News Ticker

Climate Change: Heat Waves as a Growing Threat to Human Health

Changing lives: Ante and post natal care for mums and babies in Orissa - DFID

When considering the effects of climate change there are frequent references to increased temperatures, heat waves and the impact on human health. In the last few years a considerable amount of research has been undertaken, and show would uncover have suggested a human survivability threshold based on wet bulb temperature. Wet bulb temperature is a measure of temperature and humidity, it will always be lower than the air temperature, for example an air temperature of 39°C with a humidity of 50%, and the pressure of 1000 mbar, gives a wet bulb temperature of 29.59°C. High temperatures and levels of humidity are frequently found in the Arabian / Persian Gulf during the summer months. If we take an extreme level of temperature, such as the 46°C recorded in Bandar Mahshahr, Iran on 31 July 2016, and a humidity of 49%,  this gives a wet bulb temperature of 34.6°C.

What is not sufficiently understood is that the human body has an upper limit of survivable temperature, because above a certain temperature it is impossible for a human body to regulate its own temperature, hyperthermia (heat stoke) is a medical condition where the body temperature is elevated due to a failure of thermoregulation, where the body produces or absorbs more heat than it dissipates. Unless extreme temperature elevation is treated rapidly this can result in disability or death. An essential requirement for continued normal body function is that the deep body temperature will be maintained within a very narrow limit of ± 1°C around the acceptable resting body core temperature of 37°C.[1]

According to Im et al, “human exposure to Tw [wet bulb temperature] of around 35°C for even a few hours will result in death even for the fittest humans under shaded, well ventilated conditions.”[2] They add, “while TW well below 35°C can poise dangerous conditions most humans, 35°C can be considered upper limit on human survivability in a natural (not air-conditioned) environment.”[3] In practice, any wet bulb temperature exceeding 31˚C, which lasts for any period, represents a threat to human life, particularly to the very young, the elderly and the sick.

A pioneering study by Pal and Eltahir (2016)[4] looked at future temperatures in south-west Asia, and the extent to which the wet bulb temperatures will exceed the threshold for human survivability. There are four areas on the Earth’s surface where high temperatures by the end of this century may well pose a real threat to human life, these are the area surrounding the Persian Gulf, including the interiors of Iraq and of Iran, a large part of northern India and the Indus Valley of Pakistan, together with the Indian coast of the Bay of Bengal, a third area includes parts of eastern China and Northern Vietnam, finally it is expected that parts of the Horn of Africa, may also see even hotter weather. The actual temperatures that may be experienced in these regions by the end of the 21st century will mainly be a factor on the amount of carbon released into the atmosphere, but it is likely that air temperatures may well exceed 60°C, especially given the recent extremely high temperatures in parts of Iran, Iraq and India, which have seen temperatures as high as 54°C.

The study by Pal and Eltahir considered two scenarios supported by the IPCC, RCP 4.5 and RCP 8.5. RCP 4.5 assumes approximately 650 ppm CO2E in the year 2100 without ever exceeding that value, or a rise in global temperatures of approximately 2.25˚C, slightly higher than the Paris Agreement target, but a realistic figure which could be achieved. In contrast RCP8.5 represents a “business-as-usual” scenario, which envisages a global temperature increase of between 3.8˚C –5.7˚C by 2100. This study identified a number of locations where the wet bulb temperature would exceed 30°C under either scenario, these included Dhahran, Saudi Arabia, Doha, Qatar, Dubai, Al Ain and Abu Dhabi in the UAE, Aden in the Yemen, Jeddah and Mecca in Saudi Arabia, together with Bandar Abbas and Bandar-e Mahshahr in Iran. Under the RCP8.5 scenario, it was projected that Dubai, Abu Dhabi, Doha, Dhahran and Bandar Abbas could all exceed a wet bulb temperature of 35°C, the upper limit for human survivability. They also point out in their study of South Asia, referred to below, but in the summer of 2015 the wet bulb temperature in the Bandar Mahshahr in Iran, reached the wet bulb temperature of nearly 35°C, which suggests that temperature thresholds may be breached sooner than was projected in the 2015 study.

The authors say although it may be feasible to adapt indoor activities in the rich oil countries of the region, even the most basic outdoor activities are likely to be severely impacted. In contrast, the relatively poor countries of south-west Asia with limited financial resources and declining or non-existent oil production will robably suffer both indoors and outdoors including the coastal region of Yemen around Al-Hudaydah and Aden where wet bulb temperatures are projected to reach about 33°C in extreme years under RCP8.5 and over 30˚C under RCP4.5. As a result, there can be few areas in the world which will suffer more if we fail to achieve the goal of eliminating carbon emissions.

The two authors of the 2015 study on Southwest Asia have also, with Eun-Soon Im, published a study in 2017 on the impact of rising global temperatures on South Asia. In the 2017 study the authors define South Asia as including Pakistan, Nepal, India, Bangladesh and Sri Lanka; they note that the northern part of this region is the second hottest after Southwest Asia, but also covers a larger land area, and a far larger population. Including Bhutan, the population of South Asia is, according to the United Nations, due to reach 1.8 billion by 2020, 2.27 billion by 2050, and to fall slightly to 2.24 billion by 2100. So, 1% of the South Asian population in 2020, will be over 18 million people, consequently, the threat of higher temperatures, caused by climate change, will put the lives of untold millions at risk. It is far too easy to talk glibly of climate change, as something that will be a mild inconvenience, at some time in the future. Few understand that it is already affecting our climate and that it has the potential to kill large numbers of human beings during this century, on a scale, which could exceed the number of deaths caused by the major conflicts in the 20th century. For India, in particular, dealing with climate change is the greatest threat that the country faces today, and it is particularly vulnerable to temperature increases. The country also faces related problems of changes to the pattern of the monsoon, reduced glacial water run-off from the Himalayas and rising sea levels, which will have a particular impact in the Bay of Bengal, and on the shores of the Arabian Sea.

In the 2017 study the authors note that there has been an apparent rising trend in the frequency of deadly heat waves over much of India and Pakistan, they say that the fifth deadliest heat wave in recorded history affecting large parts of India and Pakistan and claimed over 3500 lives in 2015. The areas affected during these heat waves largely coincide with the areas where wet bulb temperatures are projected to approach, or exceed the survivability threshold, under RCP 8.5. Even under RCP 4.5, large areas of India, Bangladesh and Pakistan will exceed the wet bulb temperature of 30°C, in fact under this scenario they project that Patna could see wet bulb temperatures reaching 34°C, in Pakistan Karachi, and Lahore, could both see wet bulb temperatures reaching 32°C under RCP 4.5. The authors say that under RCP 8.5 the wet bulb temperature is projected to exceed the survivability threshold in parts of north-eastern India and Bangladesh and to approach the threshold at the end of the century over most of South Asia including the Ganges River Valley, North Eastern India, Bangladesh and the eastern coast of India, the Chota Nagpur plateau, northern Sri Lanka and the Indus Valley of Pakistan. Under RCP 4.5 no regions are projected to exceed wet bulb temperature of 35°C, but “vast regions of South Asia are projected to experience episodes exceeding 31°C, which are extremely dangerous for most humans.” They add, that the geographical locations of the most extreme projected heat waves in the Indocin Ganges river valleys coincide with the locations of highly vulnerable and very large human populations in terms of population density, gross domestic product (GDP) per capita and agricultural intensity.” In particular, agricultural workers who spend significant time out of doors in the summer months, are especially vulnerable. It is, however, a matter of great concern, that it is the centres with the largest populations in India, Bangladesh and Pakistan that appear most vulnerable. Given that nearly 2 billion people live in South Asia, the numbers who are potentially at risk, are very large, but no study has yet been undertaken to estimate casualties and possible death rates. As the authors note, some of the most severe hazards associated with climate change may well fall on some of the most vulnerable populations. In South Asia air conditioning is not currently available to most of the population, unlike the situation in Southwest Asia, this increases the risk of illness and death that would be associated with extreme heat waves.

There have been other studies on the possible impact a future heat waves in India and their likely effects, including Heatwave Vulnerability Mapping for India, by Azhar et al (2017)[5] and Intensification of Future Severe Heat Waves in India and their Effect on Heat Stress and Mortality, by Murarai et al (2014).[6] Murari et al found that heatwaves are projected to be more intense in India have longer durations and occur at a higher frequency and earlier in the year, they concluded that the intensification of heat waves might lead to severe heat stress and increased mortality.

Another interesting study is Strongly increasing heat extremes in the Middle East and North Africa (MENA) in the 21st century, by Lelieveld et al (2016)[7]. Lelieveld et al concluded, “that climate change and increasingly hot weather extremes in the MENA, a region subject to economic recession, political turbulence and upheaval, may exacerbate humanitarian hardship and contribute to migration.” They also note that in the past, climate assessments have often focused on storms, floods, droughts and sea level rise, and that “It is increasingly recognised the hot weather extremes cause a loss of work capacity and aggravate says societal stresses, especially the disadvantaged people and vulnerable populations.”

It is also important to remember the parts of East Asia also will have problems with heatwaves, and even Europe, which is normally regarded as a temperate area, can also be affected by heat waves. In 2003, it has been calculated that 70,000 people died as a result of the high temperatures in Europe.[8] In fact there are a few areas of the world that will be able to avoid problems with excessive temperatures during the remainder of the 21st-century, and it is likely that it will be heatwaves, rather than flooding, that makes people aware of the severe impacts of climate change.

In some regions, notably the Middle East, but also including parts of south Asia, areas well become increasingly uninhabitable during summer months, resulting in even more pressures to migrate to cooler climates, a process which is likely to be resisted by those regions, less affected by climate change.


(c) Andrew Palmer, 2017, please do not reproduce without permission.


[1] Epstein, Yoram, and Daniel S. Moran. “Thermal comfort and the heat stress indices.” Industrial Health 44, no. 3 (2006): 388-398.

[2] Im, Eun-Soon, Jeremy S. Pal, and Elfatih AB Eltahir. “Deadly heat waves projected in the densely populated agricultural regions of South Asia.” Science Advances 3, no. 8 (2017): e1603322.

[3] Im, Eun-Soon, Jeremy S. Pal, and Elfatih AB Eltahir. “Deadly heat waves projected in the densely populated agricultural regions of South Asia.” Science Advances 3, no. 8 (2017): e1603322.

[4] Pal, Jeremy S., and Elfatih AB Eltahir. “Future temperature in southwest Asia projected to exceed a threshold for human adaptability.” Nature Climate Change 6, no. 2 (2016): 197-200.

[5] Azhar, Gulrez, Shubhayu Saha, Partha Ganguly, Dileep Mavalankar, and Jaime Madrigano. “Heat wave vulnerability mapping for India.” International Journal of Environmental Research and Public Health 14, no. 4 (2017): 357.

[6] Murari, Kamal Kumar, Subimal Ghosh, Anand Patwardhan, Edoardo Daly, and Kaustubh Salvi. “Intensification of future severe heat waves in India and their effect on heat stress and mortality.” Regional Environmental Change 15 (2015): 569-579.

[7] Lelieveld, Johannes, Y. Proestos, P. Hadjinicolaou, M. Tanarhte, E. Tyrlis, and G. Zittis. “Strongly increasing heat extremes in the Middle East and North Africa (MENA) in the 21st century.” Climatic Change 137, no. 1-2 (2016): 245-260.

[8] (Robine, et al. 2008).

1 Trackbacks & Pingbacks

  1. Wet bulb temperature exceeding 31˚C: A threat to human life | Ketheric

Leave a Reply