Urban planning is becoming more dynamic as cities try to prioritize the livelihood and wellbeing of residents while accommodating changing climates, rising sea levels, and city growth. But according to a new study by MIT, the modular way that many cities are built today insulates more heat, which renders them less sustainable, often exacerbating what is better known as the urban heat island (UHI) effect. The urban heat island effect, which is created as a result of urban areas absorbing more solar energy than nearby rural areas, makes cities hotter than surrounding exurban and rural areas. 

The study, which was published on February 22 in the Physical Review Letters journal, points to how the ‘texture’ of a city, whether cities are built with a crystalline or glass-like structure, ultimately determines the urban heat island effect in cities.

The researchers found that cities like New York and Chicago, for example, which are built according to a ‘crystalline’ or grid-like layout, increases the urban heat island effect. Cities like Boston or London, on the other hand, which are more sporadically built, with buildings placed more like atoms in disarray like in glass – hence the ‘glass-like’ structure – retain less heat. The study found that cities with a modular crystalline structures had higher chances of insulating heat as per the UHI effect than cities with glass-like layouts.

In addition to texture, the dominant materials used in building a city, like asphalt or concrete, insulate more heat. And since these cities are made up of buildings that retain heat, the excess heat released after the sun sets makes cities hotter than usual after nightfall.

Alongside MIT, the research used for the study was conducted by Roland Pellenq and T.Emig from the lab <MSE>2, professor of civil and environmental engineering Franz-Josef Ulm, M.J. Abdolhosseini Qomi and research assistant Jacob Sobstyl.

Although the study’s findings are new, the concept of the urban heat island effect dates back to 1810. The study, however, helps shed light on how the materials with which cities are built can increase the UHI effect. The dominance of materials like asphalt and concrete in city-building means that heat insulation is lessened in cities that have much more vegetation as opposed to concrete-reinforced structures or an abundance of concrete-paved roads.

In Pheonix, Arizona, the UHI effect can cause an increase in temperature of up to 10° Fahrenheit (up to ~6° Celcius) after nightfall. UHI is known to increase nighttime temperatures and pose potential health risks, including serious cardiovascular and respiratory diseases. The urban heat island effect often results in increased use of air conditioners as well, which can increase greenhouse gases, pollution, and even damage water quality.

In order to come to these findings, the researchers used statistical physics to better understand how the layout of a city and its building materials can cause or worsen an already-existing UHI effect. They used mathematical formulas to calculate how individual atoms in a building material were impacted by others around it. Once an adequate amount of data about different types of buildings was recorded, the researchers compared patterns across 47 cities and created a scale from zero to one to indicate a statistical description of how cities were laid out.

On the scale, zero represents what they are calling ‘total disorder’ and one represents ‘perfect crystalline structure.’ Once their survey of cities was finished, the researchers found that the majority of the cities fell somewhere in between 0.5 and 0.9 on their scale.

If urbanists understand how cities can be designed to insulate less heat, they can design more sustainable cities. The researchers in charge of the MIT study, however, believe the UHI effect can be used to the benefit of cities that experience harsh winters. “If you’re planning a new section of Phoenix,” Pellenq said to MIT News. “You don’t want to build on a grid, since it’s already a very hot place. But somewhere in Canada, a mayor may say no, we’ll choose to use the grid, to keep the city warmer.”

Exactly how urban heat islands work and how to curb their effect is a process that requires additional and ongoing research. For example, a 2016 study of how green roofs can mitigate the urban heat island effect in Chicago reported surprising results. While the researchers found that green roofs did reduce the overall heat island effect in Chicago, the sites where the rooftops were placed was key to determining just how effective and sustainable they would be.

While the results of the MIT study on how city layout can address heat island effect in warm climates, they do not change the fact that cities are more likely to being built with a crystalline, modular layout. This is primarily because grid-like layouts make it easier for pipes, sewage systems, and general infrastructure to be laid out across the entire city.

What the study suggests is a modification to this modular archetype of cities everywhere from China to the United States to work towards building future cities that can lower emissions, reduce costs such as those incurred by air conditioning, and create overall cooler cities.

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