Secondary Students’ Reasons for Sustainable Consumption in STEM Education

Abstract

Contemporary global challenges are characterized by their complexity and cannot be adequately understood from a single disciplinary perspective, but they require knowledge and methods from multiple fields. To prepare students for their future roles as informed decision-makers, fostering interdisciplinary knowledge is essential. In this study, an interdisciplinary learning environment was developed on the topic of sustainable consumption using apple consumption, integrating the disciplines of mathematics, geography and computer science. At the end of the learning environment, students were asked to write a critical statement on whether an apple from overseas is more climate-friendly than a local apple. Written arguments of n = 52 students were analyzed to identify the types of arguments they used and the competencies they gained. Using qualitative content analysis, the data revealed that students referred to ecological, economic, and social aspects in their reasoning. Notably, the findings demonstrate that students did not limit their reasoning to the content explicitly covered in the teaching unit. They also integrated arguments derived from their everyday experiences and personal knowledge, demonstrating the interplay between formal learning and informal understanding. Therefore, this study highlights how interdisciplinary learning settings can promote critical thinking, system thinking, and decision-making in the context of sustainability education.

1. Introduction

Contemporary global challenges such as climate change, resource scarcity, and social inequality are characterized by high complexity, uncertainty, and interdependence, and thus cannot be understood from a single disciplinary perspective, e.g., [1]. Education for Sustainable Development (ESD) responds to these challenges by aiming to foster key competencies that enable learners to critically reflect on values, cope with uncertainty, and participate responsibly in societal decision-making processes [2,3]. Within this context, STEM education (science, technology, engineering, and mathematics) is increasingly recognized as a central domain for developing learners’ abilities to analyze complex systems, interpret data, and use models to explore real-world problems, e.g., [4,5].

Sustainable consumption is a particularly salient context in which tensions between ecological, economic, and social concerns become tangible in learners’ everyday lives, as routine purchasing decisions are entangled with multiple, and sometimes conflicting, sustainability considerations [6]. Questions of production conditions, transport routes, storage practices, and individual consumer choices can be explored in school settings through the concrete and relatable case of sustainable consumption. In public and educational discourse, the popular assumption that local products are automatically more climate-friendly than imported goods has been challenged, for example when the energy demand of long-term refrigerated storage of domestic produce may offset the benefits of shorter transport distances [7]. Such controversies provide productive entry points for data-based, sustainability-related argumentation that requires students to integrate different knowledge domains and evidence types.

Against this backdrop, the present study investigates secondary students’ written arguments on the sustainability of apple consumption within an interdisciplinary STEM learning environment focused on sustainable consumption. This study examines which types of arguments students employ when evaluating this claim and how these arguments can be structured with respect to the ecological, economic, and social dimensions of sustainability, as well as the extent to which students draw on the provided learning environment versus their everyday knowledge.

2. Theoretical Background

Education for Sustainable Development (ESD) is grounded in the Brundtland definition of sustainability, defined as development that meets present needs without compromising future generations’ abilities [8]. ESD aims to empower learners to engage with complex global challenges by fostering competencies that integrate cognitive, social, emotional, and ethical dimensions of learning [2,9]. Beyond mere knowledge transmission about sustainability issues, ESD emphasizes critical reflection on values, uncertainty management, and active participation in societal decision-making processes.

Building on this foundation, interdisciplinary STEM education within ESD creates a powerful space for students to engage not only with subject-specific content but also with the ethical, social, and political dimensions of sustainability challenges [10,11]. By combining disciplinary knowledge with broader societal considerations, such approaches enable students to develop competencies for responsible decision-making and informed action in complex real-world contexts [12]. While STEM education contributes to ESD, it should be embedded within a broader interdisciplinary framework to ensure a holistic understanding of sustainability, including social, cultural, and ethical dimensions [13].

2.1. Sustainability and ESD

Central to ESD is the recognition of sustainable development as a contested and normative concept [14]. The inherent tensions between economic growth paradigms and ecological limits [15] are captured in the “Triple Bottom Line” framework of people, planet, and profit [16], which parallels the three pillars or dimensions of sustainability: social, ecological, and economic. Emerging from early critiques of unchecked growth and environmental social justice movements of the 1970s, this model gained visual prominence through Barbier’s [17] three-circle diagram, emphasizing system interdependencies over isolated goals.

In the Agenda 21 [18], the UN identified the economic, social, and environmental dimensions as the main factors for monitoring and assessing environmental conditions. The term “dimensions” underscores that ecological, social, and economic aspects are often interconnected or may overlap. This integrated and interactive perspective more accurately reflects the complex nature of sustainability and the trade-offs that arise between its dimensions. Addressing and reflecting on such trade-offs constitutes both an educational objective and a pedagogical challenge within ESD.

• The social dimension (people) encompasses aspects such as justice, human rights, participation, health, education, and fair working conditions. It aims to ensure a high quality of life for all. This includes poverty reduction, equal opportunities, social security systems, and the strengthening of communities and social cohesion.
• The ecological dimension (planet) focuses on protecting and reconstructing ecosystems, conserving natural resources, and limiting environmental pollution such as emissions, waste, and pollutants. The aim is to ensure a longer viability of the natural environment through climate protection, biodiversity conservation, sustainable land use, the expansion of renewable energies, and many more.
• The economic dimension (prosperity) describes a sustainable economic model that creates prosperity, employment, and innovation without overexploiting natural resources. Key elements include resource-efficient production, long-term investment, market stability, and the integration of environmental and social costs into economic decisions.

Building on these three dimensions, the United Nations’ 2030 Agenda for Sustainable Development expands the model to include peace and partnership, resulting in the “five Ps” of people, planet, prosperity, peace, and partnership [19]. Research revealed that the Sustainable Development Goals (SDGs) are highly interdependent [20] and can be, depending on the situation, placed into more than one of the three/five dimensions [21]. Grouping the SDGs into these dimensions serves primarily as a means of structuring and clarifying their interrelations (Figure 1).

Furthermore, real-world problems cannot typically be attributed to a single SDG. For instance, engaging with the sustainable production of food primarily addresses:

• SDG 12—Responsible Consumption and Production: This goal highlights the environmental, social, and ethical dimensions of consumption and production systems. Students are supported in developing critical thinking and foresight competencies to assess resource use, ecological footprints, and social implications of consumer behavior.
• An examination of this issue, however, also entails addressing, for example, the following SDGs:
• SDG 8—Decent Work and Economic Growth: Learning activities addressing SDG 8 encourage exploration of fair working conditions, ethical labor standards, and sustainable economic models. Students engage in evaluating the balance between economic efficiency, social well-being, and ecological boundaries, thereby strengthening decision-making and evaluative competencies.
• SDG 13—Climate Action: Educational initiatives aligned with SDG 13 focus on understanding the causes and effects of climate change as well as mitigation and adaptation strategies. By analyzing carbon footprints and evaluating climate policies, students can develop anticipatory competence and action-oriented awareness.

Integrating such examples into ESD contexts allows students not only to understand sustainability challenges, but also to experience the complexity of interrelated systems and the need for informed, ethically grounded decision-making within their personal and professional spheres. These examples demonstrate how complex global issues can be made tangible within subject-based and interdisciplinary educational contexts.

2.2. ESD and Interdisciplinary STEM Education

Sustainability challenges are inherently complex, multifaceted, and cannot be adequately addressed from a single disciplinary perspective [12]. Therefore, ESD requires an interdisciplinary approach that integrates knowledge, methods, and perspectives from multiple fields [22,23]. In school contexts, this necessitates collaborative teaching across subjects to address shared topics from different disciplinary angles. This approach fosters a more holistic comprehension of sustainability. However, the nature and intensity of integration and collaboration can vary, e.g., [24]. Interdisciplinarity is thus essential for cultivating the holistic and systemic understanding of sustainable development.

ESD competency frameworks reflect diverse theoretical foundations and differ in the extent to which competencies are operationalized in subject-specific terms [22,25,26,27]. Across these frameworks, at least three core competencies consistently emerge. First, critical thinking is central, as learners are expected to question existing norms, assumptions, and practices, critically evaluate information, and reflect on values and perspectives when engaging with sustainability challenges. Second, systems thinking plays a key role, highlighting the ability to understand complex interrelationships among social, ecological, and economic systems and to recognize how actions in one part of a system can affect others. Third, decision-making competence is embedded in the action-oriented and future-focused dimensions of the frameworks, requiring learners to weigh alternative options, manage uncertainty, anticipate long-term consequences, and translate analysis into responsible action. Together, these overlapping competencies underscore the capacity to navigate complexity, engage in value-based reflection, and act responsibly across diverse educational contexts.

Critical thinking is foundational to STEM education, enabling students to analyze problems, evaluate evidence, and make reasoned decisions across disciplines [28,29]. Integrated STEM approaches can promote higher-order cognitive skills—such as problem-solving and reflective evaluation—through multidisciplinary inquiry [30]. Dare [28] further emphasizes that such practices nurture evidence-based reasoning in real-world contexts. In addition, systems thinking helps learners to understand complex interconnections in socio-technical and environmental systems [31]. Programming-based situations, as shown by Weintrop [32], foster computational literacy alongside system thinking, bridging abstract concepts with tangible applications in engineering and science challenges. Together, these practices create rich learning environments in which students can engage meaningfully with sustainability-oriented problems.

From a competency-based perspective, interdisciplinarity is indispensable for ESD. Vorhölter et al. [33] argues that key competencies for sustainable development, such as critical thinking, responsible decision-making, and system thinking, require the integration of disciplinary knowledge, methods, and values. Consequently, this positions interdisciplinarity at the conceptual core of ESD, with STEM providing an important—though not exclusive—contribution. When embedded in interdisciplinary learning environments, STEM subjects can significantly enhance students’ ability to address complex sustainability challenges and apply knowledge responsibly in authentic contexts.

However, framing ESD too narrowly through STEM risks reducing its holistic nature. In such cases, social, cultural, and ethical dimensions may be underrepresented, and technical knowledge may become vulnerable to political or economic instrumentalization (UNESCO MGIEP, 2017) [22]. An interdisciplinary orientation counteracts these tendencies by enabling students to critically examine assumptions, evaluate evidence from multiple perspectives, and recognize the value-laden nature of knowledge. Research indicates that STEM can indeed foster interdisciplinary learning—provided that integration is purposeful and linked to authentic sustainability contexts [34]. Thus, STEM should be regarded as one contributing dimension within ESD, rather than as its dominant framework.

Studies underline the effectiveness of ESD in developing interdisciplinary competencies such as systems thinking, critical reflection, and integrative knowledge application [35,36,37]. Staats [38] demonstrates that contextualized, interdisciplinary approaches appear to enhance mathematical understanding by relating abstract concepts to real-world situations, thereby promoting deep rather than procedural learning. At the same time, mathematics frequently remains underrepresented in integrated STEM settings, often reduced to procedural calculations rather than being used as a conceptual and modeling tool [39]. Strengthening the disciplinary and connective roles of mathematics—through design thinking, interdisciplinary collaboration, and problem-based learning—is therefore essential [40,41].

Geography and informatics provide further integrative leverage points within ESD and STEM. Geography education bridges physical and human systems [42,43] through spatial thinking, geospatial analysis, and place-based inquiry, enabling students to understand human–environment interdependencies [44]. In the context of climate change [45], geography contributes the physical perspective, for example by analyzing atmospheric processes and the greenhouse effect, while from a human perspective, it situates people within the social, political, and economic constructs that shape climate change. Informatics, in turn, can support data-driven modeling, systems analysis, and decision-making across STEM fields, thereby strengthening the analytical and computational dimensions of sustainable problem-solving [46]. Teacher education must thus prepare (especially STEM and informatics) educators to design and enact interdisciplinary learning environments that reflect the systemic and ethical complexity of sustainability [47].

These educational practices correspond to Kelley and Knowles’s [48] conception of integrated STEM learning as engagement with authentic, problem-based experiences that mirror the interconnected nature of scientific and societal systems. Given that students cannot retain all content knowledge, ESD should therefore aim to cultivate sustainable ways of thinking—habits of mind that enable learners to reflect critically, act responsibly, and envision long-term consequences [49]. Experience suggests that by involving students in interdisciplinary exploration of real environmental problems, the chosen issue should connect to what students value in their environment [49]. Such integration can contribute to transformative learning by enabling students to navigate trade-offs, constraints, and uncertainties inherent in sustainability challenges.

2.3. Example of a Learning Environment Addressing Sustainable Consumption

The interdisciplinary learning environment “Sustainable Consumption”, developed within the project “Nachhaltig handeln—MINT4all”, translated to “Sustainable action—STEM4all”, integrates geography, mathematics, and computer science to especially address SDG 12 (Responsible Consumption and Production). It challenges students to evaluate whether imported apples (e.g., from South Africa to Hamburg) can be more sustainable than local German ones from the nearby ‘The Old Land’ region (a fertile region next to Hamburg), considering not only transport emissions but also the energy-intensive controlled atmosphere (CA) storage required for domestic apples’ prolonged shelf life. This confronts the common assumption that “local = better” holds universally. The overarching aim empowers students to critically assess origin-based consumption decisions, possibly fostering transferable competencies for related contexts like tomatoes or strawberries.

The structure of the learning environment embodies collaborative, multi-perspective teaching to address sustainability’s complexity [50]:

• Lessons 1–2: Introduction and global apple cultivation regions (geography: spatial classification, climate zones, import rationale).
• Lessons 3–4: Modeling greenhouse gas (GHG) emissions for South Africa–Hamburg transport (mathematics: volume/weight calculations, emission modeling).
• Lessons 5–6: Optimal transport routes using the Dijkstra algorithm (computer science: route optimization; geography: route geography).
• Lessons 7–8: Local ‘The Old Land’ cultivation and CA storage emissions (geography: regional production; mathematics: energy calculations).
• Lessons 9–10: Comparative analysis of GHG emissions and water footprints (all subjects: integrated modeling/comparison).
• Lesson 11: Reflection on findings and personal consumer behavior.

Despite the focus on subjects, the lessons were integrative in nature, as they intentionally linked subject-specific content with broader cross-disciplinary concepts and skills. The teaching materials are available in German [51].

This unit operationalizes shared ESD competencies across Rieckmann/UNESCO, GreenComp, and OR frameworks: systems thinking through modeling interdependencies (transport/storage/cultivation/water); critical thinking via questioning “local = better” assumptions and scrutinizing CA storage; normative competency balancing ecological/economic/social dimensions; anticipatory thinking forecasting long-term footprints; and action orientation via reflective consumer agency.

Mathematics transcends procedural use as conceptual modeling [39], while informatics enables optimization (Dijkstra). Geography grounds global issues locally, avoiding STEM narrowing by embedding technical work in policy contexts for holistic ESD [13].

Local relevance (‘The Old Land’) can foster place-based self-awareness [44]; interdisciplinary collaboration exemplifies Kellinghusen et al.’s [24] multi-perspective ideal. Enhancements could explicitly map reflections to ESD frameworks and deepen social dimensions (e.g., South African vs. German labor conditions), fully realizing mathematics/informatics as connective disciplines [40].

2.4. Research Questions

In ESD, sustainability is conceptualized as a normative, multidimensional concept unfolding across the three pillars—or dimensions—of ecological, social, and economic sustainability [18]. These dimensions are tightly interwoven in real-world decision-making scenarios, often generating tensions and trade-offs, such as between ecological limits and economic growth or between social equity and resource efficiency [23]. ESD therefore aims not merely at knowledge acquisition but at fostering competencies like critical thinking, systems understanding, and responsible decision-making, enabling learners to analyze and evaluate such conflicts in reasoned ways, e.g., [28,29].

Research has repeatedly emphasized the importance of argumentative engagement for fostering these competencies [36], yet empirical insights into how such argumentation unfolds in interdisciplinary learning contexts remain scarce [37]. In particular, little is known about how students draw on content knowledge gained in the learning environment when negotiating sustainability-related issues in STEM-based tasks.

Against this backdrop, this study conceptualizes students’ arguments in sustainability tasks as expressions of sustainability-related competencies, integrating cognitive, normative, and contextual dimensions. It addresses the overarching research question:

(RQ1) Which arguments do students employ when evaluating a sustainability-related problem?

More specifically, it investigates:

(RQ1a) How can these arguments be conceptualized and structured in relation to the three dimensions of sustainability—ecological, economic, and social?

(RQ1b) To what extent are students’ arguments grounded in the learning environment they worked on and in what ways do they extend beyond the opportunities provided there?

Furthermore, the following question will be examined:

(RQ2) To what extent can the ESD competencies of critical thinking, systems thinking, and decision-making be reconstructed in students’ written presentations?

3. Methods

3.1. Sample and Data Collection

To investigate the research question, data were collected through a survey conducted among students from four ninth grade classes (aged 14–15 years, n = 52) at secondary schools in Hamburg. The data collection took place over a two-week period in June 2024. Participation in the study as well as the submission of the letters were voluntary, which explains the varying distribution of students across classes (see

Table 1. Distribution of students across classes.

Class	1	2	3	4
Number of students	8	6	19	19

). The data were collected with the authorization of the institutes responsible for scientific studies in schools in Hamburg (file number e514.101.5000-002/223,043), and only those students whose parents had given their prior consent were videotaped. All students had the right to withdraw from the study at any time without facing disadvantages. At the time of data collection, the students had already acquired foundational knowledge about climate change through the teaching environment “fundamentals of climate change” and were in the final lesson (11 lesson) of the learning environment “sustainable consumption”.

Following the completion of this learning environment, students were tasked with composing a critical written statement evaluating the claim that an apple imported from overseas is more climate-friendly than a locally produced German apple. They were able to incorporate various arguments they had learned during the teaching environment as well as include their personal opinions within the context of the quote.

This task (Figure 2) involved differentiated writing assignments, ensuring that all students could contribute according to their individual levels of performance and understanding. Consent was obtained from all students prior to the data collection, and the purpose, procedure, and potential use of the date was explained. The letters are available in German [52].

3.2. Data Analysis

The student statements were analyzed using structuring qualitative content analysis according to Kuckartz and Rädiker [53] in the software MAXQDA 2024. A coding scheme was developed using a deductive–inductive approach. Deductive categories were derived from the teaching materials. During the analysis, new categories were inductively developed based on the data, enabling the inclusion of emergent categories not explicitly addressed in the learning environment. These include “economic efficiency”, “level of development”, “costs”, and “working conditions” and are marked with an asterisk in Figure 3. Following the finalization of the coding manual, the material was double-coded by two trained raters, consisting of one member of the research team and one independent coder. The first two letters were coded together, followed by three letters that were coded independently using the coding manual. This cycle was repeated once. The codings of the cycles were compared both times, leading to further refinement of the coding manual. Thereafter, a total of 28 letters were double-coded by both raters. The intercoder reliability was assessed using the coefficient according to Brennan and Prediger [54], and a high value of $\mathsf{\kappa }=0.94$ was achieved. In cases of a disagreement in the coding, categories were discussed until consensus was reached. Because of the high value of intercoder reliability, the remaining letters were coded by the member of the research team.

Once the categories had been finalized, the categories were assigned to the three overarching dimensions—ecology, economy, and social—as described in Section 2.1. Figure 3 shows the corresponding category assignment.

4. Results

A total of 18 different categories were created. Of these, 13 can be assigned to the ecological dimension (Section 4.1.1), four to the economic dimension (Section 4.1.2), and one to the social dimension (Section 4.1.3). Therefore, most of the students’ statements could be assigned to the overarching dimensions of ecology, economy, and social issues. The distribution of the overarching categories can be seen in Figure 4. The uneven distribution of categories across the three overarching dimensions likely reflects the nature of the data stemming from the content of the learning environment. This may lead to an overemphasis on ecological aspects while underestimating the presence or relevance of the other two dimensions. Consequently, conclusions regarding the relative importance of the dimensions cannot be drawn, as the distribution may partly be due to the learning environment rather than a direct reflection of students’ priorities.

The following quotations represent student responses in the letters they wrote according to the task. Students occasionally refer to their own calculations, which may result in varying numerical values across responses. This paper neither evaluates the accuracy nor the consistency of the provided figures, just as it refrains from assessing the substantive content of students’ statements.

4.1. Students’ Arguments According to the Three Pillars of Sustainability (RQ1a)

4.1.1. Ecological Dimension

The consideration of CO₂ emissions—a subcategory of the ecological dimension (see Figure 3)—plays an important role in the students’ arguments, as shown in Figure 4. This category can be divided into the area’s cultivation, transport, and storage (see Figure 5).

CO₂ emissions from cultivation are often used by the students to illustrate structural differences between the producing countries. Imported apples are often described as more emission-intensive, and in some cases a connection to other aspects is established:

“…that even during cultivation, the carbon footprint of apples from Grabow (330 tones of CO₂ per kg of apples) is more than 3 times higher than that of apples from the ‘The Old Land’ region (89 tones of CO₂ per kg of apples).” (S4, pos. 4)

“These figures show that from sowing to transporting their apples, South Africa releases far more greenhouse gases into the atmosphere than Germany, even though it (SA)does not store its apples.” (S66, pos. 3)

Some students consider the aspect of apple cultivation methods. These methods are explicitly included in their evaluation and distinguish between different agricultural practices. In particular, it is emphasized that apple cultivation in Germany is perceived as comparatively environmentally friendly, for example through integrated cultivation methods: “Integrated cultivation is used in the ‘The Old Land’ region.” (S18, pos. 3)

Transport emissions play an important role in the letters. Long transport routes are almost always highlighted as particularly harmful to the climate. Comparisons with transport emissions within Germany can sometimes be found. But transport often serves as a decisive argument with which other ecological disadvantages—in particular, the storage of German apples—are relativized: “This is not true, as cultivation, transport, etc. still consume significantly more energy than the storage of German apples. Transport alone burns 526 tones of CO₂, whereas the transport of German apples burns only 6.66 tones.” (S5, pos. 2)

Almost all of the students discuss the emissions caused by the storage of German apples. These emissions are recognized by them as relevant but are often considered in relation to transport and cultivation. Furthermore, some of the students mention that apples in Germany would have to be stored for an unrealistically long time in order to produce the same amount of CO₂ as the transport of apples from abroad: “It is true that German apples are often stored in cold storage for a long time, which consumes a lot of energy. This causes CO₂ emissions that pollute the environment.” (S30, pos. 3) “Only if an apple is stored for 20 years would it consume more CO₂e.” (S17, pos. 3)

In the letters, energy consumption is closely linked to storage. Some students argue that the cooling and long-term storage of German apples requires a high energy input, which has a negative impact on the carbon footprint. At the same time, this energy consumption is often embedded in a comparison with other sources of emissions: “It is true that German apples consume a lot of energy and cause CO₂ emissions due to their long storage period.” (S82, pos. 4)

Some of the arguments presented take into account the freshness and seasonality of apples. Students argue that seasonally available apples from the region are particularly climate-friendly. Since neither long transport routes nor storage are necessary, these apples are considered fresh. This line of argument often leads to the recommendation by the students to consume apples only during harvest season: “What would probably be best, but is likely not feasible for many, is to buy apples from a local apple farm during harvest season.” (S40, pos. 3)

Closely linked to this is the category of local and regional. Regional apples are presented in the letters as the preferred option because they are perceived as more environmentally friendly, transparent, and sustainable. These arguments combine ecological assessments with everyday consumer decisions: “It is best to eat only local apples from the ‘The Old Land’ region when they are in season.” (S16, pos. 3)

The apple variety is only explicitly mentioned in a few cases, with the note that not all varieties can be grown regionally. This puts regional availability into perspective without fundamentally questioning it: “Unfortunately, ‘The Old Land’ cannot grow all varieties of apples.” (S4, pos. 5)

The category of environmental impact extends the argument beyond the pure carbon footprint. Individual students address additional ecological consequences, such as the impact of shipping on marine ecosystems. This line of reasoning demonstrates a deeper understanding of ecological relationships within global supply chains: “Ships that endanger the habitat of marine animals also have devastating consequences for the climate.” (S85, pos. 3)

A recurring aspect, within the ecological dimension, is the consideration of environmental protection. This often serves as a summary evaluation framework. Students use it to justify their overall assessment by naming or bundling individual ecological arguments and evaluating them normatively: “Although apples from Germany are not exactly climate-friendly either, apples from Chile are even more harmful to the environment.” (S16, pos. 3)

Another line of argument in the letters concerns water. Three closely related subcategories can be identified: water consumption, water color, and water scarcity.

Many students use water consumption as a key criterion for comparing countries of origin. It is often used to argue that imported apples cause a greater environmental impact than regional products: “The water footprint in South Africa is 202 L more than in Germany, which is not necessarily more climate-friendly.” (S70, pos. 3)

Water color (green, blue, grey water) is used in the letters to illustrate qualitative differences in water consumption. Green water is rainwater stored in the ground; blue water is ground or surface water used for irrigation; and grey water is made unusable due to pollution. In particular, the high use of blue water in dry regions is repeatedly highlighted by the students as problematic: “Germany’s apples are also clean, unlike in South Africa, where they use dirty i.e., grey water, and also a lot of water from wells and rivers, i.e., blue water.” (S6, pos. 2)

In addition, water scarcity is sometimes discussed by the students, especially in relation to growing regions with limited water resources. These arguments show that students associate ecological sustainability not only with emission values, but also with the use of scarce resources: “In such areas, the water content is also lower due to less rainfall, so the apples need more blue water (groundwater), which represents a loss for the country.” (S85, pos. 3) “Despite the shortage of drinking water, water is used for agriculture (apple cultivation).” (S42, pos. 4)

4.1.2. Economic Dimension

The economic dimension occurs much less frequently than the ecological dimension (see Figure 4 and Figure 6) but complements the arguments in a specific way and is particularly interesting as it was mainly not addressed in the lesson.

Supply and demand are taken into account in one case, with the argument that the high demand for apples in Germany necessitates import structures: “In addition, the demand for apples in Germany is twice as high as the yield in the ‘The Old Land’ region.” (S4, pos. 5)

The category (economic) efficiency is used for example to portray shipping as well-organized and thereby comparatively efficient. These arguments do not completely relativize emissions, but they do place them in an economic context: “These emissions are higher, but transport by ship is very well organized.” (S28, pos. 4)

The level of development is addressed in one letter. The student points out that Germany is a highly developed country: “Germany is also more developed” (S35, pos. 2) than South Africa.

Costs also play a role in the letters, both in terms of production processes and consumer decisions. Regional apples are sometimes described by the students as cheaper or more economically viable: “[…] sometimes the process may also be cheaper and easier.” (S12, pos. 3)

4.1.3. Social Dimension

In one case, working conditions in the countries of cultivation are discussed and compared with German standards. This argument adds a social dimension to the sustainability assessment: “Working conditions in production in South Africa are probably significantly worse than in Germany.” (S68, pos. 2)

In summary, the results show that students base their opinions on this interdisciplinary, sustainable problem predominantly on ecological categories. The arguments are comparative, balanced, and often multidimensional.

4.2. Student Arguments: Connections to the Learning Environment and Independent Contributions (RQ1b)

As outlined in Section 2.3, the learning environment emphasized ecological aspects of apple consumption, particularly CO₂ emissions categorized by cultivation, transportation, and storage, alongside water usage in cultivation.

All students incorporated arguments from the ecological dimension, as anticipated (as shown in Figure 7). However, one student omitted CO₂ emissions entirely; two referenced only storage; and two mentioned only cultivation. Combinations included cultivation and transportation (n = 3 students), storage and transportation (n = 7), and cultivation and storage (n = 2).

Water-related arguments were prevalent, cited by 43 students, while 9 students did not address this topic (see Figure 7). Water scarcity was not mentioned on its own, but in combination with color and consumption (n = 5) or just consumption (n = 4). Water consumption was listed by 21 students and water color by 4 students. The combination of both was listed by 9 students.

As shown in Figure 4, arguments from the economic dimension were the second most frequent, though substantially less common than ecological ones. The unit addressed only supply and demand, yet students also mentioned economic efficiency (n = 2), level of development (n = 1), and costs (n = 4), none of which were covered in the learning environment. Supply and demand was mentioned by just one student.

Only one student introduced working conditions as a social dimension argument, notably also referencing costs beyond the learning environment’s content. In total, seven students included arguments extending beyond the learning environment’s scope.

4.3. Students’ ESD Competencies (RQ2)

The analysis of the learning environment in Section 2.3 shows that both the learning environment itself and the final reflection are suitable for examining the issue of apple consumption from multiple disciplinary perspectives. This enables an interdisciplinary engagement with the topic and allows students to adopt a critical stance on the choice of country of origin and to derive action-oriented recommendations. The available data appear to support this assessment.

The students established numerous connections between the different thematic areas addressed in the learning environment. This indicates that the competence of systems thinking was successfully addressed and applied by the majority of students. Figure 8 illustrates that four students referred to six different thematic areas, ten students to five areas, nine students to four areas, nineteen students to three areas, seven students to two areas, and one student to only one area. At the same time, it becomes evident that the students put individual foci in their argumentation. In addition, it is apparent that almost all students used arguments from the categories of CO₂ emissions, water, and environmental protection; all other categories were only used by individual students.

The figure also visualizes students’ combination of argumentation categories, with darker color representing more frequent counts. Among the 52 students, 23 different patterns can be identified. Out of these, 18 patterns can be assigned respectively to one student, leaving five patterns to be similar in two or more letters.

As the task explicitly required a critical engagement with the newspaper article, passages attributable to this competence can be identified in all students’ work. This is particularly evident in the fact that students incorporated arguments from different areas, which in turn required making trade-offs. As several of the quotations presented in Section 4.1 demonstrate, this often involved a process of weighing advantages and disadvantages.

This weighing process was achieved, among other things, through a critical reflection on the sources provided from the various thematic areas. Several students relativized the given statement by pointing out that production conditions cannot be fully generalized. Such statements indicate emerging competencies in critical source evaluation. This is illustrated particularly clearly in the following quotation: “Of course, this information does not apply to all (apple) plantations, but it does apply to most of them.” (S28, pos. 7)

Some of the students’ responses did not end solely with a decision as to whether they agreed or disagreed with the statement, but additionally included concrete recommendations for action or, at least, an explicitly articulated desire for action, as demonstrated by the cited example: “In my opinion, we should favor regional and seasonal products in order to protect the environment.” (S30, pos. 4)

Overall, the results indicate that the learning environment appears to have fostered an interdisciplinary, systems-oriented, and critically reflective engagement with the topic of apple consumption and, in some cases, supported the translation of analytical considerations into action-oriented conclusions.

5. Discussion

The learning environment was designed to foster students’ understanding of sustainability by engaging with quantifiable environmental impacts and thereby aligning with interdisciplinary educational objectives such as data interpretation and the modelling of real-world systems. Although individual lessons focused on specific subject areas, the interdisciplinary learning environment was structured so that results from one lesson were taken up and further developed in subsequent lessons. This resulted in a high degree of integration across subjects and is reflected in the students incorporating content from the learning environment into their arguments while not defining which subject area it comes from, thus showing the interdisciplinary approach and its successful incorporation of the different disciplines into the ESD context [12].

5.1. Structure of Students’ Arguments and Their Relation to the Learning Environment (RQ1)

The students’ arguments could be clearly assigned to the three established dimensions of sustainability—ecological (planet), economic (prosperity), and social (people) (see Section 2.1)—whereas the two extended dimensions, peace and partnership, defined in the 2030 Agenda for Sustainable Development [19], did not emerge in their argumentation. This absence is unsurprising, as the learning environment itself was not explicitly designed to address those transnational and governance-related aspects.

Overall, the results indicate that students predominantly drew on ecological arguments when evaluating the sustainability of local versus imported apples. Economic and social reasoning appeared less frequently and was articulated with lower differentiation. This distribution aligns with expectations, as the learning environment primarily addressed ecological aspects, thereby naturally guiding the focus of students’ argumentation. The learning environment can be most closely associated with SDG 12 (Responsible Consumption and Production), which, as Trembley et al. [21] point out, is largely attributed to the ecological dimension of sustainability.

In addition to ecological considerations, several students introduced economic aspects, particularly those related to supply and demand, efficiency, cost, and the level of development. These categories correspond to SDG 8 (Decent Work and Economic Growth) and can be situated within the economic dimension [21]. One student referred to social aspects—working conditions—which also align with SDG 8 and the social dimension as defined by the UN framework. In this way, the students’ reasoning demonstrated an emerging awareness of the interconnectedness between environmental, economic, and social systems, even though the weight of their arguments remained clearly within the ecological domain. By analyzing carbon footprints and climate change, the students build on SDG 13.

The variety of ecological arguments suggests that students were able to draw both on formal learning content and on everyday experiences. This interplay between formal and informal knowledge construction is consistent with Stevenson [49], who emphasizes that connecting disciplinary understanding with personal values and lived experiences can support deeper, more holistic reasoning. It is striking in this context that arguments that went beyond the content of the learning environment can be assigned primarily to the economic dimension, and some to the social dimension.

An interesting observation concerns the extension of students’ argumentation beyond the contents of the learning environment. The few instances in which students introduced new arguments—particularly those related to economic and, to a lesser extent, social considerations—illustrate that the chosen ESD context can foster self-directed thinking and personal engagement. This tendency reflects the kind of transformative learning processes advocated in ESD frameworks, which emphasize learners’ capacity to critically and independently extend their understanding of sustainability issues [3,22].

Thus, while students’ argumentation remained anchored in the ecological dimension due to the instructional design, their readiness to incorporate additional perspectives suggests an emerging ability to navigate multiple sustainability dimensions in an integrated manner.

5.2. Promoting Students’ ESD Competencies (RQ2)

The interdisciplinary and ESD-oriented design of the learning environment appears to foster several key competencies central to ESD.

One of the competencies possibly promoted was students’ systems thinking competence. Although individual lessons focused on specific subject areas, the learning environment was structured so that results from one lesson were taken up and further developed in subsequent lessons. This resulted in a high degree of integration across disciplines, which is reflected in students’ written arguments; statements could rarely be attributed to a single lesson or subject, as content from different lessons and subject areas was frequently combined within single claims. This integration indicates that students were able to connect different areas and to recognize their interdependencies [12], thereby demonstrating an emerging systems-oriented understanding of sustainability.

Furthermore, several students articulated arguments that extended beyond the explicitly addressed content of the learning environment, suggesting that the chosen ESD context motivated them to independently incorporate additional perspectives and subject areas. This self-directed expansion of reasoning may further support the development of systems thinking, as students moved beyond isolated facts toward more holistic, cross-disciplinary interpretations [3,55]. The analyses of carbon footprints and climate change also point to anticipatory and action-oriented competencies related to SDG 13, as students linked local consumption patterns to broader environmental consequences, thus engaging with the temporal and systemic dimensions of sustainability.

Consistent with prior research indicating that interdisciplinary ESD environments promote critical thinking (see Section 2.2), students in this study demonstrated the ability to weigh arguments for and against the consumption of imported apples and to articulate the trade-offs inherent in such decisions. They were not only able to draw on ecological reasoning but also, to a lesser extent, on economic and social considerations, thereby engaging with tensions between environmental, economic, and social goals. This engagement with competing values and constraints is characteristic of early critical thinking competence, as students identified contradictions, considered multiple perspectives, and formulated reasoned compromises [28,33,35].

Evidence of decision-making competence was evident in students’ responses. Several students concluded that regional apples should be preferred and, where possible, purchased directly after harvest. These statements indicate not only computational proficiency but also the capacity to interpret quantitative outcomes within broader ethical and ecological contexts, rather than treating them as isolated mathematical tasks. The variety of ecological arguments identified further suggests that students were able to mobilize both content explicitly taught in the unit and elements of their everyday knowledge, highlighting the interplay between formal instruction and informal understandings. This finding resonates with Stevenson [49], who emphasizes that connecting disciplinary knowledge with students’ everyday experiences and values can promote deeper, more holistic engagement. Similarly, it aligns with Staats [38], who found that contextualized interdisciplinary approaches can enhance mathematical learning, and with Meadows [44], who underscores the importance of understanding interdependencies within complex systems.

Although ethical and social dimensions were not explicitly emphasized in the learning environment, one student nonetheless referred to such aspects, lending support to Maratova et al. [47], who argue that ESD designs should explicitly integrate ethical and social considerations alongside environmental and economic ones.

6. Conclusions, Limitations, and Outlook

The findings of this study indicate that an interdisciplinary ESD learning environment appears to support lower secondary students in articulating sustainability-related arguments, particularly within the ecological dimension. Within the specific context of comparing local and imported apples, some students engaged with quantitative information and began to connect ecological, and to a lesser extent economic and social, considerations. The latter two were not in particular part of the lessons. This seems to suggest that the lessons and the assignment prompted students to raise arguments that went beyond the given scope. However, the predominance of ecological arguments and the marginal role of economic and social aspects suggest that the current design primarily foregrounds environmental perspectives, while peace- and partnership-related dimensions remained absent. By incorporating other disciplines such as politics, a shift in perspective might be possible, and the social and economic dimensions might receive more attention. In addition, the results indicate that the interdisciplinary, sustainability-oriented design of the learning environment has the potential to effectively support the development of systems thinking, critical reflection, and informed decision-making among students, thereby contributing to core ESD competencies. Given the small sample and context-bound nature of the study, these insights are not generalizable and should be interpreted as exploratory.

As with any empirical study, this study also has certain limitations.

The results of the study are based on written documents that students were asked to submit at the end of the learning environment. However, not all students were willing to provide their written statements. Consequently, the findings are derived from a limited sample of 52 letters. We cannot comment on letters and their content that were not submitted. This raises the possibility of a positive selection bias. Nevertheless, the fact that several statements were relatively brief and contained only limited argumentation suggests that such a bias is unlikely. In this regard, a more in-depth analysis of individual statements in the form of a case study would constitute a valuable direction for future research.

Furthermore, the analysis of the written documents does not allow conclusions to be drawn about students’ activities during the learning environment itself. It is possible that additional arguments, including those related to the dimensions of peace and partnership, emerged in students’ conversations and discussions but were not captured in the written material. Similarly, the analysis of students’ notes or, for example, video recordings of the working phases could have provided further insights into demonstrated ESD competencies.

Finally, the findings are limited by the specific learning environment investigated. This limitation is particularly evident in the predominance of arguments related to the ecological dimension. It would therefore be informative to analyze students’ evaluations in learning environments addressing different content areas.

The results of this study point to the potential of such learning environments and underline the need for future designs and studies that more explicitly integrate ethical, social, and governance-related perspectives in order to better understand how different ESD competencies can be fostered in classroom practice. Furthermore, future research could also examine the internal coherence of students’ arguments. At this level, it would be possible to investigate the extent to which students draw on concrete data and insights from the instructional unit, as opposed to relying on general or abstract formulations. Closely related to this is the question of whether and to what extent the learning environment fostered students’ evaluative competence, which represents a central competency within the framework of ESD.