The risk of measure more heat-related deaths is one of the most serious consequences of climate change. Heat stress can worsen existing conditions like diabetes, kidney disease, and heart disease. Senior citizens are especially vulnerable.

You may surprise to hear that recent research suggests climate change could reduce the number of deaths relate to temperature in Australia. A related study in The Lancet also found that the cold kills more people than heat in Sydney, Melbourne, and Brisbane.

My research, however, publish in Climatic change. I used a similar method to the one used in The Lancet’s study and found that heat is responsible for the majority of Australian deaths due to temperature.

Temperature-related deaths are one measure we use to evaluate the impact of climate change. It is important that we measure them consistently and accurately.

How Can Researchers Measure The Death Rate From Temperature-Related Causes?

It is important to estimate the deaths due to cold and hot weather. This done by using a baseline temperature. The reference temperature should the day when people feel comfortable in their region and their health is not affect by heat or cold. Deaths due to temperature-related causes below this temperature will be classified as cold-related. Deaths above it will consider heat-relate.

To distinguish between deaths caused by temperature and deaths that are not, we use statistical techniques. To account for seasonal factors such as flu seasons, estimates must adjust. Although flu and pneumonia deaths increase in winter, they are not directly related to the cold.

The assumptions and modelling techniques used to calculate temperature-related deaths can affect the estimates. However, the use of different reference temperatures is a major issue that can cause discrepancies in results. This affects the percentage of deaths that are attribute to heat and cold.

The Importance If The Temperature Reference

As a curve, the risk of death due to high/low temperatures can shown in relation to the reference. The figure below shows how the estimate curves, call temperature-mortality curves, can differ when the reference temperature is changed. It compares temperature-mortality curves from my latest study (the bottom row), to those from the study published in The Lancet (the top row).

The parts of the curve that are define as heat or cold are shown in red and blue shading. Arrows indicate the reference temperature that was use to calculate the curves.

Numerous studies, including that of Lancet, have calculated the death toll from heat and cold by using what is call the minimum mortality temperature (MMT), as the reference temperature.

The MMT is the lowest point of a temperature-mortality curve and is often interpret as the daily average temperature at which there’s the lowest risk of death.

Based on Australia’s findings, I am concerned that the MMT (reference temperature) used in The Lancet study is too high. A reference temperature of 22.4degC, shown in the figure above, meant that almost 90% of Melbourne’s historic daily average temperatures were consider cold. This could translate to a day with an average temperature of 31.4degC, and a night with a minimum temperature of 13.4degC.

In my most recent study, I used a different temperature as the reference temperature. As the reference temperature, I used the median of historical daily temperatures. In my study, cold days in Melbourne were those below 14.7 degrees Celsius. Any daily average temperature above 14.7degC is consider hot.

The median temperature is use as the reference temperature. This creates a 50/50 split of what is consider hot and cold.

Compare The Measure Results

To estimate the temperature-related deaths in six climate zones, I also used a different temperature reference. These range from areas experiencing a hot, humid summer in the north to areas experiencing mild/warm summers and cold seasons in Tasmania, the ACT, parts of NSW, and Victoria.

Other studies that I have mentioned included data from many cities around the globe. But only the three largest Australian capitals (Sydney and Melbourne) were use.

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