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Killer heat waves
Heat waves have killed more Australians than all other natural hazards combined. (Source: iStockphoto/Utopia_88 )

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Killer heat waves

Tuesday 25th November 2014 11:58 am

Most Australians would remember the terrible “Black Saturday” bushfires of 2009 in Victoria.

Those awesome bushfires killed 173 people. But what most Australians don’t realise is that the crippling heat wave associated with Black Saturday killed more than double that – 374 people.

In fact, heat waves have killed more Australians than all other natural hazards combined. At least 4,500 Australians have died from heat waves since the year 1900.

In the European heat wave of 2003, some 70,000 people died. The Russian heat wave of 2010 killed around 55,000 people.

First, what exactly is a heat wave?

Well, the definition varies depending on the country, and sometimes, varies from one state to another within that country – such as in the USA.

A fairly well-accepted definition comes from the World Meteorological Organisation. They start off by setting a baseline of the 30 years between 1961 and 1990. Pick one day in each of those 30 years, say the 27th of January. Add up the 30 maximum temperatures, divide by 30, and then you have the Average Maximum Temperature – for that day.

Do the same for the next four days, for the same location. According to the World Meteorological Organisation, a heat wave is when you have five days in a row, each with a daily maximum temperature five-or-more Centigrade degrees higher than the Average Maximum Temperature.

Second, what causes a heat wave?

Basically, it happens when a high pressure system in the atmosphere, instead of moving across the landscape, stays stuck in one location – for days or even weeks. In the mega-heat waves that killed tens of thousands in Europe and Russia, things were made worse by a vicious positive feedback loop between ultra-dry soil, and unexpectedly powerful high-pressure systems in the lower atmosphere. This combination trapped the heat. The heat couldn’t dissipate overnight – so the next morning started off as hot as the previous afternoon.

Third, how can you tell if a specific death is caused by a heat wave? Well, it’s hard. How you tolerate heat depends on what you are used to, and how fit you are. If you do an autopsy, there is nothing specific that points to a heat wave being the cause of death.

But you know that something very bad is happening if dead bodies start to pile up. In the heat waves of Europe in 2003, Victoria in 2009, Russia in 2010, and Victoria again in 2014, the morgues filled up. There was simply no more room for the dead bodies coming in. The overflow had to be stored in mortuaries, universities and funeral parlours.

Then, to work out how many the heat wave killed, you call in the statisticians. They compare the number of deaths during the heat wave with the number of deaths over the same time period in previous years. By the way, in Australia, the most lethal day for a heat wave death is the day after Australia Day, the 27th of January.

But what exactly kills somebody in a heat wave?

Amazingly, we still don’t fully understand what’s going on. In Paris alone in 2003, some 15,000 died – overwhelmingly elderly women living alone in the upper levels of walk-up apartments.

Excessive heat seems to be especially harmful to the very young and the very old, and to those with chronic diseases and mental illnesses. Other risk factors include being obese, very malnourished, very unfit – and drugs, both legal and illegal. Dehydration combined with alcohol consumption makes the situation worse – as does the loss of air-conditioning in poorly designed houses when the electrical power grid crashes.

I’m sorry, but I’m now going to have to use a fancy statistical phrase – it’s the “Standard Deviation”. It measures how close your data points are to the average. The average, plus or minus one standard deviation, will capture about 68% of your population. The average, plus or minus two standard deviations, will enclose about 95.5%. For three standard deviations, you’re getting about 99.7%. So anything outside three standard deviations is quite unlikely.

Back in 1961, heat waves with temperatures three standard deviations above the average covered about 1% of our planet’s land area. By 2010, this had risen to about 5%. By 2020, it’s expected to rise to 10% – and for 2040, to 20%. In other words, before the middle of this century, when heat waves do arise, they will cover about one fifth of all the land area on Earth.

So watch it – heat can pack a powerful punch, and can even literally knock the living daylights out of you.

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This blog first appeared on Dr Karl's Great Moments in Science

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