Mumbai: Not only was February 2023 the hottest February since the year 1877, but India’s minimum temperatures--used to indicate how cold a night is--were also the fifth highest in this month’s history of recorded temperatures. In simple words, nights were not as cold as they used to be.

A breakdown by region for February 2023 shows northwest, east and northeast India’s minimum temperatures were their second-highest ever. Even in March 2023, the country’s average minimum temperature was 0.59 degrees celsius (°C) above normal.

India’s minimum temperatures in February--used to indicate how cold a night is--were the fifth highest in this month’s history.
Source: IMD

In our #TIL explainer, we explain rising minimum temperatures, warm nights and why they matter.

Why it's important to track warm nights

When days are hot, nights give the human body a chance to cool down. But when nights get warmer, that does not happen, leading to increased heat stress on the body. The combined effect of a warm day and warm night is detrimental to human health. The relative mortality risk on days with hot nights could be 50% higher than on days with non-hot nights, a 2022 paper that analysed mortality across 28 cities in Japan, South Korea and China had noted.

Health risks due to daytime heatwave can substantially increase if the human body does not get a break from the heat during night, a 2017 paper by researchers from China had noted. A 2018 paper by researchers from Indian Institute of Technology, Gandhinagar, referring to existing studies on the subject, had stated, “if the stress originating due to daytime heatwave persists at night, it further exacerbates human discomfort and pre-existing health disorders. Therefore, hot day and the hot night events… may have significant implications on heat-related morbidity and mortality as observed for the 1995 Chicago heatwave”. The paper was referring to the heatwave in Chicago in 1995 when in a two-day period, the minimum apparent temperature failed to go below 31.5°C, an extremely unusual phenomenon for the region.

Warm nights also affect sleep, and the temperature effect on sleep loss is substantially larger for residents from lower-income countries, older adults and females, a 2022 paper by researchers from Denmark had noted. The researchers warned that by the end of this century, suboptimal temperatures can erode 50-58 hours of sleep per person per year.

As India braces for more frequent and intense heatwaves in the future, policymakers must also take warming nights into consideration while making heat action plans and issuing advisories and formulate policies keeping the most vulnerable in mind, experts say.

IndiaSpend reached out to the National Disaster Management Authority with questions on how it plans to address India’s increasing minimum temperatures, and its effect on health, and what steps it has taken to address the impacts of indoor heat so far. This story will be updated when they respond.

What the science says

The number of heat wave events in India have increased by about 24% during 2010-2019, as compared to 2000-2009, and associated mortality rates have increased by about 27%. Heat waves are India’s second-most disastrous extreme-weather event linked to mortality, up from the third. The number of Indian states getting affected by heat waves have climbed from nine in 2015 to 23 in 2020, the National Disaster Management Authority had noted in a report.

The India Meteorological Department’s (IMD) Hot Weather Hazard Analysis, released in 2022, maps the impact of minimum temperature (among other things such as humidity) over heat stress in different regions of the country and states that minimum temperatures have an important role in increasing the gravity of heat wave effect over different parts of the country in different months.

For example, on May 11, Jeur in Maharashtra’s Solapur district recorded a maximum temperature of 43°C when it had recorded a minimum temperature that was 4°C above normal at 25°C the previous night. On May 12, the night-time temperature in Malegaon in Maharashtra was 29°C, 6 degrees above normal. On May 25, Rajasthan’s Bikaner had a daytime temperature of 42.5°C, followed by a minimum temperature of 21.8°C the following night, six degrees above normal.

When maximum temperatures remain 40°C and above and when the minimum temperature of a region is 4.5°C to 6.4°C above normal, IMD terms it a ‘warm night’. If the minimum temperature is more than 6.4°C above normal, IMD terms it a ‘very warm night’.

“IMD has objective criteria for declaring ‘warm night’ just like heat wave criteria. If the day temperatures are on the higher side, then above normal minimum temperatures also impact humans adversely,” said Akhil Srivastava, scientist with the IMD, who co-authored the Hazard Analysis. “Minimum temperatures of any given day are generally recorded during late night or early morning hours. If the minimum temperatures are warmer than normal then they have a cascading effect on next day’s maximum temperatures…When the high daytime and night time temperatures are seen together, then they tend to increase the heat stress on humans.”

Researchers studying the mortality risks of high temperatures in Ahmedabad between 1987-2017 found a “substantial increase in the mortality rate at maximum temperature ≥ 42 °C and minimum temperature ≥ 28 °C”, which “indicates that both maximum and minimum temperature had adverse effects”.

“Our research showed that during summer, after a certain threshold, let’s say a maximum temperature of 42 and minimum temperature of 28… With every 1°C rise in maximum temperature, the risk of mortality increased by 9.6% but with every 1°C rise in minimum temperature, the risk of mortality increased by 9.8%,” said Abhiyant Tiwari, Lead of Health and Climate Resilience at NRDC India and co-author of this paper. He added that when the gap between maximums and minimums becomes small in a given day, people remain exposed to higher levels of heat energy. “We also need to understand that it is not the temperature threshold or cut-off that is important but the cumulative heat stress that people are bearing or are being exposed to.”

How bad could it get?

The frequency of warm nights has been increasing by 84.6% in Asia from one to three nights per year, a 2022 paper by two Iranian researchers looking at changes in the annual frequency of warm nights in Asia from 1970-2020 and from 2020-2070 had noted. Under different emissions scenarios, in 22% to 25% of Asia’s area, the frequency of warm nights will be more than 96 per year by 2070. “This is especially true in South and Southeast Asia, where in some areas, such as Indonesia, almost all nights of the year will be warm.” Under both scenarios, 8.7% of Asia will experience more than 308 warm nights by 2070, the paper warned.

And there is no doubt that climate change has a role to play. The paper found that if the global mean temperature rises by 1°C, the mean frequency of warm nights in Asia increases by nearly 37 nights.

The paper also says that since there is no sun at night, warming during the night can be attributed more clearly to greenhouse gases and their effects. The authors write, “studying the trend of warm nights can show the effect of increasing greenhouse gases on the climate with more accuracy than hot days”.

The authors add, “the highest intensities of warm nights are mainly in densely populated, underdeveloped or less developed areas of south and southeast Asia”. Also, because in large parts of Asia the economy is dependent on agriculture, particularly in the southeast of the continent, these communities are likely to be more vulnerable to warmer climates. "In such condition, it can be anticipated that the transformation of climatic extremes into usual events will even lead to the collapse of fragile economies in these areas.”

The 2018 paper by researchers from Indian Institute of Technology, Gandhinagar had also given a more specific warning for India. It categorised these ‘hot day followed by hot night’ events as CHDHN or Concurrent Hot Day Hot Night events.

The paper finds that the frequency of one- and three-day CHDHN events has increased in large parts of western and southern India during the post-1984 period. Night-time heat events have increased more rapidly than day-time heat events during the recent few decades. However, the frequency of one and three-day CHDHN events in the Indo-Gangetic plains and eastern parts have declined, which can be partially associated with local cooling due to irrigation and atmospheric aerosols, the researchers found.

They noted that the frequency of one- and three-day CHDHN events increased significantly due to human-caused emissions and could even increase four to 12 times of the current level by the end of the century, depending on emissions.

Researchers IndiaSpend spoke to said that warming nights should be taken into account while designing heatwave warning systems, especially for vulnerable populations. Rising minimum temperatures and their impact on indoor heat must be noted in India’s heat action plans if needed.

“Night-time heat could be more severe for residents in low-income communities if they cannot afford the additional expense of air conditioning and tend to close windows during the night time especially in locations where there is a high crime rate,” noted the 2022 paper on Japan, South Korea and China while stressing on the need for future public health policies to ensure the indoor comfort for lower-income groups and senior citizens.

(Pavan Thimmavajjala, an intern with IndiaSpend, contributed to this story.)

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