Our planet is growing warmer each day as a result of global warming and climate change. In fact, the United Nations has said that human-induced climate change is the most pervasive threat to human beings and the environment at large. One of the contributors to climate change is the urban heat island effect, which impacts thermal conditions in cities.
What is the heat island effect?
The heat island effect occurs in all urban environments. Through urban development, infrastructure, such as buildings, roads and systems, replace natural land cover and greenery. These hard, dry surfaces tend to absorb and re-emit heat more than natural features like waterbodies and vegetation. This results in cities becoming pockets of heat called “heat islands.”
Heat islands are typically an average of 1-7 degrees (Fahrenheit) warmer than surrounding rural or semi-rural areas during the day and 2-5 degrees higher at night. Urban regions that are more humid and/or more densely populated typically experience higher temperature differences than outlying areas.
Though heat islands typically compare the temperatures of cities relative to their surroundings, temperatures can also vary within cities. Certain pockets of the city may be hotter than others. These are known as intra-urban heat islands. They are caused by the uneven distribution of infrastructure to natural features. For example, residential areas incorporate facilities like parks, gardens and ponds which result in them being cooler than commercial areas like downtown.
What factors cause urban heat islands?
There are five factors that cause the heat island effect. Each factor impacts urban areas differently based on the specific geographical, infrastructural and social factors of a place.
The first factor that causes heat islands is the limited quantity of natural features in urban spaces. Vegetation and waterbodies cool environments by providing shade, evaporating surface water and transpiring water from plant leaves to enhance humidity. In cities, hardscaping and surfaces like sidewalks, roads and buildings are dry and provide less shade than natural landscapes do. This results in increased temperatures because the lack of greenery and water prevents natural cooling.
Another factor that causes the heat island effect is man-made materials used in urban environments. Infrastructure made from metals, asphalt or concrete tends to reflect solar radiation much less than other bio-based or natural materials. They also tend to absorb and emit more heat compared to vegetation, water and natural materials. This can lead to heavy absorption of solar energy during the day, which is slowly emitted throughout the evening and nighttime, causing warmer temperatures.
Urban geometry pertains to the physical form of a building and the urban fabric. Tall buildings and narrow circulation like thin streets and alleyways found in heavily developed or populated areas result in less airflow and increase absorption and reemission of heat.
Heat can also be generated from human-related activities. This includes heat emissions from transport, building systems (such as those for heating and cooling) and industrial facilities, known as “waste heat”. Alongside the waste heat produced by these activities, there are also high levels of greenhouse gases emitted. This results in more damage to the environment and further warms the atmosphere, resulting in global warming and climate change.
Finally, local weather patterns and geographical location can impact urban heat islands. Clear weather can maximize the amount of solar energy that reaches urban environments and minimizes the heat that can be extracted into the atmosphere. Conversely cloudy weather and strong winds minimize the heat island effect. Regarding geography, natural features like mountains or the ocean can impact airflow in a region, which consequently impacts heat.
How do heat islands impact societies and ecosystems?
Heat islands have several negative impacts on people and their surroundings. Higher temperatures throughout the day contribute to the rising numbers of heat-induced illnesses and heat-related deaths. Urban heat islands can also impact water quality and harm organisms in aquatic ecosystems in the area. This is because water temperatures impact the health of aquatic life and temperatures that are too high can even be fatal.
To maintain thermal comfort as a result of the heat island effect, people usually turn to HVAC systems to keep cool. Unfortunately, this can further exacerbate the heat island effect by increasing energy demands, waste heat, air pollutants and greenhouse gases that make cities warmer.
How can we mitigate the heat island effect?
Since every city will always have its own heat island, efforts can be taken to plan cities appropriately to limit its effects. Urban areas can optimize natural features like land cover and vegetation. Additionally, city planners can account for appropriate circulation and transportation to allow for appropriate airflow and lessened emissions. This way, urban spaces can maximize the effects of natural, passive cooling to lessen the heat island effect.
Greenery and waterbodies are key to maintaining thermal comfort and moisture levels in environments. These include natural elements like street trees, parks, gardens and ponds. By factoring these elements into cities and even buildings (through green infrastructures like green walls and roofs) spaces can become more comfortable to live in. This also lessens the reliance on mechanical systems for cooling, which inevitably contribute to heat and greenhouse gas emissions that warm up urban spaces and the atmosphere.
Alongside natural elements for cooling the outdoors, the use of cooling systems to regulate interior thermal comfort is required too. Energy-efficient HVAC machines help reduce electrical loads and emissions, which can exacerbate the heat island effect. Additionally, industrial, district-level cooling systems are being increasingly trialed in places like Singapore, to limit the use of individual AC condensers which are emissions-heavy and inefficient. Further, architects can incorporate passive design strategies to maximize cool breezes and limit solar gain. This way, interiors can be naturally-cooled and dependence on mechanical systems can also drop.
Surface materials are one of the key contributors to the heat island effect. Limiting the use of asphalt, concrete and certain metals can help prevent excessive heat. However, using other natural materials to substitute these may not be appropriate or applicable in certain regions. In light of this, painting exterior surfaces white (such as rooftops, walls and roads) can reduce ambient daytime temperatures by nearly 40 degrees. Lighter-colored surfaces increase the albedo of a material, which is the percentage of solar radiation reflected by a surface. Instead of absorbing and re-emitting excessive amounts of heat, lighter-colored surfaces can prevent excessive heat from being expelled into the surrounding environments. This prevents an excessive heat island effect from taking place.
Building heat resilience
As cities and their populations continue to grow, the urban heat island effect is expected to increase concurrently. By building heat resilience through context-specific urban and architectural design, heat islands in cities can be maintained at appropriate levels. This maintenance is important as otherwise, excessive heat can contribute to increasing global temperatures and climate shifts.
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