麻豆传媒

EVEN by Australian standards, last summer was a scorcher. January 2017 was the hottest ever recorded in Sydney and Brisbane,听and great swathes of the south-east endured temperatures that often exceeded 40掳C for weeks on end. In South Australia, soaring electricity demand caused an outage that left 90,000 homes sweltering through a blackout with no air conditioning. Across New South Wales, 87 bush fires blazed. It was so hot that dairy cows dropped dead in the fields.

This kind of heatwave isn鈥檛 a blip. that saw Sydney鈥檚 temperature climb to over 47掳C earlier this month 鈥� the highest recorded in the city for 79 years听 鈥� and could see both it and Melbourne experiencing mega鈥慼eatwaves with highs of 50掳C by 2040. 鈥淕oing out to 40听or 50 years, basically the summer we just had will be normal,鈥� says Sarah Perkins-Kirkpatrick at the Climate Change Research Centre of the University of New South Wales (UNSW) in Sydney. 鈥淚t hasn鈥檛 really sunken in yet in Australia.鈥�

Australians are not alone: most of us fail to听take the 鈥渨arming鈥� in global warming seriously. If you live somewhere temperate, you might even welcome a rise of a few degrees as offering more opportunity for picnics, barbecues and relaxed afternoons in听pub gardens. That is unwise. Even now, heatwaves are deadly, and as global warming听increases . Human physiology is not designed to cope with the temperatures predicted for large swathes of the globe and many areas could become uninhabitable. Fortunately, there are听things we can do to make our bodies and our environments better adapted to a warming world.

鈥淭he 2003 heatwave centred on France killed over 70,000 people鈥�

With a few notable exceptions, we are all aware that anthropogenic warming has widespread and sometimes severe consequences, so it is somewhat surprising that we are only just waking up to the fact that it can kill us. This oversight doesn鈥檛 stem from lack of evidence. In the US, extreme heat caused more fatalities between 1978 and 2003 than earthquakes, hurricanes, floods and tornadoes combined. By some estimates, the 2003 heatwave centred on France killed over 70,000. Another that struck Moscow in 2010 resulted in 10,000 deaths.

In October, featuring research by 26 global institutions including the World Health Organization and World Bank, which concluded that we face a 鈥渓ooming public health emergency鈥�. This came hot on the heels of research looking specifically at 鈥渓ethal heat鈥�. Already, 30 per cent of the world鈥檚 population experiences potentially deadly temperatures for at least 20听days every year. A team led by Camilo Mora at the University of Hawaii in Manoa reported in June that this if we do little to limit greenhouse gas emissions. So how hot is too hot?

What matters is not the air temperature, but听the temperature you experience. You can survive for a while at well above 50掳C, as long as you can sweat effectively. The problem is humidity. 鈥淭he only way you lose heat when you sweat is by turning liquid into vapour. It听has to evaporate,鈥� says Graham Bates at Curtin University in Western Australia. 鈥淲ith a humidity of 90 per cent, the air is almost saturated, and when you sweat it just drips off,听and you won鈥檛 lose heat.鈥�

The combined effect of heat and humidity, otherwise known as the apparent temperature, can be : one wrapped in a damp cloth. A听鈥渨et bulb temperature鈥� of 35掳C听鈥� equivalent to an ambient temperature of 35掳C and 100听per听cent humidity or 40掳C and 75 per cent humidity听鈥� is considered the limit for human survival. Above this, even a healthy person in the shade won鈥檛 live longer than 6听hours. Nowhere on earth has experienced it yet, although in July 2015 with the conjunction of听50听per cent humidity and 46掳C. But it is only a matter of time.

鈥淏oth temperature and humidity are going听up,鈥� says Steven Sherwood, an atmospheric scientist at UNSW. The highest risk is in places that are already humid, such as听the Amazon, the Indus valley and many tropical countries. 鈥淚t only takes a 6掳C to 7掳C increase in temperature before some of these听regions become physically uninhabitable,鈥� says Sherwood. He calculates that, unless we drastically reduce global warming, some regions will exceed this limit in 100 to 200 years.

. Research published in August 2017 showed that parts of India, Pakistan and Bangladesh could occasionally exceed a wet bulb temperature of听35掳C by the end of this century. This region is home to 1.5 billion people, about a fifth of the world鈥檚 population, most of whom are poor and exposed to the full force of lethal heat. Subsistence farmers will be disproportionately affected by crop failures. Yields of wheat, rice and maize听鈥� which together with soy generate nearly two-thirds of all calories consumed by people听鈥� are by between 3 and 7 per cent for each 1掳C rise in global temperatures. And, in a double whammy, rising temperatures also impact the productivity of rural labourers. Since 2000, global warming has effectively , according to The Lancet report.

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What鈥檚 more, even temperatures below the theorised limit for human survival could render places uninhabitable, and we will reach this long before the end of the century. In fact, the US National Weather Service classifies any wet bulb temperature above 31掳C as 鈥渆xtreme danger鈥�. That鈥檚 because your body produces its own heat. At rest, it generates some 100 watts, about the same as a small incandescent light bulb. During brief bursts of intense exercise, such as running, however, it can produce more听than 1000 watts, equivalent to the heat produced by a microwave oven. 鈥淢uscles in humans are extremely inefficient,鈥� says Bates. When you burn fuel in the engine of a car, about 35 to 45 per cent of it goes to turn the wheels. Our muscles, on the other hand, lose 90 per cent of their energy as heat. 鈥淭he minute you start walking or running in the heat, then you take on a heat load which has to be dissipated to the environment,鈥� he says.

The optimum body temperature for a human is between 36.5掳C and 37.5掳C. This is where your metabolism, specifically your enzymes and other proteins, function most effectively. Millions of years of evolution on听the African savannah have honed sophisticated cooling systems to keep our core within this narrow range. When the mercury rises, thermoregulation occurs both consciously听鈥� in behaviours such as removing clothing, drinking water and fanning yourself听鈥� and unconsciously through the autonomic nervous system. The unconscious system is triggered by thermoreceptors in your skin, muscles, stomach and other areas detecting changes in temperature. These alert听the thermostat in your brain, the hypothalamus, which then sets in motion responses including sweating and shunting blood to the surface, where it can more easily lose heat to the environment.

However, to sweat effectively you must maintain your blood volume. If you become dangerously dehydrated, or another part of the system fails, then your body temperature will start heading towards 40掳C. At this point, you go from heat stress听鈥� where your skin appears very red and sweaty听鈥� to heatstroke, where you suddenly become white with skin that is dry to the touch. This happens because blood is being diverted back to the vital organs in an attempt to limit damage due to the lack of oxygen, or 鈥渉ypoxia鈥�, caused by the blood circulating close to the skin. 鈥淎bout 70 per cent of the people that get into that situation will die or have multiple organ failure,鈥� says Bates.

Deaths from overheating are often simply recorded as heart failure, because the heart must work overtime pumping blood to the extremities, but hypoxia is another common cause of death, says Mora. In fact, his team has recently identified at least . 鈥淥ne of the main ones is your gut,鈥� he says. 鈥淭he blood goes to the skin, and several organs are deprived of blood, including the intestines.鈥� Hypoxia is particularly damaging in the gut, because it can cause the lining to disintegrate, releasing the intestine鈥檚 contents into the bloodstream and triggering a catastrophic immune response. This consists of a massive production of white blood cells that leads to clotting in major organs. Heat can kill cells directly too, when it is so high that proteins cannot function. The breakdown of muscles is another major killer because long myoglobin molecules unravel and travel via the blood, eventually clogging organs such as the kidneys, liver and lungs.

Elderly people are particularly susceptible to heatstroke because they often have weak hearts and their skin is less effective at sweating. Children are at greater risk too as their smaller bodies take less time to heat up and they have a larger surface-area-to-volume ratio, meaning they absorb heat more readily. People on medications are also vulnerable. 鈥淪ome drugs impair the capacity of the body to perceive the dangers of heat,鈥� says Mora. But nobody is immune from fatal heatstroke.

Nevertheless, our bodies听 are remarkably able to acclimatise听鈥� provided humidity is low听enough to allow effective sweating. It听takes about a week of exposure before the thermoregulatory system starts to adjust. Then, you begin to sweat earlier and more profusely in response to heat, and your sweat composition changes, so you lose less of the sodium and potassium electrolytes that maintain blood volume. Of course, there are also ways to adapt behaviour to reduce overheating (see 鈥淗ow to keep cool鈥�, below).

Given enough time, humanity might even evolve to better endure extreme heat. We already possess the raw material for evolution to work with. There is substantial variation between people in how much they sweat, as well as in the number and density of their sweat glands. Even so, there is little chance we听could adapt to cope with a wet bulb temperature of 35掳C without hardier proteins able to function at a higher optimal temperature. And that would require significant changes to our basic biology.

Time is not on our side, but if we cannot adapt our bodies, we can at least adapt our environment to reduce the impact of rising temperatures. Heatwaves are often most intense in cities, where asphalt and the dark roofs absorb more of the sun鈥檚 rays and create a 鈥渉eat island鈥� effect. Urban planners rarely consider this, even in hot, wealthy places, such as Melbourne or Abu Dhabi. But there are simple things they could do. 鈥淲e need to design our homes better. That鈥檚 a starting point,鈥� says Perkins-Kirkpatrick. She would like to see more white roofs to reflect sunlight and better insulation and double glazing so that, even without air conditioning, our homes are places we can retreat to in extreme conditions. Cities also need more shady, green spaces to help minimise the heat island effect. A trailblazer here is New York City, which has taken on a massive regreening programme in听Manhattan over the past few years. The creation of air-conditioned public refuges is another option that was discussed widely during last summer鈥檚 heatwave in Australia.

鈥淏etter-designed homes and greener spaces can ease heat pressure鈥�

Such measures would have an added benefit. In developing countries, where power听supplies are already precarious, rising temperatures will inevitably lead to more outages听鈥� even wealthy countries struggle to meet extra demand during heatwaves, as the authorities in South Australia were shocked to discover last February. In addition, heat causes problems with electricity transmission by making cables expand and sag. Better-designed homes, greener public spaces and communal refuges would all ease pressure on electricity grids struggling to cope during heatwaves. That, in turn, would result in lower greenhouse gas emissions.

This matters because with a temperature rise of just 1.5 to 2掳C听鈥� as agreed under the 2016 Paris climate change deal听鈥� summer in parts of Australia will effectively become one long heatwave by 2100. , Perkins-Kirkpatrick has found. And the situation will be far worse if greenhouse gas emissions are not curbed. She foresees 鈥渄evastating impacts if anthropogenic climate change is not constrained as soon as possible鈥�.

The heat could be well and truly on.