Discover how sweltering urban heat islands are being cooled down

Cities are experiencing the consequences of climate change and breaking records as heatwaves scorch them. The Iran Meteorological Organization recorded the highest recorded temperature ever at 53.6°C in the city of Shush. By 2030, 1.9 billion people are estimated to be exposed to heat stress and this will hit city dwellers particularly hard.

Recently published research looking 30 years into the future estimates an increase in ‘Extreme Danger Days’ – days where any property would be expected to experience dangerous levels of heat. Its analysis combines high-resolution measurements of land surface temperatures, canopy cover, impervious surfaces, land cover and proximity to water to calculate the heat exposure. It then adjusts for future forecasted emissions scenarios.

Even in moderate warming cases, severe climate hazards are likely to become more common. Worldwide, more than half of the population now live in cities at the forefront of climate risks. Cities often suffer from ‘heat islands’ – a phenomenon of temperatures being higher within cities than in neighbouring rural zones. The increased use of man-made materials in city development, such as roads and buildings displacing trees, ponds and soil, contributes to an expansion of these ‘urban heat islands’ by absorbing and re-emitting more heat, making their surroundings warmer too.

As the planet warms, urban heat islands will intensify those higher temperatures. Ecostress, the thermal radiometer on NASA's International Space Station, for example, captured an image shortly before midnight on May 5 covering the urban conglomeration of Delhi, India at its highest temperature ever recorded. This highlighted a variation of 24°C between the city of New Delhi (peaking at 39°C) and rural fields nearby, which had cooled to 15°C. Within New Delhi, however, temperatures around its iconic India Gate surrounded by leafy vegetation and high-income neighbourhoods can be 12°C lower than low-income, informal housing settlements, such as Seelampur, an area east of the Yamuna river.

As rapid urbanisation shapes more of the world with cement, glass and steel, modern cities are turning into heat sinks, but this is not an irreversible situation, says Professor Winston Chow. Chow is an Associate Professor of Urban Climate at the Singapore Management University and the lead researcher for ‘Cooling Singapore,' a multi-institutional project launched in 2017 with funding from the Singapore government to build a computer model, or digital urban climate twin, of Singapore. This will allow policymakers to analyse the effectiveness of various heat mitigation measures before spending money on solutions that might not work.

“Every city will have a heat island,” says Chow, "It’s the inadvertent consequence of converting natural and rural landscapes into urban settlements by constructing with concrete and asphalt materials and having more fuel and energy use for power, heating, cooling and transport.

Building heat resilience

Exhaustion and dehydration, power grid failures and crumbling infrastructure all result in heat stress, which requires building heat resilience. The most obvious way to fight the urban heat island effect is to reintroduce vegetation – expand green cover, plant street trees, install ‘green roofs,’ etc. Other options include introducing ‘cool roofs’ that feature bright coatings to reflect more sunlight and absorb less heat. While Zhuhai, a city of 2.4 million in the Guangdong-Hong Kong-Macao Greater Bay Area, is part of a pilot project in China promoting ‘sponge cities.’

A sponge city, according to Chinese guidelines, is one that has transformed hard surfaces, such as roads and pavements, into permeable surfaces that can absorb, seep, purify and store water and later release stored water for use. The adoption of porous bricks and porous concrete could lower pavement surface temperature by 12 and 20°C, respectively and the air temperature by up to 1°C.

Cities such as Singapore – and others that apply climate resilient development principles – are proactively reducing risks from heat islands and heatwaves. Their strategies for reducing heat impacts include more efficient physical infrastructure, such as district-level cooling that efficiently uses energy to mechanically cool large areas in cities; nature-based solutions, such as increasing the extent and density of green spaces in cities and on walls and roofs, and; integrated, inclusive planning of urban stakeholders to ensure that vulnerable urban residents are protected. For this final point, early warnings from meteorological agencies might be tied to enhanced health infrastructure to ensure elderly or medically vulnerable people have access to medical treatment or shelter, and there may be work stoppages for outdoor construction workers when heatwaves occur.

Using air conditioners and electric fans to stay cool accounts for nearly 20% of the total electricity used in buildings around the world and energy consumption for cooling has more than tripled since 1990, with significant implications for electricity grids, especially during peak demand periods and extreme heat events, according to the International Energy Agency (IEA). But encouragingly, a new study finds that switching to propane as a refrigerant could lessen the global temperature increase from space cooling, meaning that we could avoid a 0.09°C increase in global temperature by the end of the century, making a significant contribution toward keeping the global temperature rise below 1.5 °C.

"The flaw in having large heat islands intensifies from a double whammy of ignorance of nature and its non-inclusion when planning cities. An absence of green spaces and inefficient use of energy and fuel in urban infrastructure results in heat islands. While sprawling cities lacking public transportation options are often associated with more private transportation use, which generally results in more fuel usage and higher heat island intensities."

Air conditioning is a distinctive feature of cities experiencing extreme heat. This contributes to a sizeable proportion of carbon emissions and with the current energy crisis, it is also becoming unaffordable. So, how can we build more green buildings and consume less energy and stay cooler? How feasible is this model?

"While air conditioning has been, in Singapore’s founding Prime Minister’s words, one of the signal inventions in history' and 'a most important invention for Singapore,' it does have its costs with exacerbating the heat island and global warming," says Chow. "Two approaches that can work to reduce air conditioning use – especially where it is most needed in tropical cities – involve using it more efficiently.

"Firstly, district-level cooling is increasingly being utilised in Singapore in lieu of inefficient individual air conditioning condensers. As uptake of this option increases, the cost to operate these – as well as to the environment – will decrease. The second approach is to go back to applying historical tropical urban design and architectural principles in low-latitude cities dependent on air conditioning. There should be particular emphasis on using natural ventilation, artificial shading and vegetation cover to ensure that air conditioning use is significantly lowered."

Singapore has a few other innovations and radical designs that cities around the world could adopt to tackle extreme heat and avoid urban heat islands, adds Chow.

"Among the push towards more 'green' buildings and the implementation of the Singapore Green Plan 2030, is a move towards science-based policies in reducing urban heat risks," he says. "Together with colleagues in several universities in Singapore, I’ve been leading research into the Cooling Singapore Initiative since 2017.

"Our most recent project involves developing an innovative Digital Urban Climate Twin that stakeholders can use to ascertain which heat reduction approaches give the 'best' outcome for different parts of Singapore’s urban landscape. This Twin is based on coupling several environmental models that are tested and parameterised with observed data to ensure that these are representative of Singapore’s urban climate context. It is hoped that it can contribute to ensuring the urban resilience of this city-state towards climate change-related heat risks.

"I think that a blueprint, such as the recent IPCC report for human settlements, for example, that enhances adaptation and resilience can be extensively followed. Singapore – and other cities such as Ahmedabad and Semarang in Asia – have track records in dealing with climate change risks from heat, flooding and sea level rises that will, unfortunately, be more frequent and intense in the years to come.