India's Monsoon Season Is Turning Out To Be Hotter Than The Pre-monsoon Period This Year

Even as the monsoon has made its footfalls felt across the country, there seems to be little respite from the searing heat – aided by steep rises in humidity levels -- in many pockets.

The heat waves lashing India this summer are symptomatic of the anomalous temperature trends that are expected to worsen with growing climate change impacts.

Since 2016, the summer of 2022 has been the second hottest after the summer of 2010. Mega cities including Delhi, Mumbai, Kolkata and Hyderabad are much hotter than the larger region around them due to heat island effects caused by surface absorption of heat and local waste heat generated by traffic, industry and air conditioning (among other urban activities).

A new country-wide analysis by Centre for Science and Environment (CSE) indicates while extreme heat waves in the states of the northwest -- as per the classification of the India Meteorological Department (IMD) -- draw maximum public attention, the increase in overall anomalous temperature in other regions of the country have been largely neglected.

"This is a very disturbing trend as policy preparedness to mitigate rising heat due to climate change is nearly absent in India," says CSE Executive Director (Research and Advocacy) Anumita Roychowdhury.

"Without heat action plans, rising air temperature, radiating heat from land surfaces, concretisation, heat-trapping built structures, waste heat from industrial processes and air conditioners, and erosion of heat dousing forests, urban greens and waterbodies will worsen public health risks. This requires urgent time-bound mitigation," Roychowdhury adds.

"Understanding the overall temperature anomaly, extreme heat conditions, and the mixed trends in heat patterns across different regions of India has become necessary to assess the emergent risk," says CSE Senior Programme Manager (Urban Lab) Avikal Somvanshi.

"Currently, the attention is largely on the maximum daily heat levels and extreme conditions of heat waves. But it is equally important to pay attention to the overall rising temperature and humidity trends in different regions to understand the gravity of the problem,” adds Somvanshi.

The Urban Lab of CSE has analysed the temperature trends in India from January 2015 till May 2022. The effort has been to understand the warming trends by covering all three dimensions of heat stress -- surface air temperature, land surface temperature, and relative humidity (heat index) -- at the national, regional and local levels.

The trends in the metropolises of Delhi, Mumbai, Kolkata and Hyderabad (one located in each of IMD’s four homogenous regions) have also been assessed.

The analysis has studied the temperature anomaly (which refers to a departure from a reference value or long-term average or a baseline). A positive anomaly indicates that the observed temperature was warmer than the baseline, while a negative anomaly says the observed temperature was cooler.

It is generally used to report monthly, seasonal, annual, or decadal temperature changes. Heatwaves are also anomalous temperature events, but of much shorter duration and are defined in comparison to an absolute temperature threshold in addition to departure from the normal.

The Key Findings

The 2022 pre-monsoon summer heat trends overtake 2016 as the second hottest pre-monsoon season on record for India:

The seasonal average air temperature for 2022 -- pre-monsoon or summer (March, April and May as per IMD classification) -- is 1.24°C warmer than the baseline trends that relate to 1971-2000 climatology (baselines are defined based on historical timelines and may vary for different metrics; the anomaly is generally computed from the 1951-1980, 1971-2000, or 1981-2010 climatology baseline).

This is warmer than the 1.20°C anomaly noted in 2016 pre-monsoon, but lower than the 1.45°C anomaly recorded in the 2010 pre-monsoon season.

Similarly, land surface temperature anomaly has been extreme this pre-monsoon season, with a 1.46°C departure from the baseline (1971-2000). It must be noted that pre-monsoon seasonal trends in both land and air temperature are identical to the annual trends, but with more pronounced highs and lows.

Monsoon is hotter than the pre-monsoon period on an average, while winter and post-monsoon seasons are warming up faster:

At an all-India level, the monsoon season (June, July, August and September as per IMD classification) has been 0.3-0.4°C hotter than pre-monsoon (or summer) – what’s more, it is getting hotter with time.

The decadal average temperatures for pre-monsoon or summer period are now 0.49°C hotter than the long term normal (1951-80 baseline). This is a significant increase, but it pales in front of the increase noted among the decadal average temperatures for the other three seasons.

The post-monsoon period (October, November and December as per IMD classification) is hotter by 0.73°C. Similarly, the winter (January and February as per IMD classification) has been warmer by 0.68°C, and the monsoon by 0.58°C.

High heat stress and heat waves in the northwestern states:

This year, the observed average daily maximum temperature for March and April for states and Union Territories in India’s northwest (Chandigarh, Delhi, Haryana, Himachal Pradesh, Jammu and Kashmir, Ladakh, Punjab, Rajasthan, Uttar Pradesh and Uttrakhand – as per IMD classification) has been almost 4°C above the normal (compared to its baseline of 1981-2010).

This is almost twice as much as the anomaly observed at all India level, and it holds true for even average daily minimum, daily mean and land surface temperatures. Temperatures became relatively closer to normal during the month of May.

Other regions even hotter in absolute terms:

In absolute terms, most of India outside the northwestern parts was hotter, even if the number of its extreme heat wave days were lesser. Average daily maximum for northwestern states for the month of March was 30.7°C; the all India average was 33.1°C (2.4°C hotter).

The average daily minimum temperature shows an even larger (4.9°C) difference. The average daily maximum of northwestern states crossed the all-India average in the months of April and May, but only by 1-1.5°C. But the daily minimum and average temperatures continued to be higher in other regions of India.

Central India (Chhattisgarh, Dadra and Nagar Haveli, Daman and Diu, Goa, Gujarat, Madhya Pradesh, Maharashtra and Odisha, as per IMD classification) and the southern peninsular region (Andaman and Nicobar, Andhra Pradesh, Karnataka, Kerala, Lakshadweep, Puducherry, Tamil Nadu and Telangana, as per IMD classification) had higher normal temperatures compared to the northwest during the pre-monsoon or summer season.

Central India’s normal maximum was 2-7°C higher, while south peninsular India’s normal minimum was 4-10°C higher than temperatures in northwest India.

It is important to focus on both extreme heat conditions as well as overall anomalous heat conditions. The IMD criteria for heat waves says a heat wave is declared when the daily maximum temperature exceeds the normal for the season by at least 4.5°C in at least two sub-division stations on two consecutive days; in absolute terms, a heat wave is declared when the temperature exceeds 45°C.

The normal temperature for the season is defined based on a particular day at a particular station and in relation to the baseline of 1981-2010 average. Much of the heatwave conditions in March have been due to a departure from the normal.

Most deaths due to heat-stroke reported from states outside India’s northwestern region:

Data from the National Crime Records Bureau (NCRB) that also captures accidental deaths including those from climatic stress, shows that between 2015 and 2020, 2,137 people had reportedly died due to heat stroke in the states in northwest.

But the southern peninsula region had reported 2,444 deaths due to excess environmental heat, with Andhra Pradesh alone accounting for over half of the reported casualties. Delhi reported only one death for the same period.

Most deaths have been reported among working age men (30-60 year olds), usually not considered highly vulnerable to temperature anomalies. The understanding of the public health impacts of meteorological conditions like heat wave is still weak in India.

Death due to heat waves are on a decline as per official reports:

Heat waves are the second most deadly natural force in India, having killed over 20,615 people during 2000-20. Lightning, with 49,679 deaths, was the top killer. There has been a decline in number of reported deaths since 2015 when IMD reported 2,081 deaths and NCRB reported 1,908 deaths.

There were no deaths reported by IMD in 2021 and NCRB is yet to publish the 2021 edition of its annual Accidental Deaths and Suicides in India (ADSI).

The years 2016 and 2017 have reported twice the number of hazardous heat wave events compared to 2015, but reported deaths caused were less than a quarter of the 2015 toll. This year, media reports suggest about 90 deaths due to heat waves in India, but there is no official verification for the number.

It is quite possible that most of the deaths due to heat stress goes unreported as they may be attributed to other comorbidities or not get reported at all. Mostly, deaths occur among rural, low income and marginalised populations. However, these estimates need to be improved to ensure that there is no undercount in the data reported by NCRB and IMD.

No evidence of any direct correlation between heat waves and the intensity or frequency of fire instances in India:

Most instances of fire as recorded by NASA satellites were reported in the 2021 pre-monsoon season at an all-India level. Despite the massive temperature anomaly this year (over three times higher compared to 2021), fire anomaly this season is just 6 per cent of 2021 for MODIS and 32 per cent of 2021 for VIIRS (both are NASA’s satellite systems that record fire instances).

Further, this analysis is based on the product of observed fire counts and average Fire Radiative Power (FRP) of the observed fires. This matrix provides a better measure of total fire activities accounting for not just the number of fires, but also their relative strength (quantum of energy released). The anomaly in fire instances is based on a 2012-2019 baseline.

Most of the pre-monsoon fire instances in India have been reported from the forests of central India and the eastern Himalaya. The northwest region, which has been the worst affected by heat waves, is not a hotspot for natural fires as the region does not have much forest cover outside the western Himalaya.

Fire instances reported from the region are mostly farm stubble fires and these are not generally governed by weather events. The region – which constitutes about 28 per cent of India’s landmass -- accounts for only 13-14 per cent of all recorded fires in India during the pre-monsoon season.

Mega cities much hotter than the larger region around them:

The seasonal average of Delhi, Mumbai, Kolkata and Hyderabad was 1-2°C higher than the all-India average and 2.5-3.8°C higher than northwest India (that is often the reference point in public mind). This is due to the urban heat island phenomenon.

Due to excessive hard and dark surfaces, cities tend to absorb the heat coming from the sun during the day; a city also generates a lot of waste heat due to human activities which adds to the natural heat. This extra heat should dissipate after sunset, but due to pollution and continuing generation of waste heat the city fails to cool down, leading to warmer condition compared to rural areas.

Urban heat islands with massive temperature variations found in all four metros:

The study looked into the temperature and humidity data collected by the real-time air quality monitoring network and found massive variations in temperature within the cities. In terms of absolute air temperature, Hyderabad with a 7.1°C variation had the most pronounced heat islands, while Kolkata with just 1.3°C had the least pronounced ones.

Delhi had a 6.2°C variation; Mumbai’s was 5.5°C. From a heat index perspective, Mumbai leads the group with a 17.3°C variation. From the land surface temperature perspective, it is Delhi in the lead with a 24.6°C variation.

The difference among the cities can be explained by the environmental difference in the nature of the heat in their regions, as well as the differences in their topography and land use patterns. But the fact that all four cities show significant variations in their intra-city temperatures (in all three forms) is a strong evidence of urban heat island problem within the cities.

Heat hotspots in the cities:

Because of limited spatial distribution of air temperature official monitors, the study used land surface temperature to identify the heat hotspots within a city.

In Delhi, maximum surface temperature was recorded in the south-west part of the city; Najafgarh is the constant, with the highest temperature across all the time periods. Badarpur and Jaitpur are the other neighborhoods that have constant maximum temperatures above 40°C. Yamuna river and the Okhla bird sanctuary areas had the lowest temperatures.

In Mumbai, maximum surface temperature were recorded around Deonar dumping ground, followed by Chhatrapati Shivaji International Airport and its surrounding areas. Minimum surface temperatures were recorded in areas around waterbodies such as Tulsi Lake, Vihar Lake and Powai Lake, and locations close to the sea.

In Kolkata, there is not much variation in the land surface temperature within the city. Still, surface temperatures were found to be relatively high around the city center and the Netaji Shubash Chandra Bose International Airport. The area around the wetlands was the coolest.

In Hyderabad, hotspot areas were observed in the city’s south-western region, around the Rajiv Gandhi International Airport, Nadergul, Turkayamjal, Jawahar Nagar and Bollaram Industrial Area. Locations near the lakes in the city had the lowest temperatures.

Heat index that accounts for both temperature and humidity shows Mumbai, Kolkata and Hyderabad more stressed than Delhi this summer:

The daily average heat index for Kolkata has been almost 5°C higher than that of Delhi this summer. Mumbai’s summer heat index has been 4.6°C and Hyderabad 1.1°C higher than Delhi’s.

Even though the seasonal average daily maximum air temperature in Delhi has been 1.5-2.8°C higher than the other three metros, in terms of heat index Kolkata -- with almost 50°C as seasonal maximum heat index -- was significantly hotter. Delhi’s seasonal average daily maximum heat index was 45.6°C. Heat index of 41-53°C is considered dangerous, while 54°C and higher is considered extremely dangerous.

Hyderabad, though hot, is experiencing a cooler than normal summer this year:

The seasonal average air temperature of Hyderabad was about 1°C lower than normal -- even the heat index was down by almost 1.5°C. The land surface temperature has been normal as well. This is a complete contrast from the other three metros, all of which have recorded higher than normal temperatures. Delhi had the most extreme anomalies among the four metros.

What is the way ahead?

Roychowdhury says: “In this climate-constrained world, heat stress is expected to worsen. This demands urgent action to adopt direct measures to mitigate heat across the region as well as the city to reduce public health risk and occupational exposure to heat, contain energy consumption due to increased demand for cooling, prevent damage to infrastructure, and ensure overall wellbeing of the masses exposed to high and extreme heat.”

She adds: “This will require strategic interventions to reduce heat island effects in cities by conserving and expanding urban greens/forests and water bodies, adopting architectural design guidelines for the built environment to reduce the heat load on built structures, and containing concretisation of surface areas. At the same time, emergency measures would be needed to respond to the extreme heat conditions during the heat waves.”

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