Creating Climate-Smart Cities

The world is adding a new city of 1 million people every 10 days, which Seto says provides an opportunity to address climate challenges through design.

Using satellite data, Seto and a research team conducted a global analysis of urban development. They found that in the Global South urban growth tends to sprawl outward rather than climb upward (which can lead to more carbon-intense issues). The study, published in PNAS, also found that per capita, the built-up infrastructure in some countries in the Global North is more than 30 times higher than infrastructure in the Global South.

One of the most cost-effective ways to reduce emissions is to have higher densities of housing closer to higher densities of amenities and needs, such as jobs, says Seto, who was coordinating lead author for the IPCC 5th and 6th U.N. climate assessment reports and the chapters on cities. Seto, along with Arianna Salazar-Miranda, assistant professor of urban planning and data science, have studied the benefits of colocation, including the “15-minute city,” which is based on an old urban planning concept prioritizing city design around people, proximity, and walkability.

“When offices, schools, parks, and shops are located within a 15-minute walk or bike ride from people’s homes, there’s less driving, lower emissions, more green space, and more community coherence,” Seto says.

Pioneered in Paris, the 15-minute city is a characteristic of many European and East Asian urban areas, including Tokyo, Taipei, and Prague. Adapting this model to the car-centric U.S., where people typically travel long distances even to pick up a quart of milk, poses unique challenges. In a study published in Nature Human Behavior, Salazar-Miranda analyzed GPS data from 40 million mobile devices and found that only 14% of daily consumption trips in the U.S. are made locally.

“Americans have grown used to a world where even routine errands become epic road trips,” Salazar-Miranda says.

Yet, there’s a fix. Cities can incentivize walking by design. In her “Desirable Streets” study, published in the journal Computers, Environment, and Urban Systems, Salazar-Miranda constructed a desirability index for Boston based on the analysis of thousands of pedestrian trajectories. The study demonstrated that not only will people walk, but they’ll opt for even longer pedestrian alternatives if there are parks, sidewalks, and outdoor seating en route.

However, adopting the 15-minute model in the U.S. is hindered not only by a car-centric culture but also by zoning. She analyzed the zoning codes of 3,000 municipalities across the U.S. and found that they separate residential, commercial, and industrial areas, encouraging sprawl. Form-based codes, which regulate the design of buildings and their relationship to streets and public areas, improve walkability, reduce car dependency, and promote social interaction. They are being used in cities across the U.S., including Denver, Miami, and Beaufort, South Carolina.

“This is a promising avenue for city planners to explore because it shows that even in the U.S., we can reshape cities to be less car dependent and more sustainable,” she says.

Changing the way buildings are designed and constructed so they absorb less heat will also help reduce emissions, Seto notes. This requires strengthening building codes to require more climate-friendly materials, such as cross-laminated mass timber, which store carbon and have a lower carbon footprint.

In a study published in Renewable and Sustainable Energy Reviews, Rao found that in the Global South, changing the building envelope — the physical barrier that separates the inside of a building from the outside — along with painting rooftops white can reduce indoor temperatures and heat stress exposure by 98%.

“All of these passive forms of urban design can obviate the need for air conditioning and allow cites to grow efficiently in a way that enhances people’s well-being,” Rao says.

If current land use trends continue, urban energy use could increase more than threefold by 2050 from 2005 levels, Seto warns. Currently, urban areas consume between 60% and 80% of global energy resources, and three-fourths of the infrastructure that will exist by 2050 is yet to be built. Recognizing the need to reduce emissions as urban areas grow, 800 cities have made commitments to become net-zero carbon cities.

The path, however, has not been easy. The transition to decarbonization requires systems change on both macro- and micro-levels, including electrifying city and school bus fleets; building out infrastructure for EVs; encouraging public transportation; constructing energy-efficient buildings and housing units; retrofitting current units with green energy technology such as heat pumps; utilizing district energy networks that heat and cool multiple buildings through underground pipes; and harvesting industrial waste that can also heat and cool residential buildings in cities (known as urban industrial symbiosis).

Cities may not be able to achieve all this at once, but they are moving toward low-carbon solutions.

“Hundreds of experiments are happening at the local level all around the world to mitigate and adapt to climate change,” Seto says.

In Copenhagen, Denmark, and in Framingham, Massachusetts, district energy networks are heating and cooling homes with pipes that circulate water through a system of boreholes that extend deep into the earth. In San Francisco, a Hixon Center case study found that the city’s parking cap, which limited parking to one space for every two to four residential units in neighborhoods with public transit, has dramatically increased the use of public transportation there.

Transit produces almost a quarter of global carbon dioxide emissions, and as urban areas expand, 1 billion more cars will be in use. Many cities already have electrified light-rail trains and buses and are considering further electrifying transit systems to save money and reduce air pollution.

In Bogotá, Colombia, Felipe Ramírez decarbonized the bus fleet — the largest initiative outside of China — while he was serving as secretary of mobility for the city. During a Hixon Center convening of urban leaders on sustainable urban mobility in 2024, Ramírez, who is now urban mobility director at the World Resources Institute, recounted his efforts to decarbonize the city’s transit and energy sectors. His talk is part of the Hixon Center’s toolbox of case studies that it makes available to professionals, policymakers, and the public.

“Cities are still building infrastructure for cars rather than for where we all move in a massive way, which is, again, walking, biking, or in public transport systems,” Ramírez says.

Bogotá updated its fleet with 1,485 electric buses, reducing CO2 emissions by 155,000 tons per year. It established a day without cars; built out 6,000 kilometers of exclusive bike lanes with a goal of adding more; established 50,000 spots for bicycle parking; developed greenways; and initiated a plan to build five new metro train lines by 2035. Bogotá’s plan also includes developing a train system and connecting bike and pedestrian lanes.

“What we need in our cities,” says Ramírez, “is networks of multimodal and sustainable mobility at differing scales. You should not just plan for the next five years. You should plan for the future.”

In his research, Kenneth Gillingham, professor of environmental and energy economics, found that access to public transit and walkable amenities plays an important role in how much households drive. In a study of commuters in Denmark published in the Journal of Urban Economics, Gillingham compared areas with excellent public transit to regions with poor public transit. Access to public transit greatly influences the decisions by commuters about whether to switch to mass transit when fuel prices rise, he says.

If cities are looking to incentivize more electric vehicle ownership, the most cost-effective approach would be to install more public charging stations for private vehicles, he adds. Many cities, including London, have lower-cost Level 2 charging stations available in street parking, which makes owning an electric vehicle more feasible for those living in apartments, Gillingham notes.

Replacing fossil fuel-powered transit with electrification may not be the answer in every city. Globally, only 37% of people even have access to public transit. A World Resources Institute (WRI) study advised two criteria for moving forward with investments in electric power: 90% of residents must have access to electricity, and electrifying should not produce more emissions. If a grid is powered largely by fossil fuels such as coal and natural gas, greater use will only increase CO2 emissions, the study notes. Based on those criteria, WRI advised that electrification is a good strategy for Latin America, but not for sub-Saharan Africa. WRI identified 256 cities with populations over 1 million in 38 countries where electrification may not make sense.

Three Cairns Fellow Jack Omondi, a participant in YSE’s Urban Climate Leadership Certificate Program, who coordinated a study on decarbonizing transportation for the Network of African Science Academies, says that many cities in Africa lack mass transit systems.

“We found we need African leaders to take a holistic and inclusive development approach to transportation that includes pedestrian walkways and cycling lanes,” he says. “While we don’t have many cars in Africa, most government planning is developed for car owners.”

From urban forests to vertical gardens to urban parks and tree cover, green spaces are critical to reducing emissions in cities. Trees and foliage combat the heat island effect and sequester carbon in urban areas.

“Trees are one of the most undervalued solutions to urban challenges,” says Mark Ashton, the Morris K. Jesup Professor of Silviculture and Forest Ecology and senior associate dean of The Forest School.

In lower-income communities with less access to air conditioning, the lack of tree canopy has exacerbated the impact of the heat island effect. A study of 10 cities in Connecticut conducted by YSE researchers and recently published in npj Urban Sustainability found that underserved communities experienced higher overall temperatures, more sweltering days, and more significant increases in heat exposure over a 30-year period.

“We see a strong correlation between redlining and neighborhoods in cities where there are fewer trees,” says Dorceta Taylor, the Wangari Maathai Professor of Environmental Sociology. “They tend to be predominately Black and Latinx neighborhoods.”

YSE’s Urban Resources Initiative (URI) has been working with the Seto Lab to pinpoint communities in New Haven that will benefit from tree planting. The lab developed spatially explicit data that depict communities most vulnerable to increasing heat. Since its inception, URI has planted more than 10,000 trees in New Haven.

Seto is also working with the city of Freetown in Sierra Leone, where Mayor Yvonne Aki-Sawyerr has pledged to increase tree cover by 50%, transforming Freetown to “Treetown.”

Aki-Sawyerr has witnessed the impacts of climate change — from mudslides in mountain areas to flooding in coastal regions to severe heat in the city — and she is determined to take steps to prepare the city for a more resilient future. She’s committed to planting up to 1 million trees.

In May, Aki-Sawyerr met with Seto in Freetown with a group of YSE students in the “Urban Climate Solutions” capstone class who are helping to evaluate and optimize tree-planting locations using geospatial data.

“A critical piece of this project is to bridge the science divide and to learn whether what you see as working in academia, works in the field,” Seto says. “You can identify cooling corridors using data. But that’s theoretical — especially if it turns out residents don’t want trees. The question is how to make climate change an issue for people in their everyday lives.”

In the Global South, Ashton notes, trees are used for more than shade. “For urban tree planting to be effective, you need to apply the science of place to a city and how that city developed in terms of its culture, geography, and climate. In many urban settlement neighborhoods, residents plant and maintain trees for both subsistence and medicinal purposes,” he says.

While the challenges cities face can be daunting, Seto emphasizes that if the 100 cities with the largest carbon footprint could reduce their emissions through design strategies that encourage less driving and more walking, planting more shade trees, and adopting energy-efficient heating and cooling systems, cities could become powerful catalysts for climate solutions.

Seto is among a group of leaders who are calling for urgent changes in the governance of urban expansion and advocating for the creation of an urban science panel and advisory system that can support urban policymaking at a global level.

“It’s about ensuring that world leaders and policymakers have the information that they need at their fingertips to design a world that reflects and responds to humanity’s urban future,” she says.