An often-underestimated consequence of man-made climate change is heat. More extreme heatwaves than ever before have been attributed to climate change, causing health issues, fatalities, both environmental and financial damage. Cities are particularly vulnerable, as they retain significantly more heat than their non-urban surroundings. Using the example of Dhaka, Bangladesh, this blog examines the Urban Heat Island (UHI) Effect and points towards possible solutions.
Urban Heat Island Effect
The primary cause of UHI is that urban areas are largely covered by asphalt and concrete, absorbing and retaining from the sun. Additionally, buildings and infrastructure block natural air flows, trapping heat and creating a “heat island”. Other urban systems, such as transportation, industry, and air conditioning, further contribute to the overall temperature increase.
In a city like Dhaka, the world’s fourth-largest city, temperatures can be up to 7ºC higher during the day and 5 °C higher at night compared to nearby non-urban areas, due to the UHI effect. The effects in Zürich are similar. In Dhaka, where average temperatures during the hottest season are around 34 °C and extremely high humidity, perceived temperatures are even higher. During heatwaves like the one in 2024 with temperatures surpassing 40 °C, conditions become nearly unbearable.
Negative consequences of the Urban Heat Island effect
The consequences of UHI have wide-ranging implications. Heat waves in cities can be significantly more intense and prolonged than in their surrounding areas, posing an elevated risk of exhaustion, heat stress, mental stress and various heat-related illnesses. Among the most vulnerable are older people, children, individuals with preexisting medical conditions, homeless people, individuals with little financial resources and outdoor workers. The scorching heat restricts them from working outside during the daytime, depriving them of crucial income. They rarely have the possibility of taking out insurance and oftentimes live in badly isolated or poorly constructed houses in the most exposed areas. Further, they often lack access to information and are missed when spreading heat alerts. In addition to the adverse effects on human health and economic conditions, UHI also have detrimental impacts on ecosystems and wildlife, as well as loss and damage to the ecosystem, as many species struggle to adapt to the rapid changes in temperature.
Ways forward: Solutions to adapt to and mitigate the Urban Heat Island effect
Cities can proactively address the UHI effect through various adaptation and mitigation strategies. One of the most effective is increasing green cover through tree planting and creating green spaces, which provide shade and cool the air through transpiration. In this process, water evaporates from leaves and dissipates heat from the environment. However, Dhaka’s green cover is estimated at only 8%, far below the recommended 20%. Additionally, green roofs and walls can provide an extra layer of insulation, thereby cooling buildings and absorbing excess heat. In addition, reflective roofs can lower surface temperatures by reflecting solar radiation, and pervious pavements filter water, leading it effectively back to the water cycle. Reducing vehicle emissions and promoting public transportation is also essential for limiting heat buildup.
Incorporating UHI reduction measures into building codes, urban planning regulations, and heat response plans is essential for long-term climate resilience. Dhaka North City Corporation (DNCC) recently took a significant step by appointing a Chief Heat Officer, raising awareness of heat as a disaster. Although not directly tied to the UHI effect, this initiative reflects an increasing awareness of heat-related challenges. DNCC’s pledge to plant 200,000 trees is a commendable step toward heat mitigation, but a holistic approach is essential. A comprehensive heat action plan and measurable development indicators — such as trees per capita — can ensure effective and sustainable solutions to mitigate the urban heat island effect.
Sensor technology and standardized and comparable data collection and analysis can help identify urban heat hotspots and guide targeted mitigation efforts. Well-functioning early warning systems, including effective communication channels that reach the most vulnerable populations, are equally essential.
The UHI effect poses a growing challenge for cities worldwide, impacting human health and the environment. With adaptation and mitigation strategies, cities can alleviate the effects of UHI and create livable and sustainable urban environments and build a healthier, more resilient future for all.
Picture by Mirjam Grünholz with ChatGPT
