**1. Introduction**

As economies develop, they stimulate the demand for essential services that are provided by the built environment (also referred to as "material stocks") in the production and consumption of goods and services, viewed as "the material basis of societal well-being" [1]. The growth and maintenance of these material stocks occur by mobilizing material and energy flows, either from domestic sources or through imports from other societies. The larger the stocks, the greater the flows required to maintain and reproduce these stocks, creating a feedback loop that has been termed as the "material–stock–flow–service nexus" (ibid.). This process of organizing and reproducing material stocks and flows by society is referred to as "social metabolism" [2].

Focusing on a small island state in the Caribbean, this research provides evidence on the extent to which the nation's built environment is vulnerable to the impacts of climate change. Utilizing the

state-of-the-art spatially explicit material stock accounting (MSA), we highlight the economic impacts on Antigua and Barbuda's (A&B) tourism industry under a 1 m and 2 m sea level rise (SLR) scenario. Services including hotel accommodations, restaurants, real estate, yachting, and marina facilities are linked to the expansion of the tourism industry and the built infrastructure. With tourism contributing to approximately 80% to the GDP [3], the small island nation constantly experiences a heightened level of economic uncertainty due to its exposure to external economic shocks and climate-related events such as hurricanes. Based on our findings, we critique spatial planning and maladaptive practices such as coastal squeeze that need to be urgently considered for building system resilience. The Caribbean is the most tourism-reliant region in the world [4]. Tourism infrastructure and assets are mostly concentrated along the coasts and are subject to high risk from tropical storms. As such, tourism-dependent economies in the Caribbean face the highest risk worldwide [5]. Yet, growth and continued reliance on this climate-sensitive economic sector requires a continuous (re)accumulation of material stocks in buildings to meet growing demands. At the same time, the damages incurred from climate-related events are substantial in size in comparison to their economies. Ten percent of the worst impacted countries in terms of losses as a share of their GDP were Caribbean nations. For example, during the 2017 hurricane season, Sint Maarten's losses were estimated at 797% of the country's annual gross domestic product (GDP), the British Virgin Islands with 309% of their GDP, and Dominica with 259% of their GDP [6]. Sea level rise (SLR) is an imminent threat to small island developing states (SIDSs) as 26% of their land area is 5 m or less above sea level, equating to roughly 20 million people (or 30% of the SIDS population) living within these high-risk areas [7].

The high cost of loss and damage witnessed by these Caribbean islands is accompanied by excessive national debt, and the reliance on imports to supply new materials for infrastructure development and reconstruction [8,9]. Losses equate mostly to the damages caused to the built environment that delivers critical goods and services such as transport, health, food, and energy. By 2050, climate inaction is expected to cost Caribbean countries an estimated 10% of their GDP annually from hurricane damages and loss of tourism revenue alone, rising to 22% by 2100 [10]. By 2150, conservative estimates of SLR sugges<sup>t</sup> that only a fraction of the current 66 million inhabitants of the small island developing states (SIDSs) will be spared from inundation [11]. In the face of these challenges and threats, achieving island sustainability is critical for these SIDSs [12,13]. In this study, island sustainability is defined as achieving a high quality of social and human wellbeing at the lowest environmental costs. Island sustainability also implies adapting and increasing resiliency to bu ffer against the adverse impacts of global environmental change and economic instabilities while maintaining resource security and self-reliance.

This study is part of a larger e ffort to study the metabolism of islands, an emerging research field within industrial ecology, that aims to seek solutions to sustainability challenges faced by small islands (see the initiative "Metabolism of Islands" (https://metabolismofislands.org) and the special issue Metabolism of Islands, Sustainability: https://www.mdpi.com/journal/sustainability/special\_ issues/metabolism\_islands). The objectives of the research are two-fold. One is to contribute to the scant literature on the relationship of stocks and services, in particular tourism within an island context, and their exposure to climate vulnerability. We analyze these findings in the context of island sustainability by asking how can islands leverage spatial infrastructure planning as a way to build resilience and adapt to climate change, including sea level rise scenarios. Second, as a methodological contribution, this study introduces novelties to the geographical information systems (GIS)-based stock accounting method by presenting a building footprint-based identification of buildings and the use of Monte Carlo simulation in assigning material intensity typologies (MITs). In this study, the focus is directed towards classifying and estimating the material stock of construction materials including wood, non-metallic minerals, steel, and iron that are utilized for the growth and expansion of the building stocks.

#### **2. Advances in Material Stock Accounting (MSA) Research**

The 20th century has seen a massive increase in material flows that go into creating material stocks, from just over 20% in 1900 to over 50% in 2010 [14]. This has inspired several studies to focus on the dynamics and growth of anthropogenic stocks that require ever-increasing virgin materials [15]. Studies quantifying and characterizing the material composition of in-use material stocks as a potential pool of secondary resources for future material recovery are becoming important [16–19]. With the growing relevance of material stock accounting/analysis (MSA), researchers are constantly proposing additional approaches for improved material stock accounting. For example, Muller et al. [20] identified two main methodological approaches for material stock measurements, including a top-down and a bottom-up approach. Top-down approaches integrate historical data based on data availabilities on material inflows and material outflows determined by lifespan characteristics to quantify material stock [20,21]. In contrast, bottom-up approaches are data intensive and combine input parameters such as gross floor area, number of stories, and the average size of the dwelling area [22]. The bottom-up approaches require material end uses to be separated into categories sharing material intensities used to calculate material stock. This approach is mostly favored for the efficient use of GIS data [23] to explore spatial material stock accounts for local-scale studies worldwide [15,18,24–26]. Augiseau and Barles [27] analyzed thirty-one studies on material stocks and flows, along with distinguishing between six methodological approaches that were commonly used in such research.

Observing the surge of research related to the built environment within the past two decades, MSA is increasingly incorporating the use of GIS [16]. GIS comprises technology tailored to collecting, analyzing, and managing georeferenced data to produce location-specific information [23]. The application of GIS is not limited to calculating the size of the material stocks within a region but, from a spatial perspective, it can identify where material stocks have accumulated and how they are distributed within socio-economic systems. The applications of 4D-GIS and spatial data applied in previous studies focused on understanding the accumulation of material stocks on a temporal and spatial scale, in two different urban areas [24]. After a catastrophic earthquake and tsunami struck Japan, a material stock analysis (MSA) estimated the quantity of construction materials lost from the physical infrastructure (including buildings and roads) using GIS [25]. Wallsten et al. [28] combined a bottom-up material flow analysis (MFA) approach with GIS as an assessment tool to spatially characterize and examine hibernating metal stocks in urban infrastructure. Similarly, Kleemann et al. [15] utilized GIS data to quantify material stocks in buildings and map their spatial distribution within the city of Vienna through combining information about demolition activities to yield waste flow data. Mesta et al. [18] adopted a bottom-up approach to quantify the material stocks for residential buildings in Chiclayo (Peru) utilizing GIS data and data pertaining to the physical size of buildings. In Sweden, Heeren et al. [29] presented a bottom-up approach stock model with the use of geo-referenced building data to determine the building material stocks of Swiss residential buildings based on volumetric properties [29]. Symmes et al. [30] presented a bottom up GIS methodological approach to explore the vulnerability of material stock in buildings in Grenada [30]. Pott et al. [31] (paper submitted) adopt a spatial (bottom-up) GIS approach to study the relationship of in-use stocks and their services using a city and an island as cases.

#### **3. Socio-Metabolic Research on Islands**

Analyzing material stocks (MSs) plays an important and multifaceted role within socio-metabolic research, including functioning as service suppliers, wealth indicators, capital and resource repositories, and indicators for the spatial development of the built environment [32]. Small islands are ideal units of study for socio-metabolic research, with clear and distinct systems boundaries to track flows. At the same time, islands suffer from resource constraints due to their narrow resource base and limited waste absorption capabilities, causing sustainability problems both on the input side (scarcity and import dependency) and the output side (land and sea pollution) [17,33–37]. The lack of resources and relatively small populations limit the size of island economies and the ability to achieve economies of scale, thus pushing the overdependence of small island states on external markets to meet the majority of their resource needs. The openness of small island economies to external markets and their high dependency on trade to meet basic needs heighten their level of vulnerability.

Socio-metabolic research o ffers islands a unique perspective on sustainability, leveraging resource use patterns to build system resilience. Understanding the physical basis of island economies highlights areas of opportunities and constraints impacting sustainable development [38–40]. However, few socio-metabolic studies have been conducted on small islands. The first known material stock and flow account for an island was for Trinket (Nicobars, India) that portrayed the changing characteristic metabolic profile of an indigenous society subject to development programs from the Indian state [41], that was later compared with the rise in material and energy consumption due to the excessive aid following the 2004 Asian tsunami [42]. Krausmann et al. [43] focused on the application of a material flow analysis (MFA) for two high-income island states, Iceland and Trinidad and Tobago. The resource use patterns revealed that both islands heavily rely on domestic extractions, including fisheries, natural gas, and oil. Shah et al. [44] focused on institutional factors in Trinidad and Tobago and the challenges of implementing potential solutions tackling the island's waste metabolism of plastics and packaging material. Okoli [45] quantified biomass flows for Jamaica from 1961 to 2013 in the context of national food security from an island perspective. Marcos-Valls et al. [46] applied an integrated multi-scale socio-metabolic analysis for Menorca (Spain) to analyze the environmental and economic performance of major economic activities such as tourism in an island context.

Material stocks and flows were studied on the Greek island of Samothraki using a socio-metabolic approach [40,47]. More recently, Fischer-Kowalski et al. [48] focused on Samothraki's regime shift as the island transitions from an agriculture-based economy to a service (tourism) economy. In the Philippines, a material flow analysis was conducted to understand trends within a high-density country experiencing a shift from renewable to non-renewable materials as a result of ongoing development [49]. Symmes et al. [30] conducted the first material stock–flow analysis in the Caribbean, with a focus on Grenada's metabolism of construction materials, how they are distributed across the di fferent sectors of the economy, and the potential impacts as a consequence of sea level rise [30]. Pott et al. [31] (paper submitted) explore the material stock–service relationship in Grenada and examines future material stock scenarios with consequences for island sustainability. The lack of proper waste managemen<sup>t</sup> systems on islands is a growing concern as they undergo rapid development, with waste produced as a by-product of economic activities. Recently, a case study on the Faroe Islands focused on the practice of sustainable land managemen<sup>t</sup> through the lens of social metabolism within a growing local economy [50].

#### **4. Study Area: Antigua and Barbuda**

A&B is one of the Leeward Islands situated in the eastern Caribbean Sea [51]. The country's political boundary consists of three islands: Antigua, Barbuda, and Redonda. Antigua functions as the mainland territory and is the largest of the three with an area of 280 sq. km. The sister isle of Barbuda is the second largest with an area of 161 sq. km. The smallest of the three, Redonda, is the only uninhabited island with an area of 1.6 sq. km [51]. Antigua's landscape is divided into seven parishes, with St. John's being Antigua's capital city and Codrington being the capital of Barbuda.

The coupled relationship between the ecosystem and the economy has proven to be beneficial in sustaining economic growth in A&B. The country's limited natural resources, distinct ecosystems, and its rich cultural heritage are all contributing factors in sustaining the island's economy. Historically, economic growth was driven by agricultural products such as rum and sugar, but within the past decades, both agricultural and manufacturing activities have been on a drastic decline [52]. Economic driving forces have transitioned towards a more service-based economy, with the provision of services on the island contributing to 90% of the country's GDP over the past forty years, as a result of the expanding tourism industry [9,52].

The World Bank Group reports that tourism accounts for approximately 80% of GDP, 85% of the foreign exchange, and contributes to 70% of direct and indirect employment in A&B [53]. A&B experiences high temperatures all year round ranging from (23 ◦C to 27 ◦C), as the climate is influenced by northeasterly trade winds distinguished by a dry and wet season [52]. The island's tropical climate and diverse ecosystems support its dominant and flourishing sand, sun, and sea tourism industry. A&B has the highest share of tourism in GDP amongs<sup>t</sup> the other tourism-dependent islands within the SIDSs [54]. Historically from the 1960s to 2018, A&B has witnessed frequent periods of tourism booms and, as such, has experienced significant growth in tourism-related services including, but not limited to, construction, sand mining, transport, and communication [55,56].

However, this small island nation is highly vulnerable to climate change. Among the Caribbean island nations, A&B is ranked 1st in the composite vulnerability index (CVI), followed by the Bahamas (2nd), Dominica (3rd), Grenada (4th), and Jamaica (5th) [57]. By 2050, climate inaction will cost A&B 25.8% of its GDP [10]. The impacts of sea level rise and storm surges amongs<sup>t</sup> the Caribbean and the Caribbean Community (CARICOM) states introduce both short-term and long-term major threats. Global sea level rise is projected to range between 1–2 m above present levels at the end of the 21st century. The impact of the projected sea level rise within the Caribbean will be uniform, however, for smaller islands, the magnitude of economic loss in comparison to the size of the economy will be more greatly felt in St. Kitts and Nevis, Grenada, and A&B [58].
