Heat resilience in urban environments: Making cities cooler

With heatwaves becoming more frequent and currently occurring across Southern Europe and the US, how can we make our cities cooler? A panel of experts in a recent webinar discussed the challenges and solutions to soaring summer temperatures and the urban island heat effect.

Heat inequality

The elderly, young children and those with chronic conditions are particularly vulnerable to prolonged extreme heat, which places significant stress on the kidneys, heart and general cardiovascular system, says Dr Ricky Nathvani, Early Career Research Fellow, Faculty of Medicine, School of Public Health at Imperial College London. The health burden of heatwaves is especially acute when combined with poor air quality, therefore, reducing pollution will also lessen heat health impacts, he states. Furthermore, extreme heat can lead to an increase in accidents, he says.

The impact of extreme hot weather is also compounded by heat inequality. In informal settlements in Delhi, for example, there is a prevalence of metal sheet roofing. In these areas, temperatures can be around 10.8°F / 6°C higher than nearby rural areas, placing additional health stresses on already vulnerable groups says Dr Ricky Nathvani.

In the US, low-income neighbourhoods are on average about 10°F / 5.6°C hotter than more affluent parts of cities, says Greg Kats, Founder and CEO of the Smart Surfaces Coalition. He describes this heat disparity as ‘probably the single largest environmental justice issue in the US’. There are two chief causes for these temperature differences. One is a greater prevalence in poorer areas of dark impervious surfaces, which mostly absorb sunlight (around 90%), leading to surface temperatures reaching as high as 170°F / 76.7°C. The other is a lack of trees and green spaces. The retention of heat also leads to higher overnight temperatures, providing no respite for the vulnerable. Over 61,000 people died from heat-related causes during the 2022 summer heatwave in Europe according to a study published in Nature Medicine.

Cooling cities

While billions of dollars of have been spent on temporary measures to tackle urban heat islands in the US, a long-term integral approach is needed. One that doesn’t end when the funding ends, says Greg Kats.  One example is when making infrastructure decisions. Rather than choosing low-cost options such tarmac to resurface roads, cities need to take a more holistic view to mitigate the impact of sun and rain on surfaces, says Greg Kats. He advocates a right first-time approach as resurfacing roads subsequently, for example, incurs considerable additional expense.

Fast retrofitting steps cities can take include planting trees on the south and west side of buildings to keep out sunlight, and installing highly reflective roofs. Transpiration, the evaporation of water through trees, and an important cooling mechanism for cities, is another benefit of green infrastructure. To flourish, tree planting needs to be embraced by the community, counsels Greg Kats. Citing an example of a successful strategy involving the Smart Surfaces Coalition, African American churches in six metro areas are being funded to plant and care for trees.

Often, cooling in cities comes down to individuals or companies installing air conditioning and whether they can afford it, says Greg Kats. However, mass air conditioning can heat a city by 1.8 degrees Fahrenheit, adding further to global warming and putting additional strain on grid capacity at times of peak demand. He believes smart surfaces are the sustainable answer. Smart surfaces are cool roofs, green roofs, solar PV, porous pavements, urban trees, and combined surfaces. The Smart Surfaces Coalition is working with a dozen cities in the US that have started resurfacing roads, cooling the ambient temperature in adjacent residential areas by around 1.4°F / 0.8°C. The adoption of city-wide surfaces can cool a city by 5°F / 2.8°C and low-income areas by 6 - 8°F / 3.3 - 4.4°C, he says.

In Singapore, governance is very strong in implementing greenery and permeable surfaces, with trees lining streets and heavy rainfall successfully captured, says Sophia Kee, Head of Future Ready - Property & Buildings, Middle East, WSP. Cities are the right place to put such measures in place because they control a budget, agrees Greg Kats. 

Understanding how to use data is central to providing the evidence for cities to move away from a least-cost solution to a more sophisticated strategy that fully encompasses climate impacts, says Greg. Cities that fail to address climate impacts may be at risk of being economically downgraded by crediting rating agencies. This can cause the cost of borrowing to increase and the likelihood of reduced services, he warns. Weather prediction is attracting huge interest and will become extremely influential in the way we plan and prepare for climate extremes, adds Dr Ricky Nathvani.

Developing a comprehensive strategy

The most important way of protecting and designing cities against extreme heat is a comprehensive strategy, says Olga Turner Baker MRICS, Managing Director of Ekkist. It starts with city-level interventions such as reflective services, tree cover and green building envelopes, she explains. Green building envelopes (for example, greenery on walls and roofs) can have a significant cooling effect on buildings. They can also improve air quality by reducing particle pollution by 10-20% in the immediate area (according to a report by Arup) and lower urban noise levels.

When it comes to buildings themselves, good planning policies are essential in ensuring solutions are not considered in isolation but collectively, explains Olga Turner Baker. Solutions include reflective glass in buildings and the use of light exterior materials instead of dark brick, which can make the surface temperature 20°F / 11.1°C hotter than exterior surrounding temperatures. When designing buildings, she says ‘We need to think about strategies to keep heat out first before worrying about having to push it out with mechanical systems’, she says. In the Middle East, Sophia Kee’s work involves creating buildings that shield occupants from extreme temperature variations. Sophia believes the façades and outer skins are key, as well as insulating walls, which has typically been associated with cold weather. Olga Turner Baker advises the importance of having good frameworks and design briefs from the outset. These can incorporate heat prevention measures, rather than risking design teams just complying with minimum building regulations.

There are many sophisticated engineering solutions, material choices and data tools available to improve thermal safety and comfort. However, these do not reach those living in informal settlements in Sub Saharan Africa, South and East Asia. Addressing heat inequalities remains a challenging problem, points out Dr Ricky Nathvani.

Smart building systems and indoor heat

Increasingly, smart building systems are allowing active technology to be used more sustainably. For example, active management systems can direct people to hot desks in areas of buildings that are easier to sustainably cool on a particular day, says Olga Turner Baker. The way we can use sensors is becoming very clever. This can range from adjusting for occupancy, to managing people's personal preference for temperature and comfort, as well as maintaining air quality, she adds. Thermal comfort, good lighting and ventilation has become a business case for asset owners, reducing health issues and absenteeism and increasing productivity of employees, says Sophia Kee. There is also a wider perspective affecting absenteeism in the workplace, with extreme heat in cities and homes impacting most on the elderly and young children, and staff taking time off to look after family, she says.

Although we can measure outdoor temperature accurately using remote sensing and GIS data, it is much harder to measure indoor heat exposure says Dr Ricky Nathvani. The Pathways to Equitable Cities project has stakeholders in Accra in Ghana, Dhaka in Bangladesh and throughout the world. The project is looking at not only urban heat island effects but also the characteristics of individual buildings, using sensors placed with the consent of the occupiers. Heat exposure to people is a combination of both temperature and humidity, so measuring this and how it enters the home and its effects are very important, he explains. Currently the project is assessing 150 locations in Accra. The project is also applying computer vision technology to existing built environment datasets to gauge temperatures inside homes.

‘The whole process of successfully managing heat in cities has so many moving parts it’s vital to engage all stakeholders, because we can’t do it with pockets of individual expertise, there has to be a holistic approach’, concludes Sophia Kee.

With the impact of climate change becoming more pronounced and the frequency of extreme heat events increasing, cities must find ways to mitigate prolonged periods of soaring summer temperatures. Heatwaves and the Urban Heat Island effect, caused by the mass of hard surfaces of concrete, brick and asphalt in cities, place the elderly and those with chronic conditions most at risk. This webinar looks at the possible solutions from incorporating more green infrastructure and other passive options to active technologies, and how a multi-disciplinary approach by built environment professionals can make our cities cooler.