It’s the humidity: killer heat waves projected for much of south Asia

The populations of south Asia are at particular risk from projected increases in the intensity and frequency of heat waves, according to a new study that projects the combination of higher temperatures and high humidity will lead to conditions at or exceeding the upper limit of human survivability. 

While rising global temperatures associated with the continuing increase of atmospheric greenhouse gases will drive heat waves around the world, particular dangers will be faced by agricultural workers and others who will continue to work outdoors in large parts of India, Pakistan, Bangladesh and Nepal, suggests the paper published in Science Advances. More than 1.7 billion people live on the Indian subcontinent and the population continues to grow, with India expected to overtake China as the world’s most populous nation by 2025. {%recommended 1275%}

 “The most intense hazard from extreme future heat waves is concentrated around densely populated agricultural regions of the Ganges and Indus river basins,” write Eun-Soon Im, of the Hong Kong University of Science and Technology, Jeremy Pal, of Loyola Marymount University in Los Angeles, and Elfatih Eltahir, of Massachusetts Institute of Technology. “Climate change, without mitigation, presents a serious and unique risk in South Asia, a region inhabited by about one-fifth of the global human population, due to an unprecedented combination of severe natural hazard and acute vulnerability.”

The key factor issue, the authors say, is ‘wet-bulb temperature’, which is a combined measure of both ‘dry-bulb’ temperature and humidity. 

Wet-bulb temperature, or TW, is defined as the temperature an air parcel would attain if cooled at constant pressure by evaporating water within it until saturation. The higher the TW, the less difference there is between human body skin temperature and the body’s core temperature, which reduces the human body’s ability to cool itself.

“Because normal human body temperature is maintained within a very narrow limit of ±1°C, disruption of the body’s ability to regulate temperature can immediately impair physical and cognitive functions,” the authors write. If the ambient air wet-bulb temperature exceeds 35°C, which is the typical human body skin temperature under warm conditions), metabolic heat can no longer be dissipated, and more than six hours’ exposure “will result in death even for the fittest of humans under shaded, well-ventilated conditions”.

Thus while TW well below 35°C can pose dangerous conditions for most humans, they regard 35°C as the upper limit on human survivability in a natural, non-air-conditioned environment.

Spatial distribution of highest daily maximum wet-bulb temperature in modern record (1979–2015).
Spatial distribution of highest daily maximum wet-bulb temperature in modern record (1979–2015).
Im et al., 2017

The study makes TW projections using simulations based on two scenarios, or representative concentration pathways, for future greenhouse gas levels used by the Intergovernmental Panel on Climate Change. One is RCP 8.5, the highest scenario, commonly referred to as the business-as-usual scenario, though the IPCC itself no longer uses such a description. The other is RCP 4, which assumes moderate mitigation efforts that contain global warming to about 2.25°C over the century – slightly higher than the target to which most of the world’s nations have committed.

Those simulations project that, by the end of century under the RCP8.5 scenario, wet-bulb temperatures will exceed the 35°C survivability threshold at a few locations in northeastern India and Bangladesh, and approach it over most of South Asia, including the Ganges river valley, northeastern India, Bangladesh, the eastern coast of India, Chota Nagpur Plateau, northern Sri Lanka, and the Indus valley of Pakistan. Under the RCP4.5 scenario, vast regions of South Asia are projected to experience episodes exceeding 31°C, “which is considered extremely dangerous for most humans”.

“The geographical locations of the most extreme projected heat waves in the Indus and Ganges river valleys coincide largely with locations of highly vulnerable human populations in terms of population density, gross domestic product per capita and agricultural intensity,” the authors write, noting that agricultural workers in these regions spend significant time working outdoors during the seasons with the highest temperature and humidity. 

“In this regard, our findings have significant implications to the ongoing considerations regarding climate change policy. In the absence of serious mitigation, some of the most severe hazards associated with climate change will fall on some of the most vulnerable populations. In poorer regions, such as South Asia, air conditioning is not currently available as a safe haven to most of the population, increasing the risk of illness and death related to extreme heat conditions. This disparity raises important environmental justice questions beyond the scope of this study.” 

They note a particular dilemma for India, whose greenhouse gas emissions have been increasing rapidly in recent decades because of population and rapid economic growth – a trajectory that, perversely, is “likely to impose significant added human health risks to some of its most vulnerable populations”.

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