Parts of the world are approaching the boundaries of human survival due to extreme heat.

Surprisingly resilient is the human body. It can operate in a wide variety of temperatures, adapting, compensating, and perspiring its way through circumstances that our forebears would have found intolerable.

However, it has a hard limit, and we are gradually approaching it. Wet-bulb temperature is a measure of that limit. This figure combines humidity and heat to determine whether your body can truly cool itself, something that a thermometer cannot tell you on its own.

The limit of human survival

Sweat is only effective if it can evaporate, and evaporation necessitates additional moisture in the air. Even in the shade, sipping water, or doing nothing at all, that capacity is lost at a wet-bulb temperature of 35°C.

Long-term exposure is lethal because the body cannot expel heat quickly enough to survive. An estimated 500,000 people die from extreme heat each year, which is more than the average death rate.

The harm begins well before you reach the 35°C wet-bulb threshold, which is the outer limit of human survival. Dangerous effects start to manifest at considerably lower temperatures for the elderly, outdoor labourers, and anyone with an underlying medical condition.

Locations that are already risky

One of the hottest cities on Earth is Jacobabad, which is located in southern Pakistan. Since 2010, it has already surpassed the 35°C wet-bulb barrier several times. It won't be by itself for very long. According to research, if warming persists, regions of the Middle East and South Asia may frequently reach this limit by 2070.

Extreme heat is one of six global "risk tipping points" that the globe is rapidly nearing, according to the United Nations University. Tipping points are thresholds that, when exceeded, have serious, pervasive, and difficult-to-reverse effects.

Additionally, those who lack air conditioning, dependable energy, clean water, and the ability to simply stay indoors are the most vulnerable to the effects of heat. The connection between poverty and heat vulnerability is structural rather than coincidental. Additionally, it implies that those who have the fewest resources to defend themselves are also the ones who are closest to the survivability threshold.

Are we able to adjust?

One concern that frequently comes up when discussing how to adapt to excessive heat is whether or not people would eventually have to leave. There is a claim that some areas may become too hazardous to live in, leading to population displacement.

But most people are unable to just move. People are kept in by their jobs, families, finances, disabilities, and legal status. In order for adaptation to benefit the people who are staying, it must concentrate on making their living environments more livable. Much of this effort begins with urban planning. It's not always hot in cities.

Rich neighbourhoods with trees, shade, and well-insulated buildings can be significantly colder than impoverished neighbourhoods adjacent, where heat is absorbed by concrete during the day and reflected back at night. Investment can be directed to areas where it truly counts by using detailed local data about who is most exposed to the heat and where it is at its worst.

Strategies for both active and passive cooling

There are two main categories of cooling strategies: active and passive. Fans and air conditioning are examples of active cooling, although they have limitations. In addition to being costly and unavailable to many, air conditioning can overload the electrical grid during a heat wave if everyone runs their units at the same time.

Fossil fuel-powered, it also adds to the warming that necessitated it. Those disadvantages do not apply to passive cooling. Light-colored surfaces and reflective roofs absorb less heat.

The surrounding air is cooled by parks and trees. Water features, covered walks, and green areas don't need constant electricity and usually benefit entire neighbourhoods rather than just affluent households.

Some communities are going so far as to redesign around the idea of a "sponge city," which incorporates green areas to simultaneously give cooling shade, absorb rainwater, and lessen flooding.

Heat is altering how societies operate.

Extreme heat is beginning to fundamentally alter daily life outside of infrastructure. Some Persian Gulf nations currently prohibit outdoor labour between noon and three o'clock in the summer, thus outdoor employment may need to be completely moved to the early morning hours.

Heat is already having an impact on things that most people wouldn't immediately consider. Due to heat concerns, the 2022 FIFA World Cup was rescheduled from summer to winter in Qatar.

It is anticipated that there will be significant dangers associated with this year's event, which will take place in the United States, Canada, and Mexico. Approximately 25% of the matches will likely take place in hazardous conditions for both players and spectators. It's easy to overlook the importance of community responses.

Early warning systems that truly reach the most vulnerable, programs that encourage neighbours to check on one another during heatwaves, and increased access to healthcare during heat emergencies can all save lives.

Nothing modifies the fundamental issue.

Adaptation can significantly lessen harm and is real and worthwhile. Lives will be saved via reflective roofs, green areas, and changed work schedules. However, they are unable to make a world that is four or five degrees warmer safe for the billions of people who reside in the planet's most vulnerable areas.

The wet-bulb limit won't occur in a location that most people won't ever go to. In cities where people live and work, it has already been achieved several times. Stopping the warming by reducing emissions and phasing out fossil fuels is the most straightforward approach to avoid reaching it more extensively, more frequently, and in more locations. Managing the fallout from not moving quickly enough is everything else.