Mumbai experienced 65 nights, in addition to the usual, of temperatures surpassing 25 degrees Celsius each year between 2018 and 2023 due to climate change, according to a report by non-profit organisation Climate Central released on Friday. The city recorded the highest change in nighttime temperatures among all Indian metros.

Warmer nights make it difficult for individuals to recover from the day-time heat and can have severe health consequences. Hot nights are also associated with increased mortality risks. A lack of quality sleep can lead to increased risk of physical and mental health problems and impairing cognitive functioning, according to the report.

The organisation analysed the rise in nighttime warming globally due to climate change and its impact on sleep quality. Detailed analysis was conducted in India, the United States and the United Kingdom.

Climate Central’s analysis showed that Jalpaiguri and Siliguri in West Bengal and Guwahati, Silchar and Dibrugarh in Assam were the most impacted places in India due to rising temperatures. The cities have experienced an additional 80 days and 86 days with nighttime temperatures above the 25 degrees Celsius-mark.

“Approximately 50 to 80 days were added above the threshold by climate change in cities across Kerala, Karnataka, Chandrapur, Maharashtra, Tamil Nadu, Punjab, Jammu and Kashmir, and Andhra Pradesh,” said the organisation.

The organisation, comprising scientists and science journalists, conducts research and produces reports about climate change.

The changes in nighttime temperatures were analysed as a 2022 study by a team at the University of Copenhagen, Denmark stated that rising temperatures due to climate change can negatively impact sleep. It said that nighttime minimum temperatures greater than 25 degrees Celsius increase the probability of getting less than seven hours of sleep by 3.5 percentage points.

Globally, it said, an average person experienced 4.8 additional days of temperature going above 20 degrees Celsius between 2018 and 2023, driven by higher nighttime temperatures.

In the same period, an additional 11.5 days of temperatures going above 25 degrees Celsius were experienced.

Michelle Young, a climate impacts research associate at Climate Central, said that the analysis showed that over the last decade, the average person on Earth had experienced almost five more nights that were “uncomfortably or even dangerously hot” due to climate change.

“These hot nights prevent people from recovering from extreme heat during the day and are likely to have shortened and disrupted people’s sleep, with a range of serious knock-on effects on physical and mental health,” said Young. “We also know these impacts are not being experienced evenly or equally, with disparities between and within countries based on income, access to air-conditioning, age and other factors.”

She pointed out that higher minimum temperatures overnight made India’s record-breaking heatwave this year “even more deadly”.

“As nighttime temperatures continue to shoot up, there will be more and more of these sleepless nights until the world stops burning fossil fuels like coal, oil and gas and cutting down forests,” said Young.

Aarti Khosla, the director of research and consulting organisation Climate Trends, pointed out that the night temperatures have shown constant and steady rise over the last few years.

“Warm nights have been punishing this summer with several cities shattering five decades of records,” said Khosla. “It’s evident that cities will bear the highest brunt which will get worse due to urban heat island effect.”

Her opinion was echoed by Roxy Mathew Koll, a climate scientist at the Indian Institute of Tropical Meteorology in Pune.

“The urban heat island effect is most visible in the night-time temperatures,” said Koll. “Cities turn into urban heat islands when buildings, roads, and other infrastructure absorb and re-emit heat, causing cities to be several degrees hotter than surrounding rural areas.”

Koll pointed out that high-rise buildings and concrete setups in the cities do not let the excess heat escape during the night.

“Open green spaces and natural environment with trees can help release the heat faster during the night,” he said. “However, in India, we do not appreciate natural space as much as we appreciate skyscrapers.”

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