Cities are an increasingly important factor in the battle against climate change. Urban areas are responsible for the majority of the greenhouse gas (GHG) emissions around the globe, and accelerated urbanization due to the rapid migration of people to cities will only drive up these emissions.
According to a 2018 special report published by the United Nations Intergovernmental Panel on Climate Change, the world needs to become carbon neutral by 2050 to limit global temperature increase to 1.5% above pre-industrial levels. Research shows that if we don’t, there will be irreversible changes in the Earth’s climate system and major ecosystems around the world. We will see more frequent and severe heat waves on land, increasing risk of extinction for many animal and plant species, and almost complete eradication of the world’s coral reefs.
Since urban areas account for such a significant share of the global GHG output, it is essential that we optimise our cities to reduce their energy consumption and total emissions.
View from the deck of Mori building in Roponngi Hills, Tokyo. Credit: @thetalkinglens.
View from the Griffith Observatory, Los Angeles. Credit: @henning.
So, if you are a city planner, how do you make your city more energy efficient and reduce its carbon footprint? Comparing some major cities around the world is a good place to start; the existing literature on sustainable planning shows that denser cities where people live close to the city center in multi-home buildings (a prime example would be Tokyo) tend to have lower emissions compared to sprawling metroplexes where residents live in larger, suburban houses and travel long commute distances (think Los Angeles or Houston).
There are two reasons for this. First, since each vehicle kilometer traveled (VKT) has an associated carbon output linked to the fuel efficiency of the car, longer daily travel distances (unsurprisingly) increase emissions. Second, since electricity production is quite a carbon-intensive affair, larger homes with higher power consumption rates also increase emissions.
"Altering the urban form of a city becomes a necessary task...in the endeavor to reduce its GHG emissions."
So we have learned that the urban form of a city (which we can measure using parameters like population density, home size, building height, and city radius) significantly affects its carbon footprint. Altering the urban form of a city becomes a necessary task, then, in the endeavor to reduce its GHG emissions.
But a problem of this magnitude does not have a short, simple answer. Rather, it requires a multitude of strategies and policy instruments aimed at different kinds of urban form improvements, such as lowering density, increasing green spaces, promoting cycling, to name a few. To be successful, having a clear vision and the political consensus and financial power to execute it is essential.
One strategy we can use is direct land use regulation—laws prohibiting urban development beyond a certain distance from the city center and construction of buildings above a certain height. While this approach can be highly effective, cities often find that the political consensus and public support to implement such sweeping policies is not so easily fashioned.
This is where public transit comes in. Public transit (like buses, trains, or trams) can play an important role in the effort to reduce a city’s carbon footprint, not only because it reduces VKT by offering residents an alternative for their daily commute, but also because it can serve as a catalyst to organically transform the urban form of a city.
Earlier this year, we developed an analytical model at the University of Texas at Austin to quantify this very phenomenon. Setting aside land use regulation, we can actually find out how the urban form (and thereby, emissions) of a city would respond naturally to the introduction of public transit by focusing on people’s economic choices.
Credit: The University of Texas, at Austin
The model goes something like this: Imagine a circular city with a central business district where every resident commutes to work. The further out from the city center you go, houses get bigger and more spread out, simulating a common, monocentric metroplex with suburbs. Now, we introduce a single public transit line passing through the business district and extending out to the city limits, forming a diametric axis across the city. If you are a resident of this city, you now have a commuting choice— you can either (1) continue driving to work and change nothing, or (2) drive to the nearest station and take public transport to work.
Illustration of the stylized city with the central business district (CBD) in the middle and an east-west public transit axi extending from one end to another. A resident living at point P can either drive to work (r) or drive to the nearest station and take the transit line (x +y).
If we make some simplifying assumptions about the homogeneity of residents across the city, the commuting choice simply becomes a cost comparison between the two available options. If taking public transit to work is cheaper, you will switch. If not, you will keep driving. This divides the city into “public transit-preferring” and “automobile-preferring” zones.
If we use the fact that things like population density and average home size and building height depend on the residents’ commuting and housing choices and vary by location, we can solve for these measures of urban form in order to develop a picture of what the city looks like post-public transit. What we then find is exciting.
"...introducing a single linear public transit axis across the city will naturally lead to a certain degree of development along the transit line, even without any kind of regulation."
As we move further away from the city center and the public transit axis, housing prices fall and so consumers choose larger homes, while on the supply side the rental cost of land decreases and, as a result, buildings become shorter and more spread out. And as buildings get shorter and each individual dwelling contained within gets larger, it follows that fewer dwellings (therefore, people) can fit inside an acre of land. So population density decreases as we move away from the city center and transit lines.
In other words, introducing a single linear public transit axis across the city will naturally lead to a certain degree of development along the transit line, even without any kind of regulation. People will start moving towards the transit line, creating higher density corridors along the axis.
That is very promising since higher density is good for emissions, but there is some bad news. The lower transport cost offered by public transit also acts as an incentive for residents to live further out from the city center and even attracts migrants to the city from outside, extending the city area in these regions and increasing its population.
Simulations of the city before and after the introduction of public transit, with the population density pattern shown over all locations within the city. The second image is divided into public transit-preferring (PTP) and automobile-preferring (AP) regions. In the second image, notice how the city extends beyond its original limits along the transit lines.
A population increase is a problem. Simulating the carbon emissions arising from the predicted urban form shows that this unforeseen population surge ends up being quite powerful, as total city emissions actually rise after the introduction of public transit. This means that, in order to limit the resulting urban sprawl and population influx, city planners have to be mindful of two things: determining an appropriate length for public transit lines and not building too far from the city center.
"The chosen mode of public transit must be about half as carbon-intense...as driving, otherwise even per capita emissions can actually rise."
What’s more, per capita emissions (total emissions divided by total population) do decrease with public transit, but this only happens under one condition. The chosen mode of public transit must be about half as carbon-intense (twice as clean, if you will) as driving, otherwise even per capita emissions can actually rise.
This means that planners need to carefully examine the emissions intensity of their public transit mode options (alongside their investment and operational costs) before making a decision. Electric rail, for instance, would result in much greater reductions than diesel bus but it also costs a lot more.
So, what does this all mean for the real world?
To reduce our carbon output to net zero by 2050 and ensure a habitable future for all life, we need to make expedient and drastic changes across virtually every aspect of human civilization. This will include transforming the physical shape and structure of our cities to make them more sustainable and energy efficient.
We can undoubtedly say that public transit is a powerful tool for organic transformation (without the brute force of regulation), but as we’ve seen it can also encourage counterproductive urban sprawl. We need to make radical changes, and soon, but we must be measured in our approach; without careful planning and precise decision-making, even well-intentioned strategies can have negative consequences of their own.