Climate change adding 50 to 80 warmer nights each year to Indian summers, study finds

There have been at least 50 to 80 more nights with temperatures higher than 25 degrees Celsius added every year during summers since 2018 in India because of climate change.

A new report released on 21 June by Climate Central and Climate Trends titled ‘Sleepless Nights’ looks at the increase in minimum or nighttime temperatures worldwide and the role climate change plays.

Amidst the ongoing heatwave in north India with maximum temperatures crossing 45 degrees Celsius at some places, another concerning trend has been the increase in minimum temperatures across the same region.

Delhi’s Safdarjung observatory recorded an all-time high minimum temperature of 35.2 degrees Celsius on 19 June, while another north station, Alwar, recorded a minimum temperature of almost 37 degrees Celsius, which is the highest in the country.

Minimum temperatures — the lowest recorded temperature of an area in 24 hours — are usually called nighttime temperatures because they mainly occur at night when the sun has set.

The new report sheds light on the historic trend of increasing minimum temperatures and how they are tied to climate change. It also mentions how nighttime temperatures have increased much more rapidly than daytime temperatures, citing a 2016 study published in the International Journal of Climatology.

The study looks at the total number of nights with temperatures higher than 18, 20 and 25 degrees Celsius during summers in India, the US and the UK from 2018 to 2023.

It then utilises the Climate Shift Index (CSI), which is a collection of 24 different global climate models to observe how global warming and climate change have influenced changes in daily temperatures, to find out how many nights with high temperatures would have been there globally without human-induced climate change.

Using the CSI as a counterfactual, the authors calculated the total number of warmer night temperatures that were caused specifically by climate change.

“Like day temperatures, night temperatures have also shown constant and steady rise over the last few years. If we do not act now, nights would continue to be hotter, longer and sleepless, especially for the vulnerable,” said Aarti Khosla, Director of Climate Trends, in a press release.

Warmer nights in hill stations

On a global scale, the average person experienced 4.8 more days with minimum temperatures above 20 degrees Celsius because of climate change. However, the numbers differed massively across regions and countries.

The report found that most cities in India, both with and without climate change, did experience temperatures above 20 degrees Celsius during summers.

However, there was a much higher number of nights of above 20 degrees Celsius in relatively high altitude places added because of climate change. For example, Gangtok saw 50 additional days of minimum temperature above 20 degrees Celsius every year, while Shimla, Darjeeling and Mysore saw 30, 31 and 26 days respectively.

More importantly, there were many more days with minimum temperature above 25 degrees Celsius added due to climate change in states such as West Bengal and Assam. Siliguri, Jalpaiguri, Guwahati, Silchar, and Dibrugarh have seen 80-86 additional nights hotter than 25 degrees Celsius every year.

Mumbai also featured high on the list, with an additional 65 days with minimum temperature greater than 25 degrees Celsius because of climate change. Other cities in Kerala, Karnataka, Maharashtra, Jammu and Kashmir, and Punjab too saw an additional 50-60 days of hotter night temperatures every year for the last five years.

There have been multiple studies that record the devastating impact of heatwaves in India on livelihood, productivity and mortality.

A study published in February conducted a comprehensive multi-city survey on the relationship between heatwave and increasing mortality in India. It found that there were 1,116 deaths annually from 2008 to 2019 due to heatwaves.

There was also a 14 percent increase in daily mortality when temperatures remained high for over two consecutive days. Another study in 2022 by Cambridge University said that heatwaves in the country will impact the productivity of over 400 million people by 2050, and cost it 2.8 percent of its GDP.

Significance of high minimum temperatures

The temperature thresholds were selected by Climate Central because of the health impacts of high nighttime temperatures — while in the US and Europe temperatures above 20 degrees Celsius have been noted to cause physiological impacts, for tropical countries like India the threshold was 25 degrees Celsius because of studies and a WHO recommendation that 24 degrees Celsius is the ideal temperature for sleeping.

However, it is important to note that the study’s definition of summer is December-February in the Southern Hemisphere and June-August in the Northern Hemisphere, which does not exactly correspond with India’s summer months. Also, since the maximum temperature threshold it measured was 25 degrees Celsius, the report did not account for the fact that Indian cities like Delhi often reach more than 30 degrees Celsius in the nighttime during summers and the adverse impacts that result from it.

High minimum temperatures are also concerning, according to the India Meteorological Department, because of the “cascading effect on the day’s temperatures as well”. When the nighttime temperatures are high, the next day’s maximum temperature is attained earlier, and it could also last longer. When the difference between the day’s maximum and minimum temperatures reduces, its impact is felt on humans too by increasing heat stress.

Dr Sumit Ray, a critical care specialist at Holy Family Hospital in New Delhi, spoke to The Print about the importance of rest for the body during cooler nighttime temperatures.

“When night temperatures are so high, it’s akin to running a continuous marathon for your body,” he explained. “Since the body is on overdrive during the day trying to maintain its internal temperature while the ambient atmosphere is so hot, it gets exhausted fast and needs the night to cool down and rest its organs. But when night temperatures are also not cool enough for the body, there is no rest.”

The body starts to experience heat stress through symptoms such as dehydration, headaches, cramps, nausea and rashes. Excessive sweating due to the heat also leads to essential salts and water level depletion in the body, which leads to exhaustion. Heat strokes also exacerbate existing cardiovascular, renal and respiratory illnesses because of the impact on the kidneys and heart.

On 18 June, data from the National Centre for Disease Control (NCDC) showed 143 heatstroke deaths in India between March to June, with Uttar Pradesh and Delhi claiming the most lives. There have also been over 40,000 cases of heatstroke in the same period according to NCDC data.

This year, there have also been reports about animals like bats dropping dead because of the heat in Delhi and Kanpur, while other reports about Delhi’s water stress and extreme electricity demand also continue. On 19 June, Delhi reached its all-time high demand of 8,656 MW, leading discom officials to warn against the power stress in the city.