Cities warming at twice the global average

Typical A/C makes warming effect worse so we need new solutions to cool cities...
16 November 2021


A view over Jakarta.


More than 50% of the world’s population now lives in urban areas and in recent years many major cities have been hit with extreme weather events due to the effects of climate change. For example, the flooding that hit the London underground in the summer of 2021. A key reason for this might be that the urban environments themselves are exacerbating the heating effect.

Studies show that cities are warming at twice the global average due to the ‘heat island effect’. Iain Campbell, the lead author of a new UN guide on cooling cities, explains that the predominant materials in cities such as concrete, asphalt and glass “absorb greater levels of heat and hold on to it” when compared to water bodies and plants that have been cleared away to build the cities.

This effect is made worse by conventional air conditioning (A/C) systems that shift hot air from indoor spaces to outdoor spaces, increasing nighttime temperatures in cities by up to a couple of degrees Celsius. Not only that but A/C also contributes to global warming. Campbell calls this “one of the unfortunate feedback loops of cooling” because air conditioning is "very energy and [greenhouse gas] emissions intensive” due to both the carbon dioxide emitted in generating the electricity to power it and the fluorinated gas refrigerants that are emitted from the system itself. Consequently, as cities warm it is likely that they will rely more heavily on air conditioning, which inturn contributes to greater warming.

What are the best practices for cooling cities without raising urban temperatures? Campbell is quick to emphasise that there is no silver bullet, and that we need to take a holistic approach. The three key questions that need to be addressed are: can we use materials that reflect radiation and dissipate heat, can we improve the thermal efficiency of buildings with better insulation, and can we provide active cooling solutions (alternatives to A/C) that are low-carbon and do not dissipate additional heat? The guide provides case studies from around the world to address these challenges. 

One example that Campbell is keen to highlight is Paris. The city uses the river Seine as a heat sink to provide district cooling. When the temperature of the river drops, the cool water is distributed through underground pipes to all the buildings connected to the cooling system, where this water is fed through a heat exchanger to provide the required cooling. Crucially, no heat is emitted to the outside environment, no refrigerant gases are needed for operation and distribution is more efficient when the system is handled centrally, rather than building by building. Today, this system provides active cooling to about 43% of the city.

Another key solution that helps provide immediate cooling effects in the nearby vicinity is the planting of trees and other plants. A case study featured in the guide on the city of Los Angeles found that trees could reduce the local temperatures by up to 5-7 degrees Celsius due to the effect of shading and the trees using the incoming energy for moisture evaporation and photosynthesis. Campell thinks a city that has it planned out perfectly is Singapore, a city where most of the urban environment is covered in plants, and trees line most streets. An astonishing 47% of Singapore was covered in greenery as of 2016.

In comparison to national governments, city governments can be more responsive to climate threats and have often been the ones to take lead by implementing novel climate solutions. The guide highlights the best of these solutions from around the world to help inspire other built up areas and ensure we live comfortably in our high rise utopia’s for decades to come. Campbell hopes that the suggestions from the guide, which was officially released during the COP26 conference in Glasgow, will be taken seriously by city leaders from around the world.


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