Cities retain heat more than surrounding rural areas. Now strategies must be adopted to mitigate this rising heat to tackle climate change and protect human health. Previously, the widespread adoption of available technologies has been slow due to the complexity or cost involved. Cool roofs and cooling islands provide cost-effective and simple solutions that can be fundamental to tackling urban heat islands.
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Climate change is causing temperatures to rise
Since the 1850s, the average temperature on Earth has risen by more than 1°C due to climate change.
Temperatures continue to rise and the rate at which they rise is increasing.
2017 was the hottest year on record until this record was broken every year in a row.
Experts believe that unless drastic measures are taken, the global average temperature will have risen by another 5°C by the end of the century.
Cities in particular have to deal with rising temperatures
It has long been known that urban areas feel the heat more. Cities are on average about 5.5 °C warmer than their surroundings (even more during hot summers; for example, Madrid can be up to 8 °C hotter than the surrounding rural areas during this period), and this is known as the urban heat. island effect (UHI).
This phenomenon occurs as a result of a multitude of factors, such as heat-trapping asphalt and concrete, smog from traffic pollution and industry, and heat emission from buildings and vehicles that accumulate to intensity temperatures within built-up areas.
Since global temperatures are predicted to rise, even if we can meet the Paris Agreement targets to limit global warming to 2°C (ideally 1.5°C), and that more than half of As the world’s population lives in urban areas (a figure that continues to grow), it is important to tackle rising urban temperatures to tackle climate change and protect urban residents.
Vulnerable populations suffer from urban heat
The effects of exposure to high air temperatures on human health are well known and symptoms such as weakness, dizziness, nausea, headache, vomiting, diarrhea, shortness of breath and palpitations are well documented.
Some populations are more severely affected than others.
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For example, the elderly are more at risk for health complications and death from exposure to high air temperatures. In the UK, for example, summer heatwaves were responsible for an additional 900 deaths of people over 65 in 2019.
Recent research has also shown that people of color in the US are more likely to live somewhere with a higher urban heat intensity than non-Hispanic whites.
Those living in households below the poverty line are also more likely to be exposed to higher temperatures and suffer the effects of heat exposure.
Therefore, the impact of rising temperatures is not felt equally across the population, and as temperatures continue to rise, vulnerable subgroups will be hit even worse.
It is vital that solutions are designed to address the problem of urban heat islands in order to protect human health and prevent global temperatures from rising further.
What are cool roofs?
A simple idea to tackle urban heat islands is cool roofs, which can simply be a white painted roof or covered with energy-reflecting materials.
As a result, the roof absorbs less heat, reducing the temperature in the building by 2-5 °C compared to conventional roofing.
Recent research has shown that cool roofs are successful in reducing the impact of UHI by 23% (which translates into a 0.3°C reduction in population weighted temperature).
In addition, cool roofs have the potential to reduce about a quarter of heat-related deaths from the UHI.
On what scale should these cool rooftops be implemented to have positive effects across the city?
For cool roofs to have a significant impact on city temperature, they should be implemented on a city-wide scale.
To make this possible, industrial and commercial buildings, as well as residential buildings, should adopt the solution.
Some cities, such as Barcelona, Spain, offer subsidiaries to cover up to 75% of the cost of various green roof projects in the city to encourage the widespread adoption of cool roofs.
One of the reasons cities feel the heat more than their rural counterparts is that there are often fewer green spaces or even fewer gaps between buildings.
To counter this, scientists have developed the idea of cooling islands, an idea being trialled in Paris, where parks and swimming pools are laid out around the city and connected by walkways.
These spaces are intended to reduce the heat of the congested city by opening up the space and providing sources that can lower the air temperature (e.g. water and shade from trees).
Another city trying out cool islands is Medellín in Colombia, where more than 10,000 trees have been planted in low-income areas around 36 ‘green corridors‘, which led to a reduction of the surface temperature by 2 °C.
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Cities in C40’s global network have committed to making green spaces or swimming pools accessible within 15 minutes on foot or by bike by 2030.
Portland, Oregon, USA, has already started this. The city has developed ‘Complete Neighborhoods’ that realize urban justice and sustainability.
In addition, Melbourne, Australia, is developing Minute Neighborhoods, similar to 15-Minute City in Paris.
To protect cities from rising heat, these solutions need to be implemented quickly and on a large scale.
The effects of climate change will only increase unless drastic measures are taken. Cool roofs and upcoming islands offer simple and inexpensive methods of lowering city temperatures, which could help mitigate climate change.
References and further reading:
Hsu, A., Sheriff, G., Chakraborty, T., and Manya, D., 2021. Disproportionate exposure to urban heat island intensity in major US cities. nature communication, 12(1). https://www.nature.com/articles/s41467-021-22799-5#citeas
Macintyre, H. and Heaviside, C., 2019. Potential benefits of cool roofs in reducing heat-related mortality during heat waves in a European city. Environment International, 127, pp. 430-441. https://www.sciencedirect.com/science/article/pii/S0160412018319627
Nature, 2021. Cities need to protect people from extreme heat. 595 (7867), pp. 331-332. https://www.nature.com/articles/d41586-021-01903-1
Tollefson, J., 2020. How hot will the earth be by 2100?. Nature580 (7804), pp. 443-445. https://www.nature.com/articles/d41586-020-01125-x