A brief history of the research field
Since the first definition of ‘sustainable development’ put forward by the Bruntlant report i.e. “development that meets the needs of the present without compromising the ability of future generations to meet their own needs (…).” (p.41) , different scientific research fields and disciplines became concerned with understanding sustainability. Sustainability is a crosscutting concept with ecological, sociological, technological, political and economic dimensions. Towards the end of the 1990, the sustainability transitions research field emerges as an inter- and transdisciplinary research field, which rather than locked in a particular knowledge domain or scientific discipline, forms the basis for a vast body of research framed by a complex systems and co-evolutionary approach. As much as sustainability refers to a multidimensional and multi-layered concept, the notion of ‘transition’ refers to a fundamental transformation of complex societal systems. Through a variety of approaches and perspectives, the sustainability transitions research field – or, simply, transition research – aims to study and deal with complex (systemic) societal challenges, by focussing on the processes through which radical transformative changes occur. A recent synthesis by Loorbach, Frantzeskaki and Avelino, identifies two major roots of transition research: Innovation research (includes science and technology studies, evolutionary economics, innovation policy); and environmental and sustainability studies (includes integrated assessment, sustainability governance, environmental policy) . According to these authors: "The core ambition of transitions research is to better understand such transitions, to anticipate and adapt to undesirable transitions (e.g., ecosystem collapse, economic breakdown, high-impact climate change), and to explore possibilities to advance and accelerate desired transitions."(p.4)
Moving beyond an ecological modernization discourse – based on increasing optimization, efficiency or higher efficacy of technological systems -, the transition research field is concerned with understanding and fostering truly radical and transformative change. As Seyfang and Haxeltine have written: "understanding transitions is especially important when dominant 'solutions' (and the sociotechnical systems that deliver these) are locked in and contribute to unsustainable development (…), and when novel experiments might offer more sustainable alternatives or when we face persistent problems that cannot be solved using only the currently dominant approaches.” (p.383) .
Of particular significance, the notion of ‘socio-technical system’ is a core conceptual piece of this literature. Rip and Kemp´s micro, meso and macro level framework has been a fundamental pillar of transition research. The framework was proposed in the context of a publication on climate change . The authors considered that social sciences needed to take into account the co-evolutionary dynamics of society and technology, in order to properly address the climate change problem: "Central to this understanding is the link between global climate change and what we will call evolving sociotechnical landscapes, which are part and parcel of overall transformations of societies. (p.328)". Rip and Kemp thus introduce the notion of sociotechnical landscape: […] a landscape in the literal sense, something around us that we can travel through, and in a metaphorical sense, something that we are part of, that sustains us (p.334)."
Building on the work of Rip and Kemp, Frank Geels proposes the Multi-level Perspective (MLP). The MLP uses the nomenclature of socio-technical niches, regimes and landscapes  and proposes a framework to study the co-evolving processes of transformative changes between the different system levels. Geels offers the following description of MLP: "MLP views transitions as non-linear processes that result from the interplay of developments at three analytical levels: niches (the locus for radical innovations), socio-technical regimes (the locus of established practices and associated rules that stabilize existing systems), and an exogenous sociotechnical landscape […]. The regime level is of primary interest, because transitions are defined as shifts from one regime to another regime." (p.26) .
In the MLP, the concept of regimes provides an analytical framework to study how institutional configurations of use and actor practices accompany the historical path dependencies of technological innovations. These path dependencies refer to how societal systems can be locked-in particular ways of functioning, which are supported by the dominant regime. For instance, when cities were dependent on a horse-based transport system, dominant societal structures, such as rules and legislation, ways of being and experiencing city life, and even daily life practices were influenced by the horse transport system. Rip and Kemp had (1998) equally highlighted these dynamics, with the example of the automobile: "the motorcar is not an isolated artifact, but the label for part of our sociotechnical landscape, made up of steel and plastic, concrete (the roads), law (traffic rules), and culture (the value and meaning of personal mobility)" .
Since socio-technical regimes are characterized by a system state with particular dominant structures (i.e. includes physical structures, economic infrastructure,institutions, rules, regulations, organisations), cultures (i.e. values, norms, perspectives, paradigms, worldviews) and practices (i.e.social practices; behaviours) , transitions have been also defined as a long term process leading to a fundamental change in structure, culture, and practices.
A complementary definition is provided by Loorbach, Frantzeskaki and Avelino  : "Transitions in their literal sense refer to the process of change from one state to another. In transitions research, the term refers to the process of change from one system state to another via a period of nonlinear disruptive change. Such systemic change, by definition, is the result of an interplay of a variety of changes at different levels and in different domains that somehow interact and reinforce each other to produce a fundamental qualitative change in a societal system." (p.7)
As an alternative to the MLP framework, the Arenas of Development (AoD) approach has been suggested by Jorgensen (2012). Arenas are described as "actor constellations and their collective sense-making activities" (p. 997) and as "temporarily stable actor-worlds" (p.1001). The proposal offers a particular interpretation of the vertical dynamics between micro, meso and macro system levels  in transition research. It puts forward an idea of relationality, that is, boundaries between arenas are not predefined and rigid; they are continuously reconfigured through the performance of actors, who may hold multiple identities. Therefore, arenas appear as fluid and mobile (i.e. there is not a clear definition of regime, niche or landscape), and changes in socio-technical systems are the incremental outcomes of this mobility. The focus of the arenas system is on the performance of actors, who navigate through distinct configurations of actor-worlds.
While the conceptual frameworks to study transitions are still evolving, the transition research field is itself expanding. As Loorbach, Frantzeskaki and Avelino have argued, there is a growing shift “from a focus on sociotechnical systems to a recognition of socio-ecological, socio-economic, and socio-political systems as equally relevant objects of transition  (p.5)”. This expansion in focus has led to the development of new approaches, concepts, theories and methodologies that are framed by different societal problems and solutions. Three particular strands of transition research were identified and characterized by Loorbach, Frantzeskaki and Avelino:
- Socio-technical approach
This strand of transition research has its roots in Science and Technology Studies, with the already referred contributions of Rip, Kemp and Geels. The Multi-level Perspective is used to study the dynamics of path-dependencies, lock-ins and patterns of transitions, as dominant regimes are challenged by socio-technical innovations (niches). Transition pathways are explored to identify the potential trajectories transition processes may follow. Structural changes may emerge from internal dynamics within the dominant socio-technical regime; by new niches becoming increasingly empowered, or by landscape changes which create pressures on the dominant regime (e.g. new contextual and exogenous conditions).  Research in this field has also analysed the "patterns of transitions" , which relate to a set of possible configurations or patterns that characterize transitions. Based on a functionalist perspective of how socio-technical systems provide for societal needs, De Haan and Rotmans build a theoretical account of how transition processes develop over time. These processes exhibit possible patterns, which are assembled through building blocks (such as a societal system adapting to a changing climate). The Strategic Niche Management (SNM) approach relates to this strand of studies (originating from the work of Rip and Kemp). Schot and Geels  suggest SNM as a policy design for understanding and steering the dynamics of protective spaces where innovations can be tested and developed. Smith and Raven propose innovations can be shielded, nurtured and empowered in order to increase their possibility of diffusion and of entering mainstream markets . More recently, concepts of SNM have been applied to the study of 'grassroots innovation', in order to understand and promote their wider diffusion. .
- Socio-institutional approach
This strand of research draws mainly from the social sciences (i.e. studies of politics and power, sociology, governance studies and economics) to understand transformative changes. The focus is on the study of dominant cultures and practices, as regimes shift from one stabilised state to another. This body or research includes approaches to reflexive modes of governance , and studies of politics, power and empowerment issues of transitions . The Transition Management (TM) approach  is also a relevant conceptual framework for the socio-institutional approach. TM is a proposal for a new mode of governance that is concerned with influencing a new generation of long-term planning. It is an action-research oriented field that seeks to promote, influence and monitor sustainable transitions. The process of policy design develops along a cycle with four key stages: “A strategic (Problem structuring, envisioning, and establishment of the transition arena); tactical (Developing coalitions, images, and Transition agendas); reflexive (Evaluating, monitoring and Learning) and operational (Mobilizing actors, executing projects and experiments).” (p. 173) . Loorbach et al. characterize the socio-institutional field by its main focus on agency and governance in transitions, as well as on social learning experiments, cultural studies and the dynamics of social practices. Lastly, recent research in this area has analysed the role of social innovation  and Grassroots innovation  in societal transitions, exploring the socio-political, socioeconomic and sociocultural dimensions of transformative change.
- Socio-ecological approach
This research area is intrinsically linked to literature on social-ecological resilience and transformation. It builds on approaches from the natural sciences, in particular: ecology, biology, complex adaptive systems theory, ecosystem services and adaptive governance. A regime is understood as a set of quasi-stable system states. Transitions are irreversible regime changes from a steady systems’ state to another, and transformations occur when a social-ecological system (SES) loses its adaptability.. In the social-ecological resilience approach innovation is another scale within the system and an opportunity for change . Recent work on the ‘seeds of the Anthropocene’  is an example of this strand of research that analyses grassroots (social and technical) innovations as seeds for a future Earth system that is able to thrive within global and dangerous ‘tipping points’.
Lastly, the question of governance is a crosscutting theme at the heart of transition research. Different modes of governance have been proposed and tested – such as Strategic Niche Management, Transition Management and Adaptive Governance approaches . This points to a core characteristic of transition research: whether the focus is on the socio-technical, socio-institutional or socio-ecological dimensions, there is a driving intent to promote desirable sustainable transitions.
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