Challenges in accelerating net-zero transitions: insights from transport electrification in Germany and California

Addressing the climate crisis necessitates accelerating transitions towards climate-neutral systems of production and consumption, with electrification emerging as a crucial decarbonisation strategy. The acceleration of such net-zero transitions across multiple systems faces significant resistance and contestation. In this paper, we propose an extended list of challenges unique to the acceleration phase of socio-technical transitions: we introduce ‘expansion and contestation’, ‘justice’, and ‘international dynamics’ as additional challenge types to complement the already acknowledged challenge types of ‘whole systems change’, ‘interaction between multiple systems’, ‘decline and resistance’, ‘consumers and social practices’, and ‘governance’. We apply this extended analytical framework to the electrification of private-passenger vehicles and investigate the unfolding transition to e-mobility with evidence from 35 expert interviews in Germany and California. We uncover over 50 real-world challenges associated with these net-zero transitions at the beginning of the acceleration phase. Most challenges fall within the categories of ‘expansion and contestation’ and ‘governance’. While Germany and California share many real-world challenges, we also find significant variation between both jurisdictions, which we attribute to differences in their automotive incumbency, transition governance approaches, and institutional contexts. We discuss implications for future research, arguing for greater attention to the dual politics of acceleration during net-zero transitions: political conflict not only centres around the decline of old industries and future losses, but also around the expansion of the new system and associated future gains.


Introduction
Achieving climate-neutrality by mid-century necessitates a rapid decarbonisation of production and consumption systems through accelerating innovation towards net-zero solutions, particularly in energy and mobility systems as major contributors to greenhouse gas (GHG) emissions (Geels et al 2017, IPCC 2023).Implementing policy change aimed at climate-neutrality, however, faces significant resistance and contestation, making the governance of netzero transitions across multiple systems a complex and difficult endeavour (EEA 2019).An example of such a multi-system transition (Rosenbloom 2020) is the electrification of transport, characterized by increasing interactions between energy and mobility systems, and facilitated through grids, storage, and digitalisation (Canzler et al 2017).This transition requires radical socio-technical changes across multiple systems and their actors (Kanger et al 2021).
As transport electrification enters the acceleration phase in pioneering regions such as California and follower countries such as Germany (IEA 2023), new types of challenges emerge.According to Markard et al (2020) such acceleration challenges include managing whole systems change alongside single innovations (Markard andHoffmann 2016, McMeekin et al 2019), the increasingly multi-system character of transitions (Rosenbloom 2020, Ertelt andKask 2024), resistance of declining industries (Geels 2014, Rosenbloom and Rinscheid 2020), the need for major changes in consumer practises and demand patterns (Sopjani et al 2019, Whittle et al 2019), as well as the growing complexity of governing such transitions (Braams et al 2021, Song et al 2023).These challenge types, however, have not yet been systematically investigated for unfolding transitions, raising questions about their applicability and comprehensiveness.Our study aims to close this gap.Based on our analysis of transport electrification in Germany and California, we not only highlight the cross-cutting nature of real-world challenges, but also extend the five acceleration challenges introduced by Markard et al (2020) with three additional challenge types during the acceleration phase of net-zero transitions.
First, we show that the politics of transitions (Meadowcroft 2009) extends beyond declining industries (Geels 2014) and increasingly involves contestation around establishing the rules of the game of the new system (Raven et al 2016)-which we refer to as the dual politics of acceleration in net-zero transitions.This contestation around the expanding trajectory is not just about layering new policies and institutions on top of existing ones (Lockwood 2016), but radically changing or replacing these with new ones through transformative institutional work (Rohde and Hielscher 2021) and eventual institutional change (Kivimaa and Rogge 2022).Hence, we introduce an additional challenge type called 'expansion and contestation', which encompasses contestation around defining the institutional framework conditions influencing the expanding trajectory (e.g. market design, regulation, standards).These framework conditions determine the distribution of future gains among stakeholders, and are heavily contested among competing actors-incumbents and new entrants-who seek to grow their market shares in the emerging mass markets.These new rules of the game impact the expansion of the production of novel technologies, the development and success of new business models and services, as well as the product variety for future mass markets.We argue that this represents an important additional policy intervention point going beyond the destabilisation of the old regime and the acceleration of niches (Kanger et al 2020) and suggest to call it 'building the new regime' .
Second, we find that challenges associated with the global interplay of systems and their transitions (Binz and Truffer 2017) should be explicitly considered, rather than being implicitly associated with other challenge types.This dedicated attention to 'international dynamics' includes changes to global value chains and trans-national policy feedbacks, but also diminishing international market shares of incumbents as well as increased global competition, such as when new international competitors enter domestic markets (Quitzow 2015, Binz et al 2017).Such global interdependence in net-zero transitions (Meckling and Hughes 2018) has been recognized as potential driver for accelerating transitions (Kern and Rogge 2016).For example, in exportoriented industries international competition from countries seeking industrial upgrading can speed up the adoption of technology bans by countries pursuing industrial renewal of existing industries, such as in the automotive industry transitioning from internal combustion engines towards electric vehicles (Meckling and Nahm 2019).However, international dynamics can also impede acceleration, for instance through critical material constraints (Gong and Andersen 2024), industrial protectionism slowing down stringent carbon regulation (Meckling et al 2015), trade disputes increasing the costs of net-zero technologies (Hughes and Meckling 2017), or heightened geopolitical risks (IRENA 2023), for example leading to concerns about China's role in net-zero transitions (Gosens et al 2020).
Third, we suggest explicitly considering 'justice' (Heffron and McCauley 2018) as a challenge type likely to gain importance as net-zero transitions progress to the acceleration phase.Importantly, just transitions do not only relate to compensating losers in the declining trajectory (e.g.coal workers), but rather imply the need for broader transition policies (Green 2018).Such a more holistic approach to just transitions encompasses an integrated perspective covering procedural, distributive, recognition, and restorative justice (Abram et al 2022).During the acceleration phase, concerns around ensuring the affordability of new technologies and their associated infrastructures for low-income and disadvantaged households are expected to gain political salience (Sovacool et al 2019).A case in point is the affordability of battery electric vehicles (BEVs) and the need for alternatives to cheap home charging solutions for those not owning a house.Failure to address these just transition challenges adequately could result in backlash, slowing down progress towards climateneutrality and risking unsustainable outcomes (Stark et al 2023).As such, this challenge type is highly political and closely intertwined with both decline and expansion.An example for the link between justice and expansion is the challenge of building just global value chains for critical minerals (Sovacool et al 2020).
These three additional challenge types were inductively derived as new categories, with the resulting extended analytical framework guiding our analysis and discussion.Table 1 provides an overview of the extended4 list of eight challenge types associated with the acceleration phase of net-zero transitions5 .As our case, we investigate the real-world challenges arising from the electrification of privatepassenger transport 7 , which has entered the acceleration phase and promises substantial reductions of GHG emissions (IEA 2023).We selected a comparative setting that includes California, a pioneer in transport electrification and cradle of Tesla (Wesseling et al 2015), and Germany, a quickly catching up follower country with a strong incumbent automotive industry (Mazur et al 2015).Selecting two jurisdictions which are often regarded as climate policy leaders but characterized by different institutional contexts and starting points, allowed us to identify a wider spectrum of shared and context-specific acceleration challenges.Both jurisdictions have similar policy ambitions (e.g.climate-neutrality by 2045, ICE phase-out by 2035 8 ) and have seen increasing sales of electric vehicles and an expansion of their charging infrastructure (see figure 1) 9 .However, the relative 6 Data for Germany is taken from the IEA Global EV Data Explorer (www.iea.org/data-and-statistics/data-tools/global-evdata-explorer), and for California from the California Energy Commission (www.energy.ca.gov/data-reports/energy-almanac/ zero-emission-vehicle-and-infrastructure-statistics), and the US Department of Energy (https://afdc.energy.gov/stations/),accessed 10 February 2024.The light-duty zero-emission vehicle (ZEV) population for California includes battery electric (BEV), plug-in hybrid (PHEV) and fuel cell vehicles (FCEV); whereas the IEA only includes BEV and PHEV in their electric vehicle (EV) data for cars.Public charging infrastructure is a conservative figure as it does not include private charging outlets/points.Note that the EV and charging data is not standardized by population so as to show absolute figures, but for a comparison across jurisdictions relative figures (e.g. by 1,000 inhabitants) are recommended. 7Our focus on private-passenger transport electrification limits the scope of our analysis which was necessary to keep it manageable, thereby excluding the more advanced electrification of buses and less advanced electrification of heavy duty vehicles.Our case boundary also implies that we are investigating a case of technology substitution, whereas future research can extend this by investigating wider mobility transitions. 8The ICE phase-out policy originates from the EU-level, and faced some resistance from the German government, in particular its transport ministry. 9With roughly half of the population, California was only slightly below Germany's gross domestic product in 2022 (approx.US$3.6 share of EVs of the existing fleet remains very low in both jurisdictions, at approximately 4% in 2022, which is an important reminder that both jurisdictions are still at an early stage of the acceleration phase. Methodologically, our analysis is based on qualitative data collected through interviews with experts in Germany ( 16) and California (19) conducted between August 2022 and January 2023 (see annex 1).To capture as many perspectives as possible, we deliberately recruited experts from a wide range of different organizations (representing business, think tanks and society) and systems (energy, mobility and others).Interviews were implemented until they reached saturation.On average, the 35 semistructured interviews lasted 72 minutes and were mainly held online.Interviews were recorded (if allowed), fully transcribed, and coded with the qualitative content analysis software MaxQDA.Our analysis proceeded in three steps, involving multiple rounds of iterations between both analysists.First, we conducted a bottom-up coding of all real-world acceleration challenges, resulting in two comprehensive lists for Germany and California, which we clustered by themes.Second, through top-down coding, we assessed these real-world challenges using the eight challenge types presented in table 1, with cross-checks performed by both co-authors.Finally, we compared the real-world challenges to identify similarities and differences between the two jurisdictions.To enhance robustness, this final step was limited to challenges mentioned by at least five interviewees.

Overview
We identified a comparable number of real-world acceleration challenges in Germany (56, see annex 2) and California (52, see annex 3), which were categorized into the five themes 'policy' (crosscutting), 'vehicle-side' , 'charging' , 'grid-side' and 'others' (table 2).The most prominent theme in both jurisdictions is the 'vehicle-side' (19 challenges in Germany and 16 in California), followed by 'charging' (12 and 14) and 'others' (both 10), encompassing challenges in electricity generation, IT, equity and labour10 .
To ensure a robust comparison between Germany and California, we focused our comparative analysis on real-world acceleration challenges mentioned by at least five interviewees in either jurisdiction11 .Of those repeatedly mentioned challenges, our analysis reveals 19 similar ones, while 18 are unique to Germany and 5 unique to California (see table 2)12 .

Similarities between Germany and California
We identified 19 shared real-world acceleration challenges in Germany and California (see table 3).Among these, two challenges fall under the theme of 'policy' , eight pertain to the 'vehicle-side' , four relate to 'charging' , two address the 'grid-side' , and three encompass 'other' challenges.In the following, we illustrate the cross-cutting nature of these realworld acceleration challenges with three prominent examples, one from each theme of 'vehicle-side' , 'charging' and 'grid-side' .
First, in both Germany and California, a major challenge concerns establishing domestic battery production capacity [DE.V.19:8; CA.V.6:5] to reduce reliance on Asian imports {7-INT; 4-EXP; 1-WHO}.In the US, the Inflation Reduction Act (IRA) provides production subsidies and stipulates domestic content requirements, signalling a new era of industrial policy {8-GOV}.Conversely, Germany and the EU are rethinking their industrial policies in response to the changing global context [DE.P.2:8].After initial OEM reluctance, Germany now plays a pivotal role in the emerging European battery industry.However, both cases require substantial investments, innovation, streamlined permitting and favourable production conditions, such as low electricity prices {2-MSI}.Shared objectives encompass reducing global dependencies, bolstering resilience against geopolitical tensions, generating employment and welfare, and reclaiming technological leadership.A related challenge involves ensuring the availability of critical raw materials [DE.V.11:6; CA.V.5:6, CA.V.7:1], for example through diversifying source countries and increasing domestic mining {4-EXP, 7-INT}, while upholding environmental and social sustainability standards {6-JUS}.
Second, California prioritizes flexible charging, particularly passive load management [CA.C.10:14] but also active load management [CA.C.12:2].Harnessing the potential of flexible charging to alleviate grid pressures {2-MSI} is also a key concern of German experts [DE.C.10:4, DE.C.9:6].There, its economics ministry (BMWK) and Federal Network Agency (BNetzA) are tackling this challenge through regulatory changes, engaging energy and mobility system stakeholders through consultations {8-GOV, 2-MSI}.California focuses on several critical regulatory sub-challenges of passive load management, including governing load growth through determining optimal EV charging rates and demand charges {2-MSI, 8-GOV}.Active load management instead seeks to enable utilities to regulate load through  Overall, Germany and California face many similar acceleration challenges.These challenges primarily revolve around 'expansion and contestation' {4-EXP}, 'governance' {8-GOV}, 'whole systems change' {1-WHO} and 'multi-system interactions' {2-MSI}.Based on these observed patterns, we argue that the shared real-world challenges define the overall problem structure of transport electrification and will likely play an important role in other industrialized countries and regions.

Differences between Germany and California 2.3.1. Real-world challenges unique to Germany
We identified 18 real-world acceleration challenges unique to Germany and mentioned by at least 5 interviewees (see table 4) 13 .Among them, five 13 Due to our inclusion threshold of at least five mentions, some potentially important acceleration challenges unique to Germany were not analysed further.For instance, two grid-side challenges emphasize the importance of leveraging digitalization in smart grids [DE.G.4:3]   challenges fall under the 'policy' theme, seven relate to the 'vehicle-side' , five concern 'charging' and one addresses renewable energy expansion as part of 'others' .In the following, we highlight the three challenges with the most mentions.First, a critical real-world challenge unique to Germany lies in the limited implementation and reduced ambition of the transport policy strategy by the current coalition government [DE.P.1:15].Germany needs to substantiate its ambitious climateneutrality target for 2045 with aligned policy mix changes, particularly in the transport sector {8-GOV}.However, a weakening of Germany's sector-specific GHG reduction target for transport, pushed by the non-compliant transport ministry led by the probusiness party FDP {3-DEC}, reduces pressures to implement a sufficiently stringent instrument mix that not only promotes green niche creation (e.g. via BEV premiums) but also reduces the attractiveness of ICEs (e.g. through bonus-malus rules or raising carbon prices) {4-EXP}.
Second, there is significant concern about the weakening of the policy mix due to cross-party conflicts within the government [DE.P.7:13].While communication across silos has improved in the current coalition government {8-GOV}, which is crucial for multi-system transitions {2-MSI}, there is extensive political bargaining around transport electrification and decarbonisation {3-DEC, 4-EXP, 6-JUS}.This is particularly evident among the relevant ministries involved in transport electrification: the digitalisation and transport ministry (BMDV) led by the FDP, and the economics and climate ministry (BMWK) led by the Greens, but also the finance ministry (BMF) also led by the FDP.The transport ministry, in particular, is said to lack ambition and obstruct key policy measures.In contrast, many experts note a more ambitious and engaged BMWK, which faces resistance from both the BMDV and BMF implementing a more effective policy mix for accelerating transport electrification.Third, building up the charging infrastructure is identified as an important multi-faceted challenge for accelerating transport electrification in Germany [DE.C.3:11] {1-WHO, 2-MSI, 4-EXP}, not unlike in California.However, Germany's beginning acceleration phase is characterized by a unique debate about whether charging remains a bottleneck for BEV adoption {5-CON}.The automotive industry argues that insufficient charging infrastructure hampers adoption {3-DEC}, while the energy industry contends the opposite, namely that insufficient BEVs, especially competitive models, hamper the expansion of the charging network [DE.V.7:9].Regardless of this 'chicken-egg' debate, establishing public and private charging infrastructure remains a major task with various sub-challenges of technical, regulatory, economic, and systemic nature specific to the German context [DE.C.1-2,4-12].For example, in Germany it remains debated whether subsidies should be reduced or targeted at disadvantaged locations or users to ensure a more efficient use of limited public resources {4-EXP, 6-JUS, 8-GOV}.
Overall, the majority of the real-world acceleration challenges unique to Germany revolve around 'governance' issues {8-GOV} and contestation in building up the new e-mobility system {4-EXP}.They also touch upon aspects of 'decline and resistance' {3-DEC}, which is not as prominent in California.

Real-world challenges unique to California
We identified five real-world acceleration challenges unique to California frequently mentioned there, but not in Germany (see table 5) 14 .Among them, one challenge falls under the 'policy' theme, one relates to the 'vehicle-side' , and three concern 'charging' .In the following, we highlight the three challenges with the most mentions.
First, a significant challenge concerns the regulatory patchwork of the US electricity system [CA.P.7:12].While this pertains more to the US rather than to California, it has implications for multi-system business models {2-MSI} and expansion plans of actors involved in transport electrification {4-EXP}.This institutional governance challenge {8-GOV} results from electricity being regulated at the state level, primarily by Public Utilities Commissions (PUCs).Consequently, the US electricity system exhibits substantial variation across 14 Given our inclusion threshold of at least five mentions, some important but less widely recognized acceleration challenges unique to California were not included.For instance, one significant cross-cutting policy challenge is the effective and efficient implementation of already adopted policies [CA.P.1:3], and the protection of California's legal authority to set its own GHG standards [CA.P. states and PUCs, with different states having different types of systems (liberalized, vertically integrated, or, like in California, hybrid).This means that transport electrification requires policy change in 51 jurisdictions (50 states, plus Washington, DC), making it challenging for policies and business models to scale across jurisdictions, thereby requiring tailored approaches for each context and restraining acceleration.
Second, we find that business models for public charging services are weak and diverse {2-MSI, 4-EXP}, varying significantly across electric vehicle service providers (EVSPs), thereby shaping their policy preferences [CA.C.11:11].However, it remains unclear to what extent EVSPs' policy interests align with public interests {6-JUS, 8-GOV}.Several sub-challenges exist concerning business models, including disincentives to invest in maintenance, leading to low reliability, which in turn endangers consumer acceptance {5-CON}.Many charging stations in California are unreliable, discouraging vehicle adoption and potentially causing consumer dissatisfaction [CA.C.2:9].Other difficulties associated with weak business models for charging services encompass investment uncertainty, principal-agent problems between EVSPs and site hosts, conflicts over rate design and demand charges, as well as equity concerns {6-JUS}.
Third, we observe conflicts between utilities and EVSPs regarding ownership and operation of charging stations [CA.C.1:9].In California, this institutional conflict was resolved through an informal compromise known as 'make readies' , whereby utilities own and operate equipment up to the meter while EVSPs compete for the charging station {1-WHO, 2-MSI, 4-EXP, 8-GOV}.Similar conflicts are emerging in other US states, weakening the transport electrification coalition and impeding the expansion of public charging infrastructure.
Overall, these acceleration challenges unique to California revolve around 'expansion and contestation' {4-EXP} and 'governance' issues {8-GOV}, aligning with similar trends in Germany.This is followed by 'multi-system interaction' issues {2-MSI}, which also feature prominently in Germany.Notably, 'international dynamics' {7-INT} are not relevant here, whereas they were frequently considered in Germany.

Cross-cutting nature of real-world acceleration challenges
The identified real-world acceleration challenges in transport electrification in Germany and California do not neatly fit into a single challenge type but demonstrate their cross-cutting nature.On average, each real-world challenge is associated with approximately four out of the eight challenge types15 .Nevertheless, for analytical purposes for each realworld challenge we also identified its most dominant challenge type, which, however, was not always straightforward due to the interwoven nature of many challenges (see table 6).As our subsequent discussion shows, the resulting figures only provide some initial orientation, and need to be unpacked with empirical insights.

Similarities in the challenge types identified in the acceleration phase
Overall, we find a similar pattern in Germany and California.The most notable similarity is that 'expansion and contestation' {4-EXP} is ranked very highly in both Germany and California, being the number one dominant challenge type in both jurisdictions.Approximately 80% of all real-world challenges are associated with political contestations around the expanding trajectory, underlining the importance of this new category.That is, as the transition accelerates, most challenges involve designing the newly emerging multi-system regime, thereby determining 'Governance' {8-GOV} is confirmed as highly relevant challenge type in both jurisdictions, ranking first in Germany and second in California when considering multiple challenge types (and the other way around when focusing on the dominant type).This underscores the important role of the state in deliberately governing the acceleration phase, for instance by enacting policy mixes targeting creative destruction (Kivimaa and Kern 2016), but also for coordinating socio-technical change and facilitating multi-stakeholder engagement.Effective governance is essential, particularly for coordination, given the complexity of multi-system transitions [DE.P.5:6, DE.P.8:4; CA.P.6:4].
Interestingly, 'whole systems change' {1-WHO} does not appear prominently in either Germany or California when considering the dominant challenge type.Only one real-world challenge is primarily about whole systems change, namely the lock-in of the car cultures in Germany and California, with some distinct differences.Germany boosts urban mobility niche experiments and initiatives that offer avenues for loosening the socio-cultural embeddedness of cars and rethinking the future of mobility more broadly [DE.V.18:5].In contrast, the rigidity of the automobile lock-in in California makes broader mobility transitions beyond drive substitution unlikely, potentially jeopardizing broader sustainability targets in the long term [CA.V.14:4].One potential explanation for why 'whole systems change' does not appear more prominently as a dominant challenge type in either of the two jurisdictions may lie in our study's focus on transport electrification, which is mainly associated with transitioning away from cars with internal combustion engines (ICE) to battery electric vehicles (BEVs), which may not necessarily be associated with a fundamental shift away from a car-centric transportation system.When considering the cross-cutting nature of most acceleration challenges, however, approximately 55% of all real-world challenges touch upon whole systems change.
Another similarity concerns 'multi-system interactions' {2-MSI}, which is highly prevalent in both Germany and California, and indicative of the aforementioned cross-cutting nature of many real-world challenges.Its high prevalence in many real-world acceleration challenges is not surprising given that we examined the electrification of transport where interactions between the energy and mobility systems are crucial in roughly three fifths of all identified challenges.This suggests that acceleration requires actors with cross-system analytical, operational and political capacities, even if their original expertise lies in only one of the increasingly interconnected systems.Solving acceleration challenges will necessitate significant cross-system learning and multi-system capacities.For instance, enabling charging at multi-family housing requires cooperation between charging, grid, and building experts from industry, policy and civil society [DE.C.11:5,CA.C.13:11].
'Consumers and social practises' {5-CONS} received limited recognition as a dominant challenge type but featured in nearly half of all real-world acceleration challenges as one of several challenge types.However, there are nuanced differences between both jurisdictions.For example, in California the debate seems to have progressed further regarding mass-market consumers and their distinct needs compared to early adopters [CA.V.12:6].In contrast, in Germany the focus still lies on highlighting the high satisfaction of first movers to shape a more positive narrative around BEVs and charging among non-users [DE.V.19:5].
At first glance, the pattern for 'international dynamics' {7-INT} looks similar in both Germany and California, involving shared challenges associated with supply chains [DE.V9:7, DE.C.4:2; CA.V.4:5, CA.V.5:6, CA.C.3:5, CA.G.3:5] and access to critical materials [DE.V.11:6; CA.V.7:1].However, upon closer examination the jurisdictions demonstrate distinct differences.Germany is grappling with maintaining the competitiveness of its automotive industry [DE.V.16:2], while California, the cradle of Tesla, has been leading the transition to BEVs [CA.P.2:1, CA.P.3:3].This demonstrates the additional struggles Germany faces as it tries to catch up with global market developments which for a long time have been underestimated by German OEMs and politicians alike.Furthermore, industrial policy decisions at the federal level of the US, particularly the IRA, are prompting Germany and Europe to reconsider their industrial policies [DE.P.2:8], for example due to the IRA's impacts on exports [DE.P.2:8], but may also endanger acceleration speed in California, e.g. by limiting product availability [CA.C.3:5].German experts are also concerned about Chinese OEMs entering the German market, but acknowledge that this competition could help accelerate transport electrification [DE.V.14:5].This demonstrates that paying closer attention to international dynamics in the acceleration phase can generate insights on how global interplays shape transition speeds.
Finally, several challenges related to 'justice' {6-JUS} in transport electrification are shared by California and Germany, with experts raising concerns about the affordability of BEVs [DE.O.5:6;CA.O.7:1] or the slow start of used BEV markets [DE.O.6:4;CA.O.6:3].However, concerns about justice in transport electrification were generally more pronounced among Californian experts, which could be explained by California's higher living costs and more pronounced income inequality.As transitions accelerate, neglecting justice considerations might lead to backlash and hinder further electrification.To mitigate this risk, greater attention should be devoted to this challenge type.For example, policy makers could re-examine their funding programs, which in Germany were criticized for favouring affluent households [DE.O.7:3].

Differences in the challenge types identified in the acceleration phase
Overall, seven out of eight challenge types capture a basic problem structure that looks relatively similar in Germany and California, with the main exception being 'decline and resistance' {3-DEC}.Although relevant in both locations, this challenge type appears more pronounced in Germany.Ten acceleration challenges in Germany are primarily associated with resistance to decline, compared to only four in California.As a result, 'decline and resistance' ranks as the third dominant challenge type in Germany (compared to seventh in California).Similarly, nearly half of the real-world challenges in Germany contain an element of 'decline and resistance' , compared to only a third in California.
These differences could be attributed to the significant importance of the automotive industry in Germany, while California's economy appears more diversified and less dependent on its domestic oil industry.In addition, the fact that California is the cradle of Tesla-one of the world's most influential BEV manufacturers-can help explain the differences.Additional explanatory factors of the differences between the two jurisdictions will likely include California's long history of progressive environmental policymaking in the transportation sector dating back to the 1970s and California's strong regulatory institutions governing tailpipe emissions standards under the Clean Air Act.Finally, differences in sectoral institutions, such as their electricity market design and corresponding regulatory oversight serve as further explanatory factors for the observed differences.

Conclusion
We make three main contributions to the transitions literature by offering an improved understanding of the challenges associated with the acceleration phase of net-zero energy-mobility transitions.Conceptually, we extend the five challenge types proposed by Markard et al (2020) with three additional ones, namely 'expansion and contestation' , 'justice' , and 'international dynamics' .Methodologically, we conduct a comparative analysis with 'on the ground' insights from expert interviews, thus going beyond typical single case analyses, identifying similarities and differences between Germany and California.Empirically, we offer nuanced insights into multisystem transition processes for private-passenger transport electrification in Germany and California, and highlight the cross-cutting nature of real-world acceleration challenges.
Our findings reveal similar problem structures in transport electrification across Germany and California, although with nuanced differences.As the transition accelerates, however, we observe two different political modes (Breetz et al 2018).In Germany, transport electrification policy is much more politically contested, with distinct challenges stemming from the resistance of automotive incumbency.In California, the political mode appears more technocratic, with debates focused on technical aspects and regulatory conflicts in implementing transport electrification policy.This difference could potentially be explained by California's stronger regulatory institutions where climate policy is largely designed and implemented by technocrats at the California Air Resources Board, whereas in Germany this is done by technocrats and elected officials (Meckling and Nahm 2018).Future research should further unpack the underlying political economy explaining the observed differences.
Our research design is not free from limitations.First, the identified acceleration challenges are sensitive to the timing of the interviews and the expertise of interviewees in two jurisdictions that have just entered the acceleration phase.Our findings therefore only capture a specific point in time in two rapidly changing contexts, and invite future research updates and extensions to other BEV-leaders such as Norway or China.Second, by focusing on challenges mentioned by at least five interviewees, minority opinions which offer additional real-world challenges and early insights on emerging trends, are underrepresented.Third, we recognize that our method privileges expert knowledge over more bottom-up insights, to reduce the complexity of our sampling frame.To nevertheless capture as many perspectives as possible, we intentionally recruited experts from a diverse array of backgrounds.Finally, our research design focuses exclusively on the electrification of private cars to empirically contain the case and enable a meaningful comparison, but we acknowledge that reaching net-zero targets will require a broader mobility transition encompassing, for example, the electrification of trucks, the expansion of public transport, the build-up of cycling infrastructure and changes in mobility practises.
The extended analytical framework proposed in this paper-summarized in table 1-is not free from some conceptual overlaps between its eight types of acceleration challenges.For example, governance challenges are best seen as a type which permeates all other challenge types.In our view, this conceptual overlap underscores a fundamental limitation of stylized typologies rather than a flaw in our research design.Our empirical analysis clearly demonstrates the cross-cutting nature of real-world acceleration challenges, which often defy neat categorization, and emphasizes the need for research that can accommodate the nuanced nature of inductive observations.
Based on our analysis we advocate for a research agenda that pays closer attention to the role of 'expansion and contestation' , 'justice' , and 'international dynamics' for accelerating transitions, all of which are highly political and interconnected issues.By highlighting the importance of 'expansion and contestation' (understood in terms of new system building) alongside 'decline and resistance' (understood in terms of old system destabilization)-two distinct political logics that unfold concomitantly during the acceleration phase-we emphasize the dual politics of accelerating transitions, thereby contributing to the literature on the politics of acceleration (Roberts et al 2018).Without explicit attention to the political contestations around setting the new rules of the game which shape future gains, the democratic scrutiny for justice considerations in new system building might be neglected.Such justice considerations are relevant not only for compensating losers in the declining trajectory, but are equally important for ensuring the inclusion of marginalized communities in the expanding net-zero trajectory.Likewise, since netzero transitions such as transport electrification are global in nature, it will be increasingly important to consider the international dynamics in declining and expanding trajectories.Finally, whether the broader renaissance of green industrial policy and domestic content requirements either helps accelerate (e.g. through policy feedback) or impedes (e.g. through trade inefficiencies) net-zero transitions remains to be seen.
In conclusion, we argue that in the acceleration phase it becomes essential to complement the focus on old system destabilization with dedicated attention to new system building, as this is where future winners and losers are determined.The resulting dual politics of accelerating transitions will be particularly true for multi-system transitions for which the new rules of the game are currently being written by diverse actors from multiple systems, including policy makers, society, research and business actors -incumbents and new entrants alike 16 .Importantly, at the beginning of the acceleration phase, these new rules will be set in stone for the emerging new regime with expected long-lasting impacts on net-zero transitions.Ultimately, we argue that a better understanding of how political contestation shapes the new rules of our climate-neutral future will advance insights into how to globally accelerate just net-zero transitions.

Figure 1 .
Figure 1.Accelerating electrification of private-passenger transport in Germany and California (2014-2022): EV sales (left), public charging infrastructure (right).Sources: IEA, California Energy Commission and US Department of Energy 6 .
and highlight the complexity arising from the large number of distribution system operators (DSOs) [DE.G.5:4].Germany also faces distinct IT challenges, such as balancing data security demands with the delayed smart meter roll-out [DE.O.2:4], addressing shortcomings in the digital strategy for e-mobility [DE.O.4:4], and the importance of the EU data act to level the playing field in transport electrification [DE.O.3:3].Furthermore, criticism was raised regarding public funding programs mainly benefiting affluent households [DE.O.7:3].Although mentioned only by a few, these real-world challenges can still impede the acceleration of transport electrification.
3:3].Our 'other' theme encompasses additional unique challenges related to electricity generation, IT, equity, and labour.Examples include ensuring reliable and affordable electricity [CA.O.1:3], with reliability further strained by extreme weather events and forest fires [CA.O.2:2]; the exacerbation of behavioral challenges due to inequalities in California (and the US more broadly) [CA.O.8:1]; and the need to strike a balance in training requirements for workers [CA.O.10:3].

Table 1 .
Overview of extended list of different challenge types in the acceleration phase of net-zero transitions.

Table 2 .
Number of real-world acceleration challenges by theme and similarity between Germany and California.

Table 3 .
Overview of similar real-world acceleration challenges in Germany and California.
Speed up and streamline permitting processes of charging stations CA.C.6 Buildings Enabling charging solutions for EV users in multi-family housing Charging at multi-family housing ('Laternenparker') DE.C.11 Enabling charging at multi-family housing CA.C.13 (Continued.)

Table 4 .
Overview of real-world acceleration challenges unique to Germany.

Table 5 .
Overview of real-world acceleration challenges unique to California.

Table 6 .
Overview of challenge types of real-world acceleration challenges in Germany and California.The highly political nature of the acceleration phase is rooted in there being much to gain but also lots to miss out on, such as in the charging market [CA.C.1:9, DE.C.3.11] or in battery recycling [DE.V.12:3].Accordingly, actors need political capacities to shape 'the new rules of the game' to harness the opportunities afforded by transport electrification.

Annex 1 .
Overview of expert interviews.Real-world acceleration challenges in Germany (sorted by theme, sub-theme and robustness).