Cities trap heat more than surrounding rural areas. Now strategies must be adopted to reduce this rising heat in order to tackle climate change and protect human health. Previously, widespread adoption of available technologies had been slow due to the complexity or cost involved. Cool roofs and cooling islands provide cost-effective and simple solutions that may be fundamental to tackling urban heat islands.
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Climate Change is Causing Temperatures to Rise
Since the 1850s, average global temperatures have increased by more than 1°C as a result of climate change.
Temperatures continue to rise and the rate at which they are rising is accelerating.
2017 was the hottest year ever recorded until this record was smashed by each year consecutively.
Experts believe that unless significant action is taken, global average temperatures will have risen by an extra 5 °C by the end of the century.
Cities, In Particular, Will Face Soaring Temperatures
It has long been understood that urban areas feel the heat more. On average, cities are around 5.5°C warmer than their surroundings (even more during hot summers; Madrid, for example, can be up to 8°C hotter than its surrounding rural areas in this period), and this is known as the urban heat island effect (UHI).
This phenomenon occurs due to a multitude of factors such as heat-trapping asphalt and concrete, smog from traffic pollution and industry, and heat emitting from buildings and vehicles accumulating to intensity temperatures within the built-up area.
Given that global temperatures are predicted to rise, even if we can meet the Paris Agreement goals of limiting global warming to 2°C (ideally 1.5°C), and that more than half of the world’s population lives in urban areas (a figure which continues to grow), it is important to tackle rising urban temperatures in order to address climate change as well as protect urban citizens.
Vulnerable Populations Suffer from Urban Heat
The impacts of exposure to high air temperatures on human health are well understood, and symptoms such as weakness, dizziness, nausea, headaches, vomiting, diarrhea, breathlessness, and heart palpitations are well documented.
Some populations are more severely impacted than others.
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Elderly persons, for example, are more at risk of suffering health complications and death due to exposure to high air temperatures. In the UK in 2019, for example, summer heatwaves accounted for 900 excess deaths of those aged 65 or over.
Recent research has also revealed that, in the US, people of color are more likely to be living somewhere with 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 consequences 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 even worse affected.
It is vital that solutions are designed to tackle the issue of urban heat islands in order to protect human health as well as to contribute to preventing global temperatures from rising further.
What Are Cool Roofs?
A simple idea to tackle urban heat islands are cool roofs, which can simply be a roof painted white or covered with energy-reflecting materials.
The result is that the roof absorbs less heat, helping to bring down the temperature inside the building by 2–5 °C compared with conventional roofing.
Recent research has shown cool roofs to be successful in reducing the impact of UHI by 23% (which translates to a reduction by 0.3 °C of population-weighted temperature).
Additionally, cool roofs have the potential to mitigate around a quarter of heat-related mortality due to the UHI.
On What Kind of Scale Would These Cool Roofs Need to be Implemented to Have City-Wide Positive Effects?
For cool roofs to have a significant impact on city temperatures, they would need to be implemented on a city-wide scale.
To make this happen, industrial and commercial buildings, as well as residential, would have to adopt the solution.
Some cities, such as Barcelona, Spain, are offering subsidiaries to cover up to 75% of the costs of several green-roof projects in the city to encourage the widespread adoption of cool roofs.
One reason 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 that is being tried out in Paris, where parks and pools are being established around the city and linked by walkways.
These spaces are intended to reduce the heat of the congested city by opening up the space and providing sources that can reduce the air temperature (e.g. water and shade provided by trees).
Another city that is trying out cool islands is Medellín in Colombia, where over 10,000 trees have been planted in low-income areas to create 36 ‘green corridors’, which has resulted in reducing surface temperatures by 2 °C.
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Cities in the C40 global network have committed to making green spaces or pools accessible within 15 minutes by foot or bike by 2030.
Portland, Oregon, USA, has already started to implement this. The city has developed ‘Complete Neighbourhoods’ that achieve urban equity and sustainability.
Additionally, Melbourne, Australia, is developing Minute Neighborhoods, similar to Paris’s 15-Minute City.
To protect cities from rising heat, these solutions must be implemented rapidly, and on a wide scale.
The effects of climate change will only increase unless significant action is taken. Cool roofs and coming islands provide simple and cheap methods of lowering urban temperatures, which could help to mitigate climate change.
References and Further Reading:
Hsu, A., Sheriff, G., Chakraborty, T. and Manya, D., 2021. Disproportionate exposure to urban heat island intensity across major US cities. Nature Communications, 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 heatwaves in a European city. Environment International, 127, pp.430-441. https://www.sciencedirect.com/science/article/pii/S0160412018319627
Nature, 2021. Cities must 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 Earth get by 2100?. Nature, 580(7804), pp.443-445. https://www.nature.com/articles/d41586-020-01125-x