Pedestrians in Hyderabad shielding their heads from the sun during India's recent heat wave. Mahesh Kumar A./AP

The urban heat island effect is growing in large cities like Delhi and Mumbai, and will exacerbate the risks posed by heat waves.


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This June, about two-thirds of India experienced heat-wave conditions. Indian cities witnessed unusually high temperatures, which caused a spike in heat-related deaths. Although India is no stranger to heat waves, in recent years they have become more frequent, more intense, and of longer duration. This is partly because of the urban heat island effect.

An urban heat island is an area that is substantially warmer than its suburban and rural surroundings. This warming effect depends on various factors—a city’s weather conditions, its geophysical characteristics, and the heat from its buildings, vehicles, and inhabitants. The effect is more severe at night than in the daytime, and intensifies during heat waves. It can operate on different scales: It might occur around a single building, in a neighborhood, or citywide.

Due to the phenomenon, the gap between the daytime maximum temperature and the nighttime maximum temperature in major cities has been declining over the years. Or, in other words, hot cities are not only getting hotter, but retaining more heat after dark.

A NASA map shows temperatures across much of India and Pakistan on June 10, 2019. Delhi reached 48 degrees Celsius, or 118 F. (Joshua Stevens/NASA Earth Observatory)

India has been urbanizing at a very fast rate, a trend that is expected to continue in the future. At present, there are 59 urban agglomerations of more than 1 million people in the country. That number is expected to rise to 78 by 2035.

Rapid urbanization is resulting in dramatic land-use changes. Over the past four decades, the built-up area in Delhi increased by 30.6 percent, while cultivated areas decreased by 22.8 percent and dense forest by 5.3 percent. Mumbai, the financial capital of India, became almost entirely paved and concretized in the span of 40 years. In Kolkata, vegetated areas decreased from 33.6 percent of the city to 7.4 percent between 1980 and 2010. In Chennai, the built-up area tripled between 1991 and 2016, while vegetation decreased by 12 percent.

The Energy and Resources Institute, a research nonprofit based in Delhi, found in 2017 that many fast-growing cities in India have been undergoing the heat island phenomenon, some more quickly than others. (Delhi is already dotted with heat islands.) Urban development—often unplanned and in violation of local zoning laws—and the associated reduction in evapotranspiration are main drivers.

“Covering soil with concrete or asphalt prevents water from evaporating from the soil,” said Mark Jacobson, a professor of civil and environmental engineering at Stanford University. “Since evaporation is a cooling process—it takes energy to evaporate water—reducing evaporation warms the ground. In addition, urban surfaces tend to absorb more sunlight than do soil or vegetated surfaces and convert that sunlight to heat. Thus, it is not a surprise that urbanization in India turns many cities to heat islands,” Jacobson said.

Other contributing factors are sick bodies of water, pollution, and economic activity relying on dirty sources of energy. Black carbon aerosol, or soot—from vehicular emissions and the burning of coal and wood for cooking in poor households (especially in urban slums)—absorbs large amounts of solar radiation, trapping heat.

Using an array of global climate models, researchers at MIT found that, in many densely populated cities in India, heat waves are likely to breach the survivability threshold by the end of the century. The threshold is a wet-bulb temperature of 35 degree Celsius (95 degrees Fahrenheit)—a combined metric of humidity and temperature at which a few hours of exposure can cause death in even very fit people. Urban heat islands will further exacerbate thermal risks from global warming, particularly affecting the health of the roughly 24 percent of India’s urban population living in slums. Even more vulnerable are the country’s 1.8 million homeless people.

“Adding just a degree or two to a heat wave in an already vulnerable region like India can be the difference between life and death,” said Jeremy Pal, a professor of civil engineering and environmental science at Loyola Marymount University, who has authored several research papers on heat waves in South Asia.

Even if global warming is held below 2 degrees Celsius, there is likely to be a significant increase in deadly heat waves, causing many megacities to become heat-stressed. Urban heat islands can affect the severity and duration of heat waves. Death rates during heat waves are often much higher in cities than in their outlying areas, because of the effect.

Growth in air conditioning as temperatures rise compounds the problem. It leads to more greenhouse-gas emissions because India continues to rely heavily on fossil-fuel sources for electricity generation. And as urban heat islands become more intense, the use of water as well as electricity is likely to go up. That will worsen already acute water shortages in Indian cities.

In addition to the other risks to the urban population, urban heat islands also degrade the quality of air and water. Increased energy demand (for cooling) means more air pollution from power plants and more carbon emissions. Higher surface temperatures also lead to ozone formation. When water flows over hot roofs and paved surfaces, that water heats up before it reaches bodies of water, affecting the health of local ecosystems as well as increasing evaporation.

Can the intensity of urban heat islands be checked? Even amid ongoing global warming, targeted strategies could help.

Indian cities could switch to lighter-colored paving or porous green roads and cool roofs, to reflect more solar radiation. (After a severe 2010 heat wave, the city of Ahmedabad implemented a Heat Action Plan, including a cool-roofs program; research has shown this plan has prevented thousands of deaths.) Cities could increase their share of tree cover, which is significantly lower than what’s required to maintain an ecological balance.

People in urban areas could be encouraged to grow climbing plants and curtains of vegetation outside their windows. Greenbelts around cities, for wind paths, would allow the passage of exhaust heat from urban air conditioners and automobiles. Finally, air-quality standards should be enforced rigorously and continuously—not just when air pollution reaches hazardous levels.

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