Latest news with #RonitaBardhan
Forbes
5 days ago
- Science
- Forbes
Hot In The City: Why The Wrong Trees In The Wrong Places Matters
Cooling down: New York has recorded temperatures of nearly 100 degrees Fahrenheit In the last days of last month, the death toll tripled in several major European cities in the face of sweltering heat. Milan, Paris, Barcelona and London were the worst hit. With more than two thirds of the world's population expected to live in cities by 2050, the race to cool them down is on. Tree planting has become an important part of urban heat mitigation efforts. But a Cambridge University-led study shows that planting the wrong species or combinations of trees may not just reduce the benefits, but actually make things worse, elevating night time temperature. The scientists found that planting trees in urban areas can lower pedestrian-height air temperature by up to 12 degrees centigrade (53.5 F). Introducing trees reduced the highest temperature in 83% of the cities studied. However, the extent will vary, depending on urban layout and species' traits. 'Our study challenges the common perception that trees are the ultimate panacea for overheating cities worldwide,' says Professor Ronita Bardhan from the University of Cambridge. 'Trees have a crucial role to play in cooling cities down, but we need to plant them much more strategically to get the best results.' For instance, adds Bardhan, 'The cooling behaviour of a species in a rural environment may be very different in an urban setting. It also matters where these trees are placed. In more sparsely populated areas they will do well, but in heavily built-up areas, they won't.' Until now, we've known relatively little about unique tree cooling mechanisms and how these interact with different urban features. Previous research has tended to focus on specific climates or regions, relying on fragmented case studies. The first comprehensive global assessment of urban tree cooling has changed all that. The authors looked at a wide range of studies over 17 climates in 100 cities and regions for the period 2010 – 2023. Understanding why planting matters is the first step in getting green right. How Trees Cool Cities In daytime, trees help reduce temperature by blocking solar radiation while water evaporation and foliage alters the airflow. In some circumstances, however, the 'wrong' trees can also increase air temperature at night. 'When the aerodynamic resistance is high and the leaf stomata close, heat dissipation through transpiration is reduced, limiting the escape of heat energy,' Bardhan says. 'This typically happens at night. If there is a high humidity load, this humid-warm air can be trapped and re-circulate beneath the tree canopy.' The scientists found that urban trees in compact settings are most effective in cooling cities in hot and dry climates but are less effective in hot, humid ones. In tropical wet and dry climates with distinct wet and dry seasons, such as central Africa, parts of South America and northern Australia, trees can be very effective in cooling cities by day. In Nigeria, trees can cool temperatures by as much as 12 degrees centigrade. But at night, trees warmed cities the most, by up to 0.8 degrees. Evergreens beside Dubai's International Financial Centre A lush green canopy hangs over parts of Dubai City. Among the acacias, neem, olive, palm and other desert trees the evening air carries the scent of Indian jasmine. Last year the municipality planted 216,500 new trees as part of its ambitious 2023 Quality of Life Strategy. Bardhan and colleagues found that trees performed well in arid climates such as Dubai's, cooling cities by just over 9 degrees and warming them at night by 0.4 degrees. Elsewhere, however, in a tropical rain forest climate with higher humidity, daytime cooling was only 2 degrees. Closer to home, trees in more temperate climates, such as London, can cool cities by 6 degrees but warm them up by 1.5 degrees. In many hot countries, trees tend to be evergreen. By including deciduous trees in the mix, the cooling effect can often be greater. Recent planting schemes in Saudi Arabia have incorporated deciduous as well as evergreens. The nature of the local urban environment will have an impact on the potential cooling while the idea balance of temperate and deciduous trees will vary. Urban layouts such as London's are more likely to benefit from the inclusion of deciduous trees alongside evergreens, but to a lesser extent than in Saudi Arabia. 'If you plant deciduous trees in a densely populated part of London, they won't perform in the same way as in the wild,' Bardhan observes. 'It's also important where these trees are placed. Trees may perform well in sparsely populated areas, but in dense urban settings the limited cooling impact reflects failed design. "To be effective for the climate, cities need to treat nature as a fundamental design variable, rather than an afterthought.' Selecting the optimum type and location for trees in an urban setting is critical. Few cities can afford projects on the scale of Dubai's and those who may need the right trees most because of global warming – are often the poorest. Bardhan and her colleagues at Cambridge have come up with a solution. They are developing a prototype shading device, inspired by nature's way of cooling. It's already been tested out successfully in dense urban areas. 'It mimics everything that natural trees do,' explains Bardhan 'We can control humidity along with temperature unlike standard shading devices that only regulate temperature. The design also allows the release of trapped radiation at night, unlike a tree.' The shading device can resemble a tree or take other forms, such as a vending cart. Rather than just being a luxury in affluent neighbourhoods, shade and cooling can come to schoolchildren and the most vulnerable communities at relatively low cost. As Bardhan says, it's climate action put into reality. With the data provided by the Cambridge study, urban planners have an invaluable tool that could transform efforts to reduce heat. You can see the cooling effect of trees in the cities researched on the interactive map here.
Hindustan Times
16-06-2025
- Climate
- Hindustan Times
Mean summer temp in state rose 1.2°C in 70 yrs: Study
Mumbai: A long-term assessment on heat trends and vulnerabilities in Maharashtra shows that despite advances in the arrival of the monsoon in recent years, instances of extreme heat are on the rise. Maximum temperatures during summers as well as prolonged stretches with temperatures above 40° Celsius have risen substantially over the past seven decades, the assessment shows. This signals a shift from episodic to chronic heat exposure, with profound public health consequences for vulnerable groups including infants, elderly people, those with chronic illnesses, and millions of informal sector workers, said experts. As per the report compiled by the India Meteorological Department (IMD) and public health experts using 72 years of data (1951–2022), the mean summer maximum temperature in the state has risen by over 1.2°C – from 36.3°C in 1951 to 37.5°C in 2022. The number of days with temperatures exceeding 45°C have also spiked, especially in May – the state's hottest month. While such temperatures were virtually absent between 1951 and 1970, the trend escalated dramatically in the 1980s, and peaked in 2010 and 2015, when the number of extreme heat days were 65. The spike was especially pronounced in the Vidarbha region, in districts like Wardha and Yavatmal, marking them as persistent high-heat hotspots. 'We compiled this dataset from IMD and conducted a detailed region-wise analysis to understand the steady rise in temperatures and localised impacts,' said a senior official from the Maharashtra Disaster Management Department who was involved in the assessment. 'This dataset will now form the foundation for targeted climate-health policies. Since temperature patterns evolve over decades, this data will remain valid for years.' Soaring mercury According to the assessment, the mean maximum temperature in the state for May has increased from 40.17°C in 1951 to 41.19°C in 2019. Prolonged heat spells, defined by temperatures exceeding 40°C for more than two days, have also increased sharply during this period – while there were only 223 such instances in 1951, by 2022, the number had more than doubled to 451, with districts like Buldhana, Jalgaon, Hingoli, and Washim among the worst affected. Such long exposures to heat lead to electrolyte imbalances, acute kidney injury, cardiovascular events, and even heat stroke-related delirium, said experts. 'A one-degree rise in average temperature may mean an actual micro-level temperature increase of 5-6°C,' said Ronita Bardhan, professor of sustainable built environment at the University of Cambridge. 'In densely populated cities like Mumbai, indoor temperatures may soar even higher, especially in slum tenements made of metal sheets, turning homes into death traps.' Rise in the mercury raises skin temperatures and increases vulnerability, particularly for women who spend significantly more time indoors, said Bardhan. 'Biological and social factors compound risks,' she said. 'For instance, women sweat less than men and often avoid drinking water to limit bathroom visits due to lack of access, which accelerates dehydration.' Ilan Kelman, professor of disasters and health at University College, London, echoed Bardhan on gendered disparity with regards to impact of heat. 'In India, it is often socially acceptable for men to urinate in public. But women can't,' said Kelman. 'So women working away from home tend to drink less to avoid using the toilet. That puts them at higher risk of dehydration and heatstroke.' The combination of high heat and humidity was particularly dangerous, said Kelman. 'Fans blowing hot, humid air can actually accelerate dehydration, especially when nights remain hot and people don't cool down,' he said. 'Cognitive impairment is often the first sign – people stop recognising they need to rest, hydrate, or find shade. Without intervention, it progresses to multi-organ failure and death.' Urban centres with increasing asphalt sprawl, reflective rooftops and declining tree cover comprise a separate but equally urgent crisis, said experts. 'For many poor communities, artificial cooling isn't even an option,' Kelman said. 'Informal settlements lack the infrastructure to support air conditioning. Even indoor garment workers are vulnerable in poorly ventilated buildings that heat up rapidly and offer no relief.' Climatic shift Dr Dileep Mavalankar, former director of the Indian Institute of Public Health in Gandhinagar and a key architect of India's first heat action plan for Ahmedabad, said India was no longer dealing with seasonal extremes but confronting a fundamental climatic shift. 'There's no denying the intensity of Indian summers,' he said. 'Yet today's heatwaves are not seasonal fluctuations—they are the clearest signals of a changing climate. Heatwaves are arriving earlier, lasting longer, and pushing beyond previously recorded limits.' Many heat-related deaths, especially those related to cardiovascular collapse or worsening of chronic respiratory conditions, don't show up in official records, said Dr Mavalankar. 'Heat doesn't only kill through heatstroke,' he noted. 'It first impairs cognition and by the time people seek care, it's often too late.' The under-reporting of heat-related ailments and deaths is a major barrier in both preparedness and accountability, he noted. Anticipatory intervention 'Currently, we're in a reactive mode, but there's an opportunity in this urgency,' Bardhan said. 'We must integrate thermal resilience into housing schemes like 'Housing for All'. These can become invisible infrastructure, especially for women who are disproportionately exposed to indoor heat.' Though Maharashtra has introduced a state-level heat action plan, execution remains patchy and key interventions – such as public hydration booths, early warnings, shaded workspaces, and awareness campaigns – often fail to reach remote villages and urban slums. Rural health centres also often report shortages of oral rehydration salts (ORS) and cool intravenous fluids during peak summer. 'Maharashtra must urgently shift from crisis response to anticipatory governance,' said Dr Malvankar. He cited the example of implementing the heat action plan in Ahmedabad, which showed that timely early warnings, public awareness, and access to hydration could save lives. 'Unless we scale these interventions to both rural and urban districts, we will continue to grossly underestimate the toll,' he warned.
