Heat islands
What are urban heat islands and how do they affect cities
Why is it hotter in a large city than in a village just a few kilometres away? The answer lies in what are known as heat islands. This phenomenon arises from various factors: heat-emitting elements such as air-conditioning systems or the materials used such as tarmac amongst others. Furthermore, it exacerbates climate change and affects health. This phenomenon intensifies the effects of extreme heat in cities, increases energy demand and can worsen risks to health and the quality of urban life.
Climate change threatens both large population centres and natural environments. In fact, extreme weather events such as heatwaves or droughts are becoming increasingly common and cause health problems or disruptions to the water supply. It is worth remembering that more than half of the world's population already lives in urban areas, and despite a growing trend in some countries towards a return to rural areas and contact with nature, the United Nations predicts that cities will continue to see their populations grow in the coming decades. This influx is fuelling the expansion of megacities, making urban climate adaptation increasingly important.
What is an urban heat island?
Heat islands are a phenomenon that occurs in urban areas as they experience higher temperatures than the areas around them due to human activity. The main reason is the presence of structures, such as buildings, pavements and streets, that absorb more heat and release it more slowly than natural habitats such as forests, rivers and lakes. Added to this is the heat and contamination generated by traffic and industry. All of this exacerbates the consequences of climate change in towns and cities and lowers the quality of life of their inhabitants.
How is the heat island effect measured?
We can measure the heat island effect using various methodologies. The simplest method compares temperatures recorded at different observation points in the city centre and on the outskirts. Urban temperature sensors can also provide data. However, there is a risk that the samples may not be representative or may not be located at the points where temperatures are highest. One of the most accurate methods involves detecting heat using infrared satellite imagery. This provides a comprehensive and precise picture, which is not limited to recording atmospheric temperature but also surface temperature.
The urban heat island effect
The urban heat island effect is characterised by higher temperatures in cities than in their surrounding areas – villages, residential areas, agricultural areas – and is most pronounced at night, particularly in winter. The average annual air temperature in a city with a population of one million or more can be between 4°C and 6°C higher during the day than in its outskirts, while at night this difference can soar to as much as 10°C.
SEE INFOGRAPHIC: The urban heat island effect [PDF] Enlace externo, se abre en ventana nueva.
Main causes of heat islands
There are several causes that contribute to the generation of urban heat islands and raise the temperature in towns and cities. Of these, the most important are:
- Dark surfaces with high thermal conductivity. Asphalt – which, moreover, is used by a greater number of vehicles in urban areas – causes cities to absorb a greater amount of solar radiation than rural areas during the day and to release it at night.
- Lack of vegetation and green spaces. The absence of urban parks means that temperatures do not drop because of water evaporation from plants. Furthermore, fewer trees mean less shade so the ground in cities heats up more.
- Large buildings. Whether they are office blocks or residential buildings, the megastructures that dot cityscapes absorb heat and block the flow of wind, preventing cities from cooling down through air currents. This is why it is important to construct sustainable buildings that take these factors into account.
- Urban geometry. Narrow streets and tall buildings affect the flow of cooling air currents. In densely populated areas surfaces and structures become large thermal masses that do not dissipate heat efficiently.
- Human activities. Air pollution caused by factories, cars and air-conditioning units, among other sources, traps solar radiation, preventing heat from dissipating, which in turn raises the air temperature. Furthermore, anthropogenic heat is more pronounced in cities with larger populations.
Factors that can intensify heat islands
Certain factors can encourage the formation of heat islands. Clearer, calmer weather maximises the amount of solar energy reaching the city and minimises its outflow. High population density also increases the perceived temperature. Conversely, when there are strong air currents and clouds in the area the city is less prone to the heat island effect.
Consequences of heat islands
Urban heat islands have negative consequences for people's lives and the environment:
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Higher energy consumption
Temperatures in cities are higher, particularly on summer nights. The use of air conditioning increases demand for electricity, especially during heatwaves, and can put a strain on urban networks and drive up the costs associated with energy consumption.
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Impact on health
High temperatures can affect the health of city dwellers, causing general discomfort, respiratory problems, sunstroke, dehydration and fatigue, and can even lead to an increase in deaths from heatstroke.
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Increased air pollution
Heat islands worsen air quality because they increase energy consumption. As more fossil fuels are burnt, emissions of CO₂ (carbon dioxide) and other greenhouse gases such as SO₂ (sulphur dioxide) or NOx (nitrogen oxides) skyrocket, exacerbating the greenhouse effect.
This, combined with the thermal inversion – which traps greenhouse gases close to the ground – not only raises temperatures but also increases levels of photochemical smog. This haze is characterised by its brown or yellowish colour and forms when pollutants react with sunlight.
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Impact on the economy
According to a study published in Nature
External link, opens in new window. , the economic losses associated with urban heat island warming could account for 5.6% of the gross domestic product (GDP) of the affected cities by 2100. In the worst-case scenarios, losses could rise to 10.9% of GDP. -
Impact on vulnerable groups
Heat islands particularly affect older people, children, those with pre-existing health conditions and residents of neighbourhoods with limited access to green spaces, energy-efficient housing or adequate air-conditioning systems.
Solutions to heat islands
In order to minimise the impacts of urban heat islands it is necessary to commit to sustainable urban development, as has been called for by the UN via its New Urban Agenda. These solutions include:
- Bioclimatic architecture: in addition to the installation of renewable energy sources, such as solar panels, or green roofs to mitigate heat absorption, developers have created ultra-high-reflectance paints and materials that achieve passive cooling of surfaces without energy consumption.
- Sustainable infrastructure: replacing tarmac with cool, permeable pavements as well as incorporating networks of urban wetlands can harness water retention to reduce surface temperatures through evaporation.
- Eco-neighbourhoods: these are new urban developments designed to reduce environmental impact by changing residents' lifestyles and creating climate-resilient spaces with vegetation and water features. To this end they incorporate more green spaces, regenerate public spaces and promote environmental education.
- Green corridors: these areas, which also serve various social, cultural or sporting purposes, connect the city's natural areas with extensive strips of vegetation. It is also worth mentioning elements such as urban gardening.
Nature-based solutions
Sustainable urban development can also be supported by nature-based solutions, such as green roofs and green façades. However, urban heat island effects cannot be tackled solely through infrastructure renewal, but also by promoting urban renaturalisation and creating more shaded spaces. Furthermore, the restoration of riverbanks and the creation of new floodplains or urban wetlands help to mitigate heat in cities.
- Sustainable mobility: the widespread electrification of vehicles and the promotion of sustainable modes of transport such as bicycles and scooters help to reduce the emissions that exacerbate heat islands.
- Complementary green policies: taxes on CO₂ emissions, other greenhouse gases and pollutants are designed to penalise behaviour that is harmful to the environment and help offset the externalities of cities' economic development.
Reducing the urban heat island effect requires a comprehensive approach that combines thoughtful urban planning, energy-efficient infrastructure, sustainable transport and more green spaces. Beyond mitigating urban heat, these measures must ensure better health, air quality and well-being for city dwellers. Increasing urban greenery purifies the air and reduces illnesses exacerbated by heatwaves. While technological innovation and the integration of renewable energy are key to combating the urban heat island effect, infrastructure and green spaces help to restore the ecological balance of cities.