Feni Agostinho ^1* Imke De Kock ² Biagio Fernando Giannetti ¹ Cecilia Maria Villas Boas Almeida ¹ Amalia Zucaro ³

• ¹ Paulista University, São Paulo, Brazil
• ² Stellenbosch University, Stellenbosch, Western Cape, South Africa
• ³ Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), Rome, Lazio, Italy

Building sustainable cities relies on the principles of circular economy and cleaner production, which together can boost resource efficiency and environmental resilience. Redesigning and properly monitoring resource use with a focus on minimizing environmental, social and economic impacts are fundamental factors. As emphasized by Goal #11 of the 2030 Agenda (UN-SDGs, 2019), a sustainable city strives to meet the real needs of its present and future inhabitants while minimizing its impact on the environment, ensuring participatory governance processes to find effective solutions, and preserving resources for future generations. Although receiving some criticism (e.g. Giannetti et al., 2020), the SDGs are considered an important and practical guide for global governance, at various scales, to implement and monitor actions aimed at achieving sustainability. While it is recognized that each city may have its own unique needs and challenges (Zucaro et al., 2022), and that the concept of sustainability continues to evolve as new technologies and best practices emerge, there are some key characteristics and principles that can make cities more sustainable, including: a strong reliance on renewable energy, an efficient transportation system, participatory urban planning with community engagement, effective waste management, consideration of the life cycle in urban projects, water conservation strategies, availability of biodiversity and green spaces, shared economy strategies, quality education at all levels, safety and inclusiveness for diverse social classes and races, and resilience to global changes and their impacts.The classic definition of sustainability provided in the United Nations Brundtland Report (1987) allows for distinct interpretations and the development of various conceptual models. While some models place greater emphasis on economic aspects, others indicate that social and environmental aspects are equally or even more important when discussing sustainability. There are models referred to as weak, medium, and strong (e.g. Costanza et al., 1991;Daly, 1995;Ekins et al., 2003), as well as those that consider the system under study as an open system, exchanging flows between economic, social, and environmental capitals (Pulselli et al., 2015;Giannetti et al., 2019). Additionally, the model by Rockström et al. (2009) is worth mentioning, which considers a biophysical approach but relates to the impacts that humans have on the biosphere. In general, the Urban Resources Management section of Frontiers in Sustainable Cities focuses on the biophysical relationships among urbanized centers and their surrounding environment (Figure 1). This conceptual model is the same one used by Odum and Odum (2008) and Wackernagel and Rees (1996) when discussing sustainability from a biophysical perspective, commonly referred to as strong sustainability. Figure 1 illustrates the existence of an exchange of materials (food, water, minerals, wood, etc.), energy (fossil or non-fossil), information and know-how between different levels of urbanization, where more rural areas provide primary resources that sustain more urbanized centers, while also receiving concentrated waste for dilution. At the same time, the more urbanized centers serve as hubs for generating and providing information, know-how and high-tech equipment that support and govern less urbanized areas. As always, the challenge lies in finding the limits of growth for urbanized centers while respecting the biophysical carrying capacity of the surrounding environment. Identifying these limits, often referred to as sustainability, is a complex issue that can be assessed and discussed from various perspectives (methods and indicators) and scales, connecting like pieces of a large puzzle to achieve the ultimate goal of sustainability. Specifically for this Research Topic (RT), the papers focused on the following subjects (Figure 1): sustainability & smart cities; circular economy in urban systems; GHG emissions from households; waste-to-energy; and urban design for environmental Waste-to-Energy GHG emissions from households Urban Design for Environmental Services services. The main ideas behind each of the published papers are briefly presented in the following section. This RT consists of seven publications based on the subjects shown in Figure 1. The following paragraphs present the general ideas of papers, however it is strongly recommended to refer to the full papers for a better understanding of the details presented and discussed by the authors. Although the Five-Sector Sustainability Model (5SenSu) proved to be a robust method for quantifying urban sustainability -offering diagnostics, rankings, and benchmarks to support decision-makingthere is only a moderate correlation between sustainability and smart city, with Pearson and Spearman coefficients of -0.61 and -0.59, respectively. Therefore, the authors concluded that a smart city is not necessarily sustainable. This study enhances the understanding and measurement of sustainable and smart cities, informing policies aimed at fostering more sustainable urban environments. Stakeholder engagement is essential for implementing suitable innovation patterns according to the CE strategy, particularly in urban areas, where public institutions, researchers, businesses, and citizens need to work together. The authors argue that Urban Living Labs (ULLs) serve as an effective tool for stakeholder involvement, though a clear step-by-step method for implementing ULLs in CE projects is lacking. To address this, a framework for ULLs focused on co-designing CE activities is proposed, structured into four phases: (i) context analysis, (ii) exploration, (iii) participation, and (iv) execution. A detailed explanation of each phase and examples of initial applications are provided. (https://doi.org/10.3389/frsc.2024.1453829) assessed the costs, benefits, and impacts associated with incorporating GBI in urban environments using eMergy accounting as a method. They proposed a novel integrated valuation framework that includes construction and maintenance costs, ecosystem services, and impacts on human health and biodiversity. The authors found that green roofs provide significant ecosystem benefits but incur higher initial costs and dis-services. In contrast, street trees have lower costs and impacts while generating greater benefits.Despite the complexity of urban sustainability, the seven papers published in this RT cover diverse and significant aspects that support the transition toward fairer, more inclusive, equitable, and genuinely sustainable cities. It is widely recognized that 'sustainable cities' are fundamental to addressing a key part of the broader challenge of pursuing of a more sustainable world. As covered by several Sustainable Development Goals of the 2030 Agenda, specifically Goal #11, the UrbanResource Management section will continue to play its important role as a vehicle supporting highquality scientific papers for discussions on how to achieve more sustainable cities from a biophysical perspective. You are welcome to join us in this journey.