Imagine living in sauna-like temperatures for days. This situation is the reality for residents in some of world’s largest cities — in the US, several parts of East Asia, and India— during the summer, where temperatures have soared to over 40°C or 100°F.
What’s making our cities so hot? Is climate change behind this trend? How are cities in different parts of the world coping with this? It is worthwhile to explore these questions, as this issue is becoming more and more relevant for most citizens—over 50% of the global population lives in cities.
Understanding The Difference Between Urban Heat and Climate Change
While the recent incidents of extreme urban heat might tempt one to think they are due to global warming, scientists attribute this to the urban heat island effect. An urban heat island occurs when a city is warmer than its surrounding suburbs or rural areas. It is calculated by taking the difference in temperature between an urban area and its rural surroundings.
Knowing the difference between the heat island effect and climate change is essential. The increase in temperature due to climate change is a global phenomenon, whereas the urban heat island effect is local. Climate change means long-term changes in temperatures and weather patterns caused by the buildup of greenhouse gases, which gradually warm the planet. Urban heat islands are short-term increases in the temperature in the local area due to the city surfaces absorbing heat.
Factors contributing to urban heat islands include:
- Urban concrete and buildings.
- Lack of vegetation.
- Residual heat from factories, air conditioners etc.
Although urban heat islands are not caused by climate change, they can be intensified by the general increase in temperature due to global warming which means one can expect them to be more frequent and intense than before. At least two studies confirm this. A recent study by the World Bank highlighted that the severity and frequency of urban heat islands are much higher today than the historical trend, now being 23 times more frequent and 10 times more potent. Another study in Nature Climate Change estimated future temperature increases due to the combined effect of climate change and the urban heat island effect in 1,692 cities worldwide. The results show that their combined effect could make one in five cities 4°C warmer by 2050.
The Costly Combination Of Urban Heat Islands And Climate Change
The combined impact of climate change and urban heat islands could cost some countries as much as 10% of their GDP or their total economic output. The study published in Nature indicated that warming due to climate change alone could reduce the GDP of the average city in 2050 by 0.9% in high global warming scenarios. However, when combined with the urban heat island effect, this reduction could double, with the worst-off cities losing as much as 10% of their GDP by 2100.
Large cities with tropical climates are particularly vulnerable. The World Bank study analyzed data from 536 cities in Asia from 2012 to 2020 and found that warmer towns experienced a decrease in economic activity by as much as 5.5%.
So, what does this loss in economic output due to extreme heat mean for urban citizens? It could be visible in forms such as loss to infrastructure, productive hours, or even lives. For instance, in the UK in the summer of 2022, there were reports that rail tracks got buckled, causing several delays and cancellations. Tracks expand as they get hotter and can start to curve, called buckling.
How Are Cities Addressing Urban Heat Islands
Since urban heat islands are a local phenomenon, addressing them requires local solutions. Various measures are being implemented worldwide, including increasing tree and vegetative cover, installing green roofs, and using cool pavements. Some interesting initiatives include:
- Community Databases to Track Heat Island Initiatives: Databases like the Environmental Protection Agency’s community database track measures communities take to mitigate heat islands. Similarly, TreeBaltimore, a community-based organization, created a street tree inventory to inform the city’s planting map and help cool heat islands equitably.
- Science-Based Research Projects: Cooling Singapore is a government-supported research project that analyzes the effectiveness of various heat mitigation measures before implementation on computerized models. Such research allows policymakers to examine the efficacy of heat mitigation measures before spending money on them.
- Innovative Urban Infrastructure: China’s Sponge Cities is an initiative which aims to mitigate the impact of flooding by making cities more permeable to absorb, store, and release water. Zhuhai, a city in the southeast part of China, was one of the first cities where this was implemented on a pilot scale. There have been reports that this has helped the city address the flooding problem and urban heat concerns.
- Regulatory Measures: India’s building code has specifications for roof-covering materials to reflect heat. Hong Kong provides clear guidance on urban design features like stepped-back buildings, designated breezeways, and arranging buildings to channel wind.