Cities and urban heat island effect

Urban heat island makes cities hotter

Global warming is not the only warming impacting the planet. Rapidly urbanising cities are causing localised warming which is affecting our well-being. There is a need for understanding this urban heat island effect so it can be mitigated early on for ensuring sustainable urbanisation.

The transition from agrarianism to urbanism brought the ideas of economic prosperity, technological advancements and higher standard of living to the forefront. Towns emerged and rapid urbanisation spearheaded the development of densely populated and larger cities. According to Census 2011, there are 53 urban agglomerations in India with over one million population as compared to 35 in 2001. The number of million plus cities in India continues to rise due to the pace of economic growth and development. What is also becoming prominent is the creation of urban heat islands – an exclusive by-product of this rapid urbanisation.

What is urban heat island?

Urban development results in the clearing of natural surfaces such as grass, plantations, soil and water bodies. These are replaced with artificial materials such as concrete, metals and glass. Unlike natural materials, artificial materials absorb more heat from the sun during the day and slowly release the stored heat during the night. As a result, cities pop out as distinct areas (or islands) of higher average temperature than the surrounding rural areas. This can be verified through satellite imagery of the Earth’s surface.

The temperature differences between the urban and nearby rural regions have been found to range between 0.5 to 5 degrees Celsius. In some case, the upper limit of this range has been found to be as high as 10 degrees Celsius. This phenomenon of unusually high temperature in cities as compared to the nearby rural and peri-urban regions is known as urban heat island effect. Urban heat islands are more prominent during the night when the heat trapped by buildings and other urban infrastructure is slowly released to the immediate surroundings.

Is urban heat island good or bad?

When cities become several degrees hotter than their rural counterparts it causes greater thermal discomfort to the millions of its citizens. This is particularly a problem during the summer season. Moreover, development of urban heat islands during night time severely limits any relief from heat exposure faced during the day. More energy is spent by utilising cooling systems (e.g., ACs, fans, and coolers) to provide thermal comfort. A higher energy demand also puts pressure on the government to produce and procure more electricity. Effectively, inhabitants of cities exhibiting stronger urban heat island effect also have a significantly higher greenhouse gas emission footprint than if they were to live outside the city.

The situation becomes worse for outdoor workers (e.g., construction workers, traffic police, door-to-door delivery persons), the urban poor and homeless people. The more vulnerable groups such as children, elderly and those who are sick are also at greater risk. Urban heat islands are documented to cause higher incidences of heat exhaustion, dehydration, strokes, and cardiovascular issues as well as a high mortality rate among vulnerable age groups.

Some studies even note that human mortality increases by 1-3% for every one degree rise in temperature. For city dwellers, the heat stress from urban heat island effect is worsened when they have to also deal with increasing incidences of prolonged and severe heatwaves. Studies suggest that continued global warming will cause future heatwaves to become stronger, more frequent and longer in duration. India has already begun preparations for combating the impacts of heat wave especially during the summer months.

What it means for urban wildlife?

Cities are already a challenging habitat for urban wildlife such as street dogs, cats, squirrels, bats, and birds. The survival of urban wildlife is threatened more in the significantly heat stressed cities. Natural shelters like trees, wetlands and parks for urban wildlife are fewer in cities and are further reducing due to rapid urbanisation. Urban heat islands cause thermal discomfort, heat exhaustion, and severe dehydration to the urban wildlife. Many cases of urban wildlife deaths are reported during extreme summers and severe heatwave events.

Is there a solution?

The easiest solution is to think green. A growing body of evidence suggests that besides moving towards urban sustainability, integration of urban forests into cities has numerous socioeconomic, cultural, and environmental benefits. Trees and green cover cool the surrounding environment by bringing down the air temperature in its vicinity. Trees also provide shade for people, shelter and food for urban wildlife, and provide many other ecosystem services.

Preserving and developing urban forests, biodiversity parks, rooftop and vertical gardens and functional wetlands can create significantly cooler areas within a city. These can also help provide thermal comfort to urban dwellers in heat stressed cities. Additionally, in a study covering urban trees, peak power, and energy savings through cooling, it was found that gardens and street trees adjacent to buildings contribute to 27-30% lower energy demand.

Above all, inclusion of urban greening helps us to reconnect with Mother Nature. Green landscaping is a new trend in many cities of India and should continue to be promoted. Apart from urban greening, other urban heat island mitigation strategies include cool roofs, cool pavements, artificial water bodies, green architecture, and sustainable buildings.

City design is an important factor in dissipating the accumulated heat. Inclusion of evenly spaced urban greening at regular intervals, throughout the city, will provide greater thermal comfort to the city dwellers than a few isolated patches of urban forests.

Madhavi Jain

Dr. Madhavi Jain completed her Master's degree in Environmental Studies from the University of Delhi and her Ph.D. degree in Environmental Science from Jawaharlal Nehru University, Delhi. Her research work focuses on rampant urbanisation faced by cities and its manifestations on local weather.

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