Most of us have, at some point in our lives, felt either too hot or too cold. Depending on where we live, we may experience too much cold during the winter and too much heat during the summer. As we write this in late January 2023, many southern Africans are likely feeling extremely hot and exhausted; a prolonged heatwave in the region began on January 9.
Being overheated is more than simply unpleasant. Heat stress produces dehydration, headaches, and nausea; when people are exposed to high temperatures for extended periods, they run the danger of serious health consequences and may even perish. In January, at least five farm workers in the South African province of Northern Cape perished from heat stroke. At least 90 people perished in India and Pakistan during a catastrophic heatwave in May 2022.
The situation will continue to deteriorate. The Intergovernmental Panel on Climate Change of the United Nations says that by the end of the century, the percentage of the world’s population exposed to lethal heat stress is expected to rise from 30 percent now to 48 percent to 76 percent, depending on future warming levels and location.
We desired to develop a comprehensive understanding of when and where heat stress occurs in southern Africa. Using a global gridded dataset of a human thermal comfort index, we discovered that there has been a constant change in thermal comfort – the human body’s perception of the outdoor thermal environment – since the 1970s. Simply expressed, the incidence of heat stress among southern Africans has increased since 1979.
These findings are concerning given that global temperatures are projected to climb in the future years and decades. Warmer temperatures will cause places previously categorized as having “favorable” thermal comfort to be reclassified more frequently as those with “thermal stress.” It is expected that heatwaves would become more frequent and intense.
Scientists from around the world have produced the Universal Thermal Climate Index over the past two decades. It has improved our ability to model human thermal comfort, from cold stress to hot stress. Earlier thermal comfort indices often solely modeled heat stress because they primarily calculated an equivalent temperature by combining the effects of humidity and temperature.
Extreme temperatures can be hazardous to people’s health. Authors contributed.
This analogous temperature would quantify how we feel in respect to our surroundings. For example, at 5pm on 23 January, Johannesburg’s outdoor air temperature was 29˚C; relative humidity was 30%; the sky was clear and there was a gentle breeze of 16km/h.
For someone outside, the equivalent temperature would have been slightly higher than the outdoor temperature (possibly as high as 32˚C), largely due to the effect of relative humidity and limited wind chill.
The Universal Thermal Climate Index takes into account a broader range of thermal comfort-influencing parameters than its predecessors. In addition to air temperature, relative humidity and wind speed, it also includes radiant heat, a measure of how hot we feel when standing in the sun rather than in the shade.
The index is built for humans navigating the real world: it includes a clothing model and an exertion model.
During the current southern African heatwave, for instance, the model assumes that nobody is dressed in a fuzzy jersey. In winter, it assumes nobody in countries like Namibia, Botswana, Zimbabwe, Mozambique, Eswatini, Lesotho and South Africa is wearing shorts and a T-shirt.
Ultimately, the inclusion of all these factors means that the Universal Thermal Climate Index is a more accurate and realistic indicator of the level of thermal comfort (or discomfort) perceived by the human body.
SOUTHERN AFRICAN APPLICATION
To apply the Universal Thermal Climate Index to southern Africa, we drew data from the ERA5-HEAT data collection, which provides an hourly dataset, of the equivalent temperature derived from the index, for 1940 to present; it is produced by the European Centre for Medium-Range Weather Forecasts.
We zoomed into the time period 1979-2021 and considered thermal comfort at annual, seasonal and monthly scales. Over these scales, we calculated the average climatology, and investigated changes and year-to-year variability patterns in day-time, night-time and daily average equivalent temperatures across southern Africa.
We found that heat stress occurs most widely during the summer months (December to March); cold stress occurs mainly during the winter months (June to August). Heat stress was, as one would expect, most common during the day and cold stress more common at night.
Drilling further into the data, we discovered that, from September to March, more than 85% of the subcontinent experiences day-time heat stress. Over parts of the Northern Cape in South Africa, Namibia, Botswana, Zimbabwe and Mozambique, day-time heat stress can reach very strong, and potentially dangerous, heat stress levels during these months.
From May to August, our results showed that more than 80% of southern Africa experiences night-time cold stress, and over much of South Africa night-time cold stress can reach moderate cold stress. In short, it’s unusual for people in the region to feel extremely cold and fairly common in certain months to feel extremely hot, especially outside.Everyone in southern Africa is at risk of heat stress. But children, the elderly, and those with underlying comorbidities are more vulnerable.
Those working outdoors, like farm and construction workers, are especially vulnerable because there’s little that can be done to adapt to and cope with heat stress while working outdoors during the day-time. Adjusting work hours to avoid peak heat hours is one measure that could be applied.
There are also some coping mechanisms you could apply in your daily life. Limit your exposure to the sun by moving to shade or indoors to a well-ventilated or air-conditioned room. Keep hydrated (with water), avoid strenuous activities (like sports or excessive manual labour), wear lightweight protective clothing, a hat and sunblock, and, if you feel ill, seek medical attention.
»Where and when is heat stress the worst in southern Africa, where it is on the rise?«