Marios Stanitsas
email: mariossta@mail.ntua.gr

Konstantinos Kirytopoulos
email: konstantinos.kirytopoulos@unisa.edu.au

Elise Vareilles
email: elise.vareilles@mines-albi.fr

Facilitating sustainability transition through serious games: A systematic literature review

Keywords:

a b s t r a c t :

Exploring aspects of how innovative methods can truly attain a sustainable society is necessary for the future of our planet. This study focuses on serious games, and how users can increase their understanding of sustainability issues and their familiarity with sustainable development strategies. "Users" of serious games consist of all possible target groups that are interested in attaining knowledge of sustainability through the use of games that are designed for a purpose beyond entertainment, in this case for sustainability education. This paper follows the systematic literature review method to deliver a study of serious games featuring sustainable development practices and policies. In order to provide a thorough analysis of their dynamic features, 77 games were explored in this study. The findings show the growing number of serious games that seek to educate in sustainability and the categorization of these games according to the triple-bottom line of sustainability, giving clarification hints to users wishing to select the relevant tool that offers an understanding of specific sustainability issues. The purpose of such research is to reveal the contribution of serious games as effective tools in facilitating sustainability education and to group them according to their nature and direction in relation to sustainability. Limitations in their effectiveness are also identified and a research agenda for new, relevant serious games is proposed that will enhance holistic knowledge and make it easier to clarify their pedagogical basis. The recipients of the findings will be all those future users and trainers who are interested in accessing sustainability education patterns through the use of serious games. This study will enable them to select the serious game that best serves their needs.

Introduction & background

An undisputed feature of modern societies is the rapid change they are undergoing. Major social changes affect or characterize every aspect of society, and influence our ways of life, causing increasing confusion in people's daily lives [START_REF] Van Opstal | Knowledge for sustainable development: a worldviews perspective[END_REF][START_REF] Ferguson | Productivity growth as a barrier to a sustainability transition[END_REF]. Scientific techniques need to be called upon in order to provide viable solutions for perpetual sustainability concepts [START_REF] Nawaz | Development of a systematic framework for sustainability management of organizations[END_REF][START_REF] Mihelcic | Sustainability science and Engineering: the emergence of a new metadiscipline[END_REF][START_REF] Kivil€ A | Sustainable project management through project control in infrastructure projects[END_REF][START_REF] Gonz Alez | Towards a universal energy efficiency index for buildings[END_REF]. The target scenario needs to include environmental issues such as climate change, social issues such as poverty and happiness, and economical aspects such as profitable investments [START_REF] Lee | Benchmarking energy use of building environmental assessment schemes[END_REF]Komurlu et al., 2015a). Plenty of definitions of the vast field of sustainability exist. However, it is commonly accepted that all definitions should include biophysical, human and economic aspects. As [START_REF] Dick | What is socio-ecological research delivering? A literature survey across 25 international LTSER platforms[END_REF] and [START_REF] Ramcilovic-Suominen | Sustainable development e a 'selling point' of the emerging EU bioeconomy policy framework?[END_REF] indicated in their work, sustainability can be understood as the interaction of humans with the environment and with other human beings in order to achieve environmental, social and economic advantages. The World Commission on Environment and Development, also known as the Brundtland Report, shaped a definition that has been widely accepted by the scientific community [START_REF] Renoldner | Rethinking 'our common future': a physician's remarks 25 years after the release of 'Brundtland report[END_REF][START_REF] Schubert | The Literature Aftermath of the Brundtland Report 'Our Common Future' (A Scientometric Study Based[END_REF][START_REF] Oliveira | Back to the future: the potential of intergenerational justice for the achievement of the sustainable development goals[END_REF]. According to this report: "Sustainable development is a development that meets the needs of the present without jeopardizing the ability of future generations to meet their own needs" [START_REF] Wced | Our Common Future: the Bruntland Report[END_REF]. Supplementary to this philosophy are the Sustainable Development Goals (SDG) that the United Nations (UN) have set, the basis of which are the essentials of energy utilization, water and nourishment [START_REF] United Nations | Transforming Our World: the 2030 Agenda for Sustainable Development[END_REF]. As literature suggests, sustainable technologies have come a long way, driven by environmental awareness and the rising costs of fossil fuels (Marou sek, 2013; [START_REF] Mardoyan | Analysis of Czech subsidies for solid biofuels[END_REF][START_REF] Bieber | Sustainable planning of the energy-water-food nexus using decision making tools[END_REF][START_REF] Fazey | Ten essentials for actionoriented and second order energy transitions, transformations and climate change research[END_REF]. The most promising achievements concern renewable energy, sustainable living, organic agriculture, environmental economics and environmental technologies. Within these general categories, technologies like CO2 capture, water cleaning, soil improvement, sustainable design (construction) and cogeneration all add to the repository of sustainability achievements (Marou sek, 2014a; [START_REF] Koytsoumpa | The CO2 economy: review of CO2 capture and reuse technologies[END_REF]Marou sek, 2014b;[START_REF] Erez-Lombard | A review on buildings energy consumption information[END_REF][START_REF] Gurgun | Review of the LEED category in materials and resources for developing countries[END_REF][START_REF] Robichaud Lauren | Greening project management practices for sustainable construction[END_REF]. The focus seems to be on long-term issues that have environmental, social, and economic implications (Komurlu et al., 2015b). Prevention of physical waste, increase in energy efficiency and improvement of resource productivity, all help to expand profitability and enhance competitiveness in the long run. In fact, due to years of neglect, these are often high-return investment fields [START_REF] Van Opstal | Knowledge for sustainable development: a worldviews perspective[END_REF][START_REF] Janou Skov A | Closing the sustainability gap: 30 years after "Our Common Future", society lacks meaningful stories and relevant indicators to make the right decisions and build public support[END_REF][START_REF] Kakoty | Ecology, sustainability and traditional wisdom[END_REF].

Nonetheless, as the need for sustainability can no longer be overlooked, there is general agreement that the educational tools used to transmit knowledge of Sustainable Development (SD) and promote new methods to teach it need to be renovated (Dagili ut _ e et al., 2018;[START_REF] Jaca | What should consumer organizations do to drive environmental sustainability?[END_REF][START_REF] Thürer | A systematic review of the literature on integrating sustainability into engineering curricula[END_REF][START_REF] Saunila | Sustainability as a driver of green innovation investment and exploitation[END_REF][START_REF] Beumer | The sustainability of globalisation: including the 'social robustness criterion[END_REF][START_REF] Anand | Human development and economic sustainability[END_REF]. Various attempts have been made to move in this direction. A growing number of Higher Educational Institutes (HEI) are adopting and integrating SD policies and sustainable-oriented communication patterns in their educational research [START_REF] Tejedor | Transdisciplinarity in higher education for sustainability: how discourses are approached in engineering education[END_REF]. This study lists the attainment of learning outcomes as tangible and readily definable information in line with sustainable development requirements, or best practices as defined by professional bodies. It promotes Education for Sustainable Development (ESD) as vocationally vital (meeting sustainability requirements), making it worth of investigation. Serious games are a means to encourage sustainability concepts in societies. Education is a crucial element but it does not ensure a change of philosophy towards SD patterns [START_REF] Brundiers | Do we teach what we preach? An international comparison of problem-and project-based learning courses in sustainability[END_REF]. Analysis of general SD competences in several HEIs has shown that engineering students do not possess the proper skills of systemic thinking and holistic knowledge upon graduating [START_REF] Fumiyo | Dissonance in students' perceptions of sustainable development and sustainability: implications for curriculum change[END_REF][START_REF] Major | Teach it to sustain it! Environmental attitudes of Hungarian teacher training students in Serbia[END_REF]. It is crucial to achieve this element if a change of mentality is desired. [START_REF] Karel | How to educate engineers for/in sustainable development: ten years of discussion, remaining challenges[END_REF][START_REF] Fazey | Ten essentials for actionoriented and second order energy transitions, transformations and climate change research[END_REF].

During the last decades, HEIs have been conducting research on how to integrate sustainability into their educational methods [START_REF] Ragazzi | Environmental sustainability of universities: critical analysis of a green ranking[END_REF][START_REF] Tejedor | Transdisciplinarity in higher education for sustainability: how discourses are approached in engineering education[END_REF][START_REF] Holdsworth | A sustainability education academic development framework (SEAD)[END_REF]. A growing number of HEIs have created educational tools for SD, research on sustainability integration, social outreach, procedures, assessment and statements, university partnerships, institutional schemes, conventional educative programs for educators and green campus projects (Annan-Diab and Molinari, 2017). Universities, and especially engineering-related HEIs, are taking fundamental and essential steps to follow the modern-era trend of SD reformulating their educational strategies [START_REF] Beynaghi | Future sustainability scenarios for universities: moving beyond the United Nations decade of education for sustainable development[END_REF][START_REF] Luederitz | Learning through evaluation e a tentative evaluative scheme for sustainability transition experiments[END_REF]. This long and difficult path involves the crucial step of incorporating Sustainability Assessment Tools (SATs). SATs play an important role in HEIs, favoring the genesis of SD policies and the development of SD practices. They are implementers of innovation [START_REF] Dlouh A | Sustainability-oriented higher education networks: characteristics and achievements in the context of the UN DESD[END_REF]. Nevertheless, as [START_REF] Berzosa | Sustainability assessment tools for higher education: an empirical comparative analysis[END_REF] indicate, researchers have shied away from using such tools. Even though the value of sustainability is commonly recognized by academics, SD projects tend to be perceived as trivial tasks, mainly due to the difficulty of specifying clear objectives (the topic of sustainability is too broad) [START_REF] De Lange | Start-up sustainability: an insurmountable cost or a life-giving investment?[END_REF][START_REF] Ludwig | Environmental sustainability: magic, science, and religion in natural resource management[END_REF][START_REF] Silvius | Sustainability as a new school of thought in project management[END_REF][START_REF] Sierra | Bayesian network method for decision-making about the social sustainability of infrastructure projects[END_REF]. This lack of engagement gives potency to the demand for innovative educational approaches that facilitate authentic educational content and thinking in the field of SD [START_REF] Blanco-Portela | Towards the integration of sustainability in Higher Eeducation Institutions: a review of drivers of and barriers to organisational change and their comparison against those found of companies[END_REF][START_REF] Ceulemans | Sustainability reporting in higher education: interconnecting the reporting process and organisational change management for sustainability[END_REF][START_REF] Kevin | Deep learning and education for sustainability[END_REF]. In other words, integration of SD requires demanding perception of the key factors, purviews, limitations and independence of different SD projects in order to obtain a beneficial outcome [START_REF] Cairns | An environmental-economic measure of sustainable development[END_REF]. Such awareness can be established within a united, interdisciplinary framework that requires the learner to dig into compositional thinking in various stages. Such a framework constitutes deep and meaningful learning [START_REF] Major | Teach it to sustain it! Environmental attitudes of Hungarian teacher training students in Serbia[END_REF][START_REF] Kalsoom | Inquiry into sustainability issues by preservice teachers: a pedagogy to enhance sustainability consciousness[END_REF].

Fig. 1 provides an overview of the matters that need to be considered from the educator's perspective when seeking to instill SD practices through educational methods. It is based on the UNESCO report combined with the sustainable development goals that the UN set in 2015. [START_REF] Goni | Strategic alignment between sustainability and information systems: a case analysis in Malaysian public Higher Education Institutions[END_REF], developing the proposal made by [START_REF] Stephens | Higher education as a change agent for sustainability in different cultures and contexts[END_REF], outlined four points that need to be addressed for HEIs to reach the goal of a sustainable-oriented culture, through the implementation of adequate holistic educational SD tools. These points are listed as follows:

i. HEIs can become the means to integrate sustainability thinking in modern societies. This comes as a continuation of the philosophy that each one of us should practice sustainability actions individually to achieve sustainable philosophy thinking. ii. HEIs teach students the basic principles of design, project management and problem solving for them to be able to confront the need for sustainability. iii. HEIs can apply project-based learning methods, which will properly guide students through the SD path. iv. HEIs can be involved in SD practices to create curiosity amongst individuals and collaborate with other institutions or sustainability related centers. [START_REF] Jickling | Globalization and environmental education: looking beyond sustainable development[END_REF] pointed out that by implementing education for SD in HEIs, a whole range of other activities are involved, such as carrying out literature review analysis, tutoring and numerous projects that concern the structure of society. [START_REF] Melkonyan | Sustainability assessments and their implementation possibilities within the business models of companies[END_REF] acknowledged that the implementation of sustainability is a complicated philosophical pattern that leads to long processes, various challenges and risks.

It is generally accepted that Serious Games (SGs) offer great potential in the education sector, mainly due to the positive effects they have on learning outcomes [START_REF] Rossano | A serious game to promote environmental attitude[END_REF]. Focusing on sustainability concepts, the use of SGs can deliver a major increase of interest in training, project understanding and evaluation amongst users. The advantages of SGs come from the fact that they receive gameplay characteristics from common entertainment games and focus on learning or training, with concepts of applying theoretical instructions in real-life environments [START_REF] Wattanasoontorn | Serious games for health[END_REF]Lamb et al., 2018a;[START_REF] Botella | Treating cockroach phobia using a serious game on a mobile phone and augmented reality exposure: a single case study[END_REF]. SD games are the main focus of this study. SGs tap into the effects of Game -Based Learning (GBL), especially those that provide directional lines towards multi-meaning concepts like sustainability. Some reviews of SGs on SD have been conducted which analyzed the way these games help to develop the sustainable philosophy. These reviews are those of [START_REF] Katsaliaki | Edutainment for sustainable development:A survey of games in the field[END_REF] and two years later, [START_REF] Madani | Serious games on environmental management[END_REF]. Despite the fact that these two studies were published recently, their data stop at 2013. Since then, an enormous expansion of SGs on SD has been observed. Some of these newer games can be found in this study. Taking the research one step further, their categorization according to the most widely-accepted diagram of sustainability (triple-bottom line scenario of economic, social and environmental attributes) has not been attempted before and appears for the first time in this study. Adding to the same concept, the mapping of the engagement with SD practices and policies based on UNESCO and UN reports for SD has also been achieved. This study is based on the scientific hypothesis which claims that any SG on SD fully contributes to all educational attributes of sustainability's triple-bottom line (economic, social and environmental dimension). The aim of this research is to provide an overview and classification of SGs on SD and to explore their potential as educational tools. The implications of the research are two-fold. The practical implication is that practitioners and educators gain an in depth understanding of the existing serious games in relation to sustainability. Furthermore, the grouping of serious games according to the triple bottom line of sustainability and their in-depth analysis provides academics with a basis for further analysis and research into new serious games related to sustainability.

The paper unfolds as follows: A method review of SGs on SD is reported in section 2, analyzing their underlying characteristics (77 SGs are included). In section 3, the results of the study are presented, along with an extended discussion. Finally, section 4 includes the conclusions and the promising prospects for future research.

Research method

To meet the aim of the paper, a systematic literature review (SLR) was conducted. The SLR method is considered particularly useful when publishing the crucial conclusions of a large and complex body of research literature [START_REF] Sengers | Experimenting for sustainability transitions: a systematic literature review[END_REF]. It remains a method which is widely used by researchers who wish to produce eye-catching conclusions in their review papers [START_REF] Vel Asquez | Authentication schemes and methods: a systematic literature review[END_REF][START_REF] Fischer | Mindfulness and sustainable consumption: a systematic literature review of research approaches and findings[END_REF][START_REF] Guitart | Past results and future directions in urban community gardens research[END_REF][START_REF] Rodrigues | Mapping of the literature on social responsibility in the mining industry: a systematic literature review[END_REF]. Other studies in SGs have shown the path towards the application of this method, making it easier to select the proper SGs that fit the main purpose of this paper [START_REF] Katsaliaki | Edutainment for sustainable development:A survey of games in the field[END_REF][START_REF] Goni | Strategic alignment between sustainability and information systems: a case analysis in Malaysian public Higher Education Institutions[END_REF][START_REF] Chappin | Teaching sustainability to a broad audience through an entertainment game e the effect of Catan: oil Springs[END_REF][START_REF] Dlouh A | Sustainability-oriented higher education networks: characteristics and achievements in the context of the UN DESD[END_REF][START_REF] Madani | Serious games on environmental management[END_REF]. The cases studied in this review concern the use of SGs in SD as a means to incorporate holistic knowledge about sustainability issues in HEIs. In line with the suggestions of [START_REF] Thürer | A systematic review of the literature on integrating sustainability into engineering curricula[END_REF] and [START_REF] Boyle | An update to the systematic literature review of empirical evidence of the impacts and outcomes of computer games and serious games[END_REF], a systematic formula for retrieving and selecting the academic publications was used. The schema followed is presented in steps in Fig. 2. Paragraphs 2.1 to 2.3 shape the research design followed for tracing, filtering, and evaluating the documents, in that order.

Tracing the documents

Tracing the documents (documentation) of the related SGs on SD that could be used as educational tools was conducted in the form of a literature review (conference papers, journal articles, book chapters, reports from international organizations, online databases) that provides online links to accessible SGs. In this research, the subject to be analyzed is SGs on SD and their use as effective educational tools. These are thus the three main keywords of the research (refer to Table 1).

Relevant documents were identified from six online databases: Scopus, Science Direct, Web of Science, Google Scholar, Springerlink and Emeraldinsight. The selection of these specific online databases was made due to their large coverage of documents. As Table 2 suggests, so as to keep the number of documents manageable and to ensure the significance of the sources, the search was further limited to certain criteria (time period, subject area, etc.). Among 5092 results in total, using different keywords and combinations each time (refer to Table 1), 81 journal articles, 10 conference papers, 5 books, 4 reports from international organizations and 1 online database1 (plus the ones in the Appendix section) were finally included in this study. This determined the first stage of the selection method.

The study of online databases (referring to the one included in the total documentation and the rest in the Appendix section) involved reading the description of each SG as a first step, and then actually playing the game. Following this process, the opportunity to determine the classification, the relevance and the context of the games was provided. Thus, the categorization was achieved relatively easily.

The search relations also resulted from the first review and addressed the diversity of games (video games, sandbox games, etc.) that were played as well as standings for the possible outcomes of playing (assessment effects, learning, educational tools, skills, motivation).

Filtering the documents

The initial sample of 5092 documents was further reduced to 444 by reading the title and abstract. This was further reduced to 101 documents (plus the online databases provided in the Appendix section) by excluding unrelated documents after reading the full text. The vast number of unrelated documents is acceptable, as restrictions were made on the subject area (computer science, engineering, social sciences, environmental science, energy, decision sciences). This tactic was chosen mainly due to its objectivity and flexibility in the field of sustainability and educational tools like SGs.

To safeguard against relevant documents being lost, all the references were validated twice. Following this process, 4 supplementary documents were discovered and added to the total documentation sample. The final sample was 101 documents from which 31 were reviews on sustainability issues and the rest referred to SGs (on SD and as educational tools). The systematic literature investigation (filtering), including the documents' source is diagrammed in Fig. 3.

The progression of documents retrieved in scientific databases mentioned in paragraph 3.1. is shown in Fig. 4. As expected, Scopus and Science Direct were the electronic platforms that provided the most primary documents to our SLR. The platform with the highest contribution in searches was Scopus, with 2354 documents and the one that engendered the lowest contribution was Emeraldinsight, with 86 documents.

Evaluating the documents

This phase involved removing and documenting information from each of the 101 final documents (plus the online databases provided in the Appendix section). To ensure objectivity, two-time analysis and extra discussion among researchers were conducted in order to determine any possible illogicality in review results. The (2018), who developed literature reviews on sustainability and SG practices accordingly. The difficulty of evaluating the final selection of documents lies in deciding which SGs may foster education in SD issues and how they can be related to educational approaches. The integration of sustainability and SD philosophy in SGs can be understood as the outcome of specific concepts in which numerous characteristics are united. The important part of the evaluation process lies in the concept of how practices are hypothesized by the writers and how they are applied in the curricula in education. Taking these principles into consideration, the final type of document was determined (Fig. 5). The SGs identified in this study, present an educational character in sustainability topics in line with TBL philosophy. Most reviews were found in journal articles, conference papers and online databases, dependent mainly on the nature of the paper (SGs, SD).

Synthesis

Each paper individually was a source of general data on the scientific content of the paper, as presented above in Table 2. Furthermore, additional specific data required to meet the aim of the paper was collected (SGs, SD, etc.). To reach the desired outcome, an extensive review was performed (Fig. 5), to ensure that the obtained data were appropriate. This also permitted the straightforward comparison and analysis of the extracted data during the evaluation process.

The scientific database of documents was scrutinized to distinguish patterns in the literature. Three main characteristics were recognized in the results: the nature of the research and the writer, the SGs and the sustainability issues described, and the researchers involved in these practices.

Results and discussion

This section presents a research of serious games related to SD and their added value in the educational sector. The SGs were identified through web sources; the main one was CRS (2017), while others are presented in the Appendix and related paper reviews [START_REF] Katsaliaki | Edutainment for sustainable development:A survey of games in the field[END_REF][START_REF] Madani | Serious games on environmental management[END_REF]. All 101 final documents contributed in showing the "path" towards this identification. Besides the name of the SG, the year it first became available, the type, the audience it addresses, the teaching exclusivity the study of each game initially identified and the sustainable technology/SD strategy are described. The findings indicate that the increasing number of SGs featuring SD reveals a preference of the academic community for their use as educational tools to incorporate SD ideals.

Results

Serious Games (SGs) with SD themes have developed steadily in the academic world over approximately the last fifteen years (Fig. 6) and gamification has become progressively widespread since it first appeared [START_REF] Dias | Gamification and serious games in depression care: a systematic mapping study[END_REF]. The most in-depth analysis of SGs on SD was conducted by [START_REF] Katsaliaki | Edutainment for sustainable development:A survey of games in the field[END_REF] and [START_REF] Madani | Serious games on environmental management[END_REF], where the authors noted the need for further research. However, while the effectiveness and the educational value of the games analyzed is not in doubt, the empirical evidence regarding their holistic contribution to sustainability values is still emerging. After reviewing a large number of SGs on SD (as well as actually playing them to define the possible educational outcomes), it was found that SGs are one of the best means to engage in learning on sustainability. Although this is the case, findings indicate that education of this kind (theoretical background through SGs) is not enough to engage students in a holistic approach to the philosophy of sustainability. A tangible, practical solution has to be applied towards this kind of educational method as well. This is the missing part and the "Achilles' heel" of modern education systems with regard to SD.

The studies of [START_REF] Katsaliaki | Edutainment for sustainable development:A survey of games in the field[END_REF] and [START_REF] Madani | Serious games on environmental management[END_REF] constitute one of the main sources of this paper for the identification of SGs on SD. Table 3 presents the list of these SGs, along with other useful characteristics. Column 1 states the year that the SG became available. The name of the SG is presented in column 2, the type in column 3, the participants in column 4, whether the SG described was created specifically for teaching purposes (HEIs and other institutions) in column 5, while column 6 shows whether the SG was first documented by [START_REF] Katsaliaki | Edutainment for sustainable development:A survey of games in the field[END_REF] (with the indication 1) or [START_REF] Madani | Serious games on environmental management[END_REF] (with the indication 2). Finally column 7 describes the sustainable technology/SD strategy according to each game's educational content. The reference for each SG is provided in the Appendix section.

Among the 77 games analyzed, the seven categories of SG orientation (TBL) included sandbox video games, hybrid simulation games, online games, computer quiz games, computer simulation games, computer assisted games, card games, dice games, mobile apps, board games, RPGs, online sandbox games, online alternative reality games and video games. Table 3 displays the thematic breakdown of each SG, while Fig. 9 shows the type of SGs released over the period examined. An additional feature of these SGs is that many of them, even the single-player games, could be modified for use by teams of players instead of individuals. The majority of the games have been designed to educate students, SD professionals (SD prof.) and stakeholders. Some of them can be used by the general public in order to increase awareness of sustainability issues. Stakeholders are typically motivation groups of entities that need to be clearly aware of sustainability topics for their own reasons. The general public, on the other hand, are players who have no direct interest in engaging with the SD world, including all other subcategories as well. The reasons for playing and educating themselves in this direction are totally different. The virtual environment of the majority of the SGs is two dimensional. The 77 SGs that were identified in Table 3 reveal that, although the first game featuring sustainable criteria was created in 1990, the majority of the games were developed between 2010 and 2018. However, the research concerned SGs developed before February 2018, and the next five years are expected to see the release of more SGs on SD. Fig. 6 summarizes the above-mentioned information, displaying the release of all the SGs (a total of 77) over time.

The findings on game characterization, which is specified according to the TBL of sustainability, show that although the vast majority of SGs are designed to educate users in SD, their field of gravity is sometimes centered on only a part of the dimensional TBL diagram. Fig. 7 provides an overview of each game according to its orientation on the TBL. The numbers in the figure indicate that the SGs described in Table 3 follow the same numeration pattern. The majority of the games (total of 25, or 32% according to Fig. 8) have achieved the goal of educating players in SD principles by involving all three dimensions of sustainability. The second largest percentage, of 18% with a total of 14 games, belongs to the environmental aspect. Environmental management games tend to be highly popular nowadays, especially due to ecological destruction and the huge increase in the use of the Earth's natural resources [START_REF] Damania | The road to growth: measuring the tradeoffs between economic growth and ecological destruction[END_REF]. The Socio-Economic, Socio-Environmental and Eco-Efficiency dimensions are almost equally developed, with 12%, 13% and 8% respectively, which corresponds to 9, 10 and 6 games in each dimension. Adding these percentages (33%), with a total amount of 25 SGs, it is obvious that following two dimensions of the TBL achieves superior results in the educational road towards SD. Covering at least two of the selected dimensions makes it easier for educators to somehow facilitate the third one and attain holistic SD knowledge. The economic dimension is the least popular amongst these kinds of SGs with only 1 game (1%). On the other hand, the social dimension is gaining in popularity, with 12 games (16%).

Discussion

Having analyzed a large number of papers in the field of SGs on SD, it is clear that GBL in general, and the development of SGs that address sustainability issues in particular, are growing fields, and very promising for the future of sustainability education. In this paper, an analysis of a sample of recent SGs was conducted according to significant parameters like game objective and gaming type. The problems identified during this research concerns the lack of references to new SGs in the sustainability field, their categorization according to the TBL scenario and their numeration so that they can become easily accessible to users who desire to be educated towards certain aspects of sustainability values.

In order to fill the gap that has existed so far in literature reviews of SGs addressing SD issues, wide-ranging research with allembracing contributions from various scientific fields needs to be considered, in order to meticulously comprehend all the vital features that SGs on SD have to offer. Features that should be addressed include educational outcomes, ethics, aims, strategies, teaching exclusivity, the audience involved and the learning directions of sustainability issues.

One limitation identified in this study has to do with the fact that not all available SGs on SD could be found in academia or through internet sources. SGs tend to be less popular than entertainment games and thus their discovery presents obstacles.

Following the research method of the paper, a systematic literature review was conducted. Through the SLR, critical conclusions concerning the "nature" of the SGs on SD were drawn, defining their concepts and intentions towards sustainability. The analysis of the 77 SGs on SD principles involved SGs released from 1990 until February 2018. Throughout the years a progressive rise in the number of SGs has been observed (Fig. 6). While there are plenty of types of SGs on the market (board, card, online sandbox, etc.), those that have ultimately been the most popular during recent years are board games and computer games (Fig. 9). Other types, like dice games or sandbox games present a lower rate of recurrence. The thematic field of these SGs covers a variety of topics (economical aspects, social policies, environmental management) and thus their 3). categorization according to the TBL scenario of sustainability was possible (Fig. 7). To fully examine the thematic breakdown of these games, a quick play was performed in order to access the basic features of each game.

When seeking to teach specific sustainability attributes, educators are confronted with difficulties in deciding which SG is the most appropriate to their needs. Through this paper, guidance towards the desired path becomes easier than ever. Users can easily gain access to the SG of their choice by knowing beforehand if it is appropriate for their intentions (including internet links in the Appendix section). Observations through this literature review search revealed that not all SGs on SD are exclusively reserved for teaching. Among the 77 SGs presented in this study, those that seem to contribute exclusivity to teaching appear in Table 3.

The high rise in the number of SGs that address SD issues (Fig. 6) is an indicator that additional instruction and knowledge methods are expected to be developed in the future. SGs have the potential to be used as a tool for effective educational involvement. In this review, the state of SGs that give guidance in SD principles was analyzed, to provide awareness of their practicality as educational tools.

These SGs (their characteristics) have been briefly described and analyzed. The main objective of carrying out the SLR was to extract beneficial information about SGs that incorporate SD principles. The findings show that a growing number of SGs that seek to educate SD issues are being developed, especially during the last decade. The majority of these games contain major characteristics like the fact that they are available for free, making the task easier for educators and stakeholders. Another important factor is that most of them can be found on the web and are therefore easily accessible to anyone interested. Furthermore, they offer a single player option, which removes the need for team playing.

Apart from assessing the educational outcome of these games, the formation of multiple key factors is examined, including incentive, user experience and user-friendliness. Conclusions drawn from this analysis concern, the effectiveness (through their design and the audience they refer to) of the SGs mentioned in providing a holistic understanding of SD, the lack of quantitative results in some cases, and finally the increase in motivation and engagement that SGs can provide (which has valuable effects on knowledge outcomes). The reviewed SGs address a variety of learning values like motivation, socialization and understanding, as well as learning positionings, like behaviorism and humanism. There is a lack of harmony between the vital examination features of each study. Most assessments of SGs lack scientific accuracy. Based on the current analysis, it can be concluded that scientific accuracy calls for improvement in the value of SGs for educating in SD principles, in order to attain effective outcomes.

The purpose of this study was to determine the educational contribution of SGs on SD, based on the TBL of sustainability and identify their impact scenario in sustainability teaching. Based on the findings (SGs characteristics) which is consistent to the existing literature, there appears to be a lack of information in the field of GBL on the topic of SGs for sustainability education [START_REF] Annetta | The "I's" have it: a framework for serious educational game design[END_REF][START_REF] Michael | Serious Games: Games that Educate, Train, and Inform[END_REF][START_REF] Rugelj | Learning Programming with Serious Games[END_REF][START_REF] Wattanasoontorn | Serious games for health[END_REF][START_REF] Young | Our princess is in another castle: a review of trends in serious gaming for education[END_REF][START_REF] Cahier | Web 2.0, serious game: structuring knowledge for participative and educative representations of the city[END_REF][START_REF] Charsky | From edutainment to serious games: a change in the use of game characteristics[END_REF][START_REF] Corti | Games-based Learning; a Serious Business Application[END_REF][START_REF] Crookall | Serious games, debriefing, and simulation/gaming as a discipline[END_REF][START_REF] Westera | Serious Games for Higher Education: a Framework for Reducing Design Complexity[END_REF][START_REF] Madani | Serious games on environmental management[END_REF][START_REF] Argasi Nski | Affective patterns in serious games[END_REF]Lamb et al., 2018b;[START_REF] Moloney | Serious games for integral sustainable design: level 1[END_REF][START_REF] Giessen | Serious games effects: an overview[END_REF][START_REF] Allal-Ch Erif | Using serious games to manage knowledge: the SECI model perspective[END_REF]. Thus, five basic inferences can be drawn: (1) the effectiveness (teaching exclusivity) of SGs in meeting sustainability's educational requirements does not apply to all the games, but depends on the philosophy, the design and the type of each game; (2) a large number of studies of SGs (on SD or not) do not properly define their research method and lack quantitative results. Consequently, the clarity of the findings is hard to evaluate; (3) considering that the fields of SGs and sustainability are relatively new, there is a clear need to inspect the educational outcomes over time and to clarify the influence and the effectiveness of SGs, if any, on the users; (4) interviews are a significant part of GBL, permitting users to describe their understanding and comprehend the educational link that connects "pure" gaming itself and didactic objectives; and (5) education through SGs (GBL) raises users' incentive and engagement, resulting in advantageous outcomes through fast learning.

As already mentioned in the introduction sector, the scientific hypothesis of this study claims that any SG on SD fully contributes to all educational attributes of sustainability's triple-bottom line (economic, social and environmental dimension). The Systematic Literature Review (SLR) process is a formal, repeatable, recognized process, which in our case, was used for tracing, filtering and evaluating the literature relevant to SGs, sustainability and SD. By revealing the rate of recurrence of different documents in serious gaming and sustainability concepts, SLR has become a very satisfactory method to examine the original hypothesis (prove or deny) and to thoroughly gather, explore and describe the results. As [START_REF] Imtiaz | A tertiary study: experiences of conducting systematic literature reviews in software engineering[END_REF] mention in their study, after identifying 174 published SLRs between 2005 and 2011, SLR is a valuable tool for researchers seeking wide-ranging knowledge. The fact that SLR offers helpful information concerning the need for further research in older studies, further demonstrates that it is a very promising clarification method [START_REF] Hassler | Identification of SLR tool needs e results of a community workshop[END_REF]. Fig. 10 examines the research factors and their interconnections allowing deeper examination of the SLR findings.

1. Digital reality is transforming our society in fundamental ways, becoming increasingly pervasive across all sectors and being integrated in several aspects of our lives. Education, and SGs in particular, are part of this transformation [START_REF] Yu | Chapter 2 -understanding the self through the use of digitally constructed realities[END_REF]. Thinking ahead, new digital technologies are expected to create massive integration, both in the professions and in educational sectors previously considered unaffected [START_REF] Frey | The future of employment: how susceptible are jobs to computerisation?[END_REF]). 2. SGs, which are the main subject of this review, are an example of a different type of game. They are designed to develop a specific aspect of knowledge or training (sustainability issues in our case) and to be used in education [START_REF] Dias | Gamification and serious games in depression care: a systematic mapping study[END_REF]. 3. Users of digital reality are currently generating data at an unprecedented pace with information coming in many forms and directions. However, there remains a fragmented relation between the data, the environment and the human interaction involved (sustainability and SD issues). 4. Increasing attention is being paid to integration of sustainability into development patterns within a SD framework. The predominant modeling approach under the SD framework is to develop peoples' awareness of sustainability initiatives through effective educational methods (SGs) [START_REF] Halati | Intersection of economic and environmental goals of sustainable development initiatives[END_REF]. 5. The data generated nowadays usually entails a disparate and broad variety of sources, forms, locations, accuracy and velocity of collection, which results in great complexity in terms of its management and use [START_REF] Kwon | Toward Data-driven Idea Generation: Application of Wikipedia to Morphological Analysis[END_REF]. Thus, appropriate collection and handling procedures are essential, as well as particular trials for its management and application. 6. The response to these pressures has resulted in an increased interest in regulatory policies regarding procedures for the use and control of digital reality tools, like operational SGs [START_REF] Lemstra | Leadership with 5G in Europe: two contrasting images of the future, with policy and regulatory implications[END_REF]. 7. Holistic understanding of sustainability values is the most important educational outcome of a SG that addresses SD principles. Some games are getting closer to this goal, others not so much. 8. Training shows that in order for users to better acquire knowledge of sustainability issues, there is a need for the implementation of educational tools (SGs) that reinforce interdisciplinary influences. The key factor for the establishment of such methods is to acquire holistic knowledge through educational methods [START_REF] Mayorova | New educational tools to encourage high-school students' activity in stem[END_REF]. 9. Along with these lines of development, there is a need for people (users) to engage in sustainability topics and thus futureproof themselves, so as to prevent long-term economic, social and environmental damage [START_REF] Rowley | 5 -towards a business sustainability future[END_REF].

Conclusions

This study offers an understanding of the concept of 77 SGs and their contribution in sustainability matters. The analysis performed in this study shortens the gap between SD practices, policies and education (SGs), creating an extensive mapping (Fig. 1). A categorization of 77 SGs was achieved according to the TBL (Fig. 7), giving clarification hints to users wishing to select the relevant tool that offers an understanding of specific sustainability issues (TBL). After numerating the reviewed SGs on SD, they are positioned in a threedimensional diagram of sustainability, to provide easy access to users. The trend of new SGs on SD appearing over time, and the sharp increase in their development over recent years (Fig. 6), indicates that sustainability is becoming more and more essential for modern societies, and education in SD issues is seen as crucial for its proper implementation.

Considering all the above-mentioned aspects and the findings of the study, it can be concluded that the original scientific hypothesis, which claims that any SG on SD fully contributes to all educational attributes of sustainability's TBL, should be rejected. Findings indicate that there are indeed some SGs that fully contribute towards the apprehension of all of sustainability's triple-bottom line (economic, social and environmental dimension) parameters (Fig. 7), nonetheless there are plenty that do not. A large number of SGs on SD cover certain educational aspects of the TBL, such as the economic part, or in some cases, a combination of two (economicsocial). As previously mentioned, the implications of the research are two-fold. The practical implication contributes towards the understanding of the existing SGs in relation to SD. This enables users to select the SG that complies with their educational needs. Going one step further, the grouping of SGs according to the TBL of sustainability (Fig. 7) and their painstaking scrutiny offers prospect for further research on SGs that address SD values.

Despite the considerable interest in SGs as educational tools, it is important to understand that developing SGs for educational purposes can be very complex, and expensive and entail significant challenges. A possible direction for future research could be the development of experimental studies that systematically uncover key features that are useful in promoting holistic learning. Another conceivable track towards improving the layout of these SGs is to integrate realistic 3D graphics that bring users closer to reality. These graphics are impressive and thus help to raise players' motivation [START_REF] Sweetser | GameFlow: a Model for Evaluating Player Enjoyment in Games[END_REF]. Additionally, SGs should develop more intense social interaction (providing gains in the social dimension of TBL). Future research might also include a survey of the data on profiles of users and instructors, or a survey of stakeholder aspirations, including HEIs, industry and society.

Fig. 1 .

 1 Fig. 1. Engagement with SD practices and policies based on UNESCO and UN reports for sustainable development (United Nations, 2015; UNESCO, 2005).

Fig. 2 .

 2 Fig. 2. Research method (SLR) presented in steps.

Fig. 5 .

 5 Fig. 5. Final document types.

Fig. 6 .

 6 Fig. 6. SGs on over (until February 2018).

Fig. 7 .

 7 Fig. 7. Orientation of SGs on SD according to TBL (following the numeration of Table3).

Fig. 8 .

 8 Fig. 8. Percentage of SGs on SD according to their orientation policy (TBL).

Fig. 9 .

 9 Fig. 9. Type of SGs released from 1990eFebruary 2018.

Fig. 10 .

 10 Fig. 10. Research factors and interconnections.

Table 1

 1 Keywords of research.

	Serious Games (SGs)	Sustainable Development	Educational Tools
		(SD)
	Gamification	Sustainability	Educational
			technology
	Game based learning	Triple bottom line (TBL)	Holistic
	(GBL)		understanding
	Green games	Education for sustainability Effective learning
	SD games	Sustainable management	Motivation
	Environmental games	SD issues	Educational process

Table 3

 3 List of SGs on SD identified in this study.

	Year Names of SGs on SD	Type of SG	Participants	Teaching	Initially identified	Sustainable technology -SD strategy
				exclusivity	by
	1990 1. Simearth	Sandbox video game	General public	Yes	1	Air cleaning
	1994 2. Irrigation Management Game	Hybrid simulation game Students, SD prof.	Yes	2	Management of landfill
	1998 3. Geology Explorer	Online game	Students	Yes	2	Earth resources exploitation
	1999 4. Build a Prairie	Computer quiz game	Students	Yes	1	Soil improving
	2000 5. Learning SD (LSD)	Card game	Students	Yes	1	Env. Conservation & urban
						development
	6. The Great Green Web	Computer quiz game	Students,	Yes	1	Sustainable consumer behavior
			stakeholders
	7. Samba Role Play	Board game	Stakeholders	No	2	Natural resource management
	2003 8. Industrial Chlorine Transport	Card game	Students, SD prof.	Yes	2	Social technology as a SD public policy
	Metagame
	9. Slyvopast	Hybrid simulation game Students	Yes	2	Effective strategy for SD
	2004 10. Balance of the Planet	Online game	SD prof.	No	1	CO2 capture through managerial
						choices
	11. Keep Cool	Board game	General public	No	1	Sustainable process negotiation
	12. River Basin Game	Board game	Students, SD prof.	Yes	2	Water cleaning
	13. Industrial Waste Game	Card game	Students	Yes	2	Industrial waste management
	2005 14. Atollgame	Role-playing videogame	Stakeholders	No	1	Groundwater cleaning
		(RPG)
	15. MHP	RPG	Stakeholders	No	1	Strategies for SD technologies
	16. Better Business Dilemmas	Computer quiz game	Students, SD prof.	Yes	1	Decision making for SD technologies
	2006 17. Shrub Battle	Board game	Students	Yes	1	Sustainable landscape design
	18. 3rd World Farmer	Online sandbox game	General public	No	1	Sustainability of agricultural land use
	19. Climate Challenge	Online sandbox game	General public	No	1	Renewable energy sources & politics
	2007 20. Stop Disasters!	Online sandbox game	Students	Yes	1	Natural disasters prevention
	21. Energyville	Online sandbox game	General public	No	1	Sustainable energy supply
	22. Encon City	Online sandbox game	Stakeholders	Yes	1	Energy conservation
	23. Butorstar	Hybrid simulation game Students	Yes	2	Biodiversity & agricultural conservation
	24. Food Import Folly	Online sandbox game	Students	Yes	e	Sustainable resource use
	2008 25. World Without Oil	Online alternate reality	Stakeholders	No	1	Renewable energy sources (RES)
		game
	26. Environment Game	Computer quiz game	Stakeholders	Yes	1	Sustainable building design
	27. Building Game	Computer quiz game	Students	Yes	1	Sustainable building design
	28. Electrocity	Online sandbox game	Students	Yes	1	Environmental management
	29. Catchment Detox	Online sandbox game	General public	No	1	Sustainable development (TBL)
	2009 30. The Sims Adapted	Sandbox video game	General public	No	1	Energy conservation
	31. Shortfall	Online game	General public	No	1	Sustainable engineering
	32. Green City	Computer simulation	Students	Yes	1	Green urban development
		game
	33. MIT CleanStart	Online sandbox game	General public	Yes	e	Sustainable decision-making
	34. Tragedy of the Tuna	Computer simulation	General public,	Yes	1	Water resources management
		game	students
	2010 35. Enercities	Online sandbox game	Students, SD prof.	Yes	1	RES & urban development
	36. Fate of the World	Online sandbox game	General public,	Yes	1	Global warming solutions
			students
	37. Cityone	Online sandbox game	Students, SD prof.	Yes	1	Sustainable city development
	38. Oceanopolis	Online sandbox game	Stakeholders	No	1	Water management
	39. The UVA Bay Game	Computer simulation	Stakeholders	Yes	1	Sustainable products & services
		game
	40. Sustainable Delta Game	Hybrid simulation game General public	No	2	Water cleaning
	2011 41. SOS 21	Online sandbox game	Stakeholders	No	1	Sustainable living
	42. River Basin Game	Computer assisted game Students	Yes	2	Reclaimed water management
	43. Fate of the World: Tipping Point Video game	General public	No	2	Global warming solutions

Table 3

 3 (continued )

	Year Names of SGs on SD	Type of SG	Participants	Teaching	Initially identified	Sustainable technology -SD strategy
				exclusivity	by
	44. Spent	Online sandbox game	General public	No	e	Sustainable economics
	2012 45. Irrigania	Online game	Students	Yes	2	Sustainable agriculture
	46. Aqua Republica	Online game	Students	Yes	2	Water resources management
	47. Citizen Science	Online game	Students	Yes	2	Water cleaning
	48. Earthopoly	Board game	Students,	Yes	e	Resources management
			stakeholders
	2013 49. World Climate	Computer simulation	General public	No	1	Global warming decision-making
		game
	50. Climate Change Survivor	Board game	General public	No	2	Eco-innovation
	51. Papers please	Video game	General public	Yes	e	Sustainable immigration & cultural
						integration
	2014 52. Paying for Predictions	Dice game	Stakeholders	No	e	Climate-based disaster risk reduction
	53. About That Forest	Video game	General public	No	e	Sustainable forest management
	2015 54. Earth: A Primer	Mobile app	General public	No	e	Geoengineering & environmental SD
	55. Polar Eclipse	Board game	General public	Yes	e	Earth resources management
	56. EcoChains	Card game	General public	No	e	Sustainable food chains
	57. Cities: Skylines	Video game	General public	No	e	Sustainable urban design
	58. Evacuation Challenge Game	RPG	Stakeholders	Yes	e	Natural disaster & emergency planning
	59. PeaceMaker	Video game	General public	No	e	Government's role in social
						sustainability
	60. The Arcade Wire: Oil God	RPG	General public	No	e	Food sustainability & geopolitics
	61. Extreme Event: Coastal City	RPG	Stakeholders	No	e	Sustainability of natural hazard risk
	2016 62. Laudato Si	Board game	General public	Yes	e	Biodiversity & economic inequalities
	63. UrbanClimateArchitect	Online game	General public	Yes	e	Sustainable urbanism
	64. Flood Resilience Game	Board game	General public	Yes	e	Flood resilience
	65. Never Alone	Video game	General public	No	e	Resilience & sustainability
	66. Lie, Cheat & Steal	Board game	General public	No	e	Politics of sustainability
	2017 67. The Catan: Oil Springs Scenario Board game	General public	No	e	Resource management
	68. Energy Safari	Board game	Students	Yes	e	Energy conservation
	69. New Shores: A Game for	Computer simulation	General public	Yes	e	Green project management
	Democracy	game
	70. The world's future	Board game	Students,	Yes	e	Resource management
			stakeholders
	71. Nexus	Board game	Stakeholders	Yes	e	Water management for SD
	72. Gifts of Culture	Board game	General public	No	e	Cultural sustainability
	73. Lords of the Valley: board game Board game	Students,	Yes	e	Biodiversity & water management
			stakeholders
	74. Flood Control Game	Board game	General public	Yes	e	Flood disaster management
	75. Energy Transition Game	RPG	Stakeholders	Yes	e	Energy saving
	76. Lords of the Valley	Video game	Stakeholders	Yes	e	Sustainable leadership
	2018 77. ECO	Video game	Students	Yes	e	Sustainable civilization

The 1 online database was included in the final documentation sample because of the website's layout which resembles an analytical review of SGs. Other online databases (Appendix) are more like online platforms where users can play the game and read a brief description.

Scientific Content Documents that present a model, technique or literature review to explore and describe SGs on SD Documents that describe the assessment of a SG Documents that show cases of study of SD practices and policies Documents that analyze the educational contribution of SGs Documents that review existing SGs on SD Documents whose main aim is not an assessment of SGs Documents that do not describe any specs of a SG (just names) Documents that do not analyze the educational outcomes of a SG Documents that do not discuss evaluation models of the SGs prism Language English All other languages

Acknowledgements

The authors have contributed to this paper equally and declare that there's no financial/personal interest or belief that could affect their objectivity in this study. Furthermore, the authors express their gratitude to the reviewers who contributed to the improvement of this study. They are also grateful to the reviewers for their thorough and constructive comments on earlier versions of this article which greatly helped the authors to make this study as explicit and useful as possible.

Availability Available in online academic search databases

Not available online Fig. 4. Progression of documents retrieved in scientific databases.