Warmer nights, rising humidity: Why heatwaves are becoming more deadly

The Intergovernmental Panel on Climate Change (IPCC) Working Group-II, which assesses impacts of, and vulnerabilities to climate change, has noted that the urban heat island effect leads to cities experiencing air temperatures that are several degrees warmer than surrounding areas, especially during the night, “reducing [their] adaptive capacity”.

Urban heat island effect and increase in nighttime temperatures

Structures made of concrete — buildings, pavements, roads, and other infrastructure — absorb and re-emit the sun’s heat more than natural features like trees and water bodies. Cities, where these structures are concentrated, and greenery is limited, thus become “islands” of higher temperature compared to surrounding, greener, areas.

This is called the urban heat island effect. And since heat is trapped in built-up structures, greater urbanisation has a direct correlation with the rise in night temperatures.

Soumya Satyakanta Sethi and V Vinoj, scholars from the Indian Institute of Technology, Bhubaneswar, analysed nighttime temperatures across 141 Indian cities for the period 2003-2020. Almost all of them witnessed an increase in nighttime temperatures, with the mean rate for temperature increase being 0.52+/-0.19 degree Celsius, per decade. “Urbanisation alone has led to an overall 60% enhancement in warming in Indian cities,” the study published in the journal Nature stated.

Another study, by Anumita Roychowdhury, Somvanshi and Sharanjeet Kaur for the Centre for Science and Environment (CSE), found that the difference between day and night temperatures in urban areas has reduced over a period of time. Comparing the daytime and nighttime temperatures, the study found that nights were 6.2-13.2 degree Celsius cooler in the 2001-10 decade but only 6.2-11.5 degree Celsius cooler in the 2014-23 period. With the exception of Kolkata, all cities saw a decline in nighttime cooling.

Aditya Valiathan Pillai, of the Sustainable Futures Collaborative, said that long term solutions such as planting more trees and reducing the density of built areas are important to combat the urban heat island effect. “Today’s heat is yet another reminder of the terrible toll climate change will exact on India’s most vulnerable,” he said.

How humidity and warm nights impact the human body

The rise in temperatures, humidity, and warmer nights together make the summer more deadly for human beings. Humans regulate their body temperatures by sweating — the evaporation of sweat from skin cools our bodies. But high humidity prevents this from happening, which can exact a toll on the body.

Dr Dileep Mavalankar, honorary professor and former director, Indian Institute of Public Health, Gandhinagar explained further. Reacting to heat, “the heart starts pumping more blood to our skin. This activates the sweat glands which help regulate body temperature. But as a result of sweating, other organs get less blood flow,” he said.

In humid conditions, the body continues to sweat with little avail. This not only leads to dehydration and salt imbalances, but also takes a toll on organs due to reduced blood flow. And of course, it leads to the body overheating as its temperature-regulation mechanism is not working.

Dr Mavalankar added that “If the temperatures are too high, the cellular processes in the brain also get affected. This can initially lead to fainting, and in more serious cases, effects such as heat stroke and organ malfunction… At the very basic level, more humidity and warmer nights do not allow our bodies to rest and recuperate”.

A 2017 study published in the journal Science Advances found that the rise in summer mean temperatures in India (from 1960-2009) corresponds to a 146% increase in the probability of heat-related mortality events of more than 100 people.