Heat islands are cropping up across the globe. Discover more about this phenomenon and how they can wreak havoc for humans and habitats alike
By
Cities are notoriously warm. Studies have shown that surface temperatures in dense, sprawling metropolises are sometimes up to 10 to 15C higher than those in rural counterparts.
Such a difference is caused by the ‘urban heat island’ phenomenon, making temperatures in cities more sweltering when combined with the resulting warming from climate change.
But how exactly do urban heat islands work? Are they getting worse – and how exactly are cities protecting themselves against their effects?
What is an urban heat island?
An urban heat island – sometimes abbreviated to UHI – is an area within a city that is significantly warmer than the areas surrounding it. Densely-populated areas can be up to 12C warmer than their rural counterparts.
Enjoying this article? Check out our related reads:
Essentially, this phenomenon can be boiled down to several factors, one of which is the human-made materials that cities are made from. Roads and buildings – typically dark – retain heat better than natural surfaces such as grass or woodland.
This is known as albedo – a geographical term describing how reflective a surface can be. Higher albedo surfaces, such as white roofs, are reflective and absorb less heat than low albedo surfaces such as asphalt roads.
In addition to this, vegetation cools the air around it through the evaporation of water. As such, vegetation cover and the presence of albedo are two of the most important factors determining how hot an urban heat island can be.
Another factor contributing to urban heat islands is the geometry of a city. In heavily developed areas, surfaces and structures obstructed by neighbouring buildings become large thermal masses unable to release ther heat. As such, cities with many narrow streets and tal buildings are dubbed urban canyons, able to block natural wind flow that would otherwise bring cooling effects.
Human activities within cities is also a main contributor to urban heat islands. Vehicles, air-conditioning units, buildings and industrial facilities all emit heat into the urban environment.
There are two main types of urban heat islands. Firstly, there are surface heat islands, forming due to urban surfaces – such as roadways and rooftops – absorbing and emitting heat. These heat islands tend to be more intense during the day, when the sun is shining.
Atmospheric heat islands are the second type of heat island, forming as a result of warmer air in urban areas compared to cooler air in outlying areas. Atmospheric heat island vary much less in intensity than surface heat islands.
What impacts do heat islands have?
Heat islands can increase electricity demand for air conditioning, and put a strain on a region’s energy reserves.
Increased demand for air conditioning ranges from 1 per cent to 9 per cent for each two Fahrenheit uptick in temperature. The highest increases are in countries where buildings have air conditioning, such as the United States, and peaks in mid-afternoon when offices and homes are running these systems.
Heat islands can also lead to greater emissions of air pollutants and greenhouse gases. That’s because utility companies need to rely on fossil fuel power plants to meet electricity. Burning such fuels releases toxic pollutants into the atmosphere – such as carbon dioxide and fine particulate matter – and contributes towards global warming.
Along with ill-effects for the planet, heat islands can wreak havoc on human health. Areas experiencing high daytime temperatures can create conditions particularly impactful to older adults, young children, people who work outdoors and those with chronic health conditions.
How is climate change affecting heat islands?
Although urban heat islands are not caused by climate change, their intensity can be heightened by the general increase in temperature linkd to global warming.
A recent study conducted by the World Bank found that both the severity and frequency of urban heat islands are much higher today than the historical trend. Now, they occur at a rate 23 times higher and 10 times more potent than historical records.
Another study, published in Nature Climate Change estimated future temperature increases due to the dual impact of climate change and the urban heat island effect in 1,692 cities worldwide. Results showed that their combined effect could make one in five cities four degrees Celsius warmer by 2050.
Climate change and urban heat islands are projected to cost some countries as much as 10 per cent of their GDP or total economic output. The study published by Nature found warming due to climate change could reduce the GDP of a city in 2050 by 0.9 per cent.
When combined with the urban heat island effect, this reduction could double. By the end of the century, the most-affected cities could see as much as 10 per cent of their GDP lost.
In particular, large cities with tropical climates are vulnerable to the effects of urban heat islands. The World Bank study analysed data from more than 500 cities in Asia from 2012 to 2020, finding warmer cities experienced a decrease in economic activity by as much as 5.5 per cent.
How are cities fighting urban heat islands?
Across the world, various methods have been undertaken to curb the impact of urban heat islands in cities. One of the easiest solutions is to increase tree and vegetative cover. As well as this, installing green roofs and using cool pavements are other viable options for cooling a hot urban climate.
In Singapore, a government-supported research project known as Cooling Singapore analyses the effectiveness of heat mitigation methods on models before they are implemented. This research allows policymakers to see which methods perform the best prior to spending money.
Elsewhere in China, Sponge Cities is an initiative aiming to mitigate the impact of flooding by making cities more permeable to absorb, store and release water. Zhuhai in southeast China was one of the first cities to implement such a scheme, and there have been positive reports that such efforts have helped the city address both a flooding problem and urban heat concerns.
India also has made inroads in innovative solutions to combat urban heat islands. The nation’s building code has specifications for roof-covering materials to reflect heat, while in Hong Kong there is clear guidance on urban design features such as arranging buildings to channel wind, designated breezeways and stepped-back buildings.
