Why heatwaves and urban heat islands are a rising threat

9th July 2021

Wider climatic change, caused by multitudinous and varying processes, is the primary force driving rising temperatures. Over time, global temperatures have consistently risen, but this increase has accelerated in recent history. NASA quotes that “the planet’s average surface temperature has risen about 2.12 degrees Fahrenheit (1.18 degrees Celsius) since the late 19th century”, with the majority of this occurring within the last 40 years. 

This trend is especially prevalent in urbanised environments where temperatures tend to reach higher maximums than their suburban and rural neighbours. While natural surfaces, such as grassland and water bodies tend to dispense heat, man-made surfaces such as concrete and stone instead absorb heat. This is the same relationship as between light and dark surfaces.  

Urbanisation in the form of densely populated and infrastructure covered areas is therefore perpetuating rising temperatures, a climatic trend which is due to continue, if not accelerate. This poses a number of threats to human health and will likely shift the way societies operate, from urban planning to development regulations, as we are forced to adjust to increasing heat hazards. 

The effects of rising temperatures  

Rising temperatures can ultimately cause various heat events, each of which directly threaten human health. Overall, most countries across the globe are experiencing more extreme weather conditions with increased variability in temperatures. This means that when heat events do occur, they are likely to be stronger and more of a threat. 


The UK Met Office has defined a heatwave as “an extended period of hot weather relative to the expected conditions of the area at that time of year, which may be accompanied by high humidity”. This means that the nature of what makes a heatwave will vary from country to country, region to region. For example, what would be considered average temperatures in the UK compared to other areas of Europe will differ, and resultantly so will the temperatures that could be classified as a heatwave. 

Most countries will have a defined threshold which if exceeded for a preestablished number of days represents a heatwave event. Even within the UK this threshold varies between regions. According to the Met Office, this is three consecutive days where the daily maximum temperature exceeds the threshold, ranging between 25°C in northern regions to 28°C in southern cities. 

What this threshold is will depend on a country’s average temperature conditions as well as their preparedness for a heat event. As temperatures rise, some countries will be better equipped to deal with the changes than others and therefore heatwaves won’t pose the same level of risk to the population. 

For example, Southern Europe generally experiences higher temperatures than the UK on a regular basis. For this reason, air conditioning is a common, if not a standard, utility which both the public and businesses take advantage of. Similarly, the white walled buildings typical of Mediterranean countries, such as Greece are a perfect example of how hotter countries have historically accommodated for greater levels of heat.  

In contrast, while some offices have air con and temperature control, most people in the UK don’t utilise air con in their homes. This puts us in a position where we are not physically prepared with the right tools to combat heatwaves and the risks associated with them. 

Urban heat islands 

Urban heat islands represent defined urban areas of higher temperatures. The phenomenon indicates how dense cities and urban spaces show definitively higher temperatures than surrounding rural or suburban locations.   

“The precise nature of the heat island varies from urban area to urban area, and it depends on the presence of large areas of open space, rivers, the distribution of industries and the density and height of buildings. In general, the temperatures are highest in the central areas and gradually decline towards the suburbs. In some cities, a temperature cliff occurs on the edge of town.” – UK Met Office 

Urban heat islands have primarily been induced by man-made activity and can closely linked to non-natural material usage. The use of materials such as concrete, dense infrastructure, pollutant release, distance from green and blue environments, and other distinctly urban factors exacerbate the effect of rising temperatures by absorbing and trapping heat.  

This inherently means that the effects of heatwaves are increased within areas that are experiencing urban heat island effects. For example, rural populations would likely not experience as many impacts from heatwaves as populations located in city centres.  

Urban heat islands therefore pose a particular risk. Not only do they heighten the potentially devasting consequences of heat related environmental incidents, but these issues threaten much larger populations than those that exist in non-urban areas. 

The impacts on people 

Heat can be a serious health threat for people across the globe. The impacts are both direct and indirect, resulting in health issues like dehydration as well as exacerbating other pre-existing conditions such as breathing difficulties. 

Deaths and mortality 

It’s clear that excessive heat conditions as well as generally rising temperatures can directly cause deaths. In 2003 in the UK, heatwave conditions caused 2,000 total deaths (Met Office). The European Environment Agency equally estimates that mortality risk increases by up to 5.5% for every 1oC temperature increase above a given heatwave threshold.  

This is a statistic that countries need to become more aware of and integrate into their national procedures as weather conditions become more extreme, especially those like the UK that are not currently fortified against temperature change. The unfortunate truth is that many countries are not prepared for the future reality of temperature and heat hazards. 

Air quality 

Heat is also tangibly linked with air quality which when in poor condition can have severe health impacts. High temperatures are a natural factor that can trigger air pollution episodes and can also cause secondary pollutants to form. This issue is compounded in urban areas where pollution sources are already rife and heat effects, such as urban heat islands, are more prominent.  

Together these risks increase the chance of a variety of health consequences, from dehydration to exacerbated breathing conditions due to poor air quality.  

How we can combat heat hazards 

Ultimately, we cannot reverse temperature increases and so what we need to focus on is mitigating the impacts and minimising the risks.  

There are many ways to combat heat hazards but in order to effectively combat the effects of temperature rise long-term, these measures need to be sustainable. Currently, many of the approaches deployed risk making the problem worse. By using large amounts of energy to cool temperatures temporarily, we are actually adding to the temperature increase and creating a cycle of rising heat. This isn’t sustainable and so alternative more future-proofed approaches need to be considered.  

That being said, there are many nations and governments taking the initiative to implement heat mitigation strategies. One dramatic example can be seen in the Netherlands, where the city of Arnhem has introduced a 10-year plan to restructure the city to protect against heat hazards. This includes digging up asphalt, introducing ‘cooling down’ spots and creating more green space to absorb heat. 

Here are some other examples of sustainable measures which can mitigate the effects of heat and minimise heat hazards: 

  • Planting trees 
  • Implementing blue and green infrastructure  
  • Greening existing infrastructure  
  • Integrating heat considerations into urban planning and design  
  • Using passive cooling systems  
  • Identifying high-risk areas 
  • Identifying vulnerable populations  
  • Providing public advice and guidance 

What’s also important will be targeting these actions to support the most vulnerable or high-risk populations. Using satellite data mapping techniques, 4EI supported Climate Ready Clyde to gain this intelligence, helping them to understand where areas of high heat risk align with vulnerable populations in order to direct intervention action.  

Equipped with the right data, people can make more informed decisions about strategic planning action that champions sustainability. Satellite data not only helps us understand the current state of the environment and how it has changed over time, but it provides objective data about how sustainable decisions have impacted, and hopefully improved, this condition. This both informs future action and evidences how successful previous decisions have been.  

To learn more about heat hazards, visit our website or get in touch. 

by 4EI

4EI provide Applied Earth Intelligence and analytics. We gather evidence-based intelligence about the earth, using data collected through sophisticated space assets.

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