Focus on sustainable cities: urban solutions toward desired outcomes

Urbanization represents the single most impactful and long-lasting transformation of the Earth system since the dawn of civilization. Cities are simultaneously locations of innovation, social connectivity, and wealth, but they also create local-to-global environmental degradation and socioeconomic disparities. For example, food provision for cities has required significant land-use change and fertilizer input, has altered regional climate, biogeochemical cycles, and degraded marine and landscapes through biodiversity loss, algal blooms and fish kills. To maintain urban livelihoods and the provision of goods and services, cities require vast amounts of energy (e.g. to provide access to transport, cooling systems), which are massive producers of greenhouse gases, the main culprit of global change. Future urbanization will need to be transformative—wholly different from how we have designed, built, and powered cities and towns—so that they can be sustainable and resilient. Should current trends in urban expansion continue—low density and resource and energy intensive—urban energy use will triple between 2005 and 2050 (Creutzig et al 2015). Raw material demand will exceed what the planet can sustainably supply (UN IRP 2018) and the safe operating limits that have facilitated the rise of modern civilization will become increasingly at risk of being crossed, leading to a cascade of pressures that would pose dire consequences for and severely disrupt humanity (Rockström et al 2009).

The growth of future cities and towns—both newly developed and expansion of those already in existence, as well as how we maintain and retrofit existing urban centers—will determine whether humanity can transition to sustainability. Ideally, decisions about urbanization would be based on science and data-driven approaches that are grounded in specific places but are also generalizable (Bettencourt and West 2010). Although there are aspects of cities that are generalizable, current theories about urban systems are more conceptual in nature and there is limited work that takes into consideration the non-linear and non-local effects of urban systems. Nevertheless, scientific understanding today is sufficient to provide some level of meaningful input as regards benefits and consequences of urbanization. For example, the destruction of infrastructure and loss of life caused by Hurricane Betsy in New Orleans in 1965 led to critical knowledge that structural and social systems were necessary to safeguard the city against future tropical storms. However, 'development outpaced available levels of protection' (Olson 2011), minimizing the possibility of local evacuation, and eventually resulted in the devastating impacts caused by Hurricane Katrina in 2005. Some of the lessons learned post-Hurricane Betsy have yet to be incorporated post-Hurricane Katrina, diminishing the role of scientific understanding vis-à-vis the ostensible necessity for growth.

What is less known, however, is the collective and integrative effect of urbanization that cuts across disciplinary boundaries and holistically characterizes the problems that ensue, or the efficacy of solutions that are necessary, beyond the lens of any singular academic branch. For example, a frequent focus of urban climate modeling studies that examine the efficacy of temperature reduction strategies (e.g. cool and green roofs) is the ability of a particular strategy to decrease the urban heat island (UHI) (i.e. to decrease the air temperature difference between an urban and rural location). The results from such a singular focus can be misleading if it ignores other components of the urban ecosystem or lacks integration. We illustrate why by using Phoenix, AZ, as an example. Phoenix has a negligibly small, or even negative, daytime UHI, although few would argue that the fifth largest city in the US does not have a serious urban heat burden, when maximum summertime temperatures frequently exceed 43 °C (Chow et al 2012). A focus on decreasing the UHI lacks integration by neglecting closely connected environmental and human-oriented consequences that become apparent upon examination of tradeoffs and feedbacks (Seto et al 2017, Putnam et al 2018, Broadbent et al 2021). Indeed, recent work conducted in the Californian city of Los Angeles has demonstrated that the deployment of cool (i.e. highly reflective) pavements raises concerns for the undertaking of outdoor activities (e.g. reduces walkability) as a result of increased pedestrian radiant load (Middel et al 2020). This example illustrates a second key concern related to the development of meaningful metrics that are generalizable and help pave the path toward development of urban systems theory that incorporates natural and social elements. Although the utility of a particular metric may have gained traction and become popular, as the often-utilized focus on decreasing the UHI has, it may not result in actionable science. While one may argue that it does for some cities, the lack of generalizability to the majority of cities disconnects this metric from broad societal applicability (Martilli et al 2020).

This Special Issue (SI) was focused on the discovery of urban solutions aimed at accelerating the transition to economically, socially, and environmentally resilient cities through integration of desired outcomes as an organizing framework. The SI papers collectively published, totaling one research agenda-setting perspective and 15 research studies (table 1), take an important step forward by:

• (a)  
  integrating across disciplines such as hydrology, meteorology/climatology, ecology, geomorphology, economics, environmental management, environmental sciences, geography, GIScience, and urban planning, thereby explicitly acknowledging the multifaceted qualities of sustainability that each field contributes to;
• (b)  
  developing new metrics in assessing sustainability (e.g. Chertow et al 2019, Stokes and Seto 2019, Krueger et al 2020);
• (c)  
  considering an explicit multi-scalar approach when evaluating urban sustainability solutions across temporal and spatial dimensions (Aragon et al 2019, Chakraborty et al 2019, Fan et al 2019, Jeong 2019, Meerow 2019, Pregitzer et al 2019, Carmen et al 2020, Grêt-Regamey et al 2020);
• (d)  
  examining coupled interactions between various urban system aspects, such as climate-energy, food-energy, and on industrial waste with energy production and consumption, with respect to desired sustainability outcomes through detailed scenario assessment (e.g. Lipson et al 2019, Warziniack and Brown 2019, Dukes et al 2020).

Collectively, the articles in the SI also point to four gaps in knowledge:

• (a)  
  The need for more theory—Stokes and Seto (2019) reconceptualize the urban landscape in a process-based manner that is directly connected to sustainability. The explosion in data analytical tools (e.g. machine learning, data mining), data (e.g. remote sensing, cell phone records), and computing infrastructure (e.g. parallel and cloud computing) offers an unprecedented opportunity to expand this type of analysis globally, to include other processes, and paves the way toward improved characterization and monitoring of urban areas, including large-scale urban population dynamics (e.g. Tuholske et al 2019), to guide the transition to sustainability.
• (b)  
  The need for replicability—are the results consistent across urban sites using the same methods but with different data? Pregitzer et al (2019) demonstrate the importance of improved characterization of urban forests, including their composition and structure in order to better inform both policy and management, enable provision of ecosystem services and to establish a reference against which what-if (e.g. canopy expansion) scenarios can be examined.
• (c)  
  Generalizability—are the results applicable in other urban contexts? Can inferences be made in other urban areas using results from one study? For example, how and under what conditions do the findings of Chakraborty et al (2019)—who find that poorer neighborhoods experience elevated heat exposure—apply to smaller cities around the world outside of their 25 case studies? Perhaps even more important is to understand how generalizable are the underlying conditions and processes that gave rise to these outcomes. Likewise, under what conditions can urban agriculture supply 90% of annual consumption of produce, as shown in Phoenix (Aragon et al 2019) and what are the implications for local food insecurity?
• (d)  
  Scalability—there is an urgent need for solutions to be scalable. It is valuable that some case studies in the SI demonstrate innovation and provide pathways to increase resilience, but can other cities achieve similar outcomes with significantly fewer human and financial resources?

The papers also underscore the need for rethinking the metrics we are using to measure outcomes connected to urban sustainability. For example, given the potential of ISP for reusing a city's industrial waste and byproducts, there is an opportunity to develop metrics that examine a city's capacity to improve resource use efficiency. Likewise, there is a need to develop and improve tools that examine a spectrum of spatially-explicit environmental indicators, patterned after the nitrogen footprint tool (Dukes et al 2020) which permits environmental impact assessment of the aggregate activities and consumption patterns of distinct urban regions. Additionally, there is a need to develop measures of urban landscapes that link to multiple dimensions of sustainability (Grêt-Regamey et al 2020, Krueger et al 2020), including energy demand (Lipson et al 2019), flood control structures (Jeong 2019), assessment of tradeoffs and synergies that enhance urban resilience (Meerow 2019) and extend beyond the conventional focus on land use change. Understanding how city governments plan, manage and evaluate existing urban green spaces to maximize delivery of ecological, economic, and social benefits is necessary in order to make practical contributions to urban resilience (Carmen et al 2020). Finally, retrofitting existing cities and development of new urban clusters in a society based upon information technology offers prospects for breakthrough changes but also brings unique challenges with respect to privacy and cybersecurity (Moy de Vitry et al 2019).

Collectively, the articles in the SI advanced the knowledge base on cities and sustainability. They also revealed considerable opportunities for future research that advance the development of pathways to urban sustainability. Especially important is the potentially critical role of innovative policy, management, and technological solutions that facilitate locally-established desired outcomes that are generalizable, integrative, and scalable. Development of new conceptual frameworks, metrics and tools, and integration across disciplinary boundaries that holistically characterize and help prioritize science and policy concerns, can help inform a convergent research agenda that paves the way to transitioning to sustainability in an urban century.

The guest editors would like to thank the authors who contributed to this Special Issue. This collection originated from discussions at the Urban Water Innovation Network (UWIN) annual meetings. UWIN is funded by the National Science Foundation Sustainability Research Network Cooperative Agreement 1444758.