Energy transition scenarios: What policies, societal attitudes, and technology developments will realize the EU Green Deal?

The European Green Deal has been heralded as the “ Europe's man on the moon moment ” as it aims to achieve 100% GHG reductions by 2050. Achieving the decarbonization of the energy system will be driven by a combination of factors and synergies between technological development, policy exertion and societal attitudes. In this paper, we present an original set of future storylines until the year 2050 to inspire modelers, policy makers, industry actors


Introduction
In December 2019, the European Commission released their plans for tackling climate and environmental-related challengesthe European Green Deal (EGD) [1].As a response to climate change, environmental risks and pollution of forests and oceans, it "aims to transform the EU into a fair and prosperous society, with a modern, resource-efficient and competitive economy where there are no net emissions of greenhouse gases in 2050 […]" [1].
This pledge for carbon neutrality and commitment to the Paris Agreement by the European Commission comes at a time where decarbonization pathways and the question of how to reach netzero greenhouse gas (GHG) emissions by the mid of this century is extensively being analyzed by research and governmental institutions.Prominent examples are the Intergovernmental Panel on Climate Change (IPCC) Special Report on 1.5 C [2], multiple studies by the European Commission (EC) [3e5] but also from non governmental research institutions like from the Wuppertal Institut [6], the € Oko Institut [7] or by CLIMACT and ECF [8].Most of these studies combine, at least, two different methodologies, those being the fields of energy system modelling and scenario analysis.On the one hand, energy system modelling, while initially originating from energy security and cost concerns, is nowadays mainly motivated by climate change policies and the need for significant GHG reduction targets [9,10].The interpretation of results generated by energy system models can be challenging and misleading at times, especially if raw numbers are seen as the outcome of a model [11].On the other hand, scenario development and analysis proves to be the predominant way of using energy system models.Scenarios, and more specifically the relative differences between various scenario results, can help with the communication at the modeller/policy interface by highlighting the insights which can be deduced from model results [11,12].
In the case of the European energy transition, a large number of studies and projects are dedicated on decarbonization pathways which will be discussed more in-depth in Chapter 2. One such project is the open ENergy TRansition ANalysis for a low-Carbon Economy (openENTRANCE) project, which "aims at developing, using and disseminating an open, transparent and integrated modelling platform for assessing low-carbon transition pathways in Europe."<sup>1</sup>As one part of the project, four different scenarios are defined using a novel scenario generation process and results for the pan-European energy transition are computed with the help of the Global Energy System Model (GENeSYS-MOD) for each of the four scenarios [13].Since similar exercises were also conducted by other researchers and governmental institutions, the question remains if general observations and results can be found across all studies which, in turn, would help significantly in identifying policy recommendations, possible uncertainties and noregret options.
Therefore, the contribution of this paper is threefold: First, an extensive review of European energy transition pathways considering high GHG emission reduction targets for 2050 is conducted which highlights the current landscape of decarbonization scenarios (Chapter 2).Second, a novel scenario development process based on a three-dimensional approach is introduced along with a brief discussion of the results of the openENTRANCE pathways (Chapter 3).Lastly, these results are mirrored against similar other pathways which analyze European decarbonization scenarios to determine common findings and recommendations which are then, in a second step, highlighted in the context of the European Green Deal (EGD) (Chapters 4 and 5).

Review of European energy transition pathways
Assumed decarbonization targets and the socio-political and techno-economic context greatly influence pathway perspectives and their main narratives.In this regard, multiple scenario and pathways studies focus on a global, continental or country wise perspectives and their respective energy transition challenges.This chapter presents a review on existing work on defining and analyzing scenarios focused on the European energy transition.

European Commission scenarios
The European Commission Directorate-General for Energy (DG ENER) conducts its own impact assessment studies which, at times, come along with policy packages for institutions of the European Union (EU).These impact assessments are usually quantitative based analyses which are used to include key visions on possible scenarios to support the definition of policies.In the last years, the European Commission has conducted three main impact assessment studies, outlining key challenges associated with achieving the decarbonization targets declared by the EU through an analysis of policy and technology scenarios which will be summarized in the following paragraphs.

Energy Roadmap 2050 scenarios
These scenarios focus on sustainability, competitiveness and security of the EU energy system [14].The main drivers and decarbonization routes noted in the Energy Roadmap are built around four key technological developments: energy efficiency, renewable energy, nuclear energy and carbon capture and storage, which form a roadmap consisting of seven energy transition scenarios until 2050.These scenarios include assumptions on a wide portfolio of technologies, the role of consumers and investors and outlooks of existing regulatory frameworks.

Clean Energy for all Europeans package
The objective and scope of the Clean Energy for all Europeans scenarios is to analyze the feasibility of the 2030 climate targets.The scenarios mainly envision a decarbonization compatible with the 2 C climate target by modelling "[…] the achievement of the 2030 climate and energy targets as agreed by the European Council in 2014 (the first scenario with a 27% energy efficiency target and the second with a 30% energy efficiency target)" [15].With the EU reference scenario as a starting point, the following, more ambitious, EUCO scenarios aim to assess a very specific range of climate and energy targets, those being: (i) reduction of overall GHG emissions compared to 1990: 40% until 2030 and 80e85% until 2050, (ii) emissions reduction from ETS sectors: 43% in 2030 and 90% in 2050 compared to 2005, (iii) non-ETS emissions reduction: 30% in 2030 compared to 2005 and (iv) energy efficiency: reduction of primary energy demand by 27%e30% in 2030 compared to 2007.

A clean planet for all scenarios
The A Clean Planet for all study presents a long-term vision on how "Europe can lead the way to climate neutrality by investing into realistic technological solutions, empowering citizens, and aligning action in key areas such as industrial policy, finance, or research e while ensuring social fairness for a just transition."[3].Nine scenarios considering different areas of action, priorities and technological development are considered in this study.A baseline scenario is defined which, similar to the reference scenario in the Clean Energy for all Europeans study, does not meet the GHG emission reduction targets but is rather accompanied by a set of more ambitious decarbonization scenarios, distinguished by technological assumptions or emission targets.

Related EU research projects
There are multiple research projects funded by the EC that have engaged in extending the previously described work conducted by the EC.For example, the SET-Nav project ("Navigating the Roadmap for Clean, Secure and Efficient Energy Innovation") defines pathways based on two major axes of uncertainty: cooperation vs. entrenchment and decentralization vs. traditionally centralised ("path dependency"), see Crespo del Granado et al. [16] and Fig. 1.The SET-Nav pathways identify central drivers and role of these key uncertainties for a successful decarbonization of the energy system.This also entails discerning the consequences of the particular technological and political decisions that characterize each pathway.The four pathways are very diverse and therefore allow investigating a large number of drivers and uncertainties for the decarbonization of Europe.All the scenarios target 85%e95% decarbonization by 2050.As another example, the REEEM project [17] studies the "role of technologies in an energy efficient economy [through] model based analysis policy measures and transformation pathways to a sustainable energy system."In this project, the pathway definition is based on a set of priorities listed by a number of stakeholders (decision makers, market actors and consumers) and focuses on six dimensions: political, economic, social, environmental, technological and global factors.These dimensions are studied in connection with the degree of cooperation between the EU member states, and form three pathways representing the most plausible development of the aforementioned dimensions.Also, the MEDEAS project -"Modeling the renewable energy transition in Europe" [18] -defines three scenarios for describing possible future outcomes of the European energy transition.The first one is a business as usual (BAU) scenario, which is used to benchmark two alternative scenarios with stronger decarbonization policies.Similarly, the REFLEX project -"Analysis of the European Energy System" -analyses two main scenario strongly based on the PRIMES 2016 reference scenario [19] from the Clean Energy package of EC.The scenarios descriptions are based on modifications of recent projections, considering policy scenarios with ambitious decarbonization pathways [20].Two scenarios are analyzed: a moderate renewable scenario, which is comparable to a BAU scenario, and a high renewable scenario, which has higher decarbonization targets and carbon dioxide (CO2) prices to reach a 2 C target.

Other European scenario studies
Besides the European Commission scenario assessment studies considered in the previous section, several other publications analyze the transition of the European energy system.As part of the scope in this review, studies with a deep decarbonization trajectory (i.e.reducing emissions by more than 80% by 2050) and with a multi-sector perspective (i.e., taking into account at least buildings, transport and the power system) are considered.It should, however, be noticed that these scenarios often do not provide quantitative information to support their claims about long-term decarbonization goals.Moreover, scenarios that lead to drastic emission reductions in the EU do not necessarily imply similar reductions for the rest of the world.Even though there are some similarities across studies, such as the assumed economic growth ranging from 1.4% to 1.7% per year, they differ in the scenario narratives, in the modelling approach and in the assumed policies.
In Tables 1 and 2 some of the recent studies achieving around 90% GHG emission reduction by 2050 are summarized, placing these scenarios in accordance to the ambitions of the EGD.The first scenario reviewed has been published by Eurelectric in 2018 [21].The analysis shows the role of electrification when decarbonizing 80e95% of the EU economy in 2050, which is being achieved by switching from fossil-based generation to 94e96% carbon-free power, higher end-use efficiency due to electric mobility and electric heat pumps and the production of hydrogen and synthetic fuels.Another scenario has been developed by ClimAct and published by the European Climate Foundation (ECF) in 2018 with the objective of showing that a net-zero GHG emission target is technically and economically feasible by means of a distributed effort across sectors and levers (e.g. between technological change and demand-side interventions, see CLIMACT and ECF [8].Several studies have been published by the International Energy Agency (IEA) [22,23], which focus on identifying the most economical way for society to reach the long-term climate goals and technology options and aimed at providing critical analysis on trends in energy demand and supply and on their implications for energy security, environmental protection and economic development.IRENA [24] provides a study in Global Energy Transformation focused on longterm decarbonization and on the technical feasibility and socioeconomic benefits of a global energy transition.Several studies have been carried out by the Joint Research Centre (JRC) of the European Commission.In particular, the Global Energy and Climate Outlook 2018 [25] provides a comprehensive analysis of the development of the energy markets under the simultaneous interactions of economic development, technological innovation and climate policies, while the Low Carbon Energy Observatory [5] focuses on providing top-class data, analysis and intelligence on developments in low carbon energy supply technologies to support the definition of policy goals.A project initiated by nine key gas industry players named "Gas For Climate" has developed two scenarios to assess a cost-optimal way to fully decarbonize the EU energy system by 2050 and to explore the role of renewable and low-carbon gas used in conjunction with existing gas infrastructure [26].The scenarios are exogenously determined by renewable energy potentials, minimum and maximum technology shares in 2050 in end-use sectors, techno-economic parameters, energy efficiency and activity increase in final demand.A study, performed by Matthes et al. [7] aims to show a pathway that "consistently combines short and medium-term objectives with long-term objectives".It meets a EU27 (plus UK) GHG emissions budget consistent with a 2 C limit on the global temperature increase.
A large emphasis on the reduction of GHG emissions from transport is placed by the study of Sven Teske [27].The analysis looks towards reaching 100% renewable energy and near-zero emissions globally in order to meet the Paris Agreement goals, avoiding to rely on net-negative emissions.In 2018, the European wind industry's association WindEurope published a study with pathways for the decarbonization of EU energy system by electrification of industry, transport and by coupling power with heating and cooling [28].Besides the aforementioned scenario analyses, other studies have placed a particular focus on the EU electricity system.The report by Eurelectric [29] introduces and discusses possible pathways to investigate the technical and economic feasibility of achieving an 80% overall GHG emission reduction by 2050, while maintaining or improving today's levels of electricity supply reliability, energy security, economic growth and overall prosperity.In the Eurelectric report by Eurelectric [29]; two major pathways are analyzed: a reference case and the 'Power Choices' case.The scenarios base the emission reduction on the development of technology both on the supply and the demand side.The study finds that the major emission reductions happen between 2025 and 2040, and it is dependent on technological development, carbon policy and a paradigm shift in the energy demand sector fostering smart operation.In J€ agemann et al. [30] the focus is on different cost developments for power supply technology, which lead to different technological pathways.The study claims that the decarbonization of Europe's power sector is achieved at minimal costs under a stand-alone CO2 reduction target, which ensures competition between all low-carbon technologies.These cost rise significantly if investments in new nuclear and carbon capture, transport and storage (CCTS) power plants are politically restricted.In Fraunhofer Institute for Systems and Innovation Research ISI [31] there are three scenarios defined on the basis of three main aspects: development of low-carbon supply technologies, different demand projections and public acceptance.The results of the study indicate that more efficient electricity consumption is an important  element of moderating the cost of decarbonization and that large shares of variable renewable energy sources (RES) calls for significant transmission expansion.Also, in Burandt et al. [32] the focus turns into the realization of policies but with a more complete perspective on different energy carriers and multi-sector coupling effects.
Other approaches both define and assess scenarios on a global perspective.As a mention CenSES [33] focuses on shared socioeconomic pathways (SSP) to analyze both global and European power sector context.The study derives eight scenarios until 2050 and couple the emission constraints based on Representative Concentration Pathways to a European power system model.In Child et al. [34]; the authors study the Pathways to assess the feasibility of a 100% RES-based electricity generation mix by 2050 in 20 European countries and aggregated regions.According to the results of the paper, a centralized European expansion of the power grid would allow the power system to transition towards a fully renewable structure which results not only technically feasible, but also economically viable due to the reduction of the related levelized cost of electricity throughout Europe.

Scenario definition: at the crossroads of policy-technologysociety developments
Achieving net-zero GHG emissions by 2050 sparked the development of several studies to assess the technical feasibility of a fully decarbonized energy systems and its socio-economic implications.Based on this review, it is noticeable that when defining storylines most studies tend to simply introduce variations on technology development and its rate of deployment.For example, assumptions on the evolution of energy efficiency rates or the learning curves of some technologies.As part of the scenario definition this is combined with cost projections and other technology specific assumptions.Some scenario definitions might introduce some geopolitical factors and policy assumptions (mostly some form of CO2 cap) to complement the drivers in technological development.While other scenario definition, put some emphasis on how the policy outlook might shape the technological progression and choices (see policy and technology columns in Tables 1 and 2).
Other energy transition scenarios, have engaged into a more creative process on defining their storylines by assuming that these are shaped by the impact of two set of future drivers or uncertainty developments.For example, Fig. 1 illustrates three pathways definition shaped by a 2 Â 2 typology where the dimensions constructs the definition of four pathways (shared quadrants).In SET-Nav, with the degree of cooperation versus the level of decentralization, the storylines assume that policy directions will be shaped by cooperation while technology development might have different market and deployment prospects.In SUSPLAN, the intersection of societal attitudes and technology uptake provides different notions on how policy in storylines should be defined.Lastly, the wellknown shared socio-economic pathways create storylines on the dimensions of mitigation challenges versus adaptation challenges.This results in five storylines that prescribe policy and technology assumptions as a result of their positioning in the quadrant.
All in all, the different reviewed forms of storylines, scenarios and pathways definitions tend to one way or another assume an interplay between policy exertion, technological development, and societal attitudes.The latter is not much explored in the formation of storylines assumptions (see societal columns in Tables 1 and 2).In this paper, we propose an original and creative way to encompass these three drivers (or uncertainties) to form a set of storylines.That is, the European energy transition will be at the crossroads of policy-technology-society developments by understanding the trade-offs and synergies among them.

Defining scenarios for low-carbon futures: A 3D framework concept
Narrative descriptions of possible developments, structures and characteristics of future energy systems based on different storylines are suitable for systematically mapping all uncertainties ahead.In general, there is no preferred or most likely storyline.Existing literature developing climate and energy system related storylines so far has relied on methodologies enabling a differentiation of the individual narratives based on (i) the dimension of analyses topology determining the number of individual storylines, and (ii) nomination of key drivers and feature of the individual storylines highlighting the uniqueness of each one.In terms of topology, a two-dimensional approach emphasizes two key uncertainties describing possible future energy worlds, where a positive and negative expression of these two uncertainties results in four different storylines (1).In terms of key drivers and features, storytelling mainly emphasizes the questions (i) why a certain development is expected to happen and (ii) what happens.
The two main innovations of the openENTRANCE storyline approach are as follows: Firstly, the storyline topology is extended into a three-dimensional space in which each of the three dimensions determines the salience of a particular uncertain development, namely technological novelty, policy exertion, and societal attitude.The further one moves in this three-dimensional space from the origin of a coordinate system, the stronger is the expression of the respective determinant and thus the disruption from the existing energy system.In openENTRANCE, the combination of two exposed determinants, with the third being less important, results in three distinct storylines that highlight each possible pair of combinations between technology, policy and society.The fourth storyline, near the origin of the coordinate system, contains "a bit of each" of the three uncertain developments and is thus the least distinct (see Fig. 2).Secondly, openENTRANCE storytelling directly addresses the current global climate debate and attempts to better connect both disciplines, energy and climate modelling.More precisely, the expectation of climate neutrality in 2050 (i.e. a 100% reduction in GHG emissions compared to 1990) would limit the global temperature increase to 1.5 Ce2 C and avoid some of the worst climate impacts.In this context, one of the most important linking parameters between energy and climate modelling in openENTRANCE is the so-called remaining CO2-budget, notably the European fraction, which in turn is directly linked to global temperature rise.This allows us to link the different openENTRANCE storylines directly to a certain limit value for the global temperature increase.The three distinct openENTRANCE storylines at the exposed corners of the cube in Fig. 3 (Societal Commitment, Directed Transition, Techno-Friendly) correspond to the European contribution to the global 1.5 C target.The less exposed storyline close to the origin of the coordinate system (Gradual Development) refers to the less ambitious 2 C target.

Societal commitment
High societal engagement and awareness of the importance to become a low-carbon society characterizes this storyline.Individuals, communities and the overall public attitude support strong policy measures to accelerate the energy transition.Both grassroots (bottom-up) and top-down government led approaches meet to drive the strong uptake of behavioural changes in energy usage and energy choices from European citizens.Hence, "green" government initiatives drive and direct ambitious measures in decarbonizing the energy and transport sectors.However, the pathway assumes that no technological breakthroughs occur and there is a lack of major achievements in technology development.It relies on a policy mix that has wide-support from the public.The key driver of this storyline is that society as a whole embraces cleaner and smarter life styles with the public sector working with and supporting grassroots initiatives.

Directed Transition
Carbon-mitigating energy technologies emerge and require strong policy incentives for their uptake and development.The storyline assumes that the effect of grassroots and citizen-led initiatives will be minimal but that strong policy incentives can drive the required engagement of citizens to reach climate targets.This storyline is driven by a strong centralized vision on the part of policymakers and direct partnerships with industry and technology developers who respond to incentives provided by the public sector and provide broad advances in low-carbon energy-related technologies.

Techno-Friendly
A positive societal attitude towards lowering GHG emissions translates into welcoming the deployment of new technologies and changes in behavioural energy choices and grassroots movements in energy.Little resistance to adopting new technologies and openness to large-scale infrastructure projects characterize the social developments of this storyline.Centralized decision-making and policy steering are difficult to reach and hence limited in this storyline, and thus the drive of this storyline comes from grassroots initiatives and industry taking action to deliver novel technology.
The narrative centres on technological novelties complemented with sustained technology uptake by citizens such that demand for new carbon-mitigating energy technologies drives market-based development of these technologies on the part of industry actors.Partly new business models and social innovations pick up the slack from the lack of policy action.

Gradual Development
This storyline envisions that the climate target (2 C) is reached through an equal part of societal, industry/technology and policy action.Knowing that a continuation of current public policies and developments are expected to not be sufficient, significantly higher efforts are needed than the current level of commitment of several of the actors.Thus, this storyline entails ingredients of 'a little of each' of the previously described openENTRANCE storylines and therefore represents an already ambitious reference scenario in openENTRANCE.

Quantification process of the pathways with GENeSYS-MOD
The quantification of the storylines described in the previous sections is carried out with the help of GENeSYS-MOD which is based on the Open Source Energy Modelling System (OSeMOSYS) [35,36]. 2 GENeSYS-MOD is a cost-optimizing linear program, focusing on long-term pathways for the different sectors of the energy system, specifically targeting emission reduction targets, integration of RES and sector-coupling.The model minimizes the objective function which comprises total system costs Fig. 2. 3D concept for a scenario generation process.The axes represent the salience of a particular uncertain development. 2Additional information about the different storylines and their implementation in GENeSYS-MOD can be found in the Appendix A. (encompassing all costs occurring over the modeled time period) and was used in multiple case studies analyzing regional, continental or the global energy system [32,35,37e40].<sup>3</sup>Energy and energy service demands for the sectors electricity, buildings, industry and transportation are given exogenously for each time-step, with the model computing the optimal flows of energy and the resulting needs for capacity additions and storages.To achieve these results, the model can choose from a plethora of technologies spanning across the mentioned sectors, with sectorcoupling and storage options being key functionalities.Constraints, such as energy balances (ensuring all demand is met), maximum capacity additions (e.g., to limit the useable potential of RES), RES feed-in (e.g., to ensure grid stability) or emission budgets (implemented either yearly or as a total budget over the modeled horizon) are given to ensure proper functionality of the model and realistic results.
The translation of the openENTRANCE storylines into GENeSYS-MOD-input is achieved through a number of parameters and model functionalities.Features which are accounted for in all pathways such as decreasing fossil fuel prices, technology learning curves or energy demand changes are straightforward to implement, since suitable parameters already exist in the model.Other aspects are either pathway dependent, 4 require the implementation of new features such as the possibility for limited amounts of load shift during a day or are achieved through workarounds (e.g., the implementation of car-sharing is achieved through increasing annual mileage of cars, resulting in less vehicles needed to accommodate the demand). 5All results are available to the public via an open platform developed and hosted by the International Institute for Applied Systems Analysis (IIASA). 6

Results of the openENTRANCE pathways
All four pathways calculated in the openENTRANCE project show similar developments when it comes to the main indicators of the energy system.In order to decarbonize currently fossil-fuel dominated sectors (i.e., heating and industry), electricity based technologies, powered by 100% renewable electricity, are the option of choice.As a direct consequence, power capacities and production experience a significant increase until 2050, since the additional demand from all sectors outnumbers the efficiency gains of electrical appliances.In contrast, the overall demand for primary energy decreases substantially due to generally higher efficiency of these electricity based technologies (e.g., battery electric vehicles (BEVs), heat-pumps, etc.).The most notable effect on primary energy consumption can be observed in the Societal Commitment pathway and its significantly reduced final energy demand, while efficiency gains in the Techno-Friendly scenario have a higher impact on the electricity generation.
Societal Commitment, Directed Transition, and Techno-Friendly (the 1.5 C pathways) all show a (nearly) full decarbonization by 2040, while the Gradual Development pathway reaches that milestone ten years later in 2050.However, as illustrated in Fig. 4, the delayed achievement of full decarbonization comes at the costs of Fig. 3. Pathways storylines typology ¼ policy exertion x technological novelty x smart society.The three dimensions set the scene on three disruptors or uncertainties which together creates four storylines (coloured squares). 3For more information about GENeSYS-MOD including a documentation, quickstart guide, and sample dataset, the reader is referred to: https://git.tu-berlin.de/genesysmod/genesys-mod-public. 4 Examples are, among others: the consideration of overhead-trucks (Techno-Friendly), EU-wide phase-out of nuclear power plants (Societal Commitment) or setting annual emission reduction targets (Directed Transition).
higher primary energy and electricity demand for Gradual Development.
Another difference in pathway results lies in the degree of availability of CCTS technology.If the costs of CCTS are low enough to compete with other means of decarbonizing the energy system (i.e., in the Techno-Friendly and Directed Transition pathway), the primary area of application is the high-temperature industry sector and, to a lesser degree, also the power sector.Nuclear power, being another non-fossil but also non-renewable source of energy, is more sensitive to the characteristics of the respective pathways: limited societal acceptance of nuclear and fossil generated power (Societal Commitment) leads to a phase-out by 2040, while a continuation of current projections (Directed Transition and Gradual Development) results in remaining capacities in France and Eastern Europe, which in some cases are even expanded.Nevertheless, the majority of electricity originates from wind (on-and offshore) as well as photovoltaics across all four pathways.
Another interesting difference in pathway results can be observed when analyzing the speed and time frames of technology deployment: Directed Transition, with the decarbonization mainly being driven by direct policy instruments, shows a sharp increase in capacity expansion until 2040, whereas afterwards the numbers stagnate due to less technological and societal development.
Gradual Development, on the other hand, experiences the exact opposite development since the reduced action in the first years has to be compensated in the second half of the modelling period by high amounts of capacity expansions in the power sector.Similarly, this effect also carries over to hydrogen production and consumption.Key results of the four different pathways highlighted in Table 3.
Overall, these results indicate that a strong policy enforcement of climate goals in the short term does drastically affect the speed of the energy transition.Especially when considering the accumulated emissions, Directed Transition with its lower short-term emissions leads the way.While the other pathways also reach their designated climate targets, the decarbonization process moves further into the future, since break-through technologies and societal behavior require significant time to become effective.This underlines once more the importance of policy measures to ease the future energy transition.

Comparison of openENTRANCE pathways with similar pan-European pathways
The results of the four different openENTRANCE pathways illustrate how a coherent set of input assumptions can lead to insightful results in the field of energy system modelling.In the next step, these results are compared with other prominent studies which focus on high degrees of decarbonization of the European energy system by using similar tools and methodologies as in the openENTRANCE project.Tsiropuolus et al. [41] identified 26 publications between 2017 and mid-2019 on energy scenarios, mainly stemming from governmental organisations and the private sector.Out of the 67 scenarios which where analyzed in these 26 publications, eight studies reach (almost) full decarbonization of the European energy system by 2050 while at the same time achieving significant GHG emissions reduction in 2030 of at least 55% compared to 1990.These publications are of great value, since they not only allow a verification of the results of the openENTRANCE pathways but also for a more in-depth analysis on common findings, especially with regard to policy recommendations and noregret options in the energy transition.
Therefore, two other studies will be analyzed and compared to the openENTRANCE pathways.The criteria for selecting said studies where, on the one hand, the choice of methodology, which had to be similar or at least comparable to the approach used for the openENTRANCE pathways, and, on the other hand, ambitious decarbonization targets for 2030 and 2050.Two other studies perfectly matched these criteria, one being the A clean planet for all study by the European Commission [3] including two suitable scenarios (1.5 Life and 1.5 Tech) and the other one being Deployment Scenarios for Low Carbon Energy Technologies by the JRC [5] with the near ZeroCarbon scenario. 8The comparison is focused on the final energy consumption per fuel and per sector, the electrification rate within the sectors as well as the required power capacities.Since the regional coverage differs between the studies, with for example Turkey being considered in openENTRANCE, the results for the EU27 countries plus Norway, Switzerland and the UK are being compared to ensure proper comparability of the different studies.
Comparing the final energy consumption (see Fig. 5), all pathways show a total consumption within the same range of 600e1000 mtoe.However, the openENTRANCE pathways show generally higher final energy consumption in 2050 compared to the other studies.This is most likely due to a difference in the implementation of the industry sector, as well as Norway and Switzerland being included in the openENTRANCE pathways.Also, the share of ambient heat is noticeably higher in the open-ENTRANCE pathways, due to higher penetration of heat pumps in the buildings sector, and general "heat" not being listed as its own entity.The shares of hydrogen, biomass and electricity are very consistent across all pathways, with the largest discrepancy shown in the usage of fossil fuels.In the Societal Commitment pathway of the openENTRANCE project, societal attitude towards nuclear and CCTS technology leads to 0% usage of fossil fuels in 2050, while the techno-economical availability of the latter enables the use of limited amounts of fossil fuels in the Techno-Friendly pathway.
Looking at the sector-wise distribution of final energy consumption in Fig. 6, the share of the buildings sector is experiencing similar shares across all analyzed pathways.The biggest difference is in the industry and transport sectors, where some open-ENTRANCE pathways differ substantially from the other studies.This can be explained by the pathway assumptions and technology options of the model.In the Techno Friendly pathway, for example, trolley trucks powered by overhead power lines are enabled as a technology option, leading to a drastically reduced final energy demand in freight transportation due to the higher energy efficiency.The industrial sector, on the other hand, experiences a higher share due to a higher usage of hydrogen, which, compared to it's electric counterparts, requires more energy due to conversion losses.
The electrification rates (shown in Fig. 7) also confirm that development: Techno Friendly has the lowest electrification rate in industry across all pathways, but by far the highest in transport.The electrification rate for buildings is rather consistent across all seven pathways, staying within 58e72%.The same goes for transport, which sees an electrification rate of roughly 32e39%, with the aforementioned exception of the Techno Friendly pathway.The industry sector is the only one seeing a meaningful spread, with the openENTRANCE pathways experiencing a strictly higher electrification rate than the other studies.This might, again, be due to differences in the details of the representation of the industry sector (e.g. which areas are covered exactly and at what level of detail).The highest electrification rate in industry is seen in the Societal Commitment pathway with a share of close to 80% electrification.The reasons for that observation lie in the unavailability of CCTS technology, limited technological development of hydrogen Finally, the installed capacities of electricity generating technologies are compared to provide a better understanding on how these high degrees of electrification in the sectors buildings, industry and transportation are facilitated.Fig. 8 illustrates the overall installed capacities in 2030 (left) and 2050 (right) in GW across the different assessed pathways.A general observation for the 2030 values is that the openENTRANCE pathways overall show higher amounts of installed capacity, mainly being onshore wind and solar PV, which can be explained by the higher decarbonization values in 2030 which are achieved by these pathways.The amount of fossil capacities is very similar across all scenarios, with the LCEO pathway being the exception showing slightly higher remaining capacities.In 2050, the significant difference between the LCEO pathway and all others is striking.A possible explanation could be the more prominent role of hydrogen in this scenario, which is being used to a large degree in the transportation and industry sector but even goes as far as being used in the power sector.Another interesting observation is that, while in the open-ENTRANCE pathways the Techno-Friendly scenario results in the least amount of required capacities (due to higher technology efficiencies, especially in other sectors which in turn result in less  power demand), the A Clean Planet for All study shows the opposite effect, with the society and lifestyle driven pathway being less demanding on electricity.This can originate from substantially reduced demands in the 1.5LIFE pathway, while at the same time the Societal Commitment scenario in openENTRANCE does not show the utilization of nuclear or fossil fuels, which consequently leads to higher renewable capacities being required.

Discussion and policy recommendation
The comparison in the previous chapter of the different scenarios shows a large amount of similarities between the different pathways.Yet, at the same time, differences exist with the reason for said discrepancies not being necessarily apparent at first glance.In general, underlying assumptions, temporal/spatial/sectoral scope, and structure of the model are all factors which have to be considered when analyzing scenario results -especially when comparing outcomes from different models.This raises the challenge for policy makers who, therefore, rely on modelers and researchers to prepare and present model results in a comprehensive way.To address this need, the following section provides an overview of key findings and recommendations which are found in the compared scenarios, while at the same time highlighting their consideration in the EGD.
As mentioned in the introduction, the EGD aims at a wide rang of fields with regard to climate change mitigation and  sustainability.Table 4 shows a number of areas the EGD is targeting with different actions plans, defined by the [42]; and some of the action plans' key components.Coming back to the threedimensional space of technology, policy, and society, the table also highlights recommendations and key findings found across the results of the different scenarios analyzed in Section 4. If the recommendations and findings overlap for two or more dimensions, the cells are merged into a single cell for both.
Starting the analysis with the electricity sector, all studies point to a rapid phase-in of RES while at the same time a phase-out of fossil technologies is required.This is also reflected in the action plans defined in the EGD, but the also mentioned decarbonization of natural gas has to be treated with caution, since infrastructure investments into natural gas can quickly become stranded, if the technologies for producing carbon-free gas do not experience a significant reduction in costs.With high amounts of renewables and the resulting shift in the overall system, the job landscape will change with a large number of new jobs being created.Handling the implied change in requirements for employees, as well as the promotion of RES to raise the public acceptance will be one of the key challenges for a successful energy transition.
The industry sector faces the challenge of decarbonizing difficult to electrify industry branches (e.g., the steel or cement industry).Hydrogen seems to be one of the enablers of such a transformation, with CCTS only showing potential if the costs decrease substantially compared to todays development.Moreover, with capture rates of 90%e95% [43], CCTS is not suitable for a complete decarbonization of the respective processes.Apart from the required strategies to defossilize all industry branches, out of which the EGD currently only targets the steel production, early and clear regulations are necessary to provide sufficient planning security for firms.Additionally, the use of hydrogen (H2) needs to be analyzed in a holistic approach, since currently it is viewed as the main transition enabler in multiple sectors, yet its potential is limited, especially if imports of H2 are not an option.Another aspect which has to be emphasized is the promotion of sustainable consumption as well as sufficiency.Efficient recycling of materials (e.g. in the steel industry) or less demand for industrial products in general could provide a substantial part in decarbonizing the industrial sector and strategies need to be developed to support this shift.
In the buildings sector, in contrast, political targets are set but mostly fail in being achieved in reality.The renovation rate of houses and their insulation is one of the best ways of reducing building energy consumption and the EC plans to increase the target with a "Renovation Wave" initiative.Supplemented by the installation of efficient and clean heat sources, like heatpumps or other electricity-based solutions (and to a limited degree also hydrogen fueled), energy consumption and emissions in the buildings sector can be reduced significantly.However, incentives and financial support mechanisms should be put in place for private house owners, since renovations and change of heating infrastructure come hand in hand with large amounts of expenses which is another reason why the current transition progresses slower than required [44].This has to be accompanied by stopping all financial incentives to install non-renewable heating systems, as their extremely slow replacement rates would slow down the required transition.
The last of the four main energy sectors, the transportation sector, being the sector with an increase in total emissions since 1990 instead of a decrease [45], requires a fundamental overhaul which is also acknowledged in the EGD.Road transport (passenger and freight) has to be electrified (including the use of hydrogen generated with RES), the use of public transport and rail transportation in general needs to be amplified, and investments into charging infrastructure for BEVs are required to rapidly decrease the emissions in the transportation sector.However, transportation is largely influenced by the behaviour of society (e.g., travel behaviour, consumption behaviour and related freight transportation, but also car-sharing or -pooling concepts) and is likely to require a societal change as well to support the technological transition.Enabling and promoting this integrated transformation will be the key determinant to the successful decarbonization of the transportation sector, with the societal part being less explored in the current version of the EGD.This ties into the aspects of sufficiency and circular economy, as demand reductions prove to be one of the key measures to reduce overall emissions (L€ offler et al., same

Table 4
Key areas of action of the European Green deal and related recommendations and key findings found across the compared studies, distinguished by the dimensions technology, policy and society.

Coverage in EU Green Deal
Recommendations and key findings

Technology Policy Society
Electricity A power sector must be developed that is based largely on renewable sources, complemented by the rapid phasing out of coal and decarbonising gas; Increasing offshore wind production will be essential, building on regional cooperation between Member states -Renewables are the cornerstone of emission reductions in the power sector -Power demand is projected to increase and large-scale investments are needed to cover future electrification demand -Storages are key to provide flexibility for the power system -Caution needs to be placed regarding investments in gas infrastructure, as any investments into fossil fuels (including natural gas) risk being stranded -Demand reduction shows overall high effect on projected costs, emissions and electrification rates issue).Another example already envisioned by the EGD is the sustainable production of batteries and possible reuse of batteries from BEVs in the power sector -the so-called second life.
Overall, the EGD put in place many necessary plans in order to achieve the decarbonization of the European energy system, as illustrated by the scenario calculations of the openENTRANCE project but also many other institutions and projects.Yet, the speed at which the required changes are implemented will have to increase for a successful transformation, a fact also illustrated by the vote of the European Parliament in October 2020 to increase the 2030 emission reduction target to 60% compared to 1990.As a response, the EC agreed on increasing the 2030 climate target to À55% GHG emissions compared to 1990, but some of the analyzed scenarios show that a higher target would not only be possible, but also required as to not put too much stress on the years 2030e2050.
The analysis in this paper shows that a wide range of literature and studies exist in the field of energy system modeling targeting the low-carbon transition of the European energy system.The overview of related recent work and the scenario comparison carried out in this paper highlight that, while assumptions and scope of these studies might differ in some points, it is still possible to synthesize common findings and recommendations found across the literature.However, communicating said results to policy and decision makers requires additional effort, as their interpretation is, in most cases, only possible through extensive knowledge of scenario assumptions and/or comparisons.Hence, in Section 5, seven scenarios from three different modeling studies are compared which are all based on a similar set of input assumptions and methodologies.This comparison exercise clearly demonstrates that it is possible to deliver consolidated and robust recommendations for policy and decision makers.It is also noteworthy that the question of how the required strong policy enforcement can be achieved has to be analyzed in more depth.The present analysis only highlights where (and partially what) action is required, yet the particularities of the political process in the EU and its member states needs to be taken into account.
Moreover, the novel, three-dimensional scenario development methodology used for the four openENTRANCE pathways clearly shows that the success of the European energy system transformation is tightly tied to actions in the three dimensions (policy, technology and society) governing also the openENTRANCE scenario generation process.Mirroring the plans set in the EGD against the openENTRANCE results and the scenario comparisons in this paper, consequently, highlights no-regret options for the lowcarbon transition but also clearly identified/specified "construction sides" where more ambitious action is required in the different sectors.Exactly these kind of consolidated and converging recommendations and action plans is what practitioners as policy and decision makers as well as stakeholders expect from the research and modeling community.

Directed Transition
In this storyline, as a result of an indifferent public attitude and lacking societal commitment, a strong and continuous incentivebased policy push (at least in Europe) is necessary to deploy existing and novel technologies in the energy and transport sectors.At both European and country levels, active and aligned policy support is necessary to optimally exploit several potentials and available synergies.In a challenging global and European market environment industry is gaining confidence in continuous technology-specific public policy support and takes responsibility to deliver low-carbon mitigating technology portfolios in the absence of significant active societal contributions.
As for the implementation of this storyline into GENeSYS-MOD, the demand for all sectors is slightly lowered due to policy incentives for demand reduction.While this effect is not significant in most sectors, the residential heating demand sees a higher decrease in demand due to a constant process of building modernization and therefore reduced heating demand.In the industrial sector, heavy subsidies, which are implemented as decreasing costs of technologies, help in electrifying most of the process heating demand.Especially hydrogen-based solutions meeting energy services, which are already past their initial state of development today, see significant reductions in their associated costs (as a result of significant policy support) and thus considerable market deployment.
In contrast, currently not available technologies, at least from a technical point-of-view, are not being considered by the model (except CCTS technology).Net imports of hydrogen to Europe are not being considered as a result of the global market situation and reliance on energy security, and fossil fuel prices drop due to lowered global demand.CCTS is an option after the initial model periods due to heavy incentives by politics to reduce emissions, especially in the industrial sector.

Societal Commitment
This storyline mainly describes a prudent society, characterized by a sustainable life style and behavioural changes which includes a significant reduction of energy use for delivering energy and transport services, the implementation of a circular (and partly sharing) economy, as well as the exploitation of digitalization potentials to support individual and local service needs (including those of communities).Completely new business models and market solutions will emerge, partly only locally.Significant shares of local self-consumption (individual prosumers, communities of different sizes) of renewable generation (notably PV) characterizes this story.These ambitions and developments will be supported by tailor-made policymaking, not least as a result of lacking breakthrough of novel technologies (except digitalization) in the energy and transport sectors.
Overall, renewable technologies see higher potentials and market penetration than usually, due to their public support and politics focus on removing regulatory barriers.Society is willing to invest into the sustainable transformation of the energy system (with significant contributions of local self-generation at different levels), which is being implemented by adding a penalty on conventional technologies and, thus, promoting renewable solutions.In addition, the sharing nature of society is simulated by decreasing the costs of passenger vehicles and increasing their efficiency.The former characterizes the sharing nature of the society, with car sharing vehicles usually driving more kilometers per year than privately owned ones, while the latter simulates higher occupancy rates in vehicles due to pooling effects.
Regarding the technology landscape, no negative emission technologies are being allowed in this storyline.Moreover, no new nuclear power plant capacities are being considered as an option (not this is the only openENTRANCE scenario of four in total where this assumption is made).Fossil fuel prices drop due to lowered demand but a high carbon price, the highest in all four pathways, offsets this effect.This high carbon price can be explained by a widespread recognition of environmental externalities caused by greenhouse gases and leads to an almost decarbonized energy system by 2040.

Techno-Friendly
This storyline focuses on the promising breakthrough of novel technologies (incl.floating offshore wind, H2, and CCTS), being rolled out on a large-scale to meet energy and transport service needs.In addition, this storyline is characterised by the positive attitude of society towards large-scale infrastructure projects mitigating the climate challenge.A strong globalized market-pull triggers technology choice and implementation.Active policymaking is pushed into the background.As a consequence of sufficient low-carbon technology availability, energy demand reductions and active demand side participation of consumers/ prosumers are less important, but still needed.
A number of technologies is now available for the model to being used.These include CCTS capacities, significant net H2 imports to Europe, and the inclusion of certain breakthrough technologies like direct air capture (DAC) or trucks powered by an overhead power line.These technologies follow cost and efficiency projections, which usually makes them very unattractive in the current time periods, but opens up their possibility to significantly contribute in the later periods.
Additionally, as the title of the storyline already suggests, optimistic values for cost and efficiency development of technologies under consideration are implemented.Higher technological learnings can be seen for technologies, which are currently in a less mature state of development, showcasing their potential for a breakthrough.Additionally, new capacities can be built at a higher implementation rate between two periods since infrastructure investments and capacity expansions are facilitated through the storyline.A medium to high carbon price is included as is a crosssectoral and cross-regional emission trading system (ETS).Fossil fuel prices drop due to lowered demand, yet the combination of the carbon price and technology improvements of carbon-free technologies offset this reduction in prices.

Gradual Development
This storyline entails ingredients of 'a little of each' of the remaining openENTRANCE storylines and therefore represents an already ambitious reference scenario in openENTRANCE.The uniqueness of this storyline is that it describes the challenging energy transition with an equal part of societal, industry/technology, and policy action.Several of these three dimensions take responsibility and deliver tailor-made contributions to reach the least ambitious climate mitigation target (2 C; remaining storylines envisage 1.5 C).Carbon pricing in this scenario is more conservative compared to the others.
Costs and efficiencies of all technologies are changed slightly to reflect the pathway characteristics, similar to the Techno-Friendly implementation.Yet, the values are less optimistic and improvements happen at a slower rate.Also, novel and not already proven technologies are not integrated (e.g.DAC, overhead trucks, CCTS) and there is no option foreseen to have net imports of hydrogen from regions outside of Europe.Similar to Societal Commitment, this pathway is also characterized by reductions in energy demand of all different sorts.These reductions, however, are less substantial as in Societal Commitment and, additionally, the potential for demand shifting is far more limited.
Appendix B. Description of the three studies: openENTRANCE, A clean planet for all, and LCEO openENTRANCE The ambition of the openENTRANCE project is to develop, use and disseminate an open, transparent and integrated modeling platform for assessing low-carbon transition pathways of the European energy and transport system.This fully open platform will be populated with a suite of modeling tools and datasets selected to cover the multiple dimensions of this transition process.This shall facilitate and improve the dialogue between researchers, policy makers and industry when investigating key questions linked to this transition in the next decades, notably as far as the European energy and transport system is concerned.
Naturally, the European economy is not decoupled from the rest of the world.There exists a strong link and trade relationships to other regions outside Europe.Thus it is straightforward that also models and analyses tools are needed in openENTRANCE for calibration, validation and robustness tests when determining the quantitative European portion needed to achieve particular global goals, e.g.like a global warming temperature ceiling benchmark of 1.5 C or 2.0 C. Therefore, the comprehensive ensembles of datasets on existing global pathway curves embedded into the model MESSAGEix-GLOBIOM play a core role in the openENTRANCE modeling and scenario generation exercises to align the empirical foundation of several quantitative scenario and case studies carried out in this project to the status quo of knowledge in the global climate modeling community.
It is important to note, that in the openENTRANCE project from the very beginning a strong focus has been put to support policy and decision making to succeed in decarbonizing the European economy.However, since future developments in the energy and transport system can't be foreseen exactly, four different storylines have been developed (i.e.narrative descriptions of equally possible energy futures) in the openENTRANCE project.The corresponding quantitative scenario studies presented in this article directly built upon them.It is important to note that the openENTRANCE storyline descriptions are founded on a thorough analysis of already existing global and European pathway and scenario studies as well as a comprehensive review of the existing policy documents at European Commission level complying to the global climate challenges according to the Paris agreement.
A clean Planet for all [3] The aim of this long-term strategy is to confirm Europe's commitment to lead in global climate action and to present a vision that can lead to achieving net-zero greenhouse gas emissions by 2050 through a socially-fair transition in a cost-efficient manner.It underlines the opportunities that this transformation offers to European citizens and its economy, whilst identifying challenges ahead.The proposed Strategy does not intend to launch new policies, nor does the European Commission intend to revise 2030 targets.It is meant to set the direction of travel of EU climate and energy policy, and to frame what the EU considers as its long-term contribution to achieving the Paris Agreement temperature objectives in line with UN Sustainable Development Goals, which will further affect a wider set of EU policies.The Strategy opens a thorough debate involving European decision-makers and citizens at large as to how Europe should prepare itself towards a 2050 horizon and the subsequent submission of the European long-term Strategy to the UN Framework Convention on Climate Change by 2020.
The main model suite used for the scenarios presented in this assessment has a successful record of use in the Commission's energy and climate policy impact assessments.It is the same model suite used for the 2020 and 2030 climate and energy policy framework, as well as for the 2011 Commission's decarbonization Roadmaps.The model suite has been strongly enhanced over the past years in terms of more granular representation of both energy system and GHG emissions and removals, and the detail of representation of technologies.The model suite covers: The entire energy system (energy demand, supply, prices and investments to the future) and all GHG emissions and removals.Time horizon: 1990 to 2070 (5-year time steps) Geography: individually all EU Member States, EU candidate countries and, where relevant Norway, Switzerland and Bosnia and Herzegovina.Impacts: on all energy sectors (PRIMES and its satellite models on biomass and transport), agriculture (CAPRI), forestry and land use (GLOBIOM-G4M), atmospheric dispersion, health and ecosystems (acidification, eutrophication) (GAINS); macroeconomy with multiple sectors, employment and social welfare (GEM-E3).
The models are linked with each other in such a way, so as to ensure consistency in the building of scenarios.These interlinkages are necessary to provide the core of the analysis, which are interdependent energy, transport and GHG emissions trends.

LCEO [5]
The LCEO is an Administrative Arrangement being executed by JRC for RTD, to provide top-class data, analysis and intelligence on developments in low carbon energy supply technologies.Its reports give a neutral assessment on the state of the art, identification of development trends and market barriers, as well as best practices regarding use private and public funds and policy measures.The LCEO started in April 2015 and runs to 2020.
JRC experts use a broad range of sources to ensure a robust analysis.This includes data and results from EU-funded projects, from selected international, national and regional projects and from patents filings.External experts may also be contacted on specific topics.The project also uses the JRC-EU-TIMES energy system model to explore the impact of technology and market developments on future scenarios up to 2050.
The project produces the following generic reports: Techno-economic modelling results are also made available via dedicated review reports of global energy scenarios and of EU deployment scenarios.

Fig. 1 .
Fig. 1.Examples of widely-used 2 Â 2 scenario typology to combine two main dimensions of uncertainty into four storylines spanning a wide possibility space.

Fig. 5 .
Fig. 5. Comparison of final energy consumption per fuel and total energy demand in 2050 at EU27 level (including Norway, Switzerland and the UK) according to assessed pathways and scenarios.

Fig. 6 .
Fig. 6.Comparison of final energy consumption per sector in 2050 at EU27 level (including Norway, Switzerland and the UK) according to assessed pathways and scenarios.

Fig. 7 .
Fig. 7. Electrification rate of the different sectors in 2050 at EU27 level (including Norway, Switzerland and the UK) according to assessed pathways and scenarios.

Fig. 8 .
Fig. 8.Comparison of cumulative installed capacities of power generation technologies by 2050 at EU27 level (including Norway, Switzerland and the UK) according to assessed pathways and scenarios.

Table 1
An overview of existing decarbonization scenarios for EU (continues …).

Table 2
An overview of existing decarbonization scenarios for EU (continuation).

Table 3
Overview of key results of the four openENTRANCE pathways.
Technology Development Reports for each technology sector Technology Development Reports for each technology sector Technology Market Reports for each technology sector Report on Synergies for Clean Energy Technologies Annual Report on Future and Emerging Technologies (information is also systematically updated and disseminated on the online FET Database).