Heat waves, extreme weather events and climate change

Heat waves sweep across India (Image: Maxpixel, CC0 Public Domain)
Heat waves sweep across India (Image: Maxpixel, CC0 Public Domain)
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India Metrological Department (IMD) declares a heatwave based on temperature data. A heatwave is declared when the highest temperature at a station surpasses 45 degrees Celsius. If it exceeds 47 degrees Celsius it is referred to as a severe heatwave. Heat waves are defined as prolonged episodes of extreme heat (high temperature) over any region.

In the recent past, heatwave gripped parts of the United States of America and Canada for days, breaking records. Canada recorded 49.6 degrees Celsius on June 29, 2021. Scientists have warned for some time that climate change will increase the frequency and intensity of heatwaves.

A research study attributed the heatwave event to climate change. The study was published by the World Weather Attribution, an international scientific collective that conducts real-time attribution analysis of extreme weather events as they happen around the world. The researchers estimated that the extraordinary temperature rise was a one in a thousand-year event.

But, if the current Green House Gas (GHG) emissions continue, an event so extreme could start occurring every 5 to 10 years by the 2040s. People need to realise that heatwaves are killers and they are by far the deadliest extreme event says F Otto, a climate scientist and a co-author of the study. He added that heat preparation and preventing deaths during heat waves would need to be the number one priority for the authorities.

Apart from temperature, humidity is an important parameter considered for declaring heat related stress. The presence of humidity in the environment prevents the thermoregulatory mechanism of evaporative cooling of the body through perspiration thereby causing stress. During this period, there is subsidence, i.e., downward movement of air because of high pressure. The warm air is devoid of any moisture, and does not relate to any convective clouds. As the temperature rises, cold wave conditions are likely to go down because of rising temperature.

Almost all climatic models predict that deadly heatwaves will become common across South Asia even if global warming is contained to 1.5 degree Celsius. The increase in thunderstorm events due to rise in moisture content in the atmosphere points towards temperature increase while the intensity of cyclonic activities in the Arabian sea are on the rise.

Going by current projections, extreme events e.g., extreme rainfall, floods, heatwave conditions, etc., will become more frequent and more intense. In the recent past there have been multiple incidents of cloudbursts, flash floods and landslides in hilly areas of the country.

India’s climate has characteristics of the tropical and subtropical system and cyclones, thunderstorms and monsoons are a part of it. According to the India Meteorological Department, global temperatures have risen by 1.2 degrees Celsius compared to 100 years ago, while India recorded a rise of about 0.6 degrees Celsius. The rise in temperature has an impact on extreme weather events. It is getting hotter not just on the surface but also in the troposphere, increasing its moisture-holding capacity.

Studies show that with a rise of 1 degree Celsius, moisture-holding capacity increases by about 7 percent. So, the probability of occurrence of heavy rainfall has increased. Studies also show an increase in the frequency of heavy rainfall events. Such events are increasing over the tropical belt as a whole, including in India. On average, the number of light and moderate rainfall days is decreasing, while extreme rainfall events are increasing. This trend is quite significant across India's central belt.

A decrease in rainfall activity has been observed over Kerala and Jharkhand and adjoining areas, while there is an increase in West Bengal, Western Uttar Pradesh and parts of Karnataka. Increased heatwave conditions are more prominent in central and northern India.

IMD studies using 50 years (1970-2019) extreme weather events dataset have shown increasing occurrences of extreme weather events including extremely severe cyclonic storms in recent decades. Analysis of past data of cyclones during the period 1891-2020 indicates that the frequency of extremely severe cyclonic storms has increased in recent years over the Arabian Sea since 1990 and remained the same over the Bay of Bengal.

A significant rise in mean temperature across the globe is reported, which is expected to trigger more intense meteorological events e.g., intense or severe cyclonic storms. These events had significant impacts like the loss of lives and property in various regions in India. Over the last few decades, global warming and emission of carbon have altered the chemistry of the oceans and a UN report has come up with grim statistics.

This study notes that the oceans have become more acidic as seawater absorbs more carbon dioxide. Also, the upper layers of the open ocean have lost between 0.5% and 3.5% of oxygen since 1970 as temperatures have risen. In the worst-case scenario, the report cautions that if left unchecked, this could prove to be catastrophic for the world.

The Inter-Governmental Panel on Climate Change (IPCC) released the second of its three-part report in the 6th assessment cycle recently. It focuses on the impacts of climate change and its implications on vulnerability and adaptation.

The IPCC report highlights how urbanization has generated vulnerability and exposure combined with climate change hazards. The primary driver of increased heat exposure is the combination of global warming and population growth in the already warm cities in India. The urban heat island also elevates temperature within cities, which will amplify during heat waves.

Older adults, people with comorbidities and dwellers living without much access to hygienic environments will be at a much higher risk in urban areas. The report highlights that chance of high-intensity short-duration rainfall in urban areas will increase. Places like Mumbai, Chennai, Kolkata, Visakhapatnam, Goa, Andaman and Nicobar Islands and smaller coastal towns and villages are at greater risk of being flooded.

Global warming has increased the average temperature in the Himalayas which has caused glacier melt and subsequent change in the hydrological regime of the region. One of the contributing factors to glacial decline is the deposition of black carbon, which is contributed by stubble burning, brick kilns and polluting industries. Water insecurity in hill towns is becoming the order of the day. Most towns meet their water requirement using supplies from springs, ponds and lakes that are essentially interlinked systems.

IPCC warns that limiting temperature increase to 1.5 degrees Celsius above the pre-industrial level will not be possible without immediate and drastic cuts in GHG emissions. Enabling massive reductions in GHG emissions requires financial flows and technologies, a major challenge, particularly for developing countries.

Mumbai Climate Action Plan (MCAP), a policy document with a net-zero goal by 2050, warned about flood risks to the city due to the rise in sea level. The document released in March 2022 states that the city faces three risks-increasing heat and heat island effect, increasing flood risk because of sea-level rise and due to extreme rainfall events, tropical cyclones and storm surges and increasing air pollution affecting human health.

It is noted that most of the climatic models predict that deadly heatwaves will become common across South Asia even if global warming is contained to 1.5 degrees Celsius. The increase in extreme weather events e.g., extreme rainfall, floods, heatwave conditions, etc., will become more frequent and more intense.

These events have significant impacts like the loss of lives and property in various regions in India. The IPCC report stressed that India needs to readopt its traditional adaptation solutions. Better adaptation policies could lead to a safer and more sustainable future.



Dr Samar Lahiry is a former Adviser, Planning Commission, Government of India

Post By: Amita Bhaduri