Climate Change

Humid heatwaves in South Asia made 30 times more likely by climate change: Study

Trend of rising temperatures found to be lower in India & Bangladesh than Thailand, Lao PDR as well as Europe

By Akshit Sangomla
Published: Wednesday 17 May 2023
Photo: iStock

The humid heatwaves over Thailand and Lao People's Democratic Republic in mid-April 2023, would have been ‘virtually impossible’ without human-induced climate change and the ones over India and Bangladesh during the same period were made 30 times more likely by climate change, according to a study by the World Weather Attribution (WWA) released May 17, 2023. 

WWA is a global consortium of climate scientists who study the role played by human-induced climate change in the occurrence, frequency and intensity of extreme weather events such as heatwaves, droughts, cold spells, extreme rainfall, floods and storms. 

Humid heatwaves are analysed using the heat index which is a combination of heightened temperatures and relative humidity levels. It provides a better understanding of the impact of a heatwave on the human body. 

From the second to fourth week of April, many temperature records were broken in Thailand and Lao PDR and extremely high temperatures and humidity levels were observed in north, central and east India and Bangladesh. 

Though many deaths due to heatwaves were recorded in India, the long-term impact of the event will be clear only in the next few months and with a detailed study of the effects, said the authors in a press briefing on the day of the release.   

The WWA scientists used the heat index calculated by the National Oceanic and Atmospheric Administration (NOAA) of the United States but modified it to be used in the context of the Asian countries for which the study was conducted. 

The observations for heat index variables were carried out for four days from April 17-21 for south and east India and Bangladesh as one region and the whole of Thailand and Lao PDR as the other. 

The scientists found that a heatwave over Thailand and Lao PDR, which has the probability of occurring once in 200 years, will now have a heat index that is 2.3 degrees Celsius hotter than before. 

Similarly, a heatwave over India and Bangladesh with a once-in-five-years chance of occurrence will have a heat index that is 2 degrees Celsius hotter than before. 

Heat indices in the category of ‘dangerous’ (greater than 41) were common during the periods in all these countries, the findings showed. In some areas, heat indices in the ‘extremely dangerous’ category were also observed.

The chances of such events occurring in all the four countries would increase with further warming and if the GHG emissions from human activities are not brought down or stopped. The trends, though, are stronger for Thailand and Lao PDR than for India and Bangladesh, the researchers observed.

A heatwave like the recent event would be about 10 times more likely in a 0.8°C warmer world (2°C global warming over pre-industrial times), according to the authors of the report.  

In India and Bangladesh, the likelihood of this April’s event occurring again will increase by about a factor of three between today and the day global warming reaches 2°C, meaning that this humid heat event could be expected every 1-2 years, they added.  

“The increasing trend of rising temperatures and associated heatwaves with global warming is a bit lower in India and Bangladesh than in Thailand and Lao PDR. In fact it is lower than regions like Europe as well,” said Friederike Otto, of WWA and the Grantham Institute, Imperial College London, UK. 

“Though we do not understand this trend completely, it could possibly be because of the compensatory cooling impact of aerosol emissions in India and Bangladesh. There may also be certain other atmospheric circulations playing a part. But this is difficult to know or predict as the impact of aerosol emissions on temperatures is still difficult to model,” Otto added. 

Different sections of society get impacted by these humid heatwaves differently, the WWA authors also highlighted. 

This depends both on the individuals’ socioeconomic and physical vulnerability to heat and humidity. 

People with pre-existing conditions (which may get exacerbated by heat), older people, people with disabilities and those whose occupations compel them to work outdoors during such events are particularly vulnerable and most at risk. 

Further, the societal divisions based on socio-economic status, religion, caste, gender, migration and living conditions also play a part in increasing the vulnerability of people to such extreme humid heat events. 

The scientists advised that comprehensive adaptation and development interventions have to be brought in place through Heat Action Plans (HAP) to reduce the vulnerability of people to such humid heatwaves. Such measures were particularly lacking in Thailand and Lao PDR, while they needed further improvement in India and Bangladesh, noted the study authors. 

Delhi-based non-profit Centre for Policy Research analysed 37 heat action plans from India and found them to be not up to the mark, especially in terms of contextualisation to local needs.

This is another disaster that highlights the need to reduce vulnerability and think deeper about the limits to adaptation,” said Emmanuel Raju of WWA. He is also the director of Copenhagen Centre for Disaster Research at the University of Copenhagen.

“As it often happens, marginalised people are the worst-affected. Many of them are still recovering from the pandemic and from past heatwaves and cyclones, which leaves them trapped in a vicious cycle,” he added. “It is fundamental to implement mitigation and adaptation strategies to avoid visible and invisible loss and damage.”

Raju along with the other study authors advised that the HAPs should be institutionalised in all these countries that are vulnerable to humid heatwaves. 

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