New threat: city heat

Tarred roads, buildings adding to global warming

By G.S. MUDUR
  • Published 31.05.17
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Pedestrians use a plastic sheet as cover against rain in Kochi on Tuesday. The southwest monsoon hit Kerala on Tuesday, two days ahead of the scheduled arrival. (Reuters)

New Delhi, May 30: Heat trapped by tarred roads and dense clusters of buildings may have added nearly 2 degrees to temperatures in the world's most populated cities, including Calcutta, Delhi and Mumbai, over and above the effects of global warming, researchers said today.

Their study, described as the first to quantify the combined impacts of global warming and the "urban heat island effect", suggests that the overheated cities will face double the climate-change costs encountered by the rest of the world.

The urban heat island effect, which occurs when heat-trapping concrete and tarred roads replace natural vegetation and water, gives rise to local warming that is worsened by the presence of road traffic and air-conditioners.

The study, published on Monday in the journal Nature Climate Change, also suggests that city-level adaptation strategies to curb local warming may significantly amplify national and international mitigation efforts.

"If no local-level actions are taken, the benefits of global actions (on climate change) could be lost by damages caused by local temperature increase," Francisco Estrada, an environmental economist at the National Autonomous University of Mexico and study team member, told The Telegraph.

The study suggests that city-level measures such as designing pavements and rooftops that reflect sunlight, placing greenery on rooftops, and adding vegetation across city landscapes could help counter the urban heat island effect.

Estrada and his co-workers used computer simulations to determine how the urban heat island effect could influence the economy through effects such as increases in air pollution, higher energy demand for cooling and lowered worker productivity.

Their study suggests that changing only 20 per cent of rooftops and 50 per cent of pavements in a city would help reduce average air temperature by about 0.8°C.

In some of the world's most populated cities, including Calcutta, Delhi, Mumbai and Bangalore, the urban heat island effect had added an average 1.72°C to the temperature increase from global warming by 2015, and would add another 0.36°C by 2050, the study says.

About 20 per cent of the 1,692 cities worldwide considered in the study, which included over 150 cities and towns across India, could experience a total warming (over and above global warming) higher than 4°C by 2050, it says. About a quarter of these cities could warm by more than 7°C by 2100.

Estrada has cautioned that the study examined changes at a global scale and yielded only "crude estimates", and future temperature rises and economic losses for specific cities would require detailed analyses of the circumstances in each city.

However, he said, the study has placed Chennai, Hyderabad, Jaipur, Pune and Surat along with Mexico City, New York, Shanghai and Tokyo among the cities likely to experience warming higher than 4°C over the next three decades.

"But city-level adaptation strategies have important benefits," Richard Tol, professor of economics at the University of Sussex and another study team member, said in a media release.

"We have underestimated the dramatic impact local policies could make in reducing urban warming."

The researchers believe that local adaptation strategies may be "politically easier" to implement than global or national mitigation policies. They have underlined that mitigating the urban heat island effect also promises other benefits, such as improved air quality and health.