In Bangladesh and India, events like the recent humid heatwave used to occur less than once a century on average but they can now be expected around once in five years. Photo: Naveen Jalli/Wikimedia Commons

Asia’s humid heatwave in April made at least 30 times more likely due to climate change: Study

Humid heatwaves in parts of the region were once-in-a-century events or nearly impossible without the influence of climate change, finds the study

Human-caused climate change made April’s record-breaking humid heatwave in Bangladesh, India, Laos and Thailand at least 30 times more likely, according to the rapid attribution analysis by an international team of climate scientists as part of the World Weather Attribution group. The study concluded that the high vulnerability in the region, which is one of the world’s heatwave hotpots, amplified the impacts. 

Sweltering cities

Bangladesh’s Dhaka observed the highest maximum temperature recorded in decades of 40.6°C on April 15. In India, several northern and eastern cities recorded maximum temperatures above 44°C on April 18. Thailand recorded its highest-ever temperature of 45.4°C on April 15 in the city of Tak. The Sainyabuli province in Lao PDR reported 42.9°C on April 19 as its all-time national temperature record. Vientiane, the capital of Lao PDR, recorded 41.4°C on April 15, the hottest day ever for the capital. On the same day, Luan Prabang in Lao PDR reported 42.7°C. 

These extreme temperatures, combined with humidity, caused a sudden increase in heat stroke cases, roads melting and a strong surge in electricity demand in all four countries. Thirteen casualties and about 50-60 hospitalisations due to heat stroke were reported in Navi Mumbai, Maharashtra on April 16 alone, while other sources mention 650 hospitalisations. Casualties have also been reported in Thailand. The true cost to human lives will only be known months after the event. 

In India, in the states of West Bengal, Tripura and Odisha, schools closed three weeks earlier than planned due to the heat. In addition, a large number of forest fires occurred during the same time in India, Thailand and Lao PDR. 

Climate change at play

Across the world, climate change has made heatwaves more common, longer and hotter. To quantify the effect of climate change on the Asian heatwave, scientists analysed weather data and computer model simulations to compare the climate as it is today, after about 1.2°C of global warming since the late 1800s, with the climate of the past, following peer-reviewed methods. 

The analysis looked at the average maximum temperature and maximum values of a heat index for four consecutive days in April across two regions, one covering south and east India and Bangladesh, and a second one including all of Thailand and Laos. The heat index is a measure that combines temperature and humidity and reflects more accurately the impacts of heatwaves on the human body. 

(a) Heat Index  in showing 4-day average daily maximum during April 17-20, 2023 for the India-Bangladesh region (highlighted in blue). The dry, semi-arid region along the rain-shadow of the Western Ghats in India, as shown by the filled polygon is excluded from the analysis as the impacts defined by the combination of high humidity and high temperatures are not relevant here. (b) 4-day average daily maximum Heat Index during April 18-21, 2023, over the Thailand-Lao PDR region (highlighted in blue). (c) Classification maps for heat impacts based on HI, for the 4-day average daily maximum HI for the period April 17-20, 2023 for the India-Bangladesh region. (d) Classification maps for heat impacts based on HI, for the 4-day average daily maximum Heat Index for the period April 18-21, 2023, over the Thailand-Lao PDR region 

According to the study, in both regions, climate change made the humid heatwave at least 30 times more likely, with temperatures at least 2°C hotter than they would have been without climate change.

In Bangladesh and India, events like the recent humid heatwave used to occur less than once a century on average. They can now be expected around once in five years, and if temperature rise reaches 2°C , which will happen within around 30 years if emissions are not cut rapidly, events like this will occur, on average, at least once every two years, said the study.

In Laos and Thailand, the scientists found that an event like the recent record humid heatwave would have been nearly impossible without the influence of climate change, and it is still a very unusual event that can only be expected around once in 200 years, even with the influence of human-caused climate change. But if temperature rise reaches 2°C, the study warned, it will become much more common, occurring about once in 20 years. 

While high temperatures are the norm in south and southeast Asia, early heatwaves such as this one are particularly damaging. People who are most exposed to the sun and vulnerable populations are routinely the worst impacted. 

“Although we have recognized heatwaves as one of the deadliest disasters, particularly in countries like India, Bangladesh, and Thailand, there is a lack of knowledge with respect to who is vulnerable, loss and damage estimation, household coping mechanisms, and the most effective heat action plans. Except for the human casualties, other economic and non-economic loss and damage indicators are not documented. This creates a dearth in assessing the extent of risk, who is vulnerable, and also operationalizing any adaptation planning,” said Chandra Sekhar Bahinipati, Indian Institute of Technology Tirupati, India.   

The current patchwork of heatwave solutions must be improved to account for inequalities and existing vulnerabilities, the scientists said, adding that heat action plans should be inclusive and comprehensive, and ensure access to basic services, such as water, electricity and health care.

“We see again and again that climate change dramatically increases the frequency and intensity of heatwaves, one of the deadliest weather events there are. Still, heat action plans are only being introduced very slowly across the globe. They need to be an absolute priority adaptation action everywhere, but in particular in places where high humidity enhances the impacts of heatwaves,” said  Friederike Otto, senior lecturer in climate science at the Grantham Institute for Climate Change and the Environment.

No way out?

There are a range of solutions to heat-related harms from the individual to the regional level. They are currently implemented as patchwork, to various degrees, across the countries studied, with India having the most advanced heatwave planning. Solutions like self-protective action, early warning systems for heat, passive and active cooling, urban planning, and heat action plans can be effective at reducing fatalities and other negative impacts. In fact, heat-related fatalities have decreased in regions where heat action plans have been in place, e.g. in the city of Ahmedabad and the region of Odisha in India.

However, these solutions are often out of reach for the most vulnerable people, highlighting the need to improve vulnerability assessments and design interventions that account for group-specific needs. 

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