In This Issue
Spring Bridge on Sustainable Smart Cities
March 15, 2023
The world’s cities face increasing threats from natural disasters, aging infrastructure, traffic, and resource constraints. The articles in this issue examine smart infrastructure, sustainability, net zero carbon options, and autonomous driving, among other approaches to smart and sustainable cities.

Strategies for Smart Net Zero Carbon Cities

Monday, March 20, 2023

Author: Karen C. Seto

A combination of high- and low-tech strategies can help cities achieve net zero carbon emissions.

If the world is to avoid dangerous climate change, immediate, rapid, and large-scale reductions in greenhouse gas (GHG) emissions are needed within the next three decades (IPCC 2018): Global emissions must reach net zero by 2050. This will require major transitions in four systems: energy, industry, land and ecosystems, and urban and other infrastructure.

Cities in particular must transition to net zero carbon as soon as possible given that two-thirds of global GHG emissions are attributed to urban areas (Lwasa et al. 2022). Furthermore, because the urban share of national emissions is large—62 percent in 2015—and growing (Gurney et al. 2022), national or global mitigation strategies that omit urban emissions will be inadequate.

Historical Perspective

In many respects, the idea of transforming cities to make them net zero carbon is the latest manifestation of a recurring theme in history: to make cities more efficient, environmentally safe, and/or economically vibrant. These concepts are embodied in the idea of the smart city.

Smart cities are characterized by the use of technological advances to measure, manage, and improve urban decision making and functioning. Replete with sensors, they run on data and lots of it, with the goal of improving the quality, efficiency, and sustainability of urban life.

The concept of the smart and sustainable city gained ground in the early 2000s, coincident with the rise of the internet and the information revolution, but it had many forerunners. For example, the Sanitary Reform Movement, which started in the 1830s and peaked in the 1880s, was a response to tenement slums and the spread of cholera and other diseases (Ringen 1979). The Eco-City concept grew out of the environmental movement in the 1960s and 1970s and focused on efficiency and minimization of waste (Rapoport 2014).

Seen through this historical lens, the transition to net zero cities can be viewed as an evolution from sustainable and smart cities.

Conceptualizing Net Zero Carbon Cities

Net zero carbon emissions are achieved when anthropogenic carbon emissions and their removal are in balance over a particular period. At the city scale, carbon emissions include those from urban production of goods and services (whether for local consumption or export), transport, and consumption by entities such as households, governments, and commerce.

Cities offer extensive opportunities for deep decarbonization because they concentrate people, infrastructure, and activity.

However, cities are open systems that rely on nearby and distant areas for imported goods and services and waste export. A strictly territorial approach that excludes emissions that occur outside city boundaries (e.g., associated with imported food, energy, transboundary transport) can significantly underestimate urban carbon footprints. For example, a study of 79 cities found that 41 percent of their consumption-based carbon was generated outside city boundaries (Wiedmann et al. 2021). Other studies have shown that upstream emissions that occur throughout the production chain of goods consumed in cities are greater than territorial emissions (Harris et al. 2020; Minx et al. 2013).

Thus how urban carbon emissions are counted will affect the sources of emissions that are balanced under the concept of net zero cities. Territorial emissions should be complemented with accounts of upstream or embodied emissions to get a more complete picture of the emissions associated with urban consumption. With the large discrepancy between territorial and upstream urban emissions, there are increasing calls for cities to account for their supply chains in their commitments to net zero carbon emissions (Ramaswami et al. 2021; Wiedmann et al. 2021).

Given the effects of different carbon accounting systems for urban emissions, it is no surprise that there are varying definitions of what constitutes a net zero carbon city. According to one of the most common definitions, a net zero carbon city is committed to a target of at least 80 percent reduction in GHG emissions or some other decarbonization goal (CNCA 2018). The “net” component of a net zero goal implies that a city’s residual GHG emissions are offset by carbon removal.

Net zero is not the same as low carbon. For cities to achieve net zero, they must undergo systemic changes through deep decarbonization.

Urban Deep Decarbonization Strategies

Deep urban decarbonization is the process by which a city significantly reduces GHG emissions to zero or near net zero. Cities offer extensive opportunities for deep decarbonization because they concentrate people, infrastructure, and activity.

Three Strategies

To achieve deep decarbonization, cities need to undertake and integrate three broad strategies:

  1. Reduce urban demand for energy and materials.
  2. Switch urban energy supply to net zero carbon electricity, fuels, and materials.
  3. Enhance carbon sequestration and stocks and reduce emissions in urban supply chains.

Each of these strategies comprises multiple pathways. The first, for example—reducing urban demand for energy and materials—can be achieved by (i) integrating spatial planning to avoid the need for motorized transport; (ii) improving the efficiencies of individual sectors, such as buildings, transport, and wastewater treatment; and/or (iii) fostering industrial symbiosis, where wastes or byproducts from one industry are used as input for another industrial process, thereby eliminating waste and avoiding the demand and associated emissions for additional raw materials. Industrial symbiosis requires collaboration and geographic colocation (Chertow 2000).

Switching the urban energy supply would require concurrent development of a net zero carbon electricity grid; electrification of key urban activities such as mobility, heating, and cooking systems; and carbon valorization, which uses captured CO2 as the chemical feedstock to make consumer products such as plastics, fertilizers, and alcohols.[1]

In terms of mobility, an electric vehicle fleet must be an essential part of the urban mitigation portfolio. However, the planet will add another 2.5 billion residents to cities and towns over the next 30 years. If every one of the 4.5 billion existing and 2.5 billion future urban residents used an electric vehicle, that would amount to around 7 billion vehicles—and significant embodied emissions associated with manufacturing the fleet and the batteries. Therefore, electrification cannot be the only component of urban transport mitigation.

The third strategy, enhancing urban carbon uptake and stocks, can be achieved through (i) carbonation of cement materials, a slow process by which CO2 is absorbed in solid materials; and (ii) carbon sequestration by vegetation, whereby CO2 is captured and transformed into biomass through photosynthesis.

The Avoid-Shift-Improve Paradigm

Another way to conceptualize deep urban decarbonization is through the Avoid-Shift-Improve (A-S-I) paradigm. As an example of this approach, emissions may be avoided by reducing or eliminating unnecessary demand, demand for energy shifted to lower emission modes by switching travel modes from autos to bikes, and the efficiency of energy-consuming technologies and infrastructures improved by increasing energy efficiency and reducing the carbon intensity of vehicles.

First developed for the transport sector, the A-S-I approach has been applied to others—such as food, housing, and materials (Creutzig et al. 2022a)—to find potential mitigation pathways. For example, food loss and waste in the United States are estimated to account for 170 million metric tons of CO2 equivalent of GHG emissions, not including emissions from landfills (EPA 2021). The A-S-I approach can be used to help identify ways to reduce and avoid such loss and waste.

Low-Tech, Low-Cost Mitigation Pathways

Low-tech, comparatively low-cost pathways should be part of every city’s strategy to achieve net or near net zero emissions. They are less expensive and often easier to implement, especially in developing countries with rapidly growing cities and high demand for new infrastructure.

Colocate Jobs with Housing

Many studies show that higher population densities in close proximity to higher job densities are strongly correlated with lower GHG emissions (e.g., Lee and Lee 2020; Qin and Han 2013). Locating jobs and housing near each other reduces commuting distances and increases the use of both public transit and non-motorized transport, in part by making it more convenient to walk and bike to work. The associated increase in physical activity has significant health benefits, such as reductions in obesity (Ewing et al. 2014), cardio-vascular and respiratory diseases (Stevenson et al. 2016), and diabetes risk (Saunders et al. 2013).

Because of significant embodied emissions associated with manufacturing the fleet and the batteries, electrification cannot be the only component of urban transport mitigation.

For cities in the early stages of urbanization with low levels of transport infrastructure, the intentional colocation of jobs with housing can help to avoid “locking in” high energy–consuming behaviors and routines that require more effort to change.

Enhance Pedestrian and Biking Infrastructure

Locating jobs near housing is only a first step. This strategy must be supported with efforts to enhance, improve, and expand pedestrian and bicycle infrastructure to make walking and biking more attractive alternatives to motorized transport. The design of new and redesign of existing cities such that walking, biking, and public transportation can meet the needs of most urban activities is a comparatively low-cost strategy to reduce transport emissions. Bike lanes that are raised and separated from motorized vehicles improve safety and encourage cycling.

More walkable cities are good for climate change mitigation—and also make sense for economic and individual health.

The city of Seville, Spain, built a bike infrastructure network covering 77 km in just two years at a cost of €18 million ($19.6M). In contrast, the city’s first metro line cost over €630 million ($684M) to build 18 km (Marqués et al. 2015). Seville also has a bike sharing program, with approximately 2000 bikes, that reports over 8100 trips per day (Faghih-Imani et al. 2017). These types of investments in cycling infrastructure make bicycling a viable and economic form of travel across the city and encourage their use.

Making cities more walkable is good for climate change mitigation and also makes sense for economic and individual health. Reduced traffic from personal vehicle use can reduce air pollution and thus improve health. Furthermore, walkable communities have higher residential (Rauterkus and Miller 2011) and commercial property values (Pivo and Fisher 2011) and are more economically productive (Litman 2003). One study found that among the largest US metro-politan areas, the most walkable produce 49 percent more GDP than the least walkable (Leinberger and Rodriguez 2016).

Make Streets Visually and Commercially Diverse

In addition to enhancing pedestrian and biking infrastructure to improve walkability, cities must have a mix of uses and destinations in close proximity for pedestrians and cyclists. People want to be able to walk or bike to coffee shops, stores, parks, the post office or library, and work. There are two ways to achieve this in existing cities: site commerce near housing or increase housing near areas of commerce.

Improving walkability and colocating jobs and housing will also require a complementary strategy of making streets visually and commercially diverse and geared to the pedestrian. The observation of urbanist Jane Jacobs (1969) more than 50 years ago, that aesthetically diverse street life is an indicator of social and economic vitality, still holds true. Cities with vibrant street life invite people to walk and explore. In contrast, streets devoid of people or visual complexity are likely to discourage walking.

Plant Trees and Preserve Urban Natural Areas

Urban forests and street trees can help mitigate climate change directly through carbon sequestration and storage. Carbon sequestration is the process by which carbon dioxide is removed from the atmosphere and stored in carbon pools, such as the ocean, biomass, rocks, and soils. Carbon can also be stored in bio-based materials used in construction, such as mass timber and bamboo (Churkina et al. 2020).

Globally, urban trees sequester approximately 217 million tonnes of carbon annually and store approximately 7.4 billion tonnes of carbon, although the amounts are highly dependent on canopy structure, composition, extent, and biome (Lwasa et al. 2022). In the United States, annual sequestration of carbon by urban trees is approximately 25.6 million tonnes (Nowak et al. 2013). Forested natural areas in New York City store an estimated average of 263.5 megagrams of carbon per hectare, totaling as much as 1.86 teragrams of carbon citywide (Pregitzer et al. 2021).

Urban tree canopy can also mitigate climate change indirectly by providing shade that lowers surface and air temperatures. This cooling effect can reduce building energy demand for air conditioning.

Associated Costs

None of these strategies require much technology or financing compared to the high-tech solutions, but they do require vision and a strong leader. In many cases, they need supportive policies that combine more than a single objective as well as changes to zoning codes and public financing, including through possible tax incentives. The successful implementation of these strategies is highly dependent on a city’s financial and governance capability, and must be addressed simultaneously by various regulatory, management, and market-based instruments. The degree to which these can be implemented will also vary significantly in the Global North versus the Global South.

While these strategies are not expensive, they are also not cost-free. Redesigning existing streets to accommodate bike lanes requires public support and stakeholder engagement. Increasing tree canopy cover is also not without controversy. Trees produce volatile organic compounds (VOCs) that can have negative health effects. Some businesses may not want trees blocking their storefronts, and not all residents want street trees.

In all of the examples, public engagement is critical for uptake, buy-in, and successful design of infrastructure that people use.

Established vs. Rapidly Developing Cities

The opportunities and actions to transition to net zero carbon will depend on the level and stage of urban development.

For new and rapidly growing cities (most of which are in developing countries that are early in their urbanization process), there is an opportunity to design the built environment and infrastructure to avoid higher future GHG emissions. Equally important is that these emerging cities avoid infrastructural, institutional, and behavioral carbon lock-in that creates collective inertia.

In established cities, making them walkable and bikeable is not enough. To achieve deep decarbonization, they need to make systemic changes that include net zero electricity grids and electrification of transport and heating.

Major transformations in the power sector are already taking place. Many cities have incorporated district energy (centralized energy sourcing for multiple buildings in an area) in new infrastructure projects. Such a system achieves economies of scale and can also reduce GHG emissions if the source is renewable energy or waste heat. For example, Tokyo’s district energy system uses waste heat and incinerated waste. However, it can be difficult to install new underground networks in cities with established infrastructure.

Social Engagement

Ultimately, achieving deep decarbonization and net or near-net zero emissions will require behavior changes. The most recent IPCC report estimates that reducing demand through shifts in behavior and social norms can reduce global emission by as much as 40–70 percent by 2050 (Creutzig et al. 2022b).

For example, the energy use of a building is determined not only by how the building is designed and constructed but also by the behavior, norms, and culture(s) of its occupants and location. Among other things, the growing global standardization of room temperatures is shaping energy use in buildings (Shove 2003). What used to be a large range of air temperatures considered comfortable and acceptable is now narrowing as people spend more time in temperature-controlled environments such as vehicles, homes, offices, and stores.

Public awareness campaigns can make a difference in changing norms around cooling and comfort.

Public awareness campaigns can make a difference in changing norms around cooling and comfort. The “26 Degree Campaign” launched by environmental NGOs in Beijing during the summers of 2004 and 2005 successfully encouraged the public, especially hotels, restaurants, and business offices, to keep air conditioners set at 26°C (about 79°F) or higher (Xie 2011).


The clock is ticking to transform cities to net zero. To achieve this goal, cities will need to undertake multiple mitigation strategies simultaneously and as quickly as possible. Some of these will be high-tech, others will require significant political will and public engagement more than technological innovation.

The scale of the challenge is daunting. Anything short of transformative change in cities will not bend the emissions curve fast enough to avert disastrous climate consequences.


Chertow M. 2000. Industrial symbiosis: Literature and taxonomy. Annual Review of Energy & the Environment 25:313–37.

Churkina G, Organschi A, Reyer CPO, Ruff A, Vinke K, Liu Z, Reck BK, Graedel TE, Schellnhuber HJ. 2020. Buildings as a global carbon sink. Nature Sustainability 3:269–76.

CNCA [Carbon Neutral Cities Alliance]. 2018. Framework for Long-Term Deep Carbon Reduction Planning. San Francisco.

Creutzig F, Niamir L, Bai X, Callaghan M, Cullen J, Díaz-José J, Figueroa M, Grubler A, Lamb WF, Leip A, & 16 others. 2022a. Demand-side solutions to climate change mitigation consistent with high levels of well-being. Nature Climate Change 12:36–46.

Creutzig F, Roy J, Devine-Wright P, Díaz-José J, Geels FW, Grubler A, Maïzi N, Masanet E, Mulugetta Y, Onyige CD, & 3 others. 2022b. Demand, services and social aspects of mitigation. In: IPCC, 2022: Climate Change 2022: Mitigation of Climate Change, eds. Shukla PR, Skea J, Slade R, Al Khourdajie A, van Diemen R, McCollum D, Pathak M, Some S, Vyas P, Fradera R, & 5 others. Cambridge University Press.

EPA [Environmental Protection Agency]. 2021. From Farm to Kitchen: The Environmental Impacts of US Food Waste (EPA/600/R-21/171). Washington.

Ewing R, Meakins G, Hamidi S, Nelson AC. 2014. Relationship between urban sprawl and physical activity, obesity, and morbidity: Update and refinement. Health & Place 26:118–26.

Faghih-Imani A, Hampshire R, Marla L, Eluru N. 2017. An empirical analysis of bike sharing usage and rebalancing: Evidence from Barcelona and Seville. Transportation Research Part A: Policy & Practice 97:177–91.

Gurney KR, Kılkış Ş, Seto KC, Lwasa S, Moran D, Riahi K, Keller M, Rayner P, Luqman M. 2022. Greenhouse gas emissions from global cities under SSP/RCP scenarios, 1990 to 2100. Global Environmental Change 4:102478.

Harris S, Weinzettel J, Bigano A, Källmén A. 2020. Low carbon cities in 2050? GHG emissions of European cities using production-based and consumption-based emission accounting methods. Cleaner Production 248:119206.

IPCC [Intergovernmental Panel on Climate Change]. 2018. Global warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty, eds. Masson-Delmotte V, Zhai P, Pörtner HO, Roberts D, Skea J, Shukla PR, Pirani A, Moufouma-Okia W, Péan C, Pidcock R, & 9 others. Cambridge University Press.

Jacobs J. 1969. The Economy of Cities. Random House.

Lee S, Lee B. 2020. Comparing the impacts of local land use and urban spatial structure on household VMT and GHG emissions. Transport Geography 84:102694.

Leinberger CB, Rodriguez M. 2016. Foot Traffic Ahead: Ranking Walkable Urbanism in America’s Largest Metros. George Washington University Center for Real Estate and Urban Analysis.

Litman TA. 2003. Economic value of walkability. Transportation Research Record 1828(1):3–11.

Lwasa S, Seto KC, Bai X, Blanco H, Gurney KR, Kılkış Ş, Lucon O, Murakami J, Pan J, Sharifi A, Yamagata Y. 2022. Urban systems and other settlements. In: IPCC, 2022: Climate Change 2022: Mitigation of Climate Change, eds. Shukla PR, Skea J, Slade R, Al Khourdajie A, van Diemen R, McCollum D, Pathak M, Some S, Vyas P, Fradera R, & 5 others. Cambridge University Press.

Marqués R, Hernández-Herrador V, Calvo-Salazar M, García-Cebrián JA. 2015. How infrastructure can promote cycling in cities: Lessons from Seville. Research in Transportation Economics 53:31–44.

Minx J, Baiocchi G, Wiedmann T, Barrett J, Creutzig F, Feng K, Förster M, Pichler PP, Weisz H, Hubacek K. 2013. Carbon footprints of cities and other human settlements in the UK. Environmental Research Letters 8:035039.

Nowak DJ, Greenfield EJ, Hoehn RE, Lapoint E. 2013. Carbon storage and sequestration by trees in urban and community areas of the United States. Environmental Pollution 178:229–36.

Pivo G, Fisher JD. 2011. The walkability premium in commercial real estate investments. Real Estate Economics 39:185–219.

Pregitzer CC, Hanna C, Charlop-Powers S, Bradford MA. 2021. Estimating carbon storage in urban forests of New York City. Urban Ecosystems 25:617–31.

Qin B, Han SS. 2013. Planning parameters and household carbon emission: Evidence from high- and low-carbon neighborhoods in Beijing. Habitat International 37:52–60.

Ramaswami A, Tong K, Canadell JG, Jackson RB, Stokes E(K), Dhakal S, Finch M, Jittrapirom P, Singh N, Yamagata Y, & 3 others. 2021. Carbon analytics for net-zero emissions sustainable cities. Nature Sustainability 4(6):460–63.

Rapoport E. 2014. Utopian visions and real estate dreams: The Eco-City past, present and future. Geography Compass 8(2):137–49.

Rauterkus SY, Miller NG. 2011. Residential land values and walkability. Sustainable Real Estate 3(1):23–43.

Ringen K. 1979. Edwin Chadwick, the market ideology, and sanitary reform: On the nature of the 19th-century public health movement. International Journal of Health Services 9(1):107–20.

Saunders LE, Green JM, Petticrew MP, Steinbach R, Roberts H. 2013. What are the health benefits of active travel? A systematic review of trials and cohort studies. PloS One 8(8):e69912.

Seto KC, Churkina G, Hsu A, Keller M, Newman PWG, Qin B, Ramaswami A. 2021. From low- to net-zero carbon cities: The next global agenda. Annual Review of Environment & Resources 46(1):377–415.

Shove E. 2003. Comfort, Cleanliness and Convenience: The Social Organization of Normality. Berg Publishers.

Stevenson M, Thompson J, de Sá TH, Ewing R, Mohan D, McClure R, Roberts I, Tiwari G, Giles-Corti B, Sun X, & 2 others. 2016. Land use, transport, and population health: Estimating the health benefits of compact cities. Lancet 388(10062):2925–35.

Wiedmann T, Chen G, Owen A, Lenzen M, Doust M, Barrett J, Steele K. 2021. Three-scope carbon emission inventories of global cities. Industrial Ecology 25(3):735–50.

Xie L. 2011. China’s environmental activism in the age of globalization. Asian Politics & Policy 3(2):207–24.

[1]  For a comprehensive treatment of how cities can achieve net zero carbon, see Seto et al. (2021) or Ramaswami et al. (2021).

About the Author:Karen Seto (NAS) is the Frederick C. Hixon Professor of Geography and Urbanization Science, School of the Environment, Yale University.