The temperature reached 45.7°C (114 Fahrenheit) on Sunday at Prayagraj, in the East Uttar Pradesh of northern India, as a heat wave affected much of the country. Temperatures began to ease in Kolkata, a city identified by the Intergovernmental Panel on Climate Change (IPCC) as the world’s most threatened by rising urban heat, but the heat wave — and the ominous future of which it warns — has renewed concerns over how to keep India’s cities cool.

“We need policies considering the fact that heatwaves are here to stay and intensify,” says Roxy Mathew Koll, a climate scientist for the Indian Institute of Tropical Meteorology, writing for The Hindu. “We need to redesign our cities to have open spaces and trees that help release the excess heat quickly and also act as hubs for shade and cooling down.”

The National Disaster Management Authority (NDMA) promotes strategies meant to reduce urban heat island impacts from densely packed buildings, concrete and asphalt materials, and higher population numbers. They’re designed to limit temperatures that soar in the daytime but then fail to cool overnight, trapping people in cycles of heat that can (and will) prove fatal.

The NDMA recommends cooling techniques that include planting more trees to improve the amount of city protected by tree canopy. The daytime heat in a city can vary by as much as 5°C between shaded areas and parts of a city without trees, says NDMA.

Using more green roofs and green walls to reduce building heat naturally, along with other passive cooling techniques, will reduce the demand for air conditioning and related carbon emissions. And in Kolkata, where research finds temperatures have risen 4.72°C over the last 30 years due to urban sprawl, scientists want to know if these actions will best protect the more than 15 million people who now live there.

A recent study led by Ansar Khan of the University of Calcutta relied on weather research and forecasting models to assess the impact of climate mitigation actions to yield “cool city” impacts in Kolkata. The international research team, with scientists from India as well as Japan, Australia, Spain, Belgium and the United States, found that decisions to plant more trees and build green rooftops did, in fact, lower urban heat impacts, especially at night.

“Our results showed that surface and rooftop heat mitigation strategies modify the meteorological fields and the dynamics of the lower atmosphere within the city during the hot summer days,” said the researchers, who published their work in the Journal of Geophysical Research: Atmospheres.

Super-cool broadband radiative coolers (next-generation compounds and highly reflective films) appear to deliver the greatest benefits and perform better than earlier white paints, for example. They require fairly low humidity levels but had “good cooling potential despite the fact that Kolkata is a tropical wet and dry climatic city,” the authors said. The precise geography and microclimate matter, and they may not work for all buildings, but they delivered a temperature that was 1.6°C lower during a peak heating hour of 2 p.m.

Green roofs offered cooling benefits and were also likely to help improve air quality but they weren’t without their drawbacks, including the need for irrigation and the increased humidity that plants added to the city. Lawns and trees also offered urban heat benefits, notably in cooling down temperatures overnight. The significant benefits of planting in the model showed “a realistic scenario that maximizes vacant spaces” already available in the city.

Overall, the research confirmed benefits of the various technologies to lower heat in Kolkota, especially for super-cool materials and combinations of “cool city” techniques, but the authors note research will continue as India’s heat continues to climb.

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