Growing up in Bengaluru in the late 1990s, the onset of summer would mean the end of the dreadful annual exams, playing cricket on the street, access to television, and a limitless supply of cold, sugary juices or slurries.
Fast forward to 2023, the onset of summer means shorter durations spent outdoors thanks to the sweltering heat and nearly an inch of dust everywhere. The last February was the hottest such month since 1901 and the India Meteorological Department (IMD) promptly issued a warning of potential heatwaves in March, typically the start of summer.
What are heat waves?
Heatwaves are defined as a function of the departure from usual or what is considered normal temperatures for the season in any region. In India, heat waves are declared based on two conditions. 1)If the air temperatures reach a maximum temperature of 40°C in the plains and 30°C in the hills; or 2)If air temperature departures from normal are between 4.6 to 6.4° C. Heatwaves are declared if either of the two conditions is observed at least two weather stations in a meteorological sub-division on two consecutive days.
The conditions used for coastal regions are different and heatwaves are declared if temperatures are above 4.5°C provided the maximum air temperature is 37°C or more. Although heat waves are measured based on air temperatures, the volume of moisture in the air has an important effect. The temperatures perceived by organisms are dependent on the volume of humidity in the air. If the relative humidity is higher, then the perceived air temperature will be higher. This measure is called a Heat Index.
What causes heat waves?
Heatwave events are not a new phenomenon and have historically occurred due to the prevailing weather conditions. Several local and remote drivers and feedbacks result in heat waves.
“Locally, heatwaves form in response to subsidence of dry air. Subsiding air results in adiabatic compression where there is no transfer of heat, thus inhibiting convection and preventing the formation of clouds. The reduction of clouds results in a clear sky that increases shortwave radiation reaching the surface, thereby heating the region," explains Prof. Roxy Mathew Koll, who heads the Climate Research Lab at the Indian Institute of Tropical Meteorology, Pune. Some of his work has been instrumental in predicting the climate of the Indo-Pacific region.
He elaborates that at a global scale, heatwaves can occur due to anticyclonic circulation where the wind circulates a centre of high pressure or by atmospheric blocking, where a build-up of high pressures in the upper atmosphere prevents air from raising up and gradually cooling.
Koll specialises in ocean-atmosphere dynamics. He underscores the impacts of global warming by adding that “while the occurrence of anticyclones may be influenced by large-scale atmospheric circulation patterns; additional heat due to global warming also gets accumulated over these regions of subsidence/anticyclone, making the intensity of heatwaves even stronger”.
Human-induced changes
Indeed, there is mounting evidence that since the 1950’s there has been an increase in the number and duration of heatwaves across the planet. The maximum temperatures even during the colder months are higher than the average temperatures documented within the last century. The 6th assessment report by the Intergovernmental Panel on Climate Change has categorically attributed these changes to be the consequence of human influence.
Over the last couple of decades, humans have modified landscapes extensively and greenhouse gas emissions are increasing. All of this has fundamentally altered climatic processes such as atmospheric circulation. With such perturbations, there are greater incidents of high-pressure systems in the mid-latitudes as well as anomalous heating of land and ocean surfaces, especially in the equatorial regions. Such perturbations alter atmospheric stratification and result in more erratic and extreme heat waves.
Hazards of heatwaves
Heatwaves can be fatal to humans and all other life forms. The occurrence of heat waves has consequences on the economy, industry, public health, and ecosystem functioning. The impacts of heat waves are worse when the relative humidity of the air is higher. Humans, for example, cope with high temperatures by sweating. Sweat evaporates and gives us a sensation of cooling. In areas of high humidity, the air is already near saturated and evaporation rates are much lower and the process of sweating does not produce any cooling effect.
Our body experiences heat exhaustion with symptoms ranging from nausea to extreme fatigue. If the body is not hydrated or cooled, our body continues to get heatstroke. The body stops sweating and our core temperature rises beyond what we have evolved to cope with. This can result in a coma or death. It is no surprise then that a lot of attention is given to heatwaves and saving human lives. Prof. Koll highlighted that “a decline of 30–40% in the work performance is projected over India by the end of the century due to the elevated heat stress levels.”
Can heat waves be predicted?
Given the threats to the economy and life, being able to predict the occurrence of heat waves is incredibly useful. India has a wonderful network of weather stations across the country and this data is being used to make sophisticated models to make predictions that can hold over a few days to over a season.
The IMD uses a combination of models to make predictions and their self-assessment of the predictions by cross-validating with actual observations shows a high level of confidence in the predictions. Because previous temperature profiles are compared to the observed shifts in recent years, we can also predict the occurrence of heat waves spatially.
Heatwaves in new places
One of the emerging aspects from these predictions is that heatwaves are now likely to occur in areas that have historically never experienced heatwaves such as the Himalayas. Indeed, the precursor for the heatwaves may have already begun says Anand Sankar, a journalist turned entrepreneur who moved to live in a remote village in Uttarakhand a decade ago. “This past February was the hottest month he has experienced in the last decade. The snowfall has also been less this year and this will have implications on both water and crop security”. he exclaimed.
While prolonged heat might imply a longer growing season, all may not be well with cash crops like Apple. “Apple trees need snowfall in December and January so that the sub-surface soil can freeze and retain moisture till spring flowering -fruiting season. We had barely 6 inches of snowfall.” He recalls.
Dry soil tends to get heated up faster and can increase the heatwave effect and many of the snow-fed streams can dry up. The arrival of the monsoon can bring some respite to the heat, but Anand notes that “Rainfall is now erratic, and we get intense but short spells of rain including hailstorms. With intense rain, the topsoil gets washed off”. Apart from the hazards associated with heatwaves, the changes in water availability and increased risks to crops puts can worsen the regional economy as well as the quality of life, especially in the ecologically fragile mountains.
What can be done about it?
The IMD urges people to make lifestyle changes to cope with heat waves. These include the consumption of rehydration salts and not stepping out during the peak of the day. Asking people to sit indoors and not work in the hot sun is a good idea but not everyone can afford to do that. “India is now bringing up city heat action plans across the country. It is working well in Gujarat and is being exercised in other states also. Effective forecasts and early warning systems integrated with heat action plans in India have reduced the mortalities in the very recent years since 2016.” claimed Dr Koll. He urges that the problem be addressed at a systemic level by changing our policies. For instance, he says that “there are scattered attempts in Odisha and interior Maharashtra where school and work hours are adjusted. We need to start thinking about similar interventions as a policy at the state level, depending on the heatwave risks of each region”. No matter what we choose to do, we ought to do it fast because the impacts of extreme weather events are only going to increase, and we may be doing too little and too late.
(The author is an ecologist and a faculty at ATREE)