A rising body of science shows that 35°C is not merely uncomfortable; it is a physiological threshold. In Botswana and across the region, the heat is arriving earlier, staying longer, and asking more of the body than evolution prepared it for.
By August, the air over the Kalahari has already begun to thicken. In Gaborone, Botswana’s capital, the mercury regularly climbs past 35 degrees Celsius before noon. The asphalt softens. Construction crews down tools. Hospital admissions tick upward. And in the informal settlements ringing the city, where corrugated-iron roofs absorb and radiate the sun’s force long after dark, the heat does not leave when the sun goes down.
Southern Africa is no stranger to high temperatures. But what is unfolding now is different in kind, not merely degree. Average surface temperatures across the Southern African Development Community region have risen between 1.0 and 1.5 degrees Celsius since 1961, and under current emission trajectories, a further 4.5 to 5 degrees of warming is projected by midcentury. In Botswana specifically, forecasters are warning of temperatures soaring between 35 and 45 degrees Celsius; a range that, according to a growing body of physiological research, pushes the human body toward the outer edge of what it can tolerate.
A landmark consensus study released this year by the Academy of Science of South Africa – developed by an expert panel spanning Botswana, Eswatini, Lesotho, Malawi, Mauritius, South Africa and Zimbabwe – identified extreme heat as “the defining climate and health threat” for the region. The report’s language is striking in its directness: extreme heat, it argues, is no longer a distant projection but a present reality reshaping the lives of millions.
“Extreme heat is no longer a distant projection; it is already reshaping the lived realities of millions of people across Southern Africa,” said Prof. Jerome Amir Singh, chairperson of the expert panel that led the study.
The question is no longer whether the heat will come. It is what happens to the body when it does, and what can still be done about it.
What 35 degrees does to the body
The human body is a thermal machine. It generates heat as a byproduct of metabolism – even at rest, the body produces roughly as much warmth as a 100-watt incandescent bulb – and it must shed that heat continuously to maintain a core temperature near 37 degrees Celsius. At moderate ambient temperatures, this is effortless: radiation, convection, and conduction carry excess warmth away. But as the mercury climbs past 35 degrees, the environment is no longer a heat sink. It becomes a heat source.
The body’s primary cooling mechanism is sweating. As ambient temperature rises, blood vessels near the skin dilate, redirecting warm blood outward, and sweat glands switch on. When sweat evaporates, it carries heat away from the skin. This system is remarkably effective; but only up to a point.
“Extreme heat and humidity pose two major physiological threats to the human body,” said W. Larry Kenney, a professor of physiology and kinesiology at Penn State who has spent decades studying human thermoregulation. “The first is a rise in core body temperature. If that temperature climbs beyond what the body can regulate, the risk of heat-related illnesses increases; from heat exhaustion to heat stroke, which can be life-threatening. The second and more common effect is cardiovascular strain.”
That second threat is the one most people underestimate. As the body redirects blood to the skin to support cooling, the heart must pump harder and faster to maintain adequate circulation to the brain, kidneys, and other vital organs. For a healthy young adult, this cardiovascular stress is manageable; uncomfortable, but survivable. For an older adult, it can be fatal.
“Data from heat waves around the world show that the most common cause of death isn’t heat stroke or dehydration; it’s cardiovascular failure brought on by prolonged heat exposure,” Kenney said.
The wet-bulb threshold – and why it matters
For years, a widely cited 2010 paper by Sherwood and Huber suggested that the upper limit for human survivability was a wet-bulb temperature of 35 degrees Celsius; a combination of heat and humidity at which sweat can no longer evaporate and the body loses its ability to cool itself even at rest in the shade. That number became a fixture in climate science and popular reporting alike.
But research from Penn State’s HEAT Project – a multi-year, NIH-funded study placing ordinary people of all ages inside environmental chambers to test the body’s real-world limits – has revised that figure downward, dramatically. The true threshold for healthy young adults, the researchers found, is closer to a wet-bulb temperature of 31 degrees Celsius. For older adults, it is even lower.
The implications are stark. The 35-degree wet-bulb threshold has rarely, if ever, been sustained on Earth. But the 31-degree threshold has already been reached in multiple parts of the world; and parts of Southern Africa, with their combination of high heat and episodic humidity, are approaching it with increasing frequency.
This means the heat that Botswana and its neighbors are experiencing is not merely uncomfortable. It is, for vulnerable populations, physiologically dangerous at levels lower than scientists once assumed.
The cascade of harm
The Academy of Science of South Africa’s report introduces a concept that reframes the entire discussion: extreme heat as an “integrator hazard”; a multiplier that worsens every existing vulnerability simultaneously.
Consider the chain. Extreme heat dehydrates the body, straining the kidneys. It forces the heart to work harder, raising the risk of cardiac events. It worsens respiratory conditions by increasing ground-level ozone and particulate matter from dust and wildfire smoke. It reduces crop yields and nutrient quality, deepening food insecurity. It spoils perishable food faster, increasing the risk of food-borne illness — a particular concern in regions where street food and informal markets are essential to daily nutrition. It stresses electricity grids, raising the likelihood of power outages precisely when cooling and medical equipment are most needed.
And when temperatures stay high overnight, as they increasingly do in Southern Africa’s urban heat islands, the body is denied the recovery period it depends on. Overnight heat is, in the words of several researchers, the silent killer.
Certain populations bear a disproportionate burden. The ASSAf report highlights emerging evidence that extreme heat during pregnancy increases the risk of stillbirth, premature birth, and low birth weight. Infants and young children, with their limited thermoregulatory capacity, are acutely vulnerable. Older adults face the highest rates of heat-related mortality. And across much of Southern Africa, the people most exposed; agricultural workers, construction crews, miners, informal traders; are those with the fewest resources to adapt.
In several SADC member states, up to 80 to 90 percent of workers are employed informally, often without workplace protections, cooling infrastructure, or social safety nets. For them, stopping work means losing income. Continuing work means risking their lives.
What to do when the air itself is the enemy
There is no single solution to extreme heat. But there is a growing consensus – among physiologists, public health officials, and climate researchers – about a hierarchy of strategies that can reduce harm. Some are straightforward. Others require systemic change. All of them are more effective when deployed together.
Hydrate before you are thirsty
Thirst is a lagging indicator. By the time you feel it, the body is already mildly dehydrated. In extreme heat, the recommendation from the World Health Organization is unambiguous: drink water at regular intervals throughout the day, regardless of perceived thirst. Avoid alcohol and sugary drinks, which accelerate fluid loss. For those sweating heavily, oral rehydration solutions or water with a pinch of salt can replace the electrolytes; sodium, potassium; that sweat carries away.
Reschedule the day
The simplest intervention is also the most ignored: avoid exertion during the hottest hours. In Botswana and across the region, that typically means the window between 11 a.m. and 3 p.m. Where possible, shift outdoor labor, exercise, and travel to the early morning or late evening. Employers; particularly in agriculture, construction, and mining; should implement mandatory rest breaks, provide shaded rest areas, and consider adjusting shift schedules during heat alerts.
Cool the body directly
When ambient air is hotter than the skin, fans alone may not help; they can actually add heat by circulating warm air. Instead, focus on cooling the skin through evaporation: wet cloths on the neck, wrists, and forehead; cool or tepid showers; soaking feet in water. These methods exploit the body’s own evaporative cooling system and are effective even when air conditioning is unavailable.
Defend the home
For those without air conditioning, the strategy is containment: keep heat out during the day and let cool air in at night. Close curtains, blinds, and shutters on sun-facing windows during peak hours. Open windows and doors in the evening to create cross-ventilation. Avoid using stoves and ovens during the hottest part of the day. If possible, use reflective materials or light-colored paint on roofs and exterior walls; a practice that can reduce indoor temperatures by several degrees in structures typical of informal settlements.
Acclimatize gradually
Kenney’s research highlights a powerful, underappreciated tool: heat acclimatization. The body can adapt to heat over the course of seven to 14 days of gradual, increasing exposure. Spending 20 to 30 minutes a day doing light activity outdoors as temperatures rise in the spring or early summer triggers physiological changes; increased blood plasma volume, earlier onset of sweating, higher sweat rate, reduced salt loss in sweat; that dramatically improve the body’s ability to cope.
“Heat-related deaths in sports are most common in the first days of preseason practices, when athletes are not yet acclimated,” Kenney noted. “After just a few sessions, the risk drops significantly. That shows how powerful gradual adaptation can be.”
The principle applies beyond athletics. For workers returning to outdoor labor after a break, or residents facing the first heat wave of the season, gradual exposure can be lifesaving.
Watch the vulnerable
Check on elderly neighbors, particularly those living alone. Ensure that infants and young children are kept in shaded, well-ventilated spaces and are offered fluids frequently; they cannot articulate thirst. Pregnant women should take particular care to avoid prolonged heat exposure, given the documented risks to fetal outcomes. And anyone taking medications that affect hydration or thermoregulation; diuretics, antihistamines, certain blood pressure drugs – should consult a physician about heat-specific precautions.
Know the Warning Signs
Heat exhaustion manifests as heavy sweating, weakness, cold and clammy skin, a fast but weak pulse, nausea, and fainting. Move the person to a cool place, loosen clothing, apply cool cloths, and give sips of water. Heat stroke is a medical emergency: body temperature may exceed 39.4 degrees Celsius; the skin may be hot, red, and dry; the pulse rapid and strong; the person confused or unconscious. Call emergency services immediately. Do not give fluids if the person is unconscious.
The systems that must change
Individual actions can reduce risk. But as the ASSAf report makes clear, telling people to “drink more water” and “stay indoors” is insufficient – especially in communities where safe housing, reliable electricity, and clean water are not guaranteed.
The report calls for systemic interventions: improved locally appropriate early warning systems that integrate wet-bulb temperature and heat index data; better tracking of heat-related illness and mortality to guide policy; climate-resilient clinics and hospitals with reliable power and cooling; worker protections codified into labor law; urban design that incorporates shade, green space, and cooling infrastructure; and the integration of heat-health action plans into national climate strategies.
Botswana’s Ministry of Health and Wellness has already issued alerts urging residents to take precautions during extreme heat episodes. But the gap between warning and capacity remains wide – particularly in rural areas and informal settlements where the infrastructure of cooling simply does not exist.
There are promising models. South Africa has begun strengthening heat-health early warning and surveillance systems. Malawi is helping farmers adapt through climate-smart agricultural planning. Namibia has supported community-level water and resource management in heat-prone areas. Public cooling spaces; community centers, schools, clinics; can provide life-saving refuge during the worst hours.
But these are early steps in a race that is accelerating. Nineteen of the planet’s 20 hottest years have occurred since 2000. Record-breaking monthly temperatures now occur five times more frequently than they did a generation ago.
A question of survival, not comfort
There is a tendency, particularly in temperate climates, to romanticize heat – to treat it as an inconvenience, a seasonal character test, a reason to seek shade and complain. In Southern Africa, and in Botswana in particular, that framing has become untenable.
The heat arriving now is not the heat of previous generations. It is more intense, more persistent, and more dangerous – and it is arriving in a region where structural vulnerabilities prevent millions from mounting an adequate defense. The body’s ancient cooling mechanisms, refined over hundreds of thousands of years of evolutionary pressure, are being tested against conditions they were never designed to withstand.
The science is clear on what happens above 35 degrees. The cardiovascular system strains. The kidneys falter. The brain, starved of adequately cooled blood, loses clarity. Without intervention, the cascade accelerates toward organ failure and death. And the threshold at which this cascade begins is lower than scientists once believed; closer to a wet-bulb temperature of 31 degrees than the 35-degree figure long treated as the outer limit.
The solutions exist. They range from the simplicity of a wet cloth on the neck to the complexity of national heat-health action plans. What is required is the will to implement them at the scale the moment demands – before the next heat wave, before the next avoidable death, before the window of adaptation closes.
In Gaborone, the forecast for the week ahead shows temperatures above 36 degrees every afternoon. The heat is not coming. It is here.
Reporting for this article drew on the consensus study “Climate Change and Extreme Heat: Strengthening Resilience and Adaptive Capacity in the Southern African Development Community (SADC),” published by the Academy of Science of South Africa in 2026; research from the Penn State HEAT Project on human environmental age thresholds; and World Health Organization guidance on heat an