Focus on Sustainable Cities: Urban Solutions Toward Desired Outcomes

With increasing urbanisation, urban green spaces are expected to be crucial for urban resilience and sustainability, through the delivery of ecological, economic and social benefits. In practice, however, planning, management and evaluation of urban green spaces are rarely structured and evidence-based. This represents a missed opportunity to account for, track and foster the multiple benefits that green spaces are expected to deliver. To gain insight into this gap, this study assesses the availability and uptake of relevant evidence by city governments. Interviews, focus groups and quantitative surveys were applied in four medium-sized European cities: Coimbra (Portugal), Genk (Belgium), Leipzig (Germany), and Vilnius (Lithuania), covering the main governance and climatic gradients in Europe. Using straightforward data exploration and regression, we analyse which ecological, economic and social indicators are typically chosen by cities and why. Together with the city stakeholders, we derived a common set of benefit categories and key performance indicators which can be adapted to diverse local contexts. We conclude that cities tend to make pragmatic decisions when composing their indicator sets, but nevertheless cover multiple urban green space dimensions. Finally, we explore how indicator choice could be optimised towards a complementary and credible indicator set, taking into account a realistically feasible monitoring effort undertaken by the cities.

The nitrogen footprint tool (NFT) provides a novel way for communities to understand the environmental impacts of their collective activities and consumption. Reactive nitrogen (Nr; all N species except N₂) is created by the Haber–Bosch process for food production and as a by-product of fossil fuel combustion and two natural processes, biological nitrogen fixation and lightning. While it is a vital input for food production, too much Nr has a negative effect on the environment. Calculating the amount of Nr released to the environment as a result of an entity's resource consumption is the first step in reducing those Nr losses. The nitrogen (N) footprint method has previously taken this approach at the personal and institution scale. In this study, the approach is extended, for the first time, to the spatial patterns of the community nitrogen footprint within a large city, through the integration of diverse geographic information to calculate the N footprint distribution within the City of Baltimore, Maryland, USA. The total N footprint of Baltimore City was ∼19 000 MT N or 30 kg N per capita in 2016, dominated by the food production sector (73%), followed by the energy and transportation sectors (15% combined). There was geographic variability among census block groups' per capita N footprint within Baltimore City; driven primarily by economic and development factors. Several management scenarios were assessed to better understand what actions may reduce the Baltimore N footprint at the city and community scale over time. The study explored the effect and efficacy of reducing meat consumption based on differences in city consumption patterns, increasing the use of renewable energy sources, and reducing electricity consumption on the city's total N footprint. The model for the Baltimore City N footprint calculation can be applied to other communities in the United States at the spatial grain of the census block group or any country with this level of data to provide an indicator of nitrogen sustainability.

Large scale and rapid urbanization processes call for a better quality of urban planning to support human well-being. While compact cities aim to reduce land consumption, densification puts pressure on the remaining green areas, influencing ecosystem services provision and ultimately the life quality of the growing urban population. Supply of and demand for urban ecosystem services differ however greatly across the globe. In this study, we derive a set of urban typologies and their related ecosystem services bundles in both a temperate and a tropical city. We show that the supply of urban ecosystem services does not increase linearly with green area coverage, but is highly dependent on the urban form. While the surface sealed by infrastructures and the buildings themselves play a key role in influencing ecosystem services provision, we observe that the share of trees is particularly important for supporting regulating ecosystem services in built up neighborhoods. With a similar average surface-to-volume ratio, open midrise neighborhoods in Singapore provide more water flow regulation and air pollution control services than the same urban typology in Zurich. Microclimate regulation, in contrast, does not seem to be dependent on the context, but more on the amount of built up surface. Interestingly, we observe that open midrise neighborhoods synergistically support the supply of many regulating services in both case study areas, including microclimate regulation, water flow regulation and air pollution control. Large water and forest patches are unquestionably essential in both Singapore and Zurich to support bundles of ecosystem services, particularly also for recreational activities. Using open data, the approach can be transferred to other cities and support decision makers in their efforts to plan the sustainable development of cities across the world.

The security, resilience, and sustainability of urban water supply systems (UWSS) are challenged by global change pressures, including climate and land use changes, rapid urbanization, and population growth. Building on prior work on UWSS security and resilience, we quantify the sustainability of UWSS based on the performance of local sustainable governance and the size of global water and ecological footprints. We develop a new framework that integrates security, resilience, and sustainability to investigate trade-offs between these three distinct and inter-related dimensions. Security refers to the level of services, resilience is the system's ability to respond to and recover from shocks, and sustainability refers to local and global impacts, and to the long-term viability of system services. Security and resilience are both relevant at local scale (city and surroundings), while for sustainability cross-scale and -sectoral feedbacks are important. We apply the new framework to seven cities selected from diverse hydro-climatic and socio-economic settings on four continents. We find that UWSS security, resilience, and local sustainability coevolve, while global sustainability correlates negatively with security. Approaching these interdependent goals requires governance strategies that balance the three dimensions within desirable and viable operating spaces. Cities outside these boundaries risk system failure in the short-term, due to lack of security and resilience, or face long-term consequences of unsustainable governance strategies. We discuss these risks in the context of poverty and rigidity traps. Our findings have strong implications for policy-making, strategic management, and for designing systems to operate sustainably at local and global scales.

Social, technological and climatic changes will transform the way energy is consumed over the 21st century, with important implications for energy networks and greenhouse gas emissions. Here, we develop a method to efficiently explore climate-energy interactions under various scenarios of climate, urban infrastructure and technological change. We couple the Urban Climate and Energy Model with the Conformal Cubic Atmospheric Model as a full-height single column driven with a series of global climate model simulations in an ensemble approach. The framework is evaluated against observations, then a series of century-scale simulations are undertaken to examine projected climate change impacts on electricity and gas demand in the temperate/ oceanic climate of Melbourne, Australia. With air-conditioning ownership remaining at early 21st century levels, and in the absence of other changes, climate change under radiative forcing RCP 8.5 increases peak electricity demand by 10%, and decreases peak gas demand by 22% between 2000 and 2100. However, if projected increases in air-conditioning ownership are considered, peak electricity demand increases by 84%, surpassing peak gas demand in the second half of the century. These findings highlight the complex nature of changes facing energy networks. Changes will be location and scenario dependent.

A growing number of cities are investing in green infrastructure to foster urban resilience and sustainability. While these nature-based solutions are often promoted on the basis of their multifunctionality, in practice, most studies and plans focus on a single benefit, such as stormwater management. This represents a missed opportunity to strategically site green infrastructure to leverage social and ecological co-benefits. To address this gap, this paper builds on existing modeling approaches for green infrastructure planning to create a more generalizable tool for comparing spatial tradeoffs and synergistic 'hotspots' for multiple desired benefits. I apply the model to three diverse coastal megacities: New York City, Los Angeles (United States), and Manila (Philippines), enabling cross-city comparisons for the first time. Spatial multi-criteria evaluation is used to examine how strategic areas for green infrastructure development across the cities change depending on which benefit is prioritized. GIS layers corresponding to six planning priorities (managing stormwater, reducing social vulnerability, increasing access to green space, improving air quality, reducing the urban heat island effect, and increasing landscape connectivity) are mapped and spatial tradeoffs assessed. Criteria are also weighted to reflect local stakeholders' desired outcomes as determined through surveys and stakeholder meetings and combined to identify high priority areas for green infrastructure development. To extend the model's utility as a decision-support tool, an interactive web-based application is developed that allows any user to change the criteria weights and visualize the resulting hotspots in real time. The model empirically illustrates the complexities of planning green infrastructure in different urban contexts, while also demonstrating a flexible approach for more participatory, strategic, and multifunctional planning of green infrastructure in cities around the world.

Many rapidly urbanizing desert cities (RUDC) around the globe experience an acute risk of flooding. To reduce this risk, properly engineered flood control structures (FCS) must account for sediment accumulation as well as flood waters. While the Phoenix area, USA, uses regional data from non-urban, non-desert watersheds to generate sediment yield rates, the proposed desired outcome for RUDCs is to base FCS on data related to urbanization. Wolman (1967 Geogr. Ann. A 49 385–95) recognized that sediment yields spike during a relatively short period of bare-ground exposure associated with urban growth, followed by surface sealing resulting in a great reduction in sediment yield. This research presents a new analysis of empirical data where two regression models provide estimates of a more realistic sediment accumulation for arid regions and also urbanization of a desert cities: (i) linear regression between drainage area and sediment yield based on a compilation of more than 150 global sediment yield data for warm desert (BWh Köppen‐Geiger) climate; and (ii) linear regression relating percent urban growth with sediment yield using available data on urbanization-generated sediment associated with growth of a desert city. The new model can be used to predict the realistic sediment accumulation for helping provide data where few data exists in urbanizing parts of arid Africa, southwest Asia, and India.

This paper proposes a framework for measuring compactness and urban green accessibility in a high-density transit-oriented metropolis and uses Taipei City and its surrounding outskirts, New Taipei City, as a case to illustrate the measurement framework. Two indices, urban compactness index (UCI) and urban green accessibility index (UGAI), are developed to illustrate various aspects of a sustainable urban built environment, with UCI including density of residents and commercial activities, land use mix, street connectivity, access to center/subcenters, and access to transit stops, and UGAI measuring access to public urban green spaces. We found that while great spatial variations exist among different parts, our study area has a distinguished polycentric pattern of UCI index with three distinct clusters around the center and sub-centers illustrating higher index values in 2015. When compared to UCI, UGAI has a similar polycentric but more dispersed spatial pattern, as well as linear patterns along river corridors. We found that most areas of medium or high UCI values are located in areas of either plan-induced or plan-expanded development. UCI values in areas of plan-expanded development are generally higher than that of areas of plan-induced development. UCI and UGAI are spatially correlated to a certain extent. We found that most centers and one particular subcenter have high UCI and UGAI, moving towards both compactness and good green accessibility. Two subcenters with high UCI and low UGAI, i.e. Banqiao and Yonghe, call for planning to provide green spaces for residents living in these rising subcenters. UCI and UGAI can be applied and used to assess compact and green urban development of other cities and they are particularly useful to dense urban environment of large cities in Europe and Asia.

A growing literature documents the effects of heat stress on premature mortality and other adverse health outcomes. Urban heat islands (UHI) can exacerbate these adverse impacts in cities by amplifying heat exposure during the day and inhibiting the body's ability to recover at night. Since the UHI intensity varies not only across, but also within cities, intra-city variation may lead to differential impact of urban heat stress on different demographic groups. To examine these differential impacts, we combine satellite observations with census data to evaluate the relationship between distributions of both UHI and income at the neighborhood scale for 25 cities around the world. We find that in most (72%) cases, poorer neighborhoods experience elevated heat exposure, an incidental consequence of the intra-city distribution of income in cities. This finding suggests that policymakers should consider designing city-specific UHI reduction strategies to mitigate its impacts on the most socioeconomically vulnerable populations who may be less equipped to adapt to environmental stressors. Since the strongest contributor of intra-urban UHI variability among the physical characteristics considered in this study is a neighborhood's vegetation density, increasing green space in lower income neighborhoods is one strategy urban policymakers can adopt to ameliorate some of UHI's inequitable burden on economically disadvantaged residents.

With over half of the world's population living in cities, there is mounting evidence indicating that investments in urban sustainability can deliver high returns on socioeconomic and environmental fronts. Current scholarship on urban agriculture (UA) reports a wide range of benefits which have been shown to vary with the scale and type of benefit examined. Notably, most city-scale studies do not align benefits of UA with locally meaningful goals. We fill this gap by conducting a city-scale analysis for Phoenix, the fifth largest city in the USA by population, and evaluate these benefits based on their ability to contribute to select desired outcomes specified in Phoenix's 2050 Sustainability Goals: the elimination of food deserts, provision of green open space, and energy and CO₂ emissions savings from buildings. We consider three types of surfaces for UA deployment—undeveloped vacant lots, flat rooftops, and building façades—and find that the existing building stock provides 71% of available UA space in the study area. The estimated total food supply from UA is 183 000 tons per year, providing local produce in all existing food deserts of Phoenix, and meeting 90% of current annual consumption of fresh produce based on national per capita consumption patterns. UA would also add green open space and reduce by 60% the number of block groups underserved by public parks. Rooftop deployment of UA could reduce energy use in buildings and has the potential to displace more than 50 000 tons of CO₂ per year. Our work highlights the importance of combining a data-driven framework with local information to address place-based sustainability goals and can be used as a template for city-scale evaluations of UA in alternate settings.

Africa is projected to add one billion urban residents by 2050. Yet developing sustainable solutions to tackle the host of challenges posed by rapid urban population growth is stymied by a lack municipality-level population data across the continent. To fill this gap, we intersect volunteered urban settlement data from OpenStreetMap with five synthetic gridded population datasets to estimate the how Africa's urban population is distributed among over 4750 individual urban settlements across Africa. We assess how urban settlement distributions changed from 2000 to 2015 within and between countries and across moisture zones. To this end, we construct urban settlement Lorenz curves to calculate change in Gini coefficients and test the degree to which Africa's urban settlements distributions fit power law distributions exhibited by Zipf's law. Our results reveal that 77%–85% of urban settlements in Africa have fewer than 100 000 people and that at least 50% of Africa's urban population live in urban settlements with fewer than 1 million residents. Across almost all African countries, the distribution of urban population shifted towards larger cities between 2000 and 2015. However, in arid regions, our results indicate that small- and medium-sized urban settlements are absorbing a greater share of urban population growth compared to large urban settlements. While our urban population estimates vary across gridded population datasets and differ from United Nations estimates, this is the first paper to measure urban population across Africa using a consistent methodology to identify urban settlement populations. Unlike UN urban population data for Africa, our results can readily be incorporated with geolocated environmental, public health, and economic data to support efforts to monitor United Nations Sustainable Development Goals related to urban sustainability, poverty reduction, and food security across Africa's ever-growing urban settlements.

The context in which trees and forests grow in cities is highly variable and influences the provision of ecological, social, and economic benefits. Understanding the spatial extent, structure, and composition of forests is necessary to guide urban forest policy and management, yet current forest assessment methodologies vary widely in scale, sampling intensity, and focus. Current definitions of the urban forest include all trees growing in the urban environment, and have been translated to the design of urban forest assessments. However, such broad assessments may aggregate types of urban forest that differ significantly in usage and management needs. For example, street trees occur in highly developed environments, and are planted and cared for on an individual basis, whereas forested natural areas often occur in parkland, are managed at the stand level, and are primarily sustained by natural processes such as regeneration. We use multiple datasets for New York City to compare the outcomes from assessments of the entire urban forest, street trees, and forested natural areas. We find that non-stratified assessments of the entire urban forest are biased towards abundant canopy types in cities (e.g. street trees) and underestimate the condition of forested natural areas due to their uneven spatial arrangement. These natural areas account for one quarter of the city's tree canopy, but represent the majority of trees both numerically and in terms of biomass. Non-stratified assessments of urban forest canopy should be modified to accurately represent the true composition of different urban forest types to inform effective policy and management.

We examine how future changes in water yield and demand will affect the likelihood of water shortages and the efficacy of some of the most common methods for dealing with water shortages and meeting municipal demands, including improvements in water use efficiency and transfers of water between sectors of the economy. We find that more than 45.8 million people, primarily in the Southwest, central Great Plains, and southern California, would already be experiencing regular water shortages in the absence of groundwater mining. By 2060, that number would grow to over 136.2 million people. Among the reasons we find for increased likelihood of water shortages, reduced water yield is the most prevalent, affecting 80% of water basins in the US In the American West, nearly half of the water basins are projected to see an increase in shortages. We estimate future water withdrawals in the industrial and commercial and thermoelectric sectors will remain fairly steady, but withdrawals in the domestic and public sector are expected to rise. The Colorado River and Rio Grande regions see the largest percentage increases in projected domestic and public water use as well as the greatest percentage decreases in projected water yield. To cover new municipal demands, transfers from agriculture may be needed, in which case, significant impacts to agriculture will occur in northern New Mexico, parts of Utah, Nevada, and Washington where municipal demands are projected to grow to 25%–50% of agricultural water use. The situation is more extreme in northern Arizona and eastern Texas, where additional municipal demands are projected to be six times the amount used by agriculture.

If the material intensive enterprises in an urban area of several million people shared physical resources that might otherwise be wasted, what environmental and public benefits would result? This study develops an algorithm based on lifecycle assessment tools for determining a city's industrial symbiosis potential—that is, the sum of the wastes and byproducts from a city's industrial enterprises that could reasonably serve as resource inputs to other local industrial processes. Rather than report, as do many previous papers, on private benefits to firms, this investigation focuses on public benefits to cities by converting the maximum quantity of resources recoverable by local enterprises into an estimate of the capacity of municipal infrastructure conserved in terms of landfill space and water demand. The results here test this novel approach for the district of Mysuru (Mysore), India. We find that the industrial symbiosis potential calculated based on analysis of the inputs and outputs of ∼1000 urban enterprises, translates into 84 000 tons of industrial waste, greater than 74 000 tons of CO₂e, and 22 million liters per day of wastewater. The method introduced here demonstrates how industrial symbiosis links private production and public infrastructure to improve the resource efficiency of a city by creating an opportunity to extend the capacity of public infrastructure and generate public health co-benefits.

Urban areas are key to sustainability, and understanding heterogeneity in urban landscapes is important for linking development patterns to ecological, economic, and social health. Here, we characterize the urban landscape for the purpose of revealing structural variations that affect sustainability. We develop a new language and classification schema for breaking down urban areas into sub-metropolitan land units that, unlike administrative boundaries, are based on objective measures of the built and natural environment and are comparable across and within urban areas. These units capture structural differences that population density does not. The classification schema offers a process-based characterization of urban landscapes—one where 'urban' is defined by the human and biophysical interactions mediated by the urban environment and complements existing land classification systems, like those based on land use and land cover. As an example, the schema is applied here to understand transportation behaviors—a particular urban process with wide-ranging implications for urban sustainability. Using GIS, satellite, and census spatial data, we apply the classification schema in 909 US urban areas, systematically clustering development with similar structural attributes linked to transportation behaviors. In this way, an urban area is divided into a collection of smaller landscapes, larger than individual households and smaller than census tracts, that are distinct in how they function. The study shows that characterizing the urban landscape in this way can distinguish between neighborhoods with different travel behaviors. Extensions of the schema can be used to monitor and manage urban systems towards sustainability, targeting spatial planning strategies to the micro-geographies where they would be most relevant.