UA has been defined as the practice of food production within and around cities (D’Ostuni and Zaffi, 2021). This umbrella concept is used to describe a range of multifunctional and multi-purpose practices that involve different actors and imply a variety of development options. It can include commercial and non-commercial activities (D’Ostuni and Zaffi, 2021) and can be operated from intra-urban to peri-urban areas, on public or private land, employing a variety of more advanced (high-tech) to simpler (low-tech) technologies (Borges et al., 2019). UA includes, for example, community gardens, institutional gardens, guerrilla gardening, vertical farming, and zero-acreage farming (z-farming), among others.

The different SiEUGreen showcases demonstrate that the impacts of UA are highly contextual and country-specific, dependent on the actors involved, purpose, land use, property, technology, and production system (D’Ostuni and Zaffi, 2021; Nadal et al., 2018). In the Global South, UA is often seen as a means of survival and poverty alleviation, while in the Global North, motivations include reducing negative urban development impact on the environment, improving the quality of life of urban communities, as well as improving physical and mental well-being and social inclusion (Binimelis et al., 2014; Borges et al., 2019; Ferreira et al., 2018; Piorr et al., 2018).However, recent global challenges, such as the coronavirus pandemic and the Ukraine war, emphasized the fragility of food supply chains and blurred some Global North/South distinctions, raising questions concerning the need for further knowledge on more efficient food production and distribution methods in cities (International Resource Panel, 2021).

If the conceptual debate on the definition of UA, as well as its purposes, functions, and overall impacts (positive/negative), is still ongoing, it is undeniable that UA directly influences environmental and social factors, in addition to food security in cities (Ferreira et al., 2018; Pascucci, 2020; Skar et al., 2019a). However, UA as an urban sustainability practice is "when urban agriculture is circular (using regenerative practices, eliminating pollutants, recycling waste and maximising exploitation of the inputs used) and is fair and equitable for producers and consumers” (International Resource Panel, 2021).

Circular economy is an “economic system that replaces the ‘end-of-life’ concept with reducing, alternatively reusing, recycling, and recovering materials in production/distribution and consumption processes” (Kirchherr et al., 2017). Inspired by natural ecosystems, the concept conveys the need and possibilities to move away from the “linear” extraction, production, distribution, consumption, and disposal paradigm, towards a permanently regenerative economy, focusing on circular flows of reuse, restoration, and renewability and encompassing the entire value chain.

The circular economy approach has become a priority in the world policy agenda in the last decade. Countries and companies seeking to decouple economic growth and social development from resource use and waste production have adopted the circular economy approach as an operational strategy in the pathway to sustainability (WEF, 2014). However, there is some debate about the potential impact of circular economy. While some argue against its “revolutionary” potential by raising questions about its ability to deliver fundamental change (Hobson and Lynch, 2016; Skene, 2017), others stress the possible benefits of implementing circular economy strategies at different levels (e.g., macro, meso and micro levels) to achieve sustainable goals (Korhonen et al., 2018).

Despite differing opinions, the concept has received immense attention in sustainability discourse. Countries worldwide have embraced it, partly because it can help to concertize the process of setting long-term objectives and provides a framework for various regulatory initiatives (de Jesus et al., 2019). In Europe, the 2015 EU Circular Economy Action Plan included several legislative proposals ”(…) establishing an ambitious long-term path leading towards waste prevention and recycling” (EC, 2017a, p. 3) which was reinforced in 2020 by the European New Green Deal. In China, there has been explicit circular economy legislation since the Cleaner Production Promotion Law of 2002, followed by the 2008 Circular Economy Promotion Law, mainly focusing on eco-industrial parks and cleaner production practices (de Jesus et al., 2018).

Even if circularity strategies vary substantially, the most common definitions encompass activities focused on the 3R principle—reducing waste, and reusing and recycling resources (Kirchherr et al., 2017; Reike et al., 2018). This principle focuses on input minimisation and efficient use of regenerative resources; life cycle extension and systems reconceptualisation (repair, re-conditioning, and re-manufacturing options); and output/waste reduction and valorisation, namely by recycling and valuing by-products and waste (de Jesus et al., 2018; Kirchherr et al., 2017; Manríquez-Altamirano et al., 2021). The 3R priorities are accepted in academia and in circular economy practices, and they have been employed in Chinese and European policy as well as in the discourse of the European Union (EU), United Nations (UN), and Organisation for Economic Co-operation and Development (OECD). This reinforces the potential of the circular economy framework when addressing global challenges (Reike et al., 2018).

Although circular economy objectives are straightforward—limit inputs, close loops/optimise resources, and avoid/transform waste—circular economy implementation is a complex endeavour. At an urban level, different cities, such as London and Paris, are already deploying urban circular roadmaps (Prendeville et al., 2018). Around 80% of the global GDP was generated in cities in 2018, but in a linear economy, cities are often ”food deserts”, utterly dependent on production from rural areas (WEForum, 2018). The separation between “places of production” and “places of consumption” affects not only food supply but also waste management. When food and organic waste are the endpoints of a linear production-consumption-waste system, cities must contend with an expensive waste management problem (Pascucci, 2020). Introducing the circular economy concept at a city level can address these challenges, making urban environments more resilient, less dependent on external supply chains, healthier, and more resource-efficient.

FIGURE 1: Linear and Circular Economy: Perspectives

Therefore, cities are ideal test beds for the implementation of circular economy strategies. Urban environments concentrate and combine resources, knowledge, and economic activity in a limited geographical area. Cities have the potential to supply the necessary inputs (e.g., waste, by-products) to develop circularity while simultaneously implementing strategies that close the loop by recycling such materials and waste (International Resource Panel, 2021; WEForum, 2018).

There is, therefore, a case to make concerning cities' potential when implementing circular economy strategies (WEForum, 2018). Against this backdrop, UA systems can be designed considering regenerative cycles (Pascucci, 2020). UA has been found to contribute to resource-efficient food production through the application of circularity strategies such as reduced transport of food products, reduced food waste, reuse of nutrients, use of underutilised spaces, and smart water use (Artmann and Sartison, 2018; Thomaier et al., 2014). Transformative processes in UA systems can also enable circular resource flows, reincorporating resources that would otherwise be wasted (Ferreira et al., 2018).