© The Indian Express Pvt Ltd
Tags:
Journalism of Courage
The consecutive heatwaves over South Asia since March 2022 have continued the disturbing tradition of breaking historical temperature records. Mercifully, these record temperatures were not accompanied by the high mortality burden observed in previous heatwaves such as the one during 2015. Why those heatwaves were so deadly is a puzzle that we are yet to resolve.
The recent IPCC report AR6 has emphasised that humidity is also very important while estimating the physiological stress that extreme heat puts on the human body. Instead of the “dry bulb” temperature that is usually measured using a regular thermometer, an alternative metric known as the “wet bulb temperature” has been used to measure exposure to extreme heat. The report mentions that sustained exposures to wet bulb temperatures above 35°C are fatal, while sustained exposures to wet bulb temperatures above 32°C are dangerous for intense physical activity. The critical word here is “sustained”, which we will come back to later.
These projections are very welcome and are the outcome of more than a decade of work put in by the climate science community. The public health community was, of course, aware of the implications of humidity for a much longer time. Furthermore, a number of recent articles in the media have helped make the public aware of this. However, there has been a growing concern about the 35°C threshold and whether parts of South Asia will become “unsurvivable” in the coming years.
Humidity and temperature
Why is humidity such a critical factor while measuring heat exposure? Humans lose heat generated within their body by producing sweat that evaporates on the skin. The cooling effect of this evaporation is essential in maintaining a stable body temperature. As humidity rises, sweat does not evaporate —just like clothes take a long time to dry in humid locations – and makes it difficult to regulate body temperature. This is why we feel more discomfort in humid places.
The wet bulb temperature is usually lower than the dry bulb temperature, and the difference between the two increases dramatically as the air becomes dry. The variation of wet bulb temperature for a fixed dry bulb temperature is shown in the figure.
The humidity required to reach wet bulb temperatures in excess of 35°C over land is exceedingly difficult to achieve for a variety of reasons, which this article will not go into. This is the reason the AR6 says that such conditions are rarely observed nowadays. Wet bulb temperatures in excess of 35°C have been observed in Sindh in Pakistan, but such conditions occur once every three to four years, and probably for a few hours. This fails to meet the criteria of “sustained exposure”.
Newsletter | Click to get the day’s best explainers in your inbox
Just because we don’t observe such conditions in the current climate does not mean that they will be as rare in future climates. However, the research that backs the AR6 also suggests we are unlikely to experience sustained exposure to wet bulb temperatures beyond the threshold of survivability.
The hype around survivability thresholds and wet bulb temperatures obscures deeper issues, both physiological and political. Firstly, the inability of the body to stabilise its core temperature can have multiple reasons. For instance, increased strain on the heart during periods of elevated temperature could be fatal for those with pre-existing cardiac conditions and is in fact the leading cause of deaths during heatwaves. Pre-existing respiratory problems and diabetes too are potential causes of death. Such conditions impair the body’s ability to efficiently transfer heat to the environment.
A less obvious issue is that of dehydration. Many labourers, especially women, intentionally keep themselves dehydrated due to the lack of toilets in workplaces. Dehydration can lead to decreased sweat production and therefore increased vulnerability to heat stroke during heatwaves. Such public health factors can dramatically reduce the survivability thresholds and underestimate the actual vulnerability of the population.
Global and local
There is also a political issue here that often goes unnoticed. A singular focus on increasing wet bulb temperatures subtly shifts the responsibility of action from the local to the transnational arena. Increasing wet bulb temperatures are the byproduct of global climate change, and therefore keeping our population safe becomes the responsibility of international negotiators at conferences such as COP26. Very little can be done at the local level to keep wet bulb temperatures from rising if the factors controlling them are global. On the other hand, understanding the factors that increase vulnerability to heatwaves puts the onus on local actors, who have to provide better sanitation facilities, protect the elderly and those with cardiac conditions, and reduce the incidence of diabetes in the population. Such a focus will increase the pressure to improve our national health infrastructure, whose fragility has become increasingly clear to everyone over the past two years.
In summary, the recent focus on increasing temperature and humidity is a welcome step in building awareness about the impacts of climate change. However, it is important to realise that our vulnerability to heatwaves is a systemic problem that has existed for a very long time, and is not dependent only on increasing greenhouse gas emissions. Refocusing our priorities to highlight critical gaps in our urban and health infrastructure that repeatedly fail to protect the most vulnerable among us will allow us to build a more climate-resilient nation.
Joy Merwin Monteiro is a Climate Scientist working at the Department of Earth and Climate Science at IISER, Pune.
