How the urban heat island effect makes cities vulnerable to climate change

Cervest's Dr Helen Beddow reflects on the recent extreme hot weather and how the urban heat island effect exacerbates its effects.

Climate change is global. But the impacts are experienced locally, on individual people and the assets they own and use on a daily basis. Increasingly, cities are experiencing the consequences of climate change. This summer’s  UK heatwave exceeded extreme temperatures of 40℃ for the first time leading to wildfires in London and energy use and prices soaring. By 2030, it is estimated that 1.9 billion people will be exposed to heat stress, especially those in cities. 

What is an urban heat island? 

Temperatures can vary significantly even across a relatively small area. Densely populated city areas can be up to 12°C warmer than the surrounding countryside. This localized warming within cities is known as the urban heat island effect.  

Cities are covered with human-made materials such as roads and buildings which absorb and retain heat better than natural surfaces like grass or woodland. Albedo describes how reflective a surface is. High albedo surfaces, such as white roofs, are reflective and absorb less heat than low albedo surfaces such as asphalt roads. Vegetation cools the air around it through the evaporation of water. Vegetation cover and albedo are two of the most important factors which determine the strength of the urban heat island effect. 

Satellite thermal data published by the Greater London Authority shows the albedo effect in action. This map visualizes London’s daytime surface temperature, showing cooler daytime temperatures in green areas with lots of vegetation, such as Hampstead Heath, in contrast with hotspots such as Heathrow which are paved with asphalt. 

What does extreme heat do to the human body? 

The world is experiencing hotter days, more frequently and for longer consecutive periods of time. However,  heat is not the only factor that determines the impact of a heatwave.  

Humidity is a measure of how much water vapor is present in the air, and warmer air holds more water. Climate change is shifting humidity patterns, making some areas more humid, and some less humid. At 100% relative humidity (also known as wet-bulb temperature), the air is fully saturated with water, so sweat won’t evaporate. The body cannot regulate its temperature to cool itself down. At a wet-bulb temperature of 35°C, people won’t survive for more than a few hours even with shade or water.  

Managing a farm in Ghana, Cervest’s Founder and CEO, Iggy Bassi, witnessed severely hot conditions first-hand, “On extreme heat days, we knew ahead of time we had to change all of our labor rotas,” he says. “Beyond a certain temperature, the human body cannot work because the outside humidity is so high.” 

People who work outdoors, like the 3.1 million people employed by the construction sector, will literally need to down tools and stop because of the heat, increasing the cost of operational downtime. In extreme circumstances, without access to cooling, working conditions become inhumane and life-threatening. 

The impact of heat stress on employee health has been linked to lower productivity and increased absenteeism and has big financial implications for businesses. To avoid negative impacts on employee health, employers will need to absorb costs from changing energy requirements and adapting buildings to cope with increased heat. 

How does extreme heat affect buildings and critical infrastructure? 

Extreme heat impacts more than health and productivity, it can structurally damage buildings roads and infrastructure. Heat causes building materials expand and metal to rust faster. This has big implications for concrete structures, especially those internally reinforced with steel. Half the world’s buildings are constructed with concrete, housing 70% of the world’s population. Another structural problem caused by heat is soil shrinkage, making building foundations more vulnerable to subsidence. The British Geological Society (BGS) highlights this as an escalating problem in northern and central London boroughs, and in Kent in the South East due to climate change. 

Building resilience to heatwaves in the UK 

Simply put, UK cities weren’t built with the consequences of the urban island effect in mind. Compared to other hot countries, UK buildings were designed to keep heat in and rarely have air conditioning. With heatwave-induced GDP losses in Europe expected to surpass 1.14% in 2060, the cost of inaction is heating up.  

“What we are witnessing is that the surface area of risk is expanding – with new regions, cities, countries reaching new thresholds.” – Cervest Founder and CEO speaking with Sky News and BBC World News during the UK heatwave. 

Cervest’s product, EarthScan™, provides science-backed climate intelligence to help UK councils discover, quantify and share climate risks on assets they own, manage or rely on. By translating multiple datasets into decision-useful insights at the asset-level, EarthScan provides the probability of the impact of climate change between 1970 and 2100 at a portfolio and asset level.  

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Cervest is a UKGBC member and climate intelligence (CI) company putting climate at the core of every decision. The company provides personalized, dynamic and science-backed climate intelligence on any asset, anywhere, anytime — giving enterprise and government decision-makers the most comprehensive view possible of climate risk at an asset level.  

UKGBC’s Climate Resilience and Nature team are continuing to work on how our built environment can be resistance to these changes. Learn more about our work here.

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