By the end of the century, climate change could trigger heatwaves that exceed the threshold of “human survivability” in India, with the densely populated Gangetic basin at the greatest risk, finds a new study published in the journal Science Advances.

The combination of high temperature and humidity —which characterises heatwaves — increases the risk of human illness and mortality: when the combined measure of temperature and humidity (or ‘wet-bulb temperature’) is high, the human body’s ability to cool itself diminishes, impairing physical and cognitive function.

And when ambient wet-bulb temperature exceeds 35 degrees Celsius, considered an “upper limit” for human survivability, it could, even in just a few hours, “result in death even for the fittest of humans under shaded, well-ventilated conditions,” the paper explains.

Most of South Asia is projected to see maximum daily wet-bulb temperatures approach 35 degrees Celsius by 2100 under RCP8.5 (a high greenhouse gas concentration scenario). This includes the Ganges river valley, Northeastern India, the eastern coast of India, the Chota Nagpur Plateau and the Indus valley of Pakistan. Under a moderate mitigation scenario (RCP4.5) “vast regions of South Asia are projected to experience episodes exceeding 31 degrees Celsius, which is considered extremely dangerous for most humans.”

Three reasons

The Indus and Gangetic basins, where hundreds of millions live, could be at the greatest risk of severe heatwaves for three reasons: the monsoon brings warm and humid air masses into these valleys; surface air is warmer here because of their relatively low elevation; and irrigation enhances wet-bulb temperatures.

In the Gangetic plains for instance, while the current maximum daily wet-bulb temperatures hover around 30 degrees Celsius, they are projected to rise to 31 degrees Celsius by the end of the century under RCP4.5 and 33 degrees Celsius over vast swathes under RCP8.5.

Urban areas such as Patna and Lucknow are no less vulnerable. “In urban locations such heatwave events can have consequences because of the sheer number of people living there. However, in rural parts, the outdoor working conditions and poverty make populations more vulnerable,” co-author Jeremy S Pal, Professor at the Department of Civil Engineering and Environmental Science, Loyola Marymount University, Los Angeles, told The Hindu.

Rising frequency

The frequency of deadly heatwaves has been rising in India and Pakistan: in Odisha in 1998, Andhra Pradesh in 2003 and Gujarat in 2010. In 2015, the fifth deadliest heatwave in recorded history claimed around 3,500 lives in India and Pakistan.

The study looked at maximum daily wet-bulb temperatures averaged over a six-hour window in South Asia —Pakistan, Nepal, India, Bangladesh, and Sri Lanka. Using high-resolution climate change simulations, including detailed representations of topography, land surface and atmospheric physics, the paper projects that wet-bulb temperatures in South Asia “are likely to approach and, in a few locations, exceed this critical threshold [35 degrees Celsius] by the late 21st century under the business-as-usual scenario of future greenhouse gas emissions.”

These findings “may present a significant dilemma for India because the continuation of this current trajectory of rising emissions will likely impose significant added human health risks to some of its most vulnerable populations,” the authors caution.