth, the assessment proceeds to breadth where failure leads again to a ‘very likely dead-end’ or ‘inconclusive’ assessment. A pathway that passes both depth and breadth moves to timeliness, where failure results in a ‘likely dead-end’ or ‘inconclusive’ determination. Successfully passing all three tests (depth, breadth, and timeliness) suggests the pathway should be more fully considered along with other practicable options. It is important to note that there is a prompt to consider the viability of alternatives at each offramp in the second step of the framework. PPT PowerPoint slide

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TIFF original image Download: Fig 5. The assessment framework. https://doi.org/10.1371/journal.pclm.0000693.g005 For pathways which fail to pass the breadth hurdle, the assessment framework has one further step which considers whether – notwithstanding an inability to scale as initially intended – the anchor technology might nevertheless service a niche function in a net zero world. If this seems plausible, the pathway can be run through the evaluation framework with a more focused or alternative system definition to determine how it performs in this more limited context. Rescoping the system in a narrower fashion also has the intention of rightsizing resource allocation to this solution. A clear advantage of the assessment framework is, therefore, that it not only can weed out pathways which can play no useful role in attaining net zero, but also that it encourages a more careful delimitation of the contexts in which proposed options may contribute as well as corresponding reconsideration of relative investment in these options. Before demonstrating this framework through three brief case studies, it is important to consider how to address pathways that are intentionally or unintentionally vague in one or more areas. Indeed, discussion of net zero pathways is today plagued by the under-specification of (a) the anchor technology or technologies intended to underpin a net zero solution, (b) the precise applications for which the solutions are supposed to be appropriate, and (c) the timescale on which practical deployment is feasible. Ambiguity on these three elements may reflect fuzzy thinking, risk-aversion on the part of proponents given the need to protect proprietary processes, or uncertainties about the performance of their technology. However, a lack of specificity may also be part of a deliberate strategy to mislead or obfuscate [82]. This can be to win government or investor support for an option which cannot (or is unable to demonstrate that it can) achieve deep decarbonization, and/or which could only be applied in a very narrow context, and/or deployed late in the century. Or this may simply be intended to muddy the waters and delay societal investment in authentic net zero pathways which threaten to disrupt incumbent businesses. The framework handles ambiguity by reasoning forward [37] about what is implied about a transition end state based on the claims and proposals associated with a particular pathway.

5. Assessing dead-end pathways in three case studies This section harnesses the above framework to assess three case studies. The first case involves building out compressed natural gas (CNG) in the long-haul trucking system, which is being advanced in Canada. The second concerns expanding reliance on ethanol in the personal road transport system, which reflects recent decisions by the United States Environmental Protection Agency (EPA). The third case engages with the deployment of e-fuels in the personal road transport system, which is a key proposal by German automotive manufacturers as part of the European Union debate around the phase-out of internal combustion engines. Cases were selected based on a diversity of advanced industrial economic contexts and proposed solutions to show wide applicability of the framework, while holding the general system context stable to allow cases and pathways to be readily compared. The road transport system was selected as the primary focus of these cases because it represents about 15% of global emissions [80] and is an ongoing focus of climate policy and investment. As the primary purpose is to validate and demonstrate the framework, focusing the discussion on one system also simplifies exposition of multiple pathways in a condensed format. 5.1. Compressed natural gas in the long-haul trucking system Companies involved in fossil fuel production and distribution have positioned CNG as an anchor technology for a future heavy trucking transport system capable of delivering significant emissions reductions for Canada [83]. In 2024, initial investments were made to simultaneously build out a fleet of CNG-fueled semi-trailer trucks and refueling infrastructure along major highways, with plans to use this as a foundation for further deployment. In what follows, this CNG pathway for heavy trucking is assessed according to the framework. Step 1: Define the system, anchor technology, and proposed pathway The current long-haul trucking system in Canada is dominated by diesel-powered semi-trailer trucks. As of 2021, there were over half a million heavy trucks in operation on Canadian roads, travelling an average distance of 74,654 kilometers per year [84]. The primary function of this system includes transporting and delivering freight. The current system configuration includes intercity highway networks, truck supply chains, refineries, refueling stations, and regulatory frameworks (governing duty cycles, load limits, vehicle safety, etc.). CNG is an alternative fuel made by compressing natural gas to under 1% of its normal volume (i.e., at normal atmospheric pressure) and allows vehicles to be powered with reduced GHG emissions compared to conventional fuels. To run on pure CNG, specifically designed internal combustion engines are typically required. While some experimentation and initial commercialization was carried out around CNG in the 1970s and 1980s in response to the oil shocks [85], proposals to shift heavy trucks from diesel to CNG in Canada only gained traction as part of climate and air quality efforts in the early 2000s. Initial deployment of CNG trucks in Canada focused on return to base fleets (e.g., waste collection and mail services), receiving funding through various policy frameworks [86]. Interest in CNG was strengthened by declining natural gas costs brought about by the boom in shale gas development beginning in 2005. Today, proponents are seeking to advance a pathway intended to successively build out CNG-fueled heavy trucks and associated infrastructure to meet the function of road-based freight in a lower carbon fashion. Step 2: Assessment of depth, breadth, and timeliness of proposed pathway The framework can now be used to assess the depth, breadth, and timeliness of a pathway anchored in the CNG-fueled internal combustion engine in relation to a potential net zero future for Canada’s long-haul trucking system. Beginning with depth, the key diagnostic question is whether the pathway builds toward a system end state that can essentially eliminate emissions. Estimates of emissions from a CNG-based heavy road transport system vary considerably, but even avid proponents suggest no more than a 50% reduction from diesel [87] whereas others suggest that fugitive methane emissions from fuel production and distribution along with other factors could largely efface this differential [88]. Therefore, CNG for heavy trucking cannot achieve the necessary emissions reductions to pass the diagnostic test concerning depth. Since other potential anchor technologies in active development and deployment (e.g., electric battery and hydrogen fuel cells) have vastly superior emissions reductions potential, CNG can be considered a very likely dead-end pathway. Sometimes a CNG-based system is presented as a transitional or bridging option with a vaguely defined net zero fuel end state. Options floated for a gaseous internal combustion alternative (that would build on the sunk investments in CNG fueling stations and vehicles) include renewable natural gas (RNG) and e-fuels. Re-running the analysis with these end points, RNG would fail on the breadth criteria (see section 5.2 for an explanation of a biomass-based fuel scoped to a wide system function) and carbon-neutral e-fuels on the timeliness criteria (see section 5.3 for an explanation). Hydrogen is another alternative, but vehicle and fueling infrastructure are not sufficiently compatible to act as an end state for CNG system components, meaning CNG assets would need to be written off or radically retrofitted. In sum, all these options delay investment in authentic net zero pathways by at least 15 years (the average lifetime of a heavy truck) and perhaps more as investors would try to recoup the costs of natural gas fueling infrastructure. Instead, the framework would suggest that resources be funneled into maturing electric battery and or hydrogen fuel cell options which can be rolled out in differentiated segments of the heavy-duty vehicle market as technologies and cost competitiveness improve. 5.2. Ethanol in the personal road transport system The Trump administration recently required the EPA to consider allowing the use of higher ethanol blends [89]. This demand echoes earlier calls from ethanol interests in the United States, particularly industrial agriculture companies [90]. As of April 2025, the EPA announced that it will now permit the year-round sale of gasoline containing a higher percentage of ethanol. According to agriculture interest groups, the intention is for ethanol use to be ramped up further over time [91]. In what follows, we deploy the framework to assess an ethanol-based pathway for the personal transport system. Step 1: Define the system, anchor technology, and proposed pathway The current personal road transport system in the United States is dominated by the internal combustion engine-based automobile. The primary energy source in this system is motor gasoline. In 2023, 137 billion gallons of gasoline were consumed in the United States, with ethanol accounting for about 10% of fuel by volume [92]. A configuration of road networks, vehicle supply chains, refineries, and refueling stations underpin this system alongside regulatory frameworks (governing road-based travel) and entrenched user attitudes about the desirability of the automobile [93]. Pathway proposals to increase reliance on ethanol for personal automobiles can be traced back to the oil shocks of the 1970s. However, it wasn’t until deepening concern about climate change in the 2000s that ethanol was positioned as an anchor for a pathway to a future personal transport system. While the end point of this pathway is often vague, it seeks to gradually blend more and more ethanol in gasoline, displacing conventional fuel and driving down emissions from personal transport. This pathway has been encouraged through the establishment of various regulatory frameworks (e.g., the United States’ federal Renewable Fuel Standard and California’s Low Carbon Fuel Standard), which have helped propel substantial growth in ethanol and broader biofuels as well as their integration into the current system. Step 2: Assessment of depth, breadth, and timeliness of proposed pathway We can now assess the depth, breadth, and timeliness of this pathway in relation to delivering a potential net zero road transport system in the United States. Regarding the first diagnostic test, the pathway can be assessed to determine whether it can deliver on the promise of net zero personal transport at the transition end state. While current ethanol production processes can be carbon-intensive due to land use issues and energy inputs [94], some industry estimates [95] suggest that it is at least theoretically possible to bring emissions close to zero through advancements in production techniques (e.g., the use of non-emitting energy or carbon capture and storage for biorefining), farming vehicles (e.g., zero emission farming vehicles), agriculture processes (e.g., no till farming and limited fertilizer use), and land use (e.g., no deforestation). Consequently, a generous assessment would mean that the pathway does not fail on this first dimension. Concerning the second dimension, the key diagnostic test is whether the pathway can build toward a system end state that can meet its intended function at scale. Renewable sources of biomass inputs for ethanol production are limited given land, water, and energy constraints [96]. Increasing ethanol production in line with a deep conversion of the entire personal road transport system in the United States would present serious trade-offs for food and water systems [97], with some of these tensions already becoming apparent under lower ethanol blends [98]. This also says nothing about the drop in productivity from reduced fertilizer use if produced to a net zero standard or the competition for biomass and biofuels outside of road transport (e.g., for carbon removal solutions or aviation fuels). Consequently, the pathway would fail this test as it is essentially impossible to scale to the required end state. Given that there are alternative pathways available that can overcome this issue (e.g., those anchored around battery electric vehicles and diversified battery chemistries), the assessment would end here and the pathway would be considered a ‘very likely dead-end pathway’. Step 3: Review results and potentially return to step 1 and redefine the system, etc. Rescoping the system function as serving a smaller transport niche (e.g., aviation rather than personal road transport) and redefining the anchor technology in terms of sustainable aviation fuels may not deliver a dead-end determination. This demonstrates that not all pathways anchored in biofuels represent dead-ends, but that a pathway based around ethanol in the context of a full personal road transport system is very likely to lead to a dead-end. 5.3. E-fuels in the personal road transport system Interests aligned with Germany’s conventional automotive manufacturing have proposed a pathway for the road transport system anchored in e-fuels [99]. In 2023, industry advocates and German officials secured an exemption for internal combustion engines that use e-fuels as part of the European Commission’s 2035 combustion engine ban [100]. The following assesses a pathway based on e-fuels for the personal transport system. Step 1: Define the system, anchor technology, and proposed pathway Returning to the previously described personal road transport system, which we will not repeat as it shares similarities with the United States but with a different footprint, we can now further specify the proposed pathway anchored in the use of e-fuels that is garnering increasing attention [101]. E-fuels are a synthetic hydrocarbon produced through the combination of electrolytic hydrogen and carbon using the Fischer-Tropsch synthesis, which can be used as a replacement for conventional transport fuels. At present, there is essentially no e-fuel use in the road transport system in Germany, though there are a number of pilot facilities emerging across Europe to produce e-fuels for aviation applications [102]. The system end state envisioned by this pathway would allow for the continued use of the internal combustion engine supported through considerable production and use of e-fuels [103]. Step 2: Assessment of depth, breadth, and timeliness of proposed pathway The analysis can now move to assess the depth, breadth, and timeliness of this pathway in relation to delivering a potential net zero road transport system for Germany. Concerning depth, e-fuels can build toward a system end state that can essentially eliminate emissions [104]. Hydrogen inputs can be produced using electrolysis from non-emitting electricity sources and carbon can be drawn from the atmosphere using direct air capture techniques powered by non-emitting sources. The Fischer-Tropsch process can also be electrified. While the combustion of the resulting e-fuel would produce emissions, it would be carbon neutral as the carbon was initially drawn from the atmosphere as part of the aforementioned process. The pathway would pass this test and move to the next stage of the assessment. Regarding breadth, a pathway for the transport system anchored around e-fuels could in principle meet the intended function at scale. There are no constraints on the availability of carbon and hydrogen inputs. Water for the electrolytic process could also be drawn from oceans in the future [105]. Energy requirements are perhaps the only potential constraint, but not insoluble due to the abundance of non-emitting renewable energy resources. While the technology remains relatively early in its development, some optimistic perspectives project cost reductions that could make e-fuels viable [104]. Adopting a particularly charitable stance, the assessment would not fail e-fuels on this test and would move to the next dimension. With respect to timeliness, the key diagnostic question is whether the pathway can complete the transition within a timeframe that responds to net zero commitments. A pathway anchored around e-fuels relies on an immense and interconnected roll out of direct air capture, electrolytic hydrogen production, and non-emitting energy that goes well beyond that already being considered for the electrification of energy end-uses currently met through fossil fuels [106]. E-fuels are also early in their development and have yet to gain a foothold in current fuel supply chains. Taken together, research suggests that a pathway for the road transport system anchored in e-fuels is unlikely to deliver the envisioned future system in time to address climate commitments [101]. In light of this and in comparison with far more advanced pathways (e.g., those linked to battery electric vehicles), an e-fuel pathway for personal road transport would fail on the timeliness dimension and can be considered a likely dead-end. Step 3: Review results and potentially return to step 1 and redefine the system, etc. The likely dead-end pathway result could be reconsidered if the intended function of the system of interest was rescoped more narrowly to only include a niche function that could be met within a timeframe consistent with climate commitments. This might, for instance, include a focus on vintage engines (e.g., legacy automobiles in the hobby space), or some segment of the aviation fuels market, with investments scaled downward accordingly. The assessment results for the above cases are summarized in Table 2. PPT PowerPoint slide

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TIFF original image Download: Table 2. Summary of assessment results. https://doi.org/10.1371/journal.pclm.0000693.t002 These brief case studies demonstrate the merit of the framework and how it could be applied, but it is important to appreciate that assessment validity is highly sensitive to the accurate description of the anchor technologies, their development, and the proposed system scope. Shift the technological specification or area of application and the analysis must be repeated.

6. Discussion This study has outlined a novel assessment framework to conceptualize, assess, and ultimately avoid dead-end pathways. Such pathways are particularly problematic because they may lock in assets that make it more difficult to achieve climate goals, support incumbents who actively work against climate action, and divert time and resources from more viable options. This study presented three key dimensions that allow for the identification of dead-end pathways: depth, breadth, and timeliness. The framework was applied to, and illustrated through, three brief case studies, with each failing on a different dimension. Our approach demonstrates the importance of embedding near-term actions in relation to the system they are intended to transform and the subsequent steps needed to realize the transition. In this way, it underscores the importance of not taking decisions about technology support in isolation (from future steps, system end states, or other technologies), prompting consideration of which competing options in a given system context can deepen emissions reductions, scale more broadly, or complete the transition in a timelier fashion. Other things being equal, an option that can virtually eliminate emissions is preferable to an 85% reduction; one applicable to the vast bulk of a system (e.g., all heavy trucks) is worthy of more support than one that can reach a narrow niche of specialized vehicles; and one deployable in 2030 is better than one that will potentially mature in 2045. Of course, trade-offs might be required across these three dimensions and there are other factors that enter government and industry choices over preferred pathways (e.g., national or regional development opportunities, energy security, public acceptability). Nevertheless, our framework can support decision-making by helping to prioritize investment in viable pathways and avoid wasting time and resources on dead-ends (see Fig 6). PPT PowerPoint slide

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TIFF original image Download: Fig 6. Dead-end pathways waste time and resources. This figure depicts the implications of prolonging investments in a dead-end pathway. In scenario 1 (left panel), societal investment in the dead-end pathway is reallocated to the net zero pathway in a timely fashion, accelerating change processes. In scenario 2, societal investment in the dead-end pathway is extended before eventually shifting to a genuine net zero pathway, leading to delays and additional wasted resources. https://doi.org/10.1371/journal.pclm.0000693.g006 Importantly, the framework specifies how proposed pathways (fail to) build toward a net zero system in an accessible yet modular fashion, offering the potential to complement existing approaches. On the one hand, the assessment of individual dimensions can draw from data-rich and technical assessment methodologies that range from prospective life cycle analyses [66] to robust decision making [107]. Indeed, using these tools as part of the framework represents a promising direction for future research. On the other hand, the framework can be tailored to the capacity of the user (e.g., analysts in civil society organizations or government), allowing for a broader set of actors to participate in informed debate about potential pathways to net zero. The framework adopts this practical yet accessible design that differentiates it from established tools for three reasons. First, many existing approaches demand considerable capacity and resources from the user, which makes them less amenable to broad uptake, such as by civil society or community organizations involved in policy processes [8]. Second, conventional decision tools encountered regularly by the research team tend to focus on emissions quantification and carbon accounting while obscuring the relationship between individual choices (e.g., around a policy, project, or technology) and long-term system change [108,109]. Third, and perhaps most importantly, policy scholars have long acknowledged that decision-making in the face of complexity is an iterative, pragmatic process rather than a comprehensive, rational one [110]. In this context, our framework can play a valuable role in guiding action, complementing more detailed scientific analyses by offering an accessible point of entry for engagement and deliberation – a role that was further underscored by the action research carried out as part of this study. Given the accessible yet structured approach adopted here, we see several decision-making applications for the framework. It can be used as part of ex-ante evaluation of potential decisions, testing the robustness of claims by technology or project proponents in relation to net zero transitions in specific systems. As part of this, there is an opportunity to compare across multiple pathway proposals being advanced within particular systems (e.g., RNG, heat pumps, and other solutions for building heating systems). It can also be used in an ex-post fashion to call attention to problematic decisions that have already been made, showing that certain actions taken under the guise of net zero are likely inconsistent. These analyses could also be conducted in conjunction or in parallel with the other approaches mentioned above to extend, compare, and/or contrast findings. Some limitations of the framework and our study also merit discussion. First, the framework engages with cost in a more limited fashion than many traditional approaches (e.g., techno-economic modelling and abatement cost curves). Rather than restricting analysis to current costs, it emphasizes projected costs, learning rates (positive and negative), and potential diseconomies of scale as part of the breadth dimension. It does so because technological approaches typically involve learning processes that mean that the accumulation of societal resources dedicated to a technology will drive its costs down over time [77]. From a long-term perspective such as the one adopted here, cost can largely be understood as an outcome of collective decisions rather than an input to drive decisions. Second, some may argue that decisions about competing pathways are politically determined and have less to do with the development of decision frameworks. We recognize that decisions do not necessarily follow from assessment frameworks (which links to our earlier point about decision-making being a iterative, pragmatic process), but frameworks such as ours can contribute to counterbalancing incumbent positions in political contests, including attempts at greenwashing [111] and other forms of resistance and obstruction [2]. Third, some will object to this framework on the grounds that it is problematic to pick winners (or losers, in this instance) in uncertain and messy transition processes. While we appreciate the context of uncertainty in multi-decadal processes of system change, it is also critical to acknowledge that decision-makers are nevertheless faced with navigating this uncertainty as best they can in making policy and investment choices today [112]. Lastly, our study has applied the framework to three brief case studies, stopping short of in-depth application involving multiple system contexts, longitudinal analyses, or other methods (e.g., combining the framework with life cycle assessment or robust decision making). Each of these areas represent potential avenues for future research. For example, the framework might be applied to examine potential candidates for dead-end pathways in other systems, including waste combustion in cement plants, electrification of oil production, or food additives in cattle farming. Our framework can also be used in conjunction with other methods such as discourse analysis seeking to capture and then assess competing pathway proposals around consultation processes, in the media, or policy debate.

7. Conclusion Connecting with the deepening need to accelerate transitions [113] but also recognizing heightened efforts to obstruct climate action [2], the framework presented here reinforces the importance of focusing societal efforts around promising pathways and avoiding dead-ends for reaching climate commitments. While historical investments in a dead-end pathway must be written off, future investments can be wound down as quickly as possible to avoid wasting additional resources and delaying more meaningful action. How to shift away from these unhelpful approaches when some have been explicitly endorsed by governments for years, and many are backed by powerful interest coalitions presents a tricky challenge for policymakers. Nevertheless, a scarcity of time and resources available from now until mid-century demands that society rapidly abandon, and prevent further resources from being expended on, dead-end pathways.

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