Heatwave Grips Several States, New Analysis Shows April’s Extreme Humid Heat Driven By Climate Change

Amid another spell of intense heatwave in several parts of eastern and central India, a new study shows that climate change has made such events of extreme temperatures and humidity 30 times more likely in India. The rapid attribution analysis, which is yet to be peer reviewed, was led by 22 global researchers as part of the World Weather Attribution initiative.

The analysis builds on previous studies, which show that heatwaves are now becoming more severe, frequent and prolonged in countries like India. “Despite heatwaves being recognised as the deadliest disasters, there is 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,” said Chandra Sekhar Bahinipati, one of the researchers from IIT-Tirupati.

Dangers of humid heatwaves

Humid heatwave conditions prevailed over Gangetic West Bengal for close to 10 days, seven days in Bihar and for more than five days in Odisha. Such conditions exacerbate health dangers, as high humidity restricts the body’s cooling mechanism causing dehydration and heat stroke that may even prove fatal. Previous studies also indicate that a heatwave day has about 20 percent higher mortality risk than a non-heatwave day.

In order to quantify the effect of climate change on the Asian heatwave, scientists analysed weather data and computer model simulations to compare today’s climate, which has warmed by 1.2 degrees Celsius since the late 1800s, with the climate of the past. They covered two regions that reported heat-related casualties in April – south and east India, and Bangladesh as one, while Thailand and Laos as another. The team looked at the average maximum temperature and maximum values of heat index for four consecutive days (April 17 to 20) across the two regions.

Since the India Meteorological Department (IMD) does not provide a verified heat index (a combined measure of temperature and humidity), the team calculated it from ERA5 near-surface temperature and dew point temperature – the only one dataset available for the entire region.

Heat index over ‘dangerous’ levels

In both the study regions, the heat index values were found to be at the ‘danger’ level over large areas during April. “The estimated heat index values exceeded the threshold considered as ‘dangerous’ (41 degrees Celsius) over large parts of the South Asian regions studied. In a few areas, it neared the range of ‘extremely dangerous’ values (above 54 degrees Celsius) under which the body temperature is difficult to be maintained,” the team said.

In Bangladesh and India, events like the recent humid heatwave would take place less than once a century on average, they can now be expected around once in five years. If emissions are not curtailed, and temperature rise reaches 2 degrees Celsius then events like this will take place, on average, at least once every two years, the team said.

The scientists said heatwave solutions also needed to take into account inequalities and existing vulnerabilities among people. “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 from London-based Grantham Institute for Climate Change and the Environment.

The team included researchers from India, Thailand, France, Australia, Denmark, Germany, Kenya, the Netherlands, US and UK.