Number of warm nights in Indian cities leapfrogged 32% during last decade due to climate change: Global report

The number of warm nights with minimum temperature — equivalent to night-time temperature — has increased critically throughout India over the last decade due to rising climatic impacts, a global study released on June 21 showed.

Jalpaiguri in the Dooars of northern West Bengal tops the list of most impacted cities, along with cities in Assam.

The study covered around 300 Indian cities with populations of over 100,000. It found that, on average, the number of hot nights has increased around 32 per cent due to climatic impacts.

Indian cities, on average, had 718 nights during 2014-2023 when the minimum temperature reached 25 degree Celsius (°C), according to the analysis carried out by non-profit Climate Central and consulting initiative Climate Trends.

The figure is a benchmark for an unusually heated night, as sleep gets disturbed above that limit. According to the study, which considered 24 global climate assessment models, the average number of nights with 25°C or above comes down to 543-175 days and a whopping 32 per cent less if climate impact is taken out of the equation.

“Climate change has had a major influence on night-time temperatures above 25°C in India … West Bengal and Assam are the regions that have been most impacted, with cities like Jalpaiguri, Guwahati, Silchar, Dibrugarh and Siliguri experiencing between 80 and 86 additional days each year above the 25°C threshold due to climate change, on average,” reads the report, a copy of which is with this reporter.

“The urban heat island effect is most visible in the night-time temperatures. The high-rise buildings and concrete setup in the cities do not let the excess heat escape during the night,” pointed out Roxy Mathew Koll, a climate scientist with Indian Institute of Tropical Meteorology, Pune.

“The trend, which you mention, is hundred per cent correct. The cities in eastern and Northeast India are most affected by climate change as expressed through heightened night-time temperature. The latest IPCC AR 6 reports also subscribe to the trend,” said K J Ramesh, former director general of the India Meteorological Department (IMD).

An alarming trend

“Several studies have already established that by the turn of the century, without very large reductions in fossil fuel burning, night-time temperatures will not fall below 25°C in some places during hot weather … If we do not act now, nights would continue to be hotter, longer and sleepless, especially for the vulnerable,” observed Aarti Khosla, director of Climate Trends.

Climate change has had a major influence on night-time temperatures above 25°C in India, pointed out the new analysis. It added that approximately 50 to 80 days were added above this threshold by climate change each year between 2018 and 2023 in cities across Kerala, Karnataka, Maharashtra, Tamil Nadu, Punjab, Jammu and Kashmir, and Andhra Pradesh, apart from West Bengal and Assam.

The study shows that Jalpaiguri had the greatest number of unusually heated nights in India during the last decade due to the direct impact of climate change. The north Bengal city experienced night-time temperatures exceeding 25°C on 868 days during 2014-2023 due to climate change. 

Guwahati, Silchar and Dibrugarh in Assam occupied the second to fourth positions in terms of unusually hot nights in India in the last 10 years. Siliguri, Jalpaiguri’s twin city in north Bengal, came fifth with 810 nights crossing 25°C attributable to climate change. 

Overall, 16 cities crossed 25°C for 920 days during the period and 11 other cities during 919 days. The list includes metros like Chennai and Kolkata as well as cities like Port Blair, Puducherry, Puri, Surat, Visakhapatnam and Patna among others.

Several Bengal cities also are part of the list namely Haldia, Howrah, Medinipur and Uluberia. Among metro cities, Mumbai saw the highest changes in night-time temperatures. The city experienced an additional 65 days of warmer nights per year due to global warming.

IMD sources point out that the minimum temperature in Jalpaiguri shows a slightly increasing curve since the early twentieth century. However, they observed that data of a longer duration is required to establish the climate connection categorically.

However, locals vindicate the finding. “The finding is consistent with our ground level experience in north Bengal with heat and humidity combining to push up the real feel of the heat,” said Animesh Bose, a Siliguri-based environmentalist.

The report also shows that if the benchmark of 20°C of minimum temperature is considered as indicator of a warm night, then Gangtok and Darjeeling enter the list too. They had 54 and 31 additional days added every year on average over the last decade due to climatic impacts respectively. 

These findings have come up during a week that saw new records for night-time heat in several Indian cities including Delhi where, on June 19, the all-time high minimum temperature record got shattered with the mercury reaching 35.2°C overnight. On June 18, Alwar in Rajasthan had a minimum temperature of 37°C, the highest ever night-time temperature since records began in 1969.

The heatwave spell in India this year has been made hotter, frequent, and more likely by climate change, scientists found earlier.

Poor most vulnerable 

Anjal Prakash, a professor at the Indian School of Business and a former author with the Intergovernmental Panel on Climate Change, the UN scientific body working on climate change, observed:

“As global warming adds an additional 50 to 80 warm nights annually in states such as West Bengal, Assam, Kerala and others, the impact on the poor and vulnerable sections of society, without access to proper cooling mechanisms, is profound.”

“Hot nights make it harder for people to recover from the heat of the day, with wide-reaching and severe consequences. There is growing evidence that as night-time temperatures rise, human sleep is being eroded around the world (with) a range of associated impacts,” states the report.

Physician Arup Haldar, a sleep expert, pointed out that “when the environment or room temperature is warmer than usual, it’s more difficult for the body to decrease its temperature for sleep onset … that’s why it is harder to fall and stay asleep in warmer temperatures.”

Air pollution also plays a role in the trend, indicates the report.

“A clear sky paves way for cooling during night time, but with an increase in pollution or cloudy sky, energy emitted by earth in the form of longwave radiation is trapped between the base of cloud and earth surface, leading to further rise in temperature,” the report reads.