Sustainable Tourism: The Elephant in the Room

Research on sustainability and sustainable tourism has thus far avoided evaluating how tourists actually understand these terms. Instead, scholars have focused on the supply side, presuming a common and precise understanding of sustainability and sustainable tourism among all tourists and stakeholders. This study shows that most consumers link sustainability only to environmental issues, and understand sustainability differently from sustainable tourism. It finds significant interpersonal and intercultural differences regarding consumers’ conceptualisations of sustainability. The results illustrate that empirical research methodology for conceptualising consumers’ sustainability understanding frequently is doubtful or weak. This research exposes tourists’ limited understanding of sustainability, and helps tackle widespread scepticism about the effectiveness of sustainable tourism, by creating better informed sustainable tourism marketing.

Introduction
Sustainability is now a universal concept, and an integral part of policy in many sectors. Globally, the United Nations, with its 17 Sustainable Development Goals (SDG) comprising 169 targets [1] forced governments to put sustainability on their political agendas. At a European level, the European Commission decided to implement the SDGs in their 2016 policy framework [2]. Local and regional authorities take a central role in that implementation, using planning, funding, and evaluation measures, especially at a regional level [3]. In Europe, tourism policy and marketing are frequently managed regionally by public bodies or public-private partnerships. Very often the areal layout of destinations is identical to local/regional political units. Actions linked to sustainable regional development based on SDGs, therefore, have increasing influence on tourism policy, product development, and communication/marketing strategies, leading to supply-side-focused activities moving tourism towards higher levels of sustainability. However, the vitally important but unanswered question is: do consumers understand what sustainable tourism is, and do they appreciate what sustainable tourism does? The transformation of the tourism economy will only be successful if supply and demand fit together. This paper's title reflects, using an English figure of speech, that the issue of public understanding is so big, and so basic and fundamental, that it is usually ignored. Just as an elephant in a room, it is not discussed or understood: many hope that it will go away. It has largely avoided the attention of academics, the industry, the media, and governance systems [4].
Many scholars' papers about sustainable tourism over the last three decades have discussed sustainable tourism policies [5]. They predominantly focus on the supply side, especially on improving the management of tourism companies, attractions, and destinations, by product development and innovation, notably through governance processes or assessments of existing tourism's sustainability. Scientific publications rarely examine the demand side and/or consumers' understanding of sustainable tourism. Behavioural change research is a minor exception (see Journal of Sustainable Tourism special issue on marketing sustainable tourism, Volume 25, 7, 2017).
When deciding between an Alpine mountain destination and a Mediterranean seaside destination, the majority of travellers are aware of their general differences: the climate, the topography, the food, and options for activities. The same can be assumed for differences between accommodation types such as hotels or camping grounds. Many researchers and practitioners presume that consumers are able to differentiate between sustainable and unsustainable options. Consumers are also believed to have similarly precise ideas about sustainable tourism. However, do these precise notions actually exist in consumers' minds, and is this a valid assumption to make?
This paper, therefore, begins with an overview of the consumers' view of sustainability and sustainable tourism, as discussed in the literature. It goes on to satisfy two urgent requirements in tourism research and management: Firstly, testing a different questionnaire design to create more valid research findingsa theme explored recently by Dolnicar [6] and Dolnicar and Grün [7]. We used an online forum administered by a professional online research group able to provide large online European panels and allowing direct interaction with participants.
Secondly, the paper contributes findings to help promote and implement sustainable tourism, by understanding tourists' knowledge and lack of knowledge in the concept, and in the wider concept of sustainability. A better understanding of supply side knowledge can inform and direct marketing efforts, and help raise the success levels of sustainability implementation projects [8].

Consumers' Perception of Sustainable Tourism in the Literature
Sustainable tourism has been extensively analysed by researchers. Niñerola et al. [9] found over 2500 papers on Scopus having "sustainable tourism" or synonyms in their titles. Shasha et al. [10], using the Web of Science database, found similar results when reviewing eco-tourism publications; "eco-tourism" or synonyms appear in nearly 1800 papers. Reviewing sustainable tourism [5] analysed over 500 publications found by using the exact search string "sustainable tourism policy". However, these huge numbers do not prove that all aspects of sustainable tourism are treated in a balanced way [11].
To understand tourism as a system [12], three groups of interacting elements must be considered. First, the tourists, travelling from their homes for limited time periods [13] to the second element, destinations, with places and attractions. Thirdly, tour operators, travel agents, booking platforms, airlines, etc., act as intermediaries, connecting source markets and destinations. Sustainable tourism's research literature should consider all these elements and related processes along the tourism process chain.
Examining the keywords found in the 2647 papers analysed by Niñerola et al. [9], a focus emerges on the second and third group of the tourism system above: the supply side and the intermediaries. Of those elements, destinations or tourism businesses are very dominant. Consumers and tourists are first implicated by the keyword "tourist behaviour", which is ranked 32nd and "decision making" in the 34th position, with 63 and 60 mentions, respectively, out of a total of 7748 entries, indicating that past research on sustainable tourism emphasized aspects of the supply side. Papers discussing sustainable development and the management of destinations or enterprises including governance, stakeholder participation, protection of ecosystems and cultural heritage, and climate change adaptation are widely found.
Despite this relatively low ranking, recent research on the consumers of sustainable tourism, and on their individual attitudes and decision making, has gained increasing interest. After the first 25 years of publishing the Journal of Sustainable Tourism, the editors [14] identified six recent trends framing sustainable tourism issues. Three foci are on the consumer, widening the perspective. First, they see the trend towards understanding consumer behaviour much more in the context of consumers' social relations and the social norms [15] within their networks. Flight shame [16], promoted by Swedish climate activist Greta Thunberg, is a recent example. Second, and linked to the first, tourism as a part of individual consumption patterns finds more recognition and links tourism with everyday practices [17]. Sustainable tourism, therefore, is no longer seen as an isolated field of decision making; it is part of consumers' general awareness and actions. The third trend described by the editors is that of correlations between travellers' sustainable or unsustainable consumption decisions and the potential for turning the supply towards more sustainable tourism products. This third trend again concentrates more on the supply side and the need for transition management. Nevertheless, the willingness of consumers to change behaviour [8] and to perhaps pay higher prices for sustainable products [18,19] is central to this discussion. The gap between attitude and behaviour has already been described and conceptualized [20,21]. Notably, the divergence between awareness of air transport's negative impacts and its continuing use was a frequently addressed field of research [22][23][24].
Increasing numbers of consumers, aware of the need to change their consumption patterns towards sustainability, could move sustainability from the operative towards the strategic management level of the tourism industry. Sustainable tourism products can be seen as a specific product class competing with other classes that follow either traditional or innovative business models but without specifically taking into account sustainability. From the management perspective, this leads to questions regarding the role of sustainability of travel opportunities during customer journeys [25]. In the customer preparation and destination choice phase, awareness and perceptions of sustainable alternatives [26] are now an important concern. These issues have been analysed, especially for the accommodation sector [27] and for transportation to destinations [8,20]. Furthermore, the motivations and intentions of consumers [28] to choose a sustainable destination are interesting. During the visit to the destination, the perception of the overall competitive advantage of sustainable destinations compared to alternatives, is a concern. The key question is whether sustainable products are more competitive from the guests' viewpoint, and can they create higher satisfaction which, in turn, might raise loyalty. Related research can be found in the fields of nature experiences [29,30], local products, particularly food [31,32], local transport [33], the environmental management of hotels [34], restaurants [35], and ski resorts [36].

Sustainability and Sustainable Tourism: Different Perceptions, Different Interpretations
A second aspect of the literature must be considered. Many studies used terms such as sustainable, sustainability, eco, eco-friendly, or environmentally friendly in questionnaires, assuming that consumers have a common and full understanding of what these terms mean. For example, López-Sánchez et al. [37] analysed tourists' willingness to pay for sustainable destinations. They measured tourist attitudes towards destination sustainability using statements such as "it is important to work for the sustainability of tourism destinations", "I believe that this destination is sustainable", "I am willing to pay more for sustainability projects". These statements, containing the term "sustainable" or "sustainability", can be interpreted differently by different people and are typical of many studies. Lack of understanding, or differences in interpreting key terms, can invalidate results. Other studies use "sustainable tourism" as a synonym for "eco-friendly" travelling or tourism with a low environmental impact. As an example, Miller et al. [38] showed focus groups symbolic pictures of different types of travelling and discussed their carbon footprints to derive public opinion about sustainable tourism. This approach must be seen very critically, as measuring the sustainability of tourism by the perceived carbon footprint is not supported by any evidence. However, the opposite approach, offering study participants a definition of "sustainability" or "sustainable tourism" could invalidate answers because of interviewer bias induced by the definition. Interviewees' answers could mirror the given explanations rather than their own understanding. Other problems can occur when using predefined items. The answers offered to participants use the authors' perceptions: participants have no other choice.
A French study [39] used a qualitative open research approach, asking three openended questions: what a sustainable journey might be, what type of journey contradicts sustainable tourism, and what symbolizes best sustainable tourism practice (a place, a personality, an object, etc.). The study presents a typology of four components of sustainable tourism but did not consider the possibility that people have either a very personal, or simply no, idea of what sustainable tourism might be. A German report [40] about the demand for sustainable tourism allowed 7750 respondents to name freely what they associated with sustainable tourism. Environmental protection was mentioned most frequently, but environmental, social, and economic components were never named simultaneously in the same statement. Thus, the well-known triple bottom line definition of sustainability was not mentioned by anyone within this large sample. Moreover, 16% of respondents admitted not having a clear idea of what sustainable tourism is, and 25% gave no answer at all. Caruana et al. [41] found consumers' understanding of the term "responsible tourism" to be unclear. This underlines the hypothesis that terms such as "sustainable" or "responsible", describing complex multidimensional concepts used by the scientific community, might be unclear to many consumers.
Literature about consumers' understanding of sustainability is surprisingly rare. A few studies show that consumers seem to have difficulties [42,43] and when trying to explain, the focus of the description often only mentions environmental dimensions [44,45]. Related terms, such as sustainable or eco-friendly, are mainly associated with environmental characteristics [46]. Issues mentioned include longevity, economical use of natural resources, future generations, and protection of the environment, all connected with the environment. The studies above show that the social and economic dimensions of sustainability are of lower relevance in consumers' minds. Many authors attribute this biased mindset inter alia to press coverage of sustainability [39,45,47]. The press uses the term "sustainability" mainly to identify "urgent environmental problems" [48] thus undermining the concept's complexity. Sustainability is rarely (though increasingly) covered in the newspapers, possibly because it is seldom a scandalous, emotional, or dramatic issue, or linked to big personalities [49,50]. A further reason for the dominant environmental component in sustainability perception can be found in the advertising of products and image communication by companies. Kilbourne [51] argues that the advertising of environmentally friendly or green products meets the interest of target groups and can create a competitive advantage while being in line with the prevailing paradigms for growth and profit maximization. Consumers are confronted with commercials about green products often blended with some attributes of sustainability. A recent example is the "greening" of cruise ships by using LNG (liquified natural gas) technology, mixing the terms green cruising, emission free, and sustainable cruising in advertising and press releases. This overemphasises the environmental dimension, frequently using "sustainable" and "environmentally friendly" interchangeably.
In addition, intercultural differences can create different understandings of sustainability. The cultural background of consumers influences their sustainability beliefs and their evaluation of sustainability initiatives [52]: Norwegian consumers associated sustainability with the preservation of specific resources such as oil and fish [47], whereas Vincenzi et al. [53] included burning trash in backyards in their sustainability indicators for a Brazilian context.
The above literature review leads to three fundamental research questions: (1) How do consumers understand the term sustainability?
(2) Does culture influence their personal understanding of the term sustainability?
(3) Does consumers' understanding of sustainability influence their understanding and conceptualisation of the term "sustainable tourism"?
This paper provides answers to these questions by adopting a mixed qualitativequantitative approach. The UNWTO definition [13] of tourism and tourist is very general and covers all kind of trips with at least one overnight stay without differentiating by purpose and duration. The research presented here focuses on vacation tourism and, therefore, on sustainable tourism in the vacation context, which means trips of a minimum duration of four overnight stays. Because of this, business travellers as well as short trips are not part of the study. This decision was taken to keep the study participants within the same mindset when discussing the term sustainable tourism. This is fundamental when asking participants to provide positive or negative examples of sustainable tourism as they reflect in general their personal vacation experiences or those of others they have heard about.

Materials and Methods
The literature review shows that the three fundamental research questions raised above have not been addressed previously. Therefore, we applied a qualitative approach using open-ended questions to avoid interviewer bias by pre-selection of items and scales, asking participants to describe sustainability and sustainable tourism. We used an online forum for the survey conducted from 2 to 5 December 2019.
To analyse potential intercultural effects, Germany and Italy were chosen as study areas, countries with significant cultural differences [54], e.g., in context orientation [55], and with languages with different roots (Germanic/Romance). Both countries are European Union members: their national legal frameworks for environmental policy, social standards, and welfare are comparable.
Participants were recruited from two large national panels for Germany and Italy offered by Norstat (norstat. online co.uk). The recruitment was done by a two-step procedure. In a first step, participants were found using a quota plan using a screening questionnaire. This quota-plan ensured a representative sample for both countries concerning gender, age structure (18-64 years), marital status and household child numbers, income, education level, and regional distribution of residence (see Table 1). Furthermore, only active travellers that had taken holiday trips in the previous five years could take part, ensuring well-founded opinions about tourism. All had holidayed in the Alps during that time, and confirmed consideration of visiting South Tyrol in the next three years. This destinationbased sub-selection created a participant sample with similar destinations in mind when discussing sustainability issues on holidays. The final sample was well balanced on all variables except the education level; there were above-average participant numbers with an academic degree. In a total of 163 participants, 89 Germans and 74 Italians joined the study.
In the second step, the selected participants had to work on the survey itself. We used QDC-Studio (see kernwert.com/index/kernwert/software/), a software for qualitative online surveys. This offers a set of dialogue functions as forum discussions and chats but also intuitively usable touch-tools, e.g., card sorting, ranking of alternatives, or image assessment to create a survey that is varied and interesting for participants. By the direct personal interaction between the research team and the participants, immediate quality control of the reliability of data could be done.
The survey was structured in several sections. First, as an ice breaker question and to identify potential participants that were not reliably answering the questions, participants had to give a short description of their very personal destination image of South Tyrol. Second, they had to provide a description of their ideal vacation by assembling a holiday trip from different components of activities and services. These questions were used to "calibrate" the thinking of people concerning their personal way of travelling. After these tasks were completed, there followed the core questions of this study about sustainability in general and sustainable tourism in particular. Using four chat groups (two in Germany and two in Italy), additional information for the interpretation of the open-ended questions was collected. The survey ended with some more tasks dealing with pictures and specific situations during a holiday stay in South Tyrol, used for another research survey from the field of destination image formation. Participants received a payment of 5 Euros for the sustainability questions if they completely and reliably answered. All texts and posts were analysed using Grounded Theory [56], and coded independently by two researchers. We used constructivist Grounded Theory as described by Matteucci and Gnoth [57]. Contributions that were coded differently by the researchers were discussed over multiple cycles reflecting the context of single terms used in the text. This was done for posts in both languages separately. For the Italian posts, a native speaker was consulted. The authors included three native German speakers. A cultural studies researcher finally controlled and analysed the texts and posts additionally, considering intercultural aspects. The final German and Italian codes were compared and merged to one code list in English. The translation into English and backtranslation into German and Italian to verify exact meanings was supported by a native English speaker and expert from sustainable tourism research. When coding text sections about environment and nature, "environment" was used to refer to natural resources such as air, water, or soil, while "nature" was used to describe the observable natural ambience. The final codes derived from texts and posts were analysed qualitatively for content and quantitatively using SPSS 25 to support the researchers in finding differences or illustrate findings.

Consumers' Understanding of Sustainability
Participants were first asked: "What does sustainability actually mean? How would you explain it to a friend or neighbour in a simple way?" To avoid interview-bias, the question was presented without prior references to "sustainability". The answers could be posted in an open text field without any limitations. Of the 163 participants, 156 provided usable personal definitions of sustainability (87 Germans and 69 Italians) of different lengths, averaging c. 30 words. Each text was coded using Grounded Theory techniques as described above. Table 2 exemplifies the coding results for a few randomly selected postings. P64, IT: La sostenibilità è creare produrre e offrire prodotti e servizi che non inquinino l'ambiente non provochino danni potenziali a cose e persone e ci permettano di prenderci cura del nostro pianeta terra e di noi stessi Sustainability creates, produces, and offers products and services without polluting the environment, causes no potential damage to things and people, and helps take care of our planet and ourselves.
economy and environment balanced, prevention of damage, responsibility for planet, respectful social behaviour, avoiding pollution societal Finally, 34 different original aspects were found and assigned unique codes. Table 3 shows the codes derived from the 156 analysed text-postings and their link to the three dimensions of sustainability (marked by x in the corresponding column). A few are linked to two dimensions of the three-pillar concept of sustainability (economy and environment balanced, regionality (principle/prioritisation), prevention of damage) as the postings explicitly addressed two of the three pillars. The code "three pillar model" was used when participants made explicit references to environment, society and culture, and economy. Some codes could not be connected to one of the three sustainability pillars, either because they were abstract (e.g., principle/concept, process/change) or critical (buzzword/marketing). Analysis of how often each code occurred in all postings showed that codes pertaining to environmental dimensions had the highest number (see Table 3).
Each post was classified according to the participants' views about responsibility for implementing sustainability, i.e., the interviewee or other individuals personally, society, or both of those groups. The results were ranked as follows: above 50% in class ++, 25% to 50% in class +, 15% to 25% in class o. Most issues assigned to a code belonging to the social dimension tended to be seen as the responsibility of society in general. For environmental issues, the picture (see Table 3) was unclear. Here, participants mostly saw a shared responsibility between the individual and society. Only the use of environmentally friendly mobility was primarily seen as an individual responsibility. A similar pattern was found for economic issues.
The results confirm a focus on the environmental dimension of sustainability as described by Hanss and Böhm [47]: the great majority of the participants included at least one environmental dimension code (149 from 156) into their definition. Over two thirds of German and over half of Italian participants defined sustainability unilaterally, i.e., focusing only on one (19), two (23), three (12), or four to five (3) environmental aspects. When combining environmental issues with another from the two remaining pillars, the codes classified as "abstract" or "other" were dominant (61). Social or economic aspects were mentioned less frequently and were mostly considered together with the environmental dimension. Only one participant explained sustainability by using two issues from the social-cultural pillar. Especially striking was that very few described sustainability considering all three dimensions of the triple bottom line concept (16 from 156). German participants especially associated sustainability with resources. The gentle or sparse use of resources was most mentioned, followed by regeneration as a basic principle and prevention of damage (often aimed at averting damage to nature, natural resources, or fellow humans). Care dealing with the environment and nature was also frequently seen as connected with sustainability. To many Germans, individual responsibility that secures the world for future generations and preserves the status quo is part of sustainable living, especially achievable through personal responsibility in avoiding waste in daily life. For Italians, the most mentioned aspect connected to sustainability was regeneration as a basic principle followed by preventing damage. They linked careful dealing with the environment and nature to sustainability at similar levels as Germans; sparse and circular resource use received less attention. When reflecting on economic issues, Italians considered sustainability to bring economy and environment into balance. Sustainability was also seen as an individual responsibility; however, the Italian perspective was more holistic: the overall aim of individual efforts, such as avoiding pollution, was to save the entire planet.
Climate protection/climate change were hardly associated with sustainability (with just 2 participants out of 156). This was totally unexpected: the Special Eurobarometer 490 [58] and 501 [59] showed citizens of both countries to regard climate change as a major threat. This is also astonishing because 23 participants included securing the future/future generations in general in their posts, and 17 explicitly referred to responsibility for the planet.
Cluster analysis was used to identify codes with high proximity: they were frequently used together. This way we also identified codes that were never or rarely used together to define sustainability. We used hierarchical clustering with squared Euclidean distance and Ward's algorithm. The results (see Figure 1) show that the codes linked in the three largest clusters can clearly be allocated to the triple bottom line sustainability concept. Thus, even though individual participants tended only to use a small subset of codes, with some only focussing on the environment and others providing more abstract descriptions, the clustering results largely reflect the triple bottom line approach. However, the cluster of ecological issues is relatively heterogenous, underlining that participants mostly picked out one of the environmental issues to explain and illustrate their sustainability definition.
We could confirm this distinct heterogeneity of sustainability definitions with a visualisation using network analysis (see Figure 2): neither clearly central nodes nor separated subnetworks could be found using the Fruchterman-Reingold layout algorithm.

Consumers' Understanding of Sustainable Tourism
Participants' views on sustainable tourism were evaluated with a further open question. Below the headline "Does sustainable tourism exist?", participants were confronted with the following question: "There are many ideas about what sustainable tourism can be. What is your opinion on this? What does it look like? What is not compatible with sustainability and where do tourism and sustainability fit together?". Of the 158 participants who gave an explanation of the term sustainability, 116 (58 Germans and 58 Italians) responded to this question. Some composed very short texts, provided opaque answers, or focused only on one aspect (e.g., mobility): others described the issue more comprehensively and discussed it by describing various aspects of a vacation (e.g., mode of transport, accommodation, activities, consumption, etc.). As a first analytical step, we addressed "Does the consumers' personal understanding of sustainability influence their idea of sustainable tourism?". Using the codes derived from the participants' definitions of sustainability, a content analysis of sustainable tourism explanations was carried out. In a second step, we compared participants' coded sustainability definitions and their coded sustainable tourism explanations to analyse the level of conformity. As a third step, we grouped the posts according to participants' belief in the existence of sustainable tourism or not, allowing for critical or sceptical views. Finally, we evaluated the specific concepts participants used to concretise positive or negative aspects of sustainable tourism. Table 4 shows two examples as an illustration. We could confirm this distinct heterogeneity of sustainability definitions with a visualisation using network analysis (see Figure 2): neither clearly central nodes nor separated subnetworks could be found using the Fruchterman-Reingold layout algorithm. When comparing the codes from the sustainability definitions (SD) with those found in descriptions of sustainable tourism (ST), major differences became clear. Generally, few participants transferred all the codes of their sustainability definition to their sustainable tourism explanation. Within the 116 cases, there were none that showed an identical code-set; 53% showed an entirely different code-set. Only 28% of participants used all their codes from SD and supplemented them with additional codes when describing ST. Moreover, 76% of the codes used by participants in the SD description were not transferred to their ST descriptions. The overall results show that the individual understanding of the term sustainability is not strongly linked to understanding ST.

Consumers' Understanding of Sustainable Tourism
Participants' views on sustainable tourism were evaluated with a further open question. Below the headline "Does sustainable tourism exist?", participants were confronted with the following question: "There are many ideas about what sustainable tourism can be. What is your opinion on this? What does it look like? What is not compatible with sustainability and where do tourism and sustainability fit together?". Of the 158 participants who gave an explanation of the term sustainability, 116 (58 Germans and 58 Italians) responded to this question. Some composed very short texts, provided opaque answers, or focused only on one aspect (e.g., mobility): others described the issue more comprehensively and discussed it by describing various aspects of a vacation (e.g., mode of transport, accommodation, activities, consumption, etc.

Participant
Germany ID71 Italy ID80 Sustainability definition (SD) Protecting resources for the generation of tomorrow.
Sustainability is the characteristic of a process or state that can be maintained at a certain level indefinitely. The guiding principle of sustainability is sustainable development, which concerns the environmental, economic, and social spheres in an interconnected way. A majority associated sustainability in tourism with environmentally friendly mobility (56%) and felt it was the tourists' responsibility to convert a vacation into a sustainable vacation (54%). The need to renounce certain activities, e.g., to avoid flying in general or to avoid intercontinental destinations (28%), and to buy local food (20%) and regional products (18%) were seen as important for ST. Long-term, and more abstract, issues, often outlined in the SD discussion (compare Table 3), were used less frequently. When posting their SDs, gentle /sparse use of resources was mentioned by 33% of participants, but only 7% addressed this issue for ST. We also observed very different mentions of regeneration as a basic principle (SD 29%/ST 3%), resource cycle/circular economy (SD 21%/ST 6%), or preservation of the status quo (SD 11%/ST 0%). One quarter (24%) showed sceptical attitudes, feeling that sustainability was merely used as greenwash for non-sustainable tourism. In their SD, only 4% mentioned such doubts. The lower frequency of the code reducing/avoiding waste in the tourism context is in line with results from previous studies which found that "the extent of [ . . . ] pro-environmental behaviour drops in the vacation context/environment" [60] (p. 705). Social aspects which were mentioned infrequently in the SD discussion were mentioned even less in the ST. Table 5 compares the codes found for defining sustainability in general (SD) with those explaining sustainable tourism (ST).
When describing sustainable tourism, respondents tended to use concrete examples. German participants discussed unsustainable tourism largely in the context of international travel (long distance travel, Croatia, Norway), while confirming their preference for traveling outside Germany (see the German Reiseanalyse [61]). Italian contributors discussed unsustainable tourism in the context of Italian overtourism destinations (South Tyrol, Venice, Sardinia). This difference is also reflected in the two groups' actual travel behaviours [62].

Consumer Beliefs about the Existence of Sustainable Tourism
When analysing the posts concerning participants' beliefs about the existence of sustainable tourism, three different groups emerged:

1.
Participants who believe that sustainable tourism can exist, and describe it mostly based on tangible examples, for instance, using environmentally friendly mobility to reach destinations. Many are convinced that sustainable tourism is already practiced in numerous places. Some, however, also claim that fully sustainable tourism is hardly possible (28% Germany, 38% Italy).

2.
Participants who have doubts about sustainable tourism but do not oppose the concept. Many have concerns about how to implement sustainable tourism. Some heavily condemned specific tourism forms, e.g., cruise vacations (38% Germany, 52% Italy).

3.
Respondents who are sceptical about sustainable tourism, and do not believe tourism can be sustainable, assuming it is just a marketing gimmick (34% Germany, 10% Italy).
We believe there are diverse explanations for the respondents' scepticism about the existence of sustainable tourism. Some presented very strict ideas and felt that ST had to be zero-impact. From such absolute perspectives, tourism can never be sustainable, leading to very critical views.
(Participant 98, Italy) " . . . as sustainability gets defined as zero-impact, already traveling to the destination is unsustainable" Several respondents opined that the existence of ST depended on personal sacrifices which travellers were currently unwilling to make.
(Participant 63, Germany) " . . . people are not willing to renounce, sustainability is not compatible with today's lifestyles" We also observed symptoms of an attitude-behaviour gap, frequently described in the literature: even though participants exhibited a strong awareness and comprehensive understanding of sustainability, they failed to translate this into specific sustainable behaviours. Some justified lack of engagement in sustainable behaviours by describing vacations as rare break-outs from everyday routines where travellers could not be expected to accept any behavioural constraints (see also [8]).
(Participant 120, Italy) " . . . it is useless for one week of holiday a year in which a person can really rest and relax" A third of those who demonstrated disbelief in the existence of ST attributed this to external factors, including insufficient infrastructure or lack of regulations.
(Participant 149, Germany) " . . . limiting tourism and guiding tourism flows is necessary to achieve sustainability in a long-term process" Overall, German respondents were more critical regarding the existence of sustainable tourism. Many Germans and some Italians particularly expressed their concerns that sustainability might be mere greenwash.
(Participant 84, Germany) "I am of the opinion that the term sustainable tourism mainly serves marketing purposes" (Participant 131, Italy) "Surely a bio hotel that flaunts organic menus and relaxation areas with Turkish baths and saunas (often at a high price) makes sustainability only as marketing and a luxury for a few" These results agree with those of Liobikienė et al. [63] who concluded that, within the EU, Germans exhibited the lowest level of confidence in "green" products. Another difference was that Italian respondents seemed to associate ST more with loss of comfort and convenience than Germans did, even though some were aware this might be a misconception.

Symbols for Sustainable Tourism
Respondents' views on ST were not necessarily based on actual aspects of (un)sustainability, but rather on symbolic understandings of the concept. The ST descriptions tended to be short; when asked for more specific explanations of ST, participants frequently mentioned aspects of previous vacation experiences as symbolic representations of (un)sustainability. The most prominent symbols used were of transportation modes: participants felt that buses, trains, and bicycles represented sustainable options, whereas flights and cruises were almost unanimously seen as unsustainable. However, there was a contradicting usage of symbols. Some respondents believed that a bus, hotel, or cable car represented ST; others saw these symbols as unsustainable tourism. Such discrepancies were also found by Rettie et al. [64]. They showed no consensus about diet: for some, "not eating meat or fish" was a "green" activity, while for others it was not.
Some symbols showed different representational values for Germans and for Italians. A significant number of German respondents felt that railways and buses were sustainable transportation options, a view that was shared by only few Italians. Research by Ashmore et al. [65] found similar differences in the symbolic value of hybrid and electric cars across different cultures.

Discussion
Our literature review showed that researchers suppose that sustainability has a normative character, evolved from a policy makers' framework, e.g., the 17 Sustainable Development Goals of the UN [66] or the UNWTO's definition of sustainable tourism [67]. The research community presumes a clear definition, transferrable from the general level to all other areas of activity, tourism included. This present study revealed two very different findings. First, consumers' understanding of sustainability is highly individual and does not follow normative but subjective definitions. Second, the understanding of sustainability varies by context, in our case, sustainable tourism. Individuals do not transfer definitions of sustainability to other activities in a consistent way. In tourism, we found that they use symbols to define what makes travelling sustainable or unsustainable.
The understanding of sustainable tourism is highly individual to each traveller. This is reminiscent of the long-lasting discussion about destination image formation in the literature [68,69]. Tasci and Gartner [69] identified three image formation factors: demand-side perceiver characteristics (dynamic/uncontrollable), independent characteristics (semi-dynamic/semi-controllable), and supply-side destination-oriented characteristics (dynamic/controllable). The first deals with the characteristics of travellers, including demographic profile, culture, attitudes, or needs/motivation. Our findings confirm the influence of these characteristics for sustainability. We saw a different understanding and definitional focus between Germans and Italians. Thus, we conclude that sustainability can be a contribution to the destination image which influences destination brand equity [70,71]. The second factor relates to educational materials, news, word of mouth, or groups of people. If news in the mass media as well as in social media mirror societies, this factor is important in the field of sustainability. Gössling et al. [16] demonstrated connections between the public flight shame debate caused by Greta Thunberg's Fridays for Future movement and the change in social norms concerning air travel in parts of German society. This debate takes place in diverse media, primarily in social media among younger generations. Greta Thunberg, who sailed to America instead of flying, has 4.6 million followers on Twitter. The impact of this media driven debate became visible in our study. German participants frequently saw flights as symbols of unsustainable tourism and an unsustainable vacation type. The supply side, the third factor, focuses on marketing strategy, positioning, and destination promotion. In this regard we found more than a quarter of participants who linked sustainability with marketing and "greening". Many were sceptical about whether destination management was actually sustainable. The extent to which tourism marketing influences the individual understanding of, and belief in, sustainable tourism should, therefore, not be underrated. Tourism management and marketing should be aware that messages using the term sustainability can be perceived very differently by individual consumers. Inflated use of the term sustainability might lead to confusion and mistrust: the tourism industry risks converting sustainability into greenwash if people's individual conceptualisation and experience do not fit with messages and products.

Conclusions
The discussion and promotion of sustainable tourism requires a conceptualisation which is easily understandable for travellers. Respondents used straightforward symbols when conceptualising unsustainable tourism. Symbols are constructed and diffused through communication within social groups, and symbolic repertoires tend to vary across cultures. Media coverage of sustainability-which varies across cultures [72]-contributes to shaping and disseminating symbolic representations, influencing people's evaluation of how important the related concepts are [73]. When communicating the issues and strengths of sustainable tourism, journalists, practitioners, and marketers should use meaningful symbols to increase awareness and promote behavioural change among consumers and across different cultures.
Tourism researchers have to address the fact that travellers neither have a coherent understanding of what sustainable tourism is, nor are they able to apply their individual concept of sustainability to the real world of tourism. So, what happens to research where interviewees in empirical studies do not have a common and precise understanding of sustainability? One inevitable implication is that studies may have serious conceptual weaknesses. Using a term in a survey without further explanation of its definition is only valid if that term is well understood and unambiguous, i.e., if it has a normative and context-independent meaning. However, the terms "sustainable" and "sustainability" are complex, and, as this paper has proven, they are not fully or even not at all understood by many consumers and, as regards tourism, by many travellers. Explaining to study participants what sustainability or sustainable tourism is, or offering a set of items for selection or rating, must also be seen very critically [6]. Not offering the option "I don't know" could easily cause over-interpretation of results [7]. The findings and conclusions still might be valid, but they remain unproven.
Over time, a convergence of individual understandings of sustainability might develop as a result of effective political and societal debate as well as a gradual transformation of economic systems. At present, however, this study suggests that those responsible for advancing sustainability in their fields should take into account the significant individual and contextual heterogeneity of the understanding of sustainability. The European Green Deal [74] should be mentioned here. European Union member states agreed on an agenda for the transformation of the EU's economy for a sustainable future, including carbon neutrality, by 2050. Thus, over the next decades, the topic of sustainability in economies, combined with numerous measures changing daily life, should create a joint understanding of the components of sustainability. However, this understanding currently does not yet exist. In democracies with relatively short periods between elections compared to the duration of the transformation process, this divergence of competences and understanding between governments and the populations bears a significant risk in facilitating populism. The need for broadening the knowledge and consensus in societies about the concept of sustainability is now urgent.

Future General Implications
Perhaps the greatest future implication of this work is how to improve public knowledge about sustainable tourism. Academic journal articles are not read by most tourists. Newspaper travel journalists tend to promote tourism rather than discuss it: at best they arouse fears of the damage caused by crowding and overtourism. The Green Globe organization (https://greenglobe.com, accessed 16 July 2021), founded with particular help from the World Travel and Tourism Council in the 1990s, does have a brief to spread understanding about sustainable tourism to the wider world [75], but its work is very limited. There is certainly scope for future research on how to tackle this problem, including the extremely difficult question of who might pay for an effective global public education/awareness programme.

Limitations
The study had some limitations. In general, studies based on participants from online panels who are paid for taking part in a survey can create fake answers or overrepresentation of some type of personality [76]. Even though we used a two-step recruitment procedure and rigorous control of data, such effects can still partially show up. Participants had, on average, a higher education level than overall in their countries, perhaps increasing their understanding of sustainability. Our sample included only those under 65 years old. Over half of all interviewees had visited South Tyrol, and perhaps learned about sustainable tourism there (but other destinations could have had similar issues). Many other factors, such as previous travel experiences, were not investigated in depth but might influence the perception of sustainability and sustainable tourism. The role of culture which we show comparing Germany and Italy may be even more important when comparing countries with higher cultural disparities, e.g., the USA or China, or have less experience of tourism and travel.