Turning up the heat

How hot is too hot? (8 minute read)

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Winter has always been my least favourite season. I hate the short days, I’ve never liked the cold, and then there’s the rain, often blown sideways by Wellington’s wind. I’ve tried to find things to like about winter, but the best I can come up with is that it makes me appreciate the spring even more.

It would be easy, then, to be flippant about temperatures a couple of degrees warmer. From the comfortably moderate climate of Wellington, a degree or two warmer doesn’t seem that bad. In Wellington, temperatures seldom exceed 20°C, and the difference in average temperature between summer and winter is less than 10°C (it’s also less than 10°F, for those who use that scale). While we do have to worry about wind chill from our frequent strong winds, we are never really hot or really cold.

The reason that New Zealand avoids temperature extremes is complex, but one important reason is because of a property of water known as heat capacity. Water has a high heat capacity, which means it takes a lot of energy to increase the temperature of water compared with many other substances. Equally, water is slow to cool down when the heat source is removed. So, the water which surrounds New Zealand insulates us from temperature extremes. Oceanic islands closer to the equator might be warmer, but they are also protected from extreme temperatures by the water around them.

From the comfort of an equable climate, it’s easy to forget how uncomfortable and unpleasant hot temperatures can be. But it’s not so easy for those in hot climates. If you live somewhere like Singapore, you would experience year round average temperatures above 25°C, with average daily maximum temperatures just over 30°C.  If you live in Ahvaz, Iran, you would experience average summer temperatures of 37 or 38°C, with the average daily maximum over 40°C for nearly 5 months of the year. Ahvaz is one of the contenders for the world’s hottest places – in 2017 it recorded a temperature of 54°C. It’s not a remote desert either – it’s a city of over a million people.

Image credit: Chuchart Duangdaw/ Getty Images

The human body is remarkably good at tolerating extremes of temperature. Whatever the climate, it runs at a fairly constant 37°C, with a skin temperature 35°C or less. When we get too cold, our body draws blood away from the skin, creates heat with our muscles by shivering and releases hormones to speed up metabolism in ways that produce heat. We also have behavioural adaptations – wearing warm clothes and sheltering in heated buildings. When we get too hot, our body sweats and increases the blood flow to the skin (which is why people can appear red if they are too warm). Again, we have behavioural adaptations too, such as wearing light-coloured clothing, seeking shade and, if we are lucky, going somewhere with air conditioning.

But if our temperature regulation methods get overwhelmed, we can become seriously ill. We can become rapidly dehydrated from sweating, leading to heat exhaustion, with symptoms such as headaches, dizziness and nausea. More serious is heat stroke, where the body loses its ability to regulate its temperature. Someone suffering from heat stroke may experience confusion, slurred speech or seizures, and even suffer brain damage or die.

But the medical consequences of heat extend beyond direct health impacts. The effort of cooling the body places extra stress on the heart and high temperatures are associated with an increased risk of heart attacks. Exceptionally hot conditions are associated with increases in death and illness from a number of different causes, including heart and respiratory disease, pregnancy and birth complications, and mental illness. Most at risk are children, the elderly, those with chronic health conditions, and those with outdoor jobs involving physical labour.

Currently, the world is aiming to limit the global average temperature rise to 1.5°C by 2050, although if we stay on our current path, we’re not going to achieve that. By 2100, the world is likely to be from 2.9 to 3.4°C warmer. But if you read my recent article on sea level rise, you’ll remember the perils of assuming averages. Just as sea levels will rise by different amounts in different locations, so will temperatures.

New Zealand is one of the luckier countries. We are likely to experience temperature rises in line with average figures – up to 3°C under high emission scenarios. We will face greater extremes, but in our mild climate, heat-related illness is unlikely to be a major problem. But some places are warming much faster than the average. One of those places is the Middle East. There, temperatures are rising at twice the global average. It gets worse, too, because the season which will experience the greatest rise in temperature is summer – already the hottest time of the year.

Currently, the average temperature in Riyadh, Saudi Arabia, is over 30°C for three months of the year, while the average daily maximum is over 36°C for five months of the year. And Riyadh could face a rise of 4.1°C by 2084. Kuwait City already has average temperatures in excess of 35°C for three months of the year, and is seeing maximum temperatures in excess of 50°C more and more frequently.

Other hot areas are also facing a worrying rise in temperatures. In Singapore, average temperatures are predicted to rise between 1.4 and 4.6°C. India is facing more frequent heatwaves, which bring temperatures over 40°C.

But these temperatures only tell part of the story. To really understand how dangerous conditions could get, it helps to understand a concept called the “wet bulb temperature”. A wet bulb thermometer is one where the temperature sensor is encased in a wet cloth, which cools the sensor by evaporation. In a dry climate, evaporation cools the sensor very well, and there is a large gap between the actual temperature and the wet bulb temperature. In a humid climate, there is much less evaporation, and the wet bulb temperature is closer to the actual temperature.

In effect, the wet bulb temperature tells us how much we can cool our body by sweating. If the climate is humid, and the wet bulb temperature high, sweating is a much less effective way of cooling the body, which is why we find hot, humid conditions much harder to tolerate than hot, dry conditions.

If we take Singapore, the average humidity across the year is 84%. At 30oC, that would mean a wet bulb temperature of just under 28°C (check out the calculator here to work it out for yourself). However, the humidity frequently reaches 100%, meaning the wet bulb temperature is the same as the actual temperate. Under such conditions, the body’s cooling system is ineffective, and we feel every one of those 30 degrees.

In contrast, Riyadh never goes over 50% humidity, and in summer, when temperatures are at their hottest, humidity falls below 20%. So temperatures of 36°C in the Saudi summer work out to a wet bulb temperature of a very comfortable 20°C. As long as you have enough to drink, you’re not in any danger. Even temperatures of over 45°C, which are not uncommon there, still see the wet bulb temperature well below 30°C.

In fact, there’s a fairly consistent limit to the maximum wet bulb temperature in the world’s hottest climates – around 31°C. The scorching temperatures of 50 degrees are always experienced in desert climates, while extremely humid areas such as Singapore seldom go above 30°C.

But if we take the worst case scenario for Singapore, a rise of over 4.6°C, we could see wet bulb temperatures reach nearly 33°C on a day with average humidity, and over 34°C on particularly humid days. Those figure are alarming, because they’re coming dangerously close to the figure of 35°C. This is, theoretically, the maximum wet bulb temperature which humans can tolerate. Once we reach that limit, our body is unable to cool itself, even if we sit in the shade and drink plenty of water. A wet bulb temperature of 35°C is therefore considered “unliveable”.

It's not just humid tropical climates at risk either. Riyadh might be very dry in the summer months, but Kuwait City, a few hundred kilometres to the north and on the coast, is considerably more humid. There, the average summer humidity is around 40%. At that level of humidity, a temperature of 45°C is coming close to a wet bulb temperature of 33°C. If the temperature climbs to 50°C, at Kuwait’s average summer humidity, conditions reach the unliveable range.

Humans are an adaptable species. We’ve found ways to live in almost all of the world’s climates, from the poles to the tropics, and from rainforests to deserts. But climate change is forcing us to face our limits, not just the limits to our ingenuity, but the limits to what our bodies can cope with. For those in the world’s hottest and most humid areas, the temperature rises of a few degrees caused by climate change could be very dangerous indeed.

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