Why is it so Hot in Cities?

A look into the phenomenon of Urban Heat Island effect — the product of city infrastructure, architecture and climate change 

Last summer was recognized as the hottest ever recorded in Europe. This year, the climate phenomenon El Niño is making a comeback, projected to increase climate hazards like storms, floods and wildfires both in their frequency and intensity, alongside spiking the global temperatures throughout 2023 – 2024. 

Cities, as we have them, exacerbate effects of climate change and, thus, are particularly affected by certain weather extremes. Such, for instance, are heat waves that are becoming more common and more severe. Abnormally hot weather not only creates discomfort but poses huge risks to the health and wellbeing of over 4.4 billion urban residents, causing a mortality surge. Yet, at present, city infrastructure is not properly adapted to meet and mitigate the extreme temperatures. Addressing the organisation of our cities could help us understand how we can make cities cleaner, cooler and more comfortable. For this purpose, Nullker is taking a look into the phenomenon called the Urban Heat Island effect (UHI) that explains why it is so hot in cities. The invited experts Sofie Pelsmakers, Stefano Schiavon and Jonathon Taylor contributed to this article by sharing their knowledge on the Urban Heat Island effect and giving their take on what can make cities more comfortable.

Cities – Climate change – Urban residents

Climate change affects urban and rural areas differently. The disparities in cloud cover, air flow and heat release from human activities make cities and their populations vulnerable to certain impairments of climate change. The phenomenon, called the Urban Heat Island (UHI) effect, demonstrates just that: 

City organisation and infrastructure trap heat and create a particular urban climate that, amplified by climate change, results in large cities experiencing higher temperatures (1°C up to 10°C) compared to rural zones. 

The Urban Heat Island effect prolongs and intensifies heat waves, negatively affects the air quality, aggravates weather hazards, all of which have a significant impact on lives and health of city residents. Extreme temperatures create an additional stress on human physiology, worsening the existing health conditions and causing heat strokes, exhaustion, heat cramps and respiratory problems. Children, the elderly and low-income populations being particularly vulnerable to the influence of the UHI. Moreover, the UHI effect contributes to the heat-related mortality rate, which annually equates to 489 075 (global average between the years of  2000 – 2019), with 36,54% of deaths in Europe alone. 

Aside from directly affecting human health, the UHI effect indirectly worsens city conditions. High temperatures have an impact on energy demands, triggering the global surge in the cooling load to 13% – 23%. For instance, due to the UHI effect the cooling load of buildings in Athens goes double and almost triples their peak electricity demand. 

This, in turn, results in a significant increase of energy-induced CO2 emissions and other greenhouse gas emissions to power the system, further contributing to global warming and making cities more polluted and less comfortable.   

Therefore, the significance of the UHI ranges from health-related and meteorological to socio-economic impacts. Overall, its magnitude depends on many factors, such as climate, season and weather, the range of human activities, as well as, size of a city and urban density. However, particularly the architectural organisation of cities, their form and geometry, building materials and the extent of green areas have a potential to exacerbate or alleviate the UHI effect. 

Image taken from  Nuruzzaman, M. (2015). Urban heat island: causes, effects and mitigation measures-a review. International Journal of Environmental Monitoring and Analysis, 3(2), 67-73. 

How do architectural specificities of cities cause the UHI effect?

Overall, buildings are considered to be one of the largest energy consumers and greenhouse gases emitters. They consume 36% of global energy and produce 33% of global energy-related greenhouse gas emissions, which places them among the main indirect contributors to the UHI effect. The most common construction materials such as steel, aluminium, concrete and glass require large amounts of energy and resources for their production. The case study of Azadeh Sagheb et al. (2011) proposes using recyclable, locally available and more eco-friendly materials, alongside accounting for natural ventilation and light. All of which can significantly reduce the amount of CO2 emissions and make the cities less warm and less pollutant. For instance, the authors propose that using recycled glass can reduce CO2 emissions from the material by 50%, substituting stone with limestone results in a 37% reduction in emissions etc.

Additionally, concrete, stone, granite, steel, aluminium and glass, are known to absorb and store large amounts of solar radiation. Often deep dark in colour, these surfaces have low solar reflectance, significantly impeding the cooldown of cities. Specifically roof surfaces are known to play a key role. Taking up 20–25% of all urban surfaces, they can substantially reflect, absorb or alleviate solar radiation. Some studies demonstrate that reflective or, in other words, cool roofs with a reflectance higher than 0.7 showed themselves to be quite effective, decreasing indoor temperatures up to 2°C and reducing cooling loads up to 40%. 

Among other key aspects contributing to the UHI effect are city layout and geometry. As the urban population grows, increasing the need for infrastructure, cities become more dense, transforming a city into a web of narrow streets with high buildings. The produced city landscape – deep canyons and limited sky visibility (sky view factor) blocks airflow and traps heat. This, in addition, intensifies the energy crises, requiring more resources for the maintenance of thermal comfort and resulting in increased pollution. 

Moreover, UHI aggravates due to the diminishing of vegetation caused by the need for more space in the cities for habitats and infrastructure. Vegetation, however, plays a crucial role in thermoregulation, consuming solar radiation (of about 23% on average) for the process of photosynthesis, vaporising water and cooling the air. Besides, vegetation provides shade, mitigating heat stress, and absorbs CO2 largely produced by cars, construction, industrial sector etc. 

It has been found that expanding green areas in high-density residential zones by 10% could help maintain surface temperatures at or even below the baseline 1961–1990 level, while increasing the greenery by 30% could prevent around 2644 premature deaths in Europe. 

Sofie Pelsmakers, a chartered architect and environmental designer, a writer, a co-founder of an NGO and currently an Assistant Professor at the Aarhus School of Architecture in Denmark, believes in the need for cities to adjust – in rewilding of cities at all scales, passive cooling measures and cool zones, as well as in educating people on how to adapt to and act during heatwaves.

“The big issue here is that we need more green, permeable surfaces, more ponds / bodies of water, and open spaces, and more spaces between buildings so that hot air can dissipate instead of linger and build up between buildings, preventing dispersal of heat and pollutants. We need more reflective materials on buildings too. This is because the dark road surfaces and dark building colours absorb and later give back the heat that was absorbed during the day. Light and green surfaces reflect solar radiation back, and greenery and water bodies contribute to evaporative cooling and create different wind patterns; trees also give shade. This all can reduce temperatures by 2 – 3°C in green areas vs the ‘ungreen’ city”

According to Sofie Pelsmakers, the benefits of increasing areas of greenery expands beyond reducing the UHI effect:

“It [greenery] not only reduces UHI, but also therefore reduces active measures and mitigates climate change, while adapting to it. (E.g. if located when taking the prevailing wind into account it can also reduce winter heat loss as greenery can buffer and protect from cold winds). Greenery and soil can store carbon too”

“And, of course, there are also biodiversity and other well being benefits as access to green [...] encourages physical activity, social interaction and meets our biophilic needs of connecting with nature – good for psychological well being” 

Aside from rewilding cities, Dr Pelsmakers advocates for “passive cooling measures”: 

“Our work has shown, however, that many passive measures can reduce and prevent overheating risk (e.g. solar shading of windows, night-time cooling enabled by better openable windows, a simple fan that can help move air around can also increase comfort). And, of course, a well insulated and airtight building also reduces heat transfer in summer, not just in winter (but we need solar shading to ensure that the sun does not come inside through windows). 

The provision of safe, cool zones is also important during hot periods (e.g. a library that is cooled where people can take shelter in heatwaves – this is the case in hot regions in the USA, for example). The issue with active cooling measures is that it can further exacerbate climate change (as it uses electrical energy that is often fossil fuelled) and can also further exacerbate hot temperatures”

Jonathon Taylor, an Associate Professor in Urban Physics at Tampere University, based in the department of Civil Engineering, co-investigator in various projects and a contributor to the 2019, 2020 and 2021 Lancet Countdown on Climate Change and Health, believes that, at present, for the already built-up cities passive measures such as “cool roofs are the most effective, but with the caveat that this may depend very much on the city built form and the climate”. 

“[...]Cool roofs can reduce the outdoor temperatures as well as indoor temperatures by reflecting the solar radiation that would otherwise be conducted as heat into the building. Now, if we are going to be improving the energy efficiency of buildings the way that we should be, we would be adding more insulation to roofs. Then the insulation would limit the amount of heat that gets conducted indoors and the primary benefit from a cool roof would be outdoor temperatures. Whereas, perhaps greenspace doesn’t reduce the UHI quite as much, but it could provide shading that stops solar radiation from coming indoors. So, I am not sure we quite understand the cool roof vs greening argument that well in terms of the indoor environment, anyway. But, regarding the UHI – I think greenspaces are less effective than cool roofs for reducing the UHI, but we need to properly understand the benefits and disadvantages year-round, as well as the other consequences. Greening has so many other benefits, there are few disadvantages in trying to add as much greenspace as possible in cities” 

Dr Taylor broadly considers the UHI effect, taking into account energy efficiency, our understanding of sufficiency limits, needs for housing adaptations as well as benefits of the UHI for some cities.  

“If you take an active cooling approach using air conditioning systems, then you need energy efficient buildings to stop the loss of cooled air and a clean energy supply – otherwise you increase GHG emissions which will worsen the climate. The advantage of A/C is that it is the only solution that can ensure the building remains comfortable during really hot weather. The disadvantage is that it removes heat to the outdoor environment (worsening the UHI), the GHG emissions, and the fact that not everybody can afford it – leading to big inequalities in heat exposure. Otherwise, passive measures including shading and overhangs, cool roofs, shutters, green roofs, and energy efficiency adaptations. Energy efficiency has huge co-benefits – it helps reduce energy and GHG emissions from the building stock, reduces exposure to winter cold, and can lead to health benefits for occupants. Clean energy sources are also really important to reduce air pollution from fossil fuel burning power stations. These passive measures can be used alongside active measures to help reduce the demand for A/C – so they are not mutually exclusive. Greening is perhaps not as effective as other adaptations for reducing the UHI, but still offers significant benefits for health and wellbeing in cities”

“I think we need to be cautious about sweeping generalisations about rethinking our cities to combat the UHI. For example, in many cities in temperate and cold areas, cold is the greater risk to health than heat. Paradoxically, the UHI may actually provide some winter benefits for energy and health that might outweigh disbenefits from heat. So, for things like cool roofs, you need to properly understand the advantages and disadvantages for your city before you act. But other things like greening have a wide range of other benefits for health and biodiversity that it may be useful. If you were designing a new city from scratch, you would want to think about things like having plenty of greenspace and urban ventilation, which would provide benefits for more than just heat. But I think a focus on housing adaptations – e.g. through building regulations – might be more reasonable in existing cities”

“A lot of focus has been on urban-level adaptations that reduce the UHI, but the design of buildings has an important role to play in reducing heat exposure, given the large amount of time we spend indoors. In particular, adaptations could be specifically targeted to the population groups most vulnerable to the negative health effects of heat – such as the very old, those with pre-existing health conditions, and the very young. I also wonder about how we define heat and comfort. This can depend on the country and regulations, and there are various methods to do so. But, a lot of the focus has been on efficiency – achieving these thermal comfort standards (which are sometimes arbitrary) using as little energy as possible. Perhaps we can think more about sufficiency – how much are we willing to tolerate some periods of being too hot in order to save energy? Obviously, because heat can be a big health risk for some people, this may not work for everyone”

Stefano Schiavon, a Professor of Architecture and Civil and Environmental Engineering at UC Berkeley and the Associate Director of the Center for Environmental Design Research (CEDR) believes that increasing green and blue areas is an effective strategy for urban overheating, but the core one should be the energy transition:

 “Moving away from fossil fuels is the first step towards sustainability. Given that buildings are responsible for ~40% of the greenhouses gas emissions, we need to electrify them and produce electricity with renewable energies. Another important step is to increase the energy efficiency of buildings; we have been working on this topic for fifty years (since the Oil Crises in 1973). We have achieved many notable goals, from energy efficiency standards and incentives to effective solutions as LED and heat pumps”

In order to keep people comfortable outdoors, “the best ways [...] are to block the sun via shading and increase the air speed by allowing wind to flow or by using fans in semi-outdoor spaces”. Particularly, dr. Schiavon advocates for “fans as an alternative to air conditioning”:

“We spend most of our time in built spaces that substantially affect our health, well-being, and productivity. Conditioning the built environment has a large influence on climate change, most of which comes from the energy used to create indoor comfort. The need for cooling has been increasing globally, with most of it happening in tropical countries because of their economic and demographic growth. We need solutions to reduce the impact of air conditioning. Electric fans can be both an alternative to and augmentation of air conditioning. In a remarkable number of ways, they have the potential to simultaneously reduce energy use and increase thermal comfort. I summarized the work in this presentation and here there is a free online guidebook on this topic”

At the end, we asked Stefano Schiavon about the extent of potential of green areas. 

“Green areas have many benefits beyond the mitigation of UHI. The cooling effect of green areas is present but probably not the most important reason we want to increase green areas. Besides the ecological benefits, there are health benefits for people. One of the three pillars of health is physical activity, having green corridors increase the opportunities for outdoor activities. An excellent example are the "park connectors" used in Singapore”

Perhaps, in their emergence and expansion, cities feel utterly separate from the natural world. Nevertheless, urban environments have a direct influence on nature, largely contributing to climate change and exacerbating detriments of it. The UHI effect is one of the most studied climatic phenomena within urban environments. It has a broad range of impact on human lives, ranging from socio-economic, to health-related and meteorological effects. City organisation and architecture significantly contribute to the UHI, hence, there is a need to adapt and adjust our cities. The materials we use, the density of infrastructure, city layout and geometry as well as the amount of green space, all play a crucial role in determining what is to come for the climate, cities and their inhabitants. What we need is to rethink our cities, how we build them and what for. Most of all, we need to make nature an integral part of a city rather than a privilege. 

Do you feel like your city gets warmer and warmer? How do you adapt? 

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