Unless we act soon, these extreme temperatures may mark the beginning of a descent into a heat crazed hell for the world’s cities and their citizens. Firstly, excessive heat reduces worker productivity and hurt economies, according to the International Labour Organization.
Worse, urban heat compromises human health and comfort, states the US Environmental Protection Agency, placing people increasingly at risk of heat-related illness - or even death. Globally, heat causes more deaths each year than other weather-related killers, such as floods, tornadoes, or hurricanes. Heat can kill directly via heatstroke or indirectly through an increased risk of heart attack or stroke.
Some will be worse affected by rising urban temperatures than others. The urban poor are particularly at risk from exposure to heat, with many overheating not just outdoors but in their homes. Those who live on the upper floors of buildings, work outdoors, and don’t have air conditioning are likewise susceptible. So too are the old, the young and those suffering from ill health and chronic health conditions, like heart disease and obesity, states a study by Shilu Tong, professor of epidemiology at the Department of Clinical Epidemiology and Biostatistics and colleagues at Shanghai Jiao Tong University School of Medicine, China
Rapid urbanisation is a significant factor fuelling these extreme heat-related disasters, as ever more of the world’s population swarms into cities, particularly in sub-Saharan Africa and southeast Asia. Urbanisation is not going to stop either. According to the United Nations, 68% of the world population will live in urban areas by 2050, rising from 55% in 2018.
The average air temperature in a major city like London can be between 4°C and 10°C higher than the surrounding suburban and rural areas. So, why precisely are built-up areas so much hotter? The way cities are designed means that they trap more heat. Traditional building materials, such as metal, concrete, and brick, are designed to insulate, and as a result, they easily absorb and store heat from the sun. This heat has nowhere to go and lingers in and between buildings, creating areas of increased temperature, or urban heat islands (UHIs).
These heat islands can be observed around cities all over the world. They are typically found where there are lots of asphalt streets, tarred or other hard, dark surfaces. In addition, a lack of greenery in cities limits ‘evapotranspiration.’ This is the cooling effect of evaporation from the soil plus transpiration from plants. On top of this, human activities, such as transport, lighting and air conditioning, all release heat, which is then trapped.
Turning up the air conditioning doesn’t do much good either: the energy generated by electric fans and air conditioning ends up contributing to an even hotter UHI, resulting in a vicious circle. UHIs also become more noticeable at night because buildings, pavements and carparks block the heat coming from the ground from rising into the cold night sky, with the heat then trapped on lower levels.
Cutting greenhouse gas emissions to meet the 1.5°C target could prevent hundreds to thousands of annual heatrelated deaths per American city during extreme events, according to Eunice Lo of the University of Bristol. Her work shows the 1-in-30-year heat deaths in the studied cities in (outer circle) the 3°C scenario, and the difference in deaths between (inner circle) 1.5°C and 3°C, and (innermost circle) 2°C and 3°C. A larger circle indicates a higher number of deaths. (https://bit.ly/EuniceLo)
UHIs present enormous challenges for humans, says Henry Ibitolu, Ph.D student in Future Cities Engineering at the Universities of Edinburgh and Glasgow. His research focuses on mitigating UHI effects by making cities of the future more sustainable and energy-efficient.
“First, urbanisation means natural surfaces are converted into built-up structures that alter the thermal dynamics of the immediate environment. Second, urbanisation is often accompanied by densification. This increases the number of occupants of structures without increasing their footprint. This is seen in expanding cities, where suburban dwellings are often replaced with duplex houses or low-rise apartment blocks to meet housing demand.”
So, how can we turn down the temperature? Most cities are already built-up, meaning we can’t radically alter the urban geometry, continues Ibitolu. Plus, there is limited space to add more greenery. There are solutions, however. These include using lighter-coloured materials and reflective coatings to reflect more sunlight and absorb less heat. White-washing black asphalt or roofs can help.
We can also look to the past to find suitable solutions for the future, adds Ibitolu. “For centuries, ancient architects and builders have relied on the thermal efficiency of earth materials (earthen bricks and rammed earth) for building. These earth materials have significant thermal properties and present a low-embodied carbon alternative to conventional cement construction.”
Ibitolu explains that his research investigates the extent to which cement building materials can be replaced with thermally massive earthen materials. “By doing so, we can mitigate emerging UHI effects and alleviate the need for energy-intensive air conditioning.”
Adding external facades can improve indoor thermal and energy efficiency. Using green roofs, namely roofs of buildings covered in plants, and increasing the amount of greenery around a city or simply adding more trees for shade helps absorb carbon dioxide, a leading pollutant, and lower temperatures. We all know the temperature difference between a tree-shaded park during the summer months compared with an exposed, sun-baked car park.
Varying the differences in building heights can also create more air circulation, which has a cooling effect.
In other words, with the right degree of urban planning and the right amount of investment and effort by architects and the real estate industry, we can mitigate urban warming.
Essentially, in future, architects, city designers and urban planners need to construct buildings with energy performance in mind, continues Ibitolu. “They should also consider how their performance will influence structures around them and the nearby urban microclimate. The outcome is to contribute towards achieving the UN sustainable goals and to ensure a sustainable city for all.”
The COP26 UN Climate Conference has kept the Paris Agreement targets alive, gathering commitment to cut emissions to stay within the global warming limit of 1.5°C. Achieving a threshold of 1.5°C could avoid between 110 and 2,720 annual heat-related deaths per US city over the next 30 years, according to research by Eunice Lo, a climate scientist at the University of Bristol.
By planning more efficient infrastructure and investing in future-proof buildings, cities can limit the worst effects of dangerous heat increases, lower their carbon footprint and adapt to a warming world. The solutions are there. It’s simply up to us to use them.
Eunice Lo, climate scientist, University of Bristol