The future of cities may be hot, humid and deadly

Informal settlements are going to bear the brunt of the increased heat stress of urban areas.

Back in September, NASA researchers announced that the months of June, July and August 2023 (otherwise known as the Northern Hemisphere summer) were the warmest since global records began. Down here in the Southern Hemisphere, we see a continuation of these trends. A strong El Nino in the Pacific Ocean, combined with a positive Indian Ocean Dipole – both amplified by climate change – are currently driving summer temperatures recordwith even worse.

Record-breaking summer heatwaves are sometimes greeted with enthusiasm, but the reality is that they can have dire consequences, threatening lives and livelihoods around the world. Heatwave in Europe in 2022 led to deaths more than 61,000 people. In the USA in 2023 it caused extreme heat corn and cotton harvests to wither, while in Australia, wheat and barley production is declining 9-15% average. The same UN report on food and agriculture shows that in Ethiopia, about 20.1 million people face “severe acute food insecurity, mainly due to the prolonged impact of drought conditions”.

As climate change continues to warm our planet, our exposure to extreme heat is increasing. While humans are well designed to shed heat (thanks to our bipedal locomotion, lack of fur and covering of sweat glands), there comes a point where normal daily activity becomes impossible, even for the young, fit and healthy among us. Recent research shows that large, densely populated cities and informal settlements are well on their way to becoming dangerous hot spots.

Thermometers tell only part of this story. We also need to consider humidity. The temperature reported in weather forecasts is more formally known as η dry bulb temperature – literally, the temperature of the air as measured by a dry thermometer protected from the sun. There is also a measurement called the wet bulb temperature, which is measured using a thermometer wrapped in a wet cloth*. As the air temperature rises, water evaporates from the fabric, causing cooling and lowering the temperature. This is similar to how our body sweats to keep us cool. However, if the air around the thermometer (or our body) is humid, this evaporation slows down significantly. At 100% relative humidity, it stops completely.

In 2010, climate change researchers at the University of New South Wales was theorized that a wet bulb temperature of 35°C represented an upper “adaptability limit” for the human body. They said that above this threshold, the human body can no longer cool itself through the evaporation of sweat, and the body’s core temperatures begin to rise steadily. For a long time, such conditions can be fatal. The researchers suggested that climate change would make such conditions more common, rendering some parts of the world uninhabitable.

Physiologists from Pennsylvania State University (PSU) put this theory to the test in 2022, through a pioneering experimental study. In it, a group of young, healthy men and women sat in an environmental room, “movement it’s enough to simulate the minimal activities of daily life, such as showering, cooking and eating.” As they did this, the researchers slowly increased either the temperature or the humidity in the chamber while monitoring the participants’ core temperature through a small thermometer pill to swallow they had each been swallowed.

They wanted to determine when participants fell into unbearable heat stress. the point at which their bodies could no longer thermoregulate, putting them at risk of heat-related illness. They found it well below the theoretical limit of 35°C. For example, at 50% relative humidity, it occurred as low as 31°C. And they say that for people who do vigorous activity or people who are more vulnerable to heat (due to age, body size, co-morbidities, medication use or physiological disorders), the threshold is likely even lower.

If you think those temperatures don’t seem too high, listen to its editors a 2020 document on the role of climate change in increasing extreme bulb temperatures. Like them explain, “Outside of a steam bath, few people have encountered anything close to 35°C….. When the UK sweltered with a record dry-bulb temperature of 38.7°C in July 2019, the wet-bulb temperature in Cambridge was no more than 24°C. Even in the killer heat wave of Karachi in 2015, the wet bulb temperature remained below 30°C. However, things are changing. These same researchers showed that extreme wet heat events have more than doubled in frequency since 1979. Their analysis suggests that if global temperatures rise by 2.5°C over the next few decades, vast areas of South Asia and the southern Persian Gulf will regularly experience wet bulb temperatures above 35°C.

Other studies, incl one from the PSU team, have reached similar conclusions. In 2023 they wrote, “Parts of the Middle East and the Indus River Valley show brief overshoots with warming of only 1.5 °C. More widespread, but brief, dangerous heat stress occurs in a +2 °C climate, including eastern China and sub-Saharan Africa, while the US Midwest emerges as a wet heat stress hotspot in a +3 °C climate. As part of their analysis, they looked at the number of “annual warm hours” that people living in large cities would experience under different climate projections. In the worst case scenario – climate +4°C – Tokyo (population: 13.5 million) would see extreme moisture events for 4.6 days per year. In Delhi (population: 35 million) would be more like ~70 days.

ONE new perspective piecepublished last week in the magazine An Earth, argues that informal settlements (sometimes referred to as “shacks”) are even more vulnerable to the effects of extreme heat-humidity events. Currently home in more than a billion people globally, with another 2 billion expected by 2050, urban informal settlements fall outside formal planning rules and regulations and often lack reliable access to services such as electricity, safe drinking water and sanitation. Homes in these areas are typically built with low-quality materials, and many residents work outdoors. All of these factors combine to mean that residents are already at increased risk of heat stress compared to the average city dweller, with limited options for escaping the conditions. And given that 85% of all informal settlements are in sub-Saharan Africa and Asia – the same regions predicted to experience the most extreme wet bulb temperatures – the outlook is bleak.

As lead author of the study, Dr Emma Ramsay explains, There is also a lack of climate monitoring in these areas, which could lead to these communities being misrecognized and underserved. Overhaul of building codes so that housing and materials emphasize efficiency and resilience, providing green infrastructure to counteract the urban heat island effect and develop efficient and equitable early warning systems they are all promising ways to adapt our cities to a hot, humid future. But unless we really know which areas of our cities – and which groups of the urban population – are most vulnerable to human heat exposure, such interventions will have limited impact.

To close this piece, I wanted to include an excerpt from Ramsay’s paper. She writes, “Informal settlements are currently a blind spot in global climate monitoring. Assessing the true magnitude and extent of exposure to moist heat, accurately and at a local scale, is essential for risk management. Such efforts must be prioritized to protect the most vulnerable from the existential threat of humid heat.”

* (this reddit comment is one of the most comprehensive descriptions of wet and dry bulb temperatures I’ve seen)