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Reality Check: Are the hottest cities in the world becoming uninhabitable thanks to climate change? © Getty Images
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Reality Check: Are the hottest cities in the world becoming uninhabitable thanks to climate change?

Published: 09th June, 2022 at 15:40
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A recent heatwave in Jacobabad in Pakistan saw temperatures reaching a record-breaking 51˚C – a figure dangerously close to the limit of human survivability.

More-and-more heatwave records are shattered every year. Last month we saw temperatures max out at around 51˚C in central Pakistan. And last year we saw a nearly 5˚C toppling of the previous temperature record in Western Canada. For some of India’s current heatwaves, we have already estimated that climate change has made them a hundred times more likely, and by the end of the century, we are predicting that heat waves temperatures of 50˚C will be an annual occurrence in the region. The exact numbers vary depending where you are on the globe, but one thing we can be sure about, more extremes are coming.

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The feeling of being hot or cold doesn’t simply depend on temperature, but a range of weather conditions. Humidity, wind speed, and solar radiation can all add or subtract from our perceived temperature, and in certain combinations, they can be deadly.

At the heart of this combination is ‘dry-bulb temperature’, that is what we normally think of when we say temperature, and what you would measure if you stick a thermometer above your head and read off the mercury. But meteorologists often prefer to use a ‘felt temperature’, known as wet-bulb temperature, which is also measured with a thermometer but wrapped in a wet cloth to mimic many of the same heat transfers that are regulated by our bodies.

Most of us know the uncomfortable feeling of being unable to cool down during a particularly humid night - the same mechanisms are at play here. Physiologically, this makes sense. Humans, and indeed all mammals, sweat to cool down. If the humidity is too high, the sweat cannot move from our skin, so this cooldown mechanism is prohibited.

We actually know the human tolerance level pretty well - it is at 35˚C wet-bulb temperature. At this point humans cannot survive for more than a couple of hours because we can no longer transfer heat from our body to the environment. This might not sound like much, but the numerical value of the wet-bulb is always lower than that of the dry-bulb (except at 100 per cent relative humidity). This means 35˚C wet-bulb temperature can easily equate to over 50˚C dry-bulb temperature, even at moderate humidity levels.

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So, have we ever exceeded a 35˚C wet-bulb temperature threshold before? The answer is yes, but it is extremely rare. There have only been around ten reported cases around the world, and those were in the Middle East, in and around India, in Australia and in Mexico.

Even when wet-bulb temperatures do not reach this high, the large, densely packed cities in these regions compound the heat-health problem, and so it is commonplace to see hundreds of heat-related deaths every year in some of the major urban centres.

In general, the richer and more modern the city, the less of a problem this is, and in some of the larger Middle Eastern cities they have learnt to adapt very well. Indeed, the locals of Dubai and Abu Dhabi know to dress up warm in the height of summer, because they travel by car from building-to-building, with the AC maxed out, and without ever needing to walk outdoors. The poorer and more rural-based citizens of the world do not have this luxury.

But will climate change make this worse? Given the increase in temperatures, we expect more exceedances of the 35˚C survivability threshold in the future, but these cases are likely to remain rare and only occur for a few hours at a time. We expect them to be limited to locations in the tropics and subtropics, and even then only during certain years. We predict that the likelihood of these events significantly diminishes if we can adhere to the Paris Agreement climate goals - that is limiting global averaged increases in temperatures to well below 2˚C.

While temperatures might not exceed survivability limits, heat still kills. Adapting to new heat-norms is inevitable, but the good news story is that we already have a wealth of heat-reducing strategies available to us. Even in Europe, we see many streets surrounded by tall rows of buildings, able to shade inhabitants from the penetrating sunlight. We also see buildings painted in lighter colours, reflecting the Sun’s heat and providing a cooler environment.

For countries closer to the equator, more drastic measures have been implemented. AC, where financially viable, is an excellent option, but many of the poorer countries do not have the power infrastructure to make this technology reliable. One near-universal strategy that has been shown to work is introducing more nature-based spaces, trees, and water bodies in cities.

While not always feasible if your climate is not conducive to such ecosystems, where it is implemented, it has been shown to include a wealth of physical and mental health benefits. In this world of ever-increasing urbanisation, we have spent decades paving over nature - now is the time to let nature claim some of that back.

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Authors

Professor Dann Mitchell is a professor of climate science at the University of Bristol. Using data on climate trends from the last 100 years, he looks at how low emission scenarios, like those in line with the Paris Agreement on climate change, will impact human health.

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