10 December 2021

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

 Fran Bartolić

By Fran Bartolić

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

In the first of a two-part blog series exploring cities and extreme heat, researcher Fran Bartolić explains how and why our cities are vulnerable to increasing temperatures. Fran is part of Cervest’s Research Residency program and has just completed a 6-month project with our science team focussing on modeling urban heat islands.

Climate change is global. But the impacts are experienced locally, on individual assets, and the consequences are suffered by individual owners. By 2030, it is estimated that 1.9 billion people will be exposed to heat stress. The potential for loss and damage is extreme, and anyone without access to cooling equipment (and the power to run it) is very vulnerable.

In some cities, all buildings and infrastructure will need manual cooling technology like air conditioning in place. At certain times, people's lives will depend on this. Cities need to be wary of creating single dependencies. What happens if the power infrastructure fails? Cities might be entirely dependent on the stability of a highly centralized electrical grid. If there is a single point of failure, then it’s not truly climate resilient.

Why we need to think beyond global average temperature

As global temperatures increase, it’s vital to keep in mind that these increases will not be uniform. Discussions around climate change often focus on curbing emissions and preventing global average temperature from rising above certain limits, such as the 1.5°C best case scenario established in the Paris Agreement.

Global temperature averages are a critical yardstick for Net Zero commitments, but less helpful for adaptation planning. Global averages don’t convey the potential range of regional change, which, by 2100, could be up to 10℃ in areas like the Arctic. Different regions will need very different resilience strategies.

What is an urban heat island?

Temperatures can vary significantly even across a relatively small area. This is particularly true of cities, which can be up to 12°C warmer than the surrounding countryside. Even within individual cities, some neighborhoods are hotter than others. This localized warming within cities is known as the urban heat island effect. Riding a bicycle with an air temperature sensor, you’ll find significant variation across the city.

Cities are covered with human-made materials such as roads and buildings which absorb and retain heat better than natural surfaces. Vegetation cover and albedo are two of the most important factors which determine the strength of the urban heat island effect.

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’s cooling effect is highly localized, and the hottest parts of cities tend to be areas with little to no vegetation. Densely populated areas with high-rise buildings are often warmer than leafy suburbs.

City surfaces such as asphalt roads absorb a lot of heat during the day and slowly release it in the evening and throughout the night, which is why the urban heat island effect has bigger impacts later in the day. In a thermal image of a large built-up city like Los Angeles, even at 3 AM you can see highways and airports glowing because they haven't cooled off yet.

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, it’s not just heat we need to be aware of. While many of us know our planet is warming, we are less aware of how climate change is affecting humidity.

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 areas less humid. This is important because extreme heat and humidity can be a lethal combination. The key term to know is “wet-bulb temperature” - a combined measure of temperature and humidity.

Sweat cools us down when it evaporates, regulating body temperature. This can also make us dehydrated, putting pressure on our internal organs. Chronic heat stress can lead to kidney failure and heart problems.

At 100% relative humidity, the air is fully saturated with water, so sweat won’t evaporate. We can’t regulate body temperature and cool down. Even people used to working in the heat can’t do normal outdoor activities past a wet-bulb temperature of 32°C. At a wet-bulb temperature of 35°C, people won’t survive even with shade or water. We’ve only reached these heat and humidity extremes for an hour or two at a time at a few specific locations, but the fact that we have hit it at all is worrying. Climate models indicate that by 2100, combined heat and humidity events will pass this limit in the Persian Gulf region, Indian subcontinent and eastern China.

How does extreme heat affect city economies and critical infrastructure?

Extreme heat impacts more than health. Building materials expand, causing damage to buildings, roads and infrastructure. If it’s hot enough, tarmac starts to melt. Metal rusts faster, with big implications for concrete structures internally reinforced with steel. The foundations of buildings are vulnerable to subsidence and soil shrinkage, particularly in clay areas. Warmer temperatures also expand the habitats of insects that can carry disease, destroy crops or structurally damage buildings. Heat has even been linked to increases in violent crime.

“Your whole economic model changes at a certain temperature,” Cervest’s Founder and CEO, Iggy Bassi, recently told The Financial Times. Iggy witnessed severely hot conditions first-hand while managing a farm in Ghana, “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.”

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.

In some industries, there will be work that can’t happen at all in extreme heat. The cost of operational downtime will increase because people who work outdoors in industries like food production and construction will literally need to down tools and stop. In extreme circumstances, without access to cooling, working conditions become inhumane and life-threatening.

This is why adaptation planning is so important. To become climate-resilient, cities must put essential adaptation plans in place to protect their critical infrastructure and the lives and livelihoods of their communities. Understanding the risk to our assets is the starting place; access to on-demand Climate Intelligence (CI) with EarthScan™ is the way forward.

In part II of this blog, we will explore building resilience to cope with climate change and how CI is the secret to adaptation.

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