More trees doesn’t always mean less heat: IIT-Gn study

Ahmedabad: Planting more trees alone may not be enough to cool Indian cities battling rising heat, according to a new study by researchers at IIT Gandhinagar (IIT-Gn). Most urban cooling plans include greening as a major component, as tree canopies not only block direct sunlight but also significantly reduce ambient temperatures. However, not all trees have the same benefits. Researchers found that the cooling impact of trees depends on humidity, airflow, urban density, and the type of climate a city experiences, according to a recent study published in the journal Nature Communications.The research — carried out by Angana Borah, Adrija Datta, Ashish S Kumar, Raviraj Dave and Udit Bhatia — analysed 138 Indian cities between 2003 and 2020 across different climate categories, including tropical savannah, semi-arid steppe and humid subtropical regions.Researchers found that while tree cover and vegetation generally help lower temperatures, the benefits are not uniform everywhere. “Greening is essential for climate adaptation, and shade gives people immediate relief. But one-size-fits-all plantation targets do not address the entire problem,” said Udit Bhatia, associate professor at IIT-Gn.“Cities need greening strategies that are designed together for shade, ventilation and moisture balance,” he added.Angana Borah, a research graduate and co-author of the study, said the key question is not just about planting more trees, but about understanding which type of greenery is suitable, where it should be planted, and how much is needed. “In dry cities, vegetation can provide strong cooling effects. But in humid and compact neighbourhoods, planners also need to think about airflow and moisture build-up,” she said.The researchers recommended integrated urban cooling plans that incorporate parks, roadside plantation, open spaces, shade trees, and ventilation corridors.Unlike many conventional urban heat studies that rely mainly on land surface temperature, the IIT-Gn team reconstructed the ‘Heat Index’, which combines temperature and humidity to better reflect how heat is actually experienced by the human body.To conduct the study, researchers used multiple satellite datasets and urban indicators, including vegetation indices, canopy density measurements, local climate zone mapping and night-time light data. Using explainable artificial intelligence methods, the team generated one-kilometre Heat Index maps to identify urban features associated with heat stress across Indian cities.The study found that vegetation cover and canopy density were generally associated with lower Heat Index values once greenery exceeded certain thresholds. However, in highly humid, densely packed urban areas, extremely high canopy activity can sometimes worsen heat discomfort by trapping humidity.The researchers said better-designed urban greening plans could significantly reduce long-term heat vulnerability for millions of people living in rapidly warming Indian cities.