Informal settlements set to bear the brunt of increased urban heat stress.

Back in September, NASA researchers announced that the months of June, July, and August 2023 (otherwise known as Northern Hemisphere summer) were the hottest since global records began. Down here in the Southern Hemisphere, we’re seeing a continuation of those trends. A strong El Niño in the Pacific Ocean, coupled with a positive Indian Ocean Dipole – both amplified by climate change – are currently driving summer temperatures to record highs, with worse yet to come.

Record-setting summer heatwaves are sometimes greeted with excitement, but the reality is that they can have dire consequences, threatening lives and livelihoods across the globe. Europe’s blistering heatwave of 2022 led to the deaths of more than 61,000 people. In the US in 2023, extreme heat caused corn and cotton harvests to wilt, while in Australia, wheat and barley production are down 9-15% on average. That same UN food and agriculture report shows that in Ethiopia, some 20.1 million people face “severe acute food insecurity, mainly due to the lingering impact of drought conditions”.

As climate change continues to warm our planet, our exposure to excess heat increases. 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 at which normal daily activity becomes impossible, even for the young, fit and healthy amongst us. Recent research suggests that large, densely populated cities and informal settlements are on track to becoming dangerously hot places to be.

Thermometers only tell part of this story. We also have to consider humidity. The temperature reported in weather forecasts is more formally known as the drybulb temperature – literally, the temperature of the air as measured by a dry thermometer sheltered from the sun. There is also a metric called the wetbulb temperature, which is measured via a thermometer wrapped in a wet cloth*. As the air temperature rises, water evaporates from the cloth, causing a cooling effect and reducing the temperature. This is similar to the way our bodies sweat in order to keep us cool. However, if the air around the thermometer (or our body) is humid, that evaporation slows down considerably. At 100% relative humidity, it ceases altogether.

In 2010, climate change researchers at the University of New South Wales theorized that a wetbulb 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 core body temperatures begin to steadily rise. Over an extended period of time, such conditions can be lethal. The researchers suggested that climate change would make such conditions more common, rendering some parts of the world unhabitable.

Physiologists from Pennsylvania State University (PSU) put that theory to the test in 2022, via a groundbreaking experimental study. In it, a cohort of young, healthy men and women sat in an environmental chamber, moving just enough to simulate the minimal activities of daily living, such as showering, cooking and eating.” As they did this, researchers slowly increased either the temperature or the humidity in the chamber, while tracking the participants’ core temperature via a small, ingestible thermometer pill they’d each swallowed.

They wanted to identify when participants tipped over into uncompensable heat stress; the point at which their bodies could no longer thermoregulate, putting them at risk of heat-related illnesses. They found it far below the theorised 35°C threshold. For example, at 50% relative humidity, it occurred at just 31°C. And they say that for people doing strenuous activity, or those more vulnerable to heat (due to their age, body size, comorbidities, medication use, or physiological impairments), the threshold is likely even lower.

If you’re thinking that these temperatures don’t seem very high, listen to the authors of a 2020 paper on the role of climate change in driving extreme wetbulb temperatures. As they explain, “outside a steam room, few people have encountered anything close to 35°C….. When the UK sweltered with a record drybulb temperature of 38.7°C in July 2019, the wetbulb temperature in Cambridge was no more than 24°C. Even in Karachi’s killer heatwave of 2015, the wetbulb temperature stayed below 30°C.” However, things are changing. These same researchers showed that extreme humid heat events have more than doubled in frequency since 1979. Their analysis suggests that if global temperatures increase by 2.5°C in the coming decades, huge swathes of South Asia and the southern Persian Gulf will regularly experience wetbulb temperatures in excess of 35°C.

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

A new perspective piece, published last week in the journal One Earth, argues that informal settlements (sometimes referred to as ‘slums’) are even more vulnerable to the effects of extreme humid heat events. Currently home to more than a billion people worldwide, with a further 2 billion expected by 2050, urban informal settlements fall outside formal planning rules and regulations, and often lack reliable access to services including electricity, safe drinking water and sanitation. Housing in these areas is typically constructed using low quality materials, and many residents work outdoors. All of these factors combined mean that residents are already exposed to an elevated 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 found in sub-Saharan Africa and Asia – the same areas projected to experience the most extreme wetbulb temperatures – the outlook is bleak.

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

To close out this piece, I wanted to include a quote from Ramsay’s paper. She writes, “Informal settlements currently constitute a blind spot in global climate monitoring. Assessing the true magnitude and extent of humid heat exposure, accurately and at local scales, 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 complete descriptions of wet and dry bulb temperatures I’ve seen)



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