New Delhi:

With Mumbai seeing the highest changes in the nighttime temperatures, India continues to suffer a severe heatwave, a new analysis on Friday showed climate change added nearly 50 to 80 nights each year where the temperature exceeded 25 degrees Celsius, with serious impacts on sleep and health.

Climate change is leading to a rise in nighttime warming, which is impacting sleep quality and human health in India and across the globe, said the analysis by Climate Central and Climate Trends.

Nighttime temperatures have increased even more rapidly than daytime temperatures as the world heats up due to climate change, primarily caused by burning fossil fuels like coal, oil, and gas.

As one of the countries most vulnerable to the impacts of the climate crisis, India has experienced a significant rise in minimum nighttime temperatures over the last decade due to climate change.

The national capital on June 18 experienced its warmest night in at least 12 years, with the mercury touching 35.2 degrees Celsius, says the India Meteorological Department. This is the city’s highest minimum temperature since 1969.

The analysis shows that nearly 50 to 80 days each year were added above this threshold by climate change in cities across Kerala, Karnataka, Maharashtra, Tamil Nadu, Punjab, Jammu and Kashmir, and Andhra Pradesh between 2018 and 2023.

Among the metro cities, Mumbai has seen the highest changes in the nighttime temperatures, with the city experiencing an additional 65 days of warmer nights due to global warming.

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 degrees threshold due to climate change, on average.

Several cities saw between 15 and 50 additional days where the minimum temperatures exceeded 25 degrees due to the influence of climate change, including Jaipur, with an additional 19 hot nights attributable to climate change.

Meanwhile, in both observations and in the counterfactual climate, the nighttime summer temperatures across India often exceed 20 degrees over the entire summer period.

The cities that had the largest number of days where the minimum temperature exceeded 20 degrees due to climate change are Gangtok, Darjeeling, Shimla, and Mysore, with an average of 54, 31, 30, and 26 days added by climate change, respectively.

Higher nighttime temperatures can cause physiological discomfort and impact human health by preventing body temperature from cooling off during the night, increasing mortality risks.

There is also a growing body of evidence that as nighttime temperatures rise, it is adversely affecting the quality and length of sleep.

Poor sleep adversely affects physical and mental health, cognitive functioning and even life expectancy. Hot nights can have disproportionate impacts on vulnerable groups, including the elderly and people, who do not have access to proper cooling mechanisms.

These findings come during a week that saw new records for nighttime heat in several Indian cities.

On June 19, Delhi shattered the all-time high minimum temperature record, with the mercury reaching 35.2 degrees overnight. Delhi recorded almost four numbers of additional nights over 25 degrees between 2018 and 2023, according to the Climate Central analysis.

On June 18, Alwar in Rajasthan had a minimum temperature of 37 degrees, the highest-ever nighttime temperature since records began in 1969.

Alwar experienced almost nine additional nights over 25 degrees that are attributable to climate change between 2018 and 2023.

In Uttar Pradesh, Lakhimpur Kheri, Shahjahanpur and Varanasi also witnessed their highest recorded minimum temperatures at 33 degrees, 33 and 33.6 degrees respectively this week.

Varanasi saw four additional nights over 25 degrees due to climate change from 2018 to 2023.

These increasingly frequent extreme nighttime temperatures are contributing to heat stress, exhaustion and heat-related deaths.

The current ongoing heatwave in India has been made hotter, frequent and more likely by climate change, according to scientific studies by World Weather Attribution and ClimaMeter.

Roxy Mathew Koll, Climate Scientist, Indian Institute of Tropical Meteorology, Pune, “The urban heat island effect is most visible in the nighttime temperatures. 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.”

“During the day, the sun’s rays reach as shortwave radiation and heat the earth’s surface. At night, the heat escapes as longwave radiation. While shortwave radiation can easily penetrate through and reach the surface, the longwave gets trapped easily by concrete and clouds.”

Aarti Khosla, Director, of Climate Trends, said, “Like day temperatures, night temperatures have also 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. Cities will bear the highest brunt which will get worse due to the urban heat island effect.”

“Several studies have already established that by the turn of the century, without very large reductions in fossil fuel burning, nighttime temperatures will not fall below 25 degrees in some places during hot weather impacting one’s ability to recover for the next day. If we do not act now, nights will continue to be hotter, longer and sleepless, especially for the vulnerable.”

(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)

A visualisation of all tropical cyclone tracks over South and Southeast Asia that formed in 1985-2005, in six-hour intervals. The colour scheme denotes wind intensities on the Saffir-Simpson scale. Storms remnants are not shown.
| Photo Credit: Nilfanion/NASA, Public domain

Numerous studies have reported trends in various climate variables over the Indian subcontinent. A decreasing trend in the amount of monsoon rainfall for more than six decades is one. Others include intensifying trends in the occurrence of extreme rainfall events, droughts, heatwaves, and cyclones. The period over which these trends have been estimated vary but global warming has always been invoked as the prime suspect.

A question that isn’t getting as much attention as it deserves to in this milieu is: are these really trends, or are they shifts or decadal cycles?

(A shift is a jump from one state to another, such as a quick transition from one amount of rainfall to another. The best example is seasonal monsoon rainfall: it tends to remain above the long-period average (LPA) for about 20 years and then shifts to a state of less rainfall than the LPA for a similar duration.)

The question matters because trends, shifts and decadal cycles portend important differences in the way we plan the use of our resources, including water, crops, energy, etc.

Scientists’ glossary of change

A common term used by climate scientists these days is ‘anthropogenic trend’. ‘Trend’ of course implies that there are climate variables moving in one direction, like the continuous increase in temperature. The ‘anthropogenic’ suffix presumes that these trends are occurring within human lifetimes. As such, the duration over which a variable needs to evolve for its behaviour to be called a ‘trend’ is not always clear.

Climate scientists also use the term ‘secular trend’, which is to say that a variable has been increasing for a certain period within a longer span, such as for 30 years in a 100-year period.

Then there is ‘decadal variability’, a common term that isn’t entirely distinct from a shift. Decadal variability refers to an oscillation from a positive to a negative phase on the order of tens of years. On the other hand, a shift can mean an irreversible jump or just a rapid transition that will later return to a prior/older state.

On the whole, without observing a particular variable for sufficiently long periods of time, climate scientists tend to be less than rigorous about their choice of descriptor to describe the variable’s observed behaviour.

In this context, it is critical to understand whether cyclones are becoming common and/or more intense, if they are a part of a decadal oscillation or if their numbers have jumped to a new state.

A rapid increase

A study just published in the journal Climate and Atmospheric Science (of which I am a part) reported a sharp change in the potential for cyclones to form over the Arabian Sea during the late 1990s.

Cyclone-genesis – or cyclogenesis – is an indicator that denotes the chance of a cyclone forming. It depends on some parameters, including the sea surface temperature, the ocean heat content, change in winds from the surface into the upper atmosphere (or the vertical shear), and rotation of winds near the surface. If these conditions line up, they will sow the seed for a cyclone, but we still don’t fully understand why some seeds sprout and grow into cyclones and some don’t.

This said, all these factors except for wind rotation have seemingly favoured a higher cyclone formation potential since the 1990s. The crucial question is why this switch – a rapid increase – occurred around this time. The present study notes that the rapid increase in the cyclogenesis potential over the Arabian Sea coincides with a shift in the so-called ‘Warm Arctic, Cold Eurasian’, or WACE, pattern. Again: a shift rather than a trend.

WACE is a pattern of warm surface temperatures over the Arctic and a large blob of cold surface temperatures over Eurasia. This pattern is associated with upper level circulation changes that reach into the Indian Ocean sector.

Global warming also experienced a slowdown around the same time (although this continues to be debated). More interestingly, scientists have argued that a so-called ‘regime shift’ occurred in the same period as well. Such shifts are not unheard of; a similar event was noted in the mid-1970s.

A new why

The causes of such changes are not fully understood – but they do raise some remarkable questions for India. Irrespective of whether they are ‘shifts’ or ‘decadal cycles’, it is important that we understand whether the decrease in the monsoon’s intensity, increase in the amount of extreme rainfall, and greater prevalence of  heatwaves are trends that will continue in the coming decades – or if they are parts of longer processes that will revive the monsoon, reduce the number of cyclones, and ameliorate heatwaves and extreme rainfall.

The expectations with which we invest in resources to adapt to future climate risks are vexed by many difficulties, including those arising from uncertainties in climate risk at the level of specific regions across the country, vis-a-vis sea-level rise, heavy rain, drought, heatwaves, and cyclones. Of course, given our limited financial resources, climate adaptation remains a considerably monumental socioeconomic and political challenge.

As a result, climate scientists have our work cut out for us. Instead of always focusing on predicting what climate change will look like in 2100 or training our tunnel vision on global warming targets, we need to better understand the natural variability in our own neighbourhood – especially since natural variability itself is modulated by global warming.

As the new study indicates, a monsoon decadal cycle that used to last for around 20 years earlier may now last for longer. Why?

Raghu Murtugudde is a visiting professor at IIT Bombay and an emeritus professor at the University of Maryland.