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Heatwaves are more deadly than bushfires and they're going to get worse

In 2009, the terrible Black Saturday bushfires killed 173 people. What most Australians don't realise is that the crippling heat around the horrendous bushfires killed 374 people.

In the European heatwave of 2003, 50,000-70,000 people died between June and August. The Russian heatwave of 2010 killed about 55,000 people.

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Heatwaves explained

What are heatwaves, and how do they affect us?

A bushfire leaves obvious signs of the cause of death (burns, blisters, etc), but a heatwave does not. Deaths from heatwaves are revealed indirectly. In Victoria in 2009, the first sign that the heatwaves were killing people was the morgues filling up. The unexpected extra corpses had to be stored in universities, mortuaries, funeral parlours and the like.

There are many definitions of a heatwave. The one from the World Meteorological Organisation (WMO) is widely accepted. According to the WMO, a heatwave happens when you have five days in a row, each with a daily maximum temperature five-or-more degrees higher than the average maximum temperature. It uses the 30 years between 1961 and 1990 as the baseline reference. For 30 years, maximum temperatures (usually after midday) are recorded. So over that time, you have 30 temperatures recorded for every day, which lets you work out the average maximum temperature, over that 30 years.

A heatwave happens when a high pressure system stays stuck in one location – for days or weeks – instead of moving across the landscape,

In the mega-heatwaves that killed more than 100,000 people in Europe and Russia, there was another factor. A vicious positive feedback loop between ultra-dry soil, and unexpectedly powerful high-pressure systems in the lowest level of the atmosphere made the heatwaves much worse. The heat was trapped, and couldn't escape overnight – so the following morning started off as hot as the previous afternoon. The cycle intensified each day. Ultimately, it created a blanket of hot air, four kilometres thick.

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We still don't fully understand what kills somebody in a heatwave.

In Paris alone in 2003, some 15,000 people died from heat. They were overwhelmingly elderly women, living alone, and in the upper levels of apartments. "Elderly" often implies smaller reserves of strength. "Living alone" meant that another person wasn't available to help. "Upper levels" meant that their apartments were getting the full brunt of the heat. Another factor in Europe is that houses are designed to keep the heat in, not out.

Excessive heat is especially harmful to the very young and the very old – and to those with chronic diseases and mental illnesses. Other risk factors are being obese, very malnourished, or very unfit. Drugs – both legal and illegal – can worsen your risk. Dehydration from alcohol can contribute.

If the electrical grid crashes, and you lose air-conditioning, the heat in poorly designed houses can be fatal.

The science is quite clear that climate change (which has been accepted as real since in 1988, and yes, we caused it) is worsening heatwaves. Dr Thomas Knutson and colleagues from the US Geophysical Fluids Dynamics Laboratory in Princeton University wrote a paper showing that (with a very high degree of confidence) climate change caused the extreme heat Australia experienced in 2013.

In Australia, heatwaves now arrive earlier, are hotter, and last longer.

In Sydney, the heatwave of February 2011 pushed 595 people into hospital emergency departments, and killed 96 people. In Melbourne, in 2014 during the Australian Open tennis tournament, the heatwave brought four consecutive days with temperatures over 41 degrees. More than 1000 spectators were treated for heat stress.

The prediction is for more heatwaves. In the early 1960s, in any calendar year, heatwaves covered 1 per cent of the land area of our planet. This rose to 5 per cent in 2010. The trend shows a rise to 10 per cent in 2020, and 20 per cent by 2040.

Heat stress causes loss of production – for crops and people. Our most lethal day for heatwave deaths is the day after Australia Day – January 27.

So take care – heat packs a powerful punch, sometimes knocking the living daylights out of you.

Dr Karl Kruszelnicki's 40th book, The Doctor, is out now

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