A recent study by the Delhi-based think tank Centre for Science and Environment (CSE) has found that heat waves are intensifying in India's major cities due to rising temperatures and increasing humidity, posing a significant threat to public health and urban life
A new study by Delhi-based think tank Centre for Science and Environment (CSE) has uncovered a pressing issue in India’s major cities: Heat waves are intensifying due to a combination of rising temperatures, increasing humidity and urban expansion. The research reveals a disturbing trend: cities are not cooling down at night as much as they did during 2001-2010. This trend poses a serious threat to public health and urban life.
The study, Decoding the Urban Heat Stress among Indian cities, analysed data from six major cities – Delhi, Mumbai, Kolkata, Hyderabad, Chennai and Bengaluru — over 23 years (January 2001- April 2024). It found that while air temperatures haven’t risen significantly in some cities, increased humidity is worsening the heat stress people experience.
Heat stress, a condition resulting from a combination of air temperature, land surface temperature, and relative humidity, is not just about discomfort. It leads to acute thermal discomfort and increases the risk of heat-related diseases in cities. Even if air temperatures vary, high relative humidity and land surface temperatures can exacerbate this discomfort, posing a serious threat to human health.
The United States National Weather Service states that the heat index, which includes humidity, indicates how hot it feels, with a heat index of 41 degrees Celsius being dangerous for human health.
Assessing the changing trend in heat, relative humidity, land surface temperature, and day and nighttime temperatures is necessary to develop a comprehensive heat management plan for urban centres.
Further, it is necessary to implement emergency measures during heat waves to protect public health. She also added that she would develop longer-term strategies to mitigate heat by increasing green areas and waterbodies, improving thermal comfort in buildings, and reducing waste heat from vehicles, air conditioners, and industries.
As a result, people can suffer heat stress and illness, occasionally with fatal consequences even at much lower ambient temperatures. Interestingly, nighttime temperature remains elevated in cities, he added.
The culprit behind rising humidity? Uncontrolled urban sprawl. The study found a direct correlation between an increase in built-up area and a rise in urban heat stress in all six cities. For example, Chennai has seen its green cover shrink by nearly 14 percent in the last two decades, while concretisation has doubled.
This trend creates a harmful cycle as concrete absorbs and retains heat, contributing to the "urban heat island effect" in cities, making them significantly warmer than surrounding areas.
Hotter nights are another worrying finding. While cities used to cool down considerably at night, the study shows that nighttime cooling has significantly reduced in recent years. This means people get little respite from the daytime heat, further impacting their health.
The consequences of this deadly combination are dire. The study has flagged a surge in the number of days with a heat index exceeding 41°C (considered dangerous for human health) in cities like Kolkata and Chennai. Moreover, monsoons are no longer providing relief, with some cities experiencing hotter and more humid conditions during this period compared to pre-monsoon months. This underscores the urgent need for action and the potentially catastrophic outcomes of inaction.
The CSE study underscores the need for immediate action to combat this escalating threat. It advocates for the implementation of city-specific heat management plans that extend beyond emergency response during heat waves. These plans should focus on reducing heat generation from buildings, vehicles, and industries and increasing green cover and water bodies in cities to create a cooling effect. The time for action is now.
The study also called for reversing land use by increasing green cover and water bodies in cities to create a cooling effect. Strengthening scientific monitoring of heat-related factors like land use and surface temperatures and developing emergency action to shield vulnerable communities from heat waves is also important.