HEAT WAVES IN INDIA
A CSE Media Briefing Note
How bad has this year’s heat wave been in India?
The early heat waves of 2022 that began on March 11 have impacted 15 Indian states and Union territories (as of April 24), according to data from the India Meteorological Department (IMD) that was analysed by Down To Earth. Rajasthan and Madhya Pradesh have suffered the most among the states, with 25 heat wave and severe heat wave days each during this period.
The IMD says a heat wave happens when the temperature of a place crosses 40oC in the plains, 37oC in coastal areas, and 30oC in the hills. The weather agency declares a heat wave when a place registers a temperature that is 4.5 to 6.4oC more than the normal temperature for the region on that day. If the temperature is over 6.4oC more than the normal, the IMD declares a ‘severe’ heat wave.
The IMD also uses another criteria to declare a heat wave which is based on absolute recorded temperatures. If the temperature crosses the 45oC mark, the Department declares a heat wave; when it crosses 47, a ‘severe’ heat wave is declared.
Surprisingly, after Rajasthan and Madhya Pradesh, the mountainous state of Himachal Pradesh has been the most affected by heat waves this year — with 21 heat wave and severe heat wave days. One anomalous point in the IMD’s data is that the weather agency has officially declared only one heat wave day for Odisha, even though Down To Earth has recently reported higher than 40oC temperatures being recorded across the state on April 24 and rising temperatures since the beginning of April.
TABLE: State-wise number of heat wave days between March 11 and April 24
| State | Number of heat wave days |
| Gujarat |
19 |
| Goa |
2 |
| Rajasthan |
25 |
| Himachal Pradesh |
21 |
| Uttarakhand |
4 |
| Maharashtra |
6 |
| Madhya Pradesh |
25 |
| Odisha |
1 |
| Jammu and Kashmir |
16 |
| Haryana |
15 |
| Delhi NCR |
15 |
| Uttar Pradesh |
11 |
| Jharkhand |
11 |
| Bihar |
2 |
| Punjab |
7 |
D Sivananda Pai of the Kottayam-based Institute for Climate Change Studies says that anti-cyclones over western parts of Rajasthan in March and the absence of rain-bearing Western disturbances had triggered the early and extreme heat waves. Anticyclones cause hot and dry weather by sinking winds around high pressure systems in the atmosphere.
Raghu Murtugudde, a climate scientist at the University of Maryland explains that a north-south pressure pattern, associated with the La Nina phenomenon in eastern and central Pacific Ocean that happens during winters in India, has persisted longer than expected and interacted with warm waves coming in from a rapidly warming Arctic region, leading to the heat waves. The sea surface temperatures over east and central Pacific Ocean become cooler-than-average during La Niña. This affects the trade winds flowing over the ocean surface through changes in wind stress. The trade winds carry this weather disturbance elsewhere and affect large parts of the world. In India, the phenomenon is mostly associated with wet and cold winters. Therefore, the current impact of La Niña on the spring and summer season in India is completely unexpected. Murtugudde says that the heat waves may continue till at least the monsoon season begins in June.
What global evidence do we have of heat waves happening across the world?
In the first instalment of the Sixth Assessment Report, the IPCC asserted that human activities have warmed the planet at a rate never seen before in the planet’s long history, and the Earth’s global surface temperature has warmed by 1.09oC compared to the pre-industrial period of 1850-1900. Human influence is the main driver of hot weather extremes (which have become more frequent and intense since 1950s). Improvements in climate models and analytics have enabled scientists to identify “fingerprints” of human influence on climate change by observing records of rainfall, temperature, and other factors. In the past two decades, scientists have published more than 350 scientific papers and assessments analysing the role of anthropogenic GHG emissions in individual extreme events.
The IPCC report says that every additional 0.5oC of warming will increase hot weather extremes, along with extreme precipitation and drought. Heat waves in India are likely to “last 25 times longer by 2036-2065” if carbon emissions remain high and push global temperature rise to 4oC by the end of the century, according to an international climate report published October 28, 2021, covering the G20 countries.
What is ‘wet-bulb’ and why does it matter?
The world is not only getting hotter but also wetter or more humid. We usually consider the dry-bulb temperature – the ambient air temperature — to describe how hot or cold a place is. Scientists, however, have been stressing upon considering humidity and other factors to assess how weather conditions will affect human health and activities. Wet-bulb temperature is a measure of humidity in the air. Factoring in humidity along with the heat, called the heat index, helps us determine what the temperature actually ‘feels like’.
Humidity combined with heat is deadlier for human health and wellbeing. Humans with their sweat-based cooling system, have been well-designed to beat the heat. But there is a limit to the level of heat and humidity we can cope with. A wet-bulb temperature of 35°C is considered the maximum limit of humidity that humans can handle. Beyond this, the body can no longer effectively cool itself via perspiration.
In the latest report by the IPCC, focused on the impacts of climate change, “deadly heat waves projected in some of the 14 densely populated agricultural regions of South Asia (such as the Ganges and Indus river basins)”. These regions are likely to exceed the critical threshold of wet-bulb temperature of 35°C under the business-as-usual scenario, if greenhouse gas emissions are not reduced.
What have been the impacts of heat waves?
Heat waves exert enormous impacts on health, agriculture and availability of water – all often related to each other in complex ways. Even though the number of deaths due to heat waves in India has decreased over the years, research shows that the general physical and mental wellbeing of people does get affected by extreme temperatures. On the other hand, agricultural yields get impacted as well. For instance, the wheat crop in the current rabi season in Punjab, Haryana and Uttar Pradesh has been impacted by heat waves. Many farmers have reported losses between 20 and 60 per cent in these states. This happened because the heat waves were early this year and the temperatures affected the wheat plants during their growth stage, leading to shriveled grains which fetch lower prices in the market, resulting in losses. To reduce agricultural losses due to heat waves, heat-tolerant varieties of wheat need to be developed.
Similarly, heat-resistant varieties of other rabi crops also need to be developed. Apart from direct heat, agricultural yields may also get impacted by droughts or drought-like conditions that are often associated with heat waves. This mainly occurs because of non-availability of water for irrigation during drought conditions.
The unlikely impact of the current heat waves would occur in the Himalayan regions of Himachal Pradesh, Jammu and Uttarakhand that are not used to heat waves and not well adapted to the extreme temperatures. One major impact in these regions would be on the accelerated melting of glaciers due to extreme temperatures which are the main source of water for the people living there.
What is being done globally, and what else needs to be done?
Cities need a combination of early warning systems, preparedness and training among government officials; community outreach programmes to assist during a heat wave; and proactive urban planning to avoid the worst impacts of extreme heat.
The first Heat Action Plan (HAP) was developed for Ahmedabad in 2013. Under this, people will receive weather alerts through SMS on mobile phones and medical professionals will be trained to increase preparedness. The initiative avoided 1,190 deaths a year, according to a study published in 2018, that evaluated the plan’s impact on death rates.
Several cities around the world are hiring dedicated officials to address extreme heat. The first such official was hired in the Miami-Dade County in the United States – Jane Gilbert. In less than a year since Gibert’s appointment, four more cities have followed. In July 2021, Athens, the capital of Greece, named its former deputy mayor Eleni Myrivili as chief heat officer. In October, Phoenix city in Arizona, US, and Freetown in Sierra Leone, Africa, named their chief heat officers. On March 3, 2022, Santiago, the capital of Chile, appointed urban planner Cristina Huidobro as the world’s fifth chief heat officer. Los Angeles in California, US, has also advertised a vacancy for the role.
The new-found urgency with which cities are hiring heat officers reflects a reckoning on heat unfolding across the world. Heat waves claimed over 166,000 lives between 1998 and 2017, according to the World Health Organization.
The third instalment of the IPCC’s Sixth Assessment Report (AR 6) focuses on the importance of urban green and blue infrastructure such as green walls, greenways, street trees, urban forests, green roofs, blue spaces, to cool urban areas. Urban trees can mitigate some of the impacts of climate change by reducing the urban heat island effect and heat stress. In addition, they can reduce stormwater runoff, improving air quality, and supporting health and well-being in areas where the majority of the world’s population resides.