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How Cities are facing a double whammy of Global Warming and Urban Heat Island Effect?

A conceptual image showing the urban heat island effect in a city
A conceptual image showing the urban heat island effect in a city

Last year the United Nations chief António Guterres in his news brief highlighted the hottest month recorded on Earth and urged that the era of global warming has ended and the global boiling has arrived at our door. Global warming is an anomalous average global temperature rise of the planet Earth caused by the surplus rise in the concentration of greenhouse gases (carbon dioxide, methane, nitrous oxide, hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), ozone etc.) where carbon dioxide (CO2) has the highest proportion. On the 3rd of July 2023 the average global temperature reached 17.01° Celsius (62.62° Fahrenheit), the highest ever recorded, (ref: U.S. National Centre for Environmental Prediction). 

The Urban heat island (UHI) effect is the phenomenon of heat accumulation due to urban development and human activity. This effect has widely created a local pocket of unprecedented heat zones in the urban areas during summer. The UHI phenomenon enhances the temperatures in metropolitan regions than adjacent non-urban areas. Lake Howard, an English academic who studied the urban climate in London at the beginning of the 1800s, was the first to measure and explain the UHI impact. Since then, a large number of academics from all around the world have carried out in-depth studies on the features of the UHI effect.

The combination of global warming and the heat island effect makes most of our cities an unwanted place to live during summer. IMD's latest briefs indicate that northern India is officially facing the second heat wave spell in April. Because extreme weather events like heat waves, storms, and floods are becoming more often and intense, climate change is expected to exacerbate urban health hazards and economic inequality and will eventually lead to disruption of urban ecosystem services. The majority of large Indian cities face heat waves every summer that cause heat exhaustion, heat stroke, heat syncope (fainting), and even fatalities in its inhabitants, particularly those living in impoverished neighbourhoods.

Urban heat islands: why do they form?

Three distinct levels are used to measure the urban heat island effect: the ground level, the canopy level, and the urban level. The UHI effect is specifically influenced by the thermal behaviour of surface materials and their exposure to sun radiation. Numerous features contribute to the formation of urban heat islands.  First, a city's reduced ability to cool itself through evapotranspiration is a result of the loss of tree cover. Buildings, particularly tall ones, offer various surfaces that both reflect and absorb solar heat. The most common materials in cities are concrete for most structures and asphalt for roads, both of which have scientific properties that allow them to absorb far more heat than the surrounding rural areas. All these phenomena led to the development of an urban microclimate in the city. The climate conditions in the lower atmosphere of urban regions, which are impacted by land use, building geometry, urban shape, and human activity, are referred to as the urban microclimate. The effect of land use transformation can be directly observed in urban areas as a change in the microclimate of the particular cities leads to an increase in the albedo, which impacts the warming of cities.

How to reduce the UHI effect?

Reducing urban heat islands with vegetation is an easy and efficient solution, as trees and other plants aid in cooling the surroundings. Evapotranspiration from trees and forests reduces air temperature and increases relative humidity, significantly altering the microclimate and thus cooling cities. According to research, the average temperature of urban forests is 2.9°F lower than that of unforested metropolitan regions. Trees and forests can reduce our energy usage by directly providing shade, improving air quality, help in the sequestration of excess carbon dioxide (greenhouse gases) produced in the city. Making cool islands of water bodies in the cities. Because water bodies have a higher specific heat capacity than other materials on the surface of the land, they warm more slowly than their surroundings. The cooler air on the water bodies convects to the surrounding areas through air exchange, producing a cooling effect and on occasion, this cooling distance can reach one thousand meters. These water bodies will also help in groundwater recharging and can address the issue of drinking water in a city. Recently in India Bengaluru in India faced a severe water crisis, which is a consequence of losing its lakes and ponds due to urban development. Cities are economic hubs for a nation which needs to be developed and evolve always but having a sustainable and greener approach can benefit a lot in the mitigation of heat island effects as well as global warming. The use of building materials with poor optical and thermal properties, rising anthropogenic heat, and increased air pollution are the major factors influencing the urban microclimate. Green infrastructure like the rooftop of a building that is partially or fully covered with vegetation and a substrate for plant growth, vertical walls partially or fully covered with plants. Coating material for building purposes should be designed to mitigate the heat island effect. Utilization of coating materials with the ability to both store and release latent heat to reduce peak surface temperatures and increase the perceived thermal capacity of buildings and urban structures can be beneficial.

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