Why Cities are Hotter than the Rest of the World

Do you know why big cities are usually most sensitive to climate issues? Naturally, it makes sense to argue that this is because most people live in cities. From there, it is quite logical to assume that a larger number of people will suffer compared to rural areas. Although this is definitely true, it is still a bit too simple of an explanation. In case of flooding, for example, we also need to take into account that many metropolitan areas are located close to rivers and other flood-sensitive areas. There are also geographical factors that we shouldn’t forget about.

When we think of flooding, intensified heat waves and drought are on the other side of the coin. There is another weather phenomenon that we simply don’t discuss as often: the urban heat island effect. This is why cities are hotter. Bigger metropoles are getting significantly warmer than other parts of the world, and it’s becoming a serious problem. Places like Delhi, Beijing, Johannesburg, New York, and Paris are already about 1-3 degrees (Celcius) hotter than surrounding areas.

Why cities are hotter than the rest of the world
Central Barcelona on a sunny day (Photo: Roselinde Bon)

Urban Heat Island: Why We Can’t Ignore Architecture

Sometimes it is easy to underestimate the power of architecture. Our constructions can strongly influence the atmosphere, without us even realising the potential damage. We think about the polluted fumes from dirty factories and our carbon footprints when we take flights around the world. Rarely do we realise how architecture also plays a vital role in the ecosystem.

Houses, shops, and offices are excellent at trapping heat inside the city. What’s worse, skyscrapers are even better at it.

Human activity and urban heat is strongly connected to the number of buildings. Of course, urban areas contain more buildings than national parks or rain forests, that much is clear. But it’s not just about constructing a lot of buildings in one place, it’s also about how they have been designed. Modern buildings are mostly made to be very well insulated. Unfortunately, this also means that houses, shops, and offices are excellent at trapping heat inside the city. The fact that cities are full of immensely tall buildings, like skyscrapers, means the heat is trapped for even longer. It has become nearly impossible for the hot air to escape in either horizontal or vertical directions.

The Climate Curse of Concrete and Metal Materials

Clearly, the problem of trapped heat is closely connected to the type of materials we use to create our buildings. When we try to wrap our heads around global warming, these materials are even more crucial than you might suspect. The main cause for urban heat is actually the vast amount of concrete and metal. We use it for our roads, walls, roofs, gutters, and pavement. You name it. The issue with these particular materials is the fact that they are extremely good at absorbing the sun’s heat. This heat is then re-radiated in the urban atmosphere at night. Darker concrete material, like many highways, has an even lower albedo (the amount of light reflected), retaining the most amount of heat.

Darker concrete materials, like our roads and highways, trap an insane amount of urban heat.

Why cities are hotter than other parts of the world
Hot summer day in Venice, Italy (Photo: Roselinde Bon)

The Urban Desert: How A City Blocks out Water

Today, when we build or grow a city, most of the soil and natural vegetation is replaced by concrete. Apart from an obvious decrease in air quality (since less plants means less absorption of harmful fumes), this also means that water cannot reach through the pavement, for example. This leads to decreased evapotranspiration: concrete and metal structures limit vegetation cover, so the transferral of water vapour into the atmosphere is blocked by urban materials. This can ultimately result in increasingly hotter and dried up cities.

Possible solutions: Should We Transform into Forest Cities?

If you are wondering how we might cool down our cities, you are not quite the first. What could be our most ideal solution? The best option, of course, is if every country worked together to slow the process global warming. This political problem, however, is proving to be more difficult than we anticipated.

There are other architectural strategies that cities can implement on their own. Experts have suggested that rooftop farming may offer a solution, as it functions as natural insulation. The farms can cool and humidify the air around the building through local evapotranspiration. We would need to spend less energy on air conditioning, which would have otherwise produced even more excess heat.

China is taking this idea to ‘re-plantify’ the city to a whole other level with its new mission to build a Forest City.

China is taking this idea to ‘re-plantify’ the city to a whole other level with its new mission to build a Forest City. This ultra green city will hopefully absorb many more pollutants than a regular city and generate tonnes of extra oxygen. There are ambitious plans to finish the Liuzhou Forest City by 2020, located near Guangxi. The new city would include homes for 30,000 residents, public transport, schools, and a hospital.

Park in Beijing on a summer day
Urban park at the Beijing Normal University campus in summer (Photo: Roselinde Bon)

What do you think might be a solution? Are forest cities a step too far, or is the concept an excellent example of how our cities need a serious makeover? Have you ever consciously thought about growing more plants? Let’s discuss and let us know what you think in the comment section.

Sources: [1],[2],[3]

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