In a world of sprawling growth and exurban development, urban planners and environmentalists typically praise dense downtowns—full of people and skyscrapers—as models of efficiency and sustainability. More people on a smaller piece of land wastes less space, reduces the energy needed for transportation, and centralizes the provision of goods and services. Overall, it makes for a more environmentally friendly way to live.
But taller and denser isn’t necessarily better for the environment, according to a new study published in the journal npj Urban Sustainability. By studying the full lifecycle greenhouse gas emissions of urban development—from the production and transportation of the building materials to the energy required to use and live in buildings over time—an international team of researchers has found that high-rise cities are actually producing more total emissions than shorter, but still dense, urban areas.
Taking into account the full lifecycle emissions of urban development, the study finds that high-density low-rise cities are more environmentally friendly than high density high-rise cities. (They’re both still better than the suburbs.) Paris, for example, with its mostly five- and six-story buildings, produces fewer overall emissions than both sprawling exurbs and skyscraper cities like Hong Kong. It’s the Goldilocks zone of urban density and height—just enough to get the efficiencies of urban living but not so much that the resulting emissions wipe out the other sustainability benefits.
“We’ve always been looking at this problem from a building perspective,” says Francesco Pomponi, the study’s lead author and a professor at Edinburgh Napier University. “If you look at the building perspective and you analyze the footprint, of course a tall building is better. The high-rise building houses more people. But when you start looking at the bigger picture, you realize you cannot put two high-rise buildings as close as you can two low-rise buildings . . . To build tall, you need heavier structures, chunkier foundations and also, for a lot of good reasons like privacy, ventilation, and daylighting, high-rise buildings need to be further apart.” Given the land required to build tall buildings and the carbon-intense building materials like aluminum and steel it takes to construct them, a neighborhood of skyscrapers would result in about 140% more total emissions than a Paris-like lower-rise area with the same population.
“If we need more materials and the buildings need to be further apart, maybe it’s not so straightforward that we need to be packing high-rise buildings together,” Pomponi says.
The study compared the full lifecycle greenhouse gas emissions of four types of city developments, with either high or low population densities and high or low building heights. Using data from real European cities, and an algorithm to simulate 5,000 urban areas of various population densities and land areas, the study found that relatively short buildings of less than 10 stories, built in large quantities, are the optimal choice. Over the full lifecycle of the buildings, or about 60 years, this scale of development would incur about 365 tons of carbon dioxide per person less than a high-density high-rise alternative, Pomponi says. And even though more of these lower-rise buildings would be needed to match the population capacity of skyscrapers, using extra land to match that population still results in lower carbon emissions than what it would take to build taller. Pomponi and his coauthors acknowledge that their model only goes so far; future studies should include the detailed emissions impact from transportation and other issues, they say.
Pomponi says these findings could be critical for ensuring that future urban development meets the needs of growing populations while reducing the overall environmental impact of building.
“Most of the global population that we’ll be housing through the end of the century is not going to be in Europe or the U.S., it’s going to be in Asia, China, India, and most parts of Africa,” he says. “If we start planning for neighborhoods that are more dense and not high-rise, that will create cities over time that are on average more dense but not high-rise. It’s important to start embedding this in urban planning.”
These findings are not an argument for turning every city in the developing world into a version of central Paris, with dense blocks of repetitive six-story buildings, but Pomponi argues that having a deeper understanding of the long-term environmental impacts of urban development should begin to influence what gets built or not.
“Each building should not be identical to the next with a very fixed and prescribed height. It’s more about having an upper threshold that, unless you’ve got a really, really good reason, it should not be exceeded,” he says. “This will ensure that over time we get a diverse-built environment that doesn’t trigger the environmental costs of building tall.”