Temperature and Mortality

We are all going to die, nothing is surer. “Nobody knows the day or the hour”, but one thing is clear: we are more likely to die in winter than in summer.

Death by unnatural causes (suicide, accident, bushfire, disaster, even acute illness) can come to otherwise healthy people of any age. Death by natural causes is more predictable.

Those vulnerable to death are the elderly, very young babies, those with chronic illness (e.g. asthma, diabetes) and weakened immunity, and those with respiratory and circulatory illness.

Analysing mortality is made difficult because the sample population is always changing. Excess deaths in one month may be followed by further excess deaths in the following month, or because so many vulnerable people have already died, there will be fewer than expected deaths in the next month or months, or even the next couple of winters. Similarly, if fewer than expected deaths occur, there will be a larger cohort of the vulnerable in the following months, getting older and with probably poorer health. Population growth, aging, migration, improved vaccines, and public education programs all play a part as well.

In this analysis, I use mortality and population data from the Australian Bureau of Statistics (ABS), and temperature data from the Bureau of Meteorology (BOM), for Victoria, as it is a small and compact state which is subject to large temperature changes and also severe heat waves. Monthly mortality data are difficult to find, so this study is restricted to the period January 2002 to December 2011. A 10 year period is hardly sufficient for meaningful averages, however some useful insights can be found.

Mortality statistics are available by month, but population figures are by quarter, therefore I interpolated estimated monthly population figures based on three month growth.

Firstly, this plot shows the total deaths for every month from January 2002 to December 2011.

Fig. 1:

act D per mnth
Note the seasonal spikes and dips. The apparent increase in deaths can be compared with Victoria’s population increase:


Population Vic
By dividing the total deaths by the population in thousands we can calculate the death rate:

Fig. 3:

Death rate per yr

Note the mortality rate has decreased, and that, in spite of heatwaves, bushfires, and flu pandemics, 2009 had a lower death rate than 2008.

Because months have varying numbers of days, a better analysis can be made by calculating the Daily Death Rate for each month (by dividing each monthly rate by 31, 30, 29, or 28 days).

Fig. 4:

mortality per month

For the state of Victoria for the 10 years to 2011, on average more deaths occurred for each day in August than for any other month. The lowest Daily Death Rate was in February.

Now compare with monthly averages (2002 to 2011) for maximum and minimum temperatures:

Fig. 5:

Tmax Tmin avg

The death rate peak lags July temperature by about a month. Cooler months (June to September) are deadlier than warmer (December to April).

The relationship with temperature can be shown with scatter plots:

Fig. 6:

DDR v Tmax

Fig. 7:

DDR v Tmin

Which merely reinforce that deaths are more likely in winter!

Now we look at the question of estimating how many deaths are likely in a given period, by multiplying the average daily death rate for each month by the number of days in each month and by the estimated total population for each month. By subtracting this figure from the actual number of deaths we get a mortality “anomaly”.  The following graph shows this anomaly for each year:

Fig. 8:

Act minus exp deaths per year

And each month:

Fig. 9:

Diff act minus exp Deaths per mnth

Note the peaks in the winters of 2002 and 2003, and also in the summer of 2008-2009. Note also that both graphs show that in spite of a killer heatwave, the Black Saturday bushfire, and the swine flu pandemic, deaths in 2009 were below what could be expected.

To put the anomaly for January 2009 into context, we can compare actual daily deaths per 1,000 population for all months from 2002 to 2011:

Fig. 10:

act daily D per mnth

Note that the extreme figure for January 2009, while extremely high for January, is still below those of the lowest extremes of June, July, and August.

Perhaps higher mortality in the winter months is coincidence and due to some other factor than temperature- seasonal flu incidence for example. I now look at the month of August with the highest average mortality rate:

Fig. 11:

Act minus exp deaths vs Tmin August

There is fairly decent correlation showing that for every degree warmer in minima, the August death toll will be around 150 less than expected.

February, with the lowest rate:

Fig. 12:

Act minus exp deaths vs Tmin Feb

Even in summer, warmer minima mean fewer deaths.

In summer, do higher maxima cause more deaths?

Fig. 13:

Act minus exp deaths vs Tmax Feb

Even including the 173 deaths in the Black Saturday bushfires in the 200 extra deaths for February 2009, there is no trend.

January, whose data include the 2009 heatwave:

Fig. 14:

Act minus exp deaths vs Tmax Jan

A very small trend, but the 2009 heatwave outlier is obvious and skews the data. (Victorian health authorities say there were 374 excess deaths in the week to 1 February 2009).

Extreme heatwaves are indeed killers. Normal hot summers up to two degrees above average are not.


Improved public health measures, influenza vaccines, and improved public awareness – plus warmer winters- have led to a decrease in the Victorian mortality rate in the period 2002-2011.

Extreme heatwaves are dangerous in Victoria and cause hundreds of extra deaths especially amongst the elderly (>75 years old). However, these are rare events. Severe and Extreme Heatwaves are newsworthy precisely because they are unusual.

Normal Victorian winters are even more dangerous with on average 17.5% more deaths in winter than summer every year, but because this is normal and expected, this regular annual spike in deaths is unremarkable and not newsworthy- much less regarded as a natural disaster. While 374 excess deaths in a week in a heatwave is shocking, even with these included, the highest January’s Daily Death Rate (in 2009) is below that of the lowest of any winter month.

Warmer minimum temperatures are associated with lower death rates at all times of the year, but especially in August in Victoria, where for every degree of extra warmth, about 150 fewer deaths can be expected. I hope, for the sake of those who are sick or elderly, that we have a warm winter this year.

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9 Responses to “Temperature and Mortality”

  1. MikeR Says:

    Hi Ken,

    Another interesting and though provoking contribution but I have some reservations about your conclusions.

    As you have shown ,the death rate has dropped from 6.9 to 5.5 per 1000 during the period of 2002 to 2011. At the same time the temperatures have increased over this period so you naturally get a correlation between mortality rates and temperatures.
    However, if you simply want to attribute this decrease as being due to temperature increases then you need to discount the numerous other factors that have affected mortality rates ( assuming all causes) over the same period.

    These are the the obvious ones such as the reduction in cardiovascular disease due to the reduction in smoking, the significant drop in motor vehicle deaths due to improvements in car safety and more stringent policing. Other important factors that would have contributed to the reduction are better medical care with improved treatments and diagnostic techniques along with the reduction in deaths in the elderly due to the increase in flu vaccination rates , reduction is SIDS due to new advice regarding sleeping positions, etc. etc..

    To attribute the decreased mortality to increases in temperatures without considering these confounding factors is a prime example of correlation being confused with causation.

    Finally it would be interesting to see the correlation between Tmax, Tmin and mortality for tropical third world countries, if such data exists. The correlation may be very different as the heat stress, combined with lack of available resources for cooling and inadequate protection from the elements, may give a very different picture.

    We do live in a very luck country and we should not forget about those that are less fortunate , see – http://www.telegraph.co.uk/news/worldnews/asia/india/11634228/Blistering-heatwave-sweeping-India-kills-at-least-1100.html .

  2. kenskingdom Says:

    Hi Mike, thank you for your comment.
    It seems you have misinterpreted my conclusions. I did not “attribute the decreased mortality to increases in temperatures” alone. I actually wrote that the decrease in mortality was due to: “Improved public health measures, influenza vaccines, and improved public awareness- plus warmer winters”.
    Over a 10 year period the impact of improvements/ reductions in CV disease, smoking, MV accidents, SIDS, and medical diagnosis and care is relatively small, and certainly can be discounted when comparing winter months with summer months.
    Re: the Indian heatwave and 1100 deaths- terrible as this is, how many deaths in week or even a day would you expect in north India? I suspect many more than 1100. Instead of newsworthy stories of extreme weather events, consider Daily Death Rate per 1,000, or Actual minus Expected deaths.

  3. catweazle666 Says:

    If you have not already come across it you might find this study from ‘The Lancet’ interesting, Ken.

    Mortality risk attributable to high and low ambient temperature: a multicountry observational study


  4. kenskingdom Says:

    Most of the temperature-related mortality burden was attributable to the contribution of cold. The effect of days of extreme temperature was substantially less than that attributable to milder but non-optimum weather. ” Exactly as I found for Victoria.

  5. MikeR Says:

    You are quit right about the relatively small contribution of deaths due to motor accidents and SIDS to the overall mortality rate.

    Cardiovascular disease (heart disease and strokes), along with cancer, are the predominant contributors to the mortality rate In Australia. The major reduction in deaths due to cardiovascular disease in Australia has been the major contributor to the reduction overall mortality. For coronary events the mortality has been decreasing at a remarkable rate of 3 to 4% per year over many years see -http://www.aihw.gov.au/WorkArea/DownloadAsset.aspx?id=60129547044 and http://heartfoundation.org.au/images/uploads/publications/HeartStats_2014_web.pdf.

    The mortality rate due to cardiovascular disease dropped from 253 to 170 deaths per 100,000 population from 2002 to 2011 (see table 1.6a n the above Heart Foundation article).

    So it is unsurprising that the overall mortality rate has dropped in Victoria over the period 2002 to 2011. Possibly, the just less than 0.3 degree C increase in temperature, has contributed but there are a range of more plausible reasons for this such as 1. Lifestyle changes such as reduction in smoking 2. Improved treatments such as cholesterol lowering drugs , better medications for control of blood pressure. 3. Better monitoring of these factors , 4. Improved imaging and diagnostic techniques, 5. Improved public awareness of the symptoms of heart attacks and strokes 6. Improved treatments in emergency departments for heart attack and stroke victims and 7. Refinements in bypass surgery and in the use of stents.

    It is difficult to assess what the relative contribution of these factors are but I hazard a guess the effect of small increase in temperatures would be relatively miniscule, unless Ken you have some other information at your disposal.

    catweazle666 has provided an interesting link to a Lancet article regarding deaths due to cold relative to deaths due to heat.

    It is clear after reading the Lancet study that deaths from cold far outweighed those from heat for the populations that were studied. But as the authors have stated below this is for predominantly urban populations which presumably have greater access to protection from the elements than rural populations and have greater access to heating and cooling equipment
    i.e. from the discussion section (page 8) of the Lancet study-

    “Some limitations must be acknowledged. First, although this investigation comprises populations with markedly different characteristics and living in a wide range of climates, the findings cannot be interpreted as globally representative. Entire regions such as Africa or the Middle East are not included, and the assessment is mainly limited to urban populations. In addition, although results indicate a substantial inter-country variation in attributable risk to both heat and cold, the analysis does not characterise these differences, identifying determinants of vulnerability or resilience to the effects of temperature. These limitations will be addressed in future research, by extending the data set to populations living in other regions, and by collecting standardized measures of meta-variables on location-specific characteristics to be included in the second-stage metaregression. Results from these analyses will complement the evidence provided in this contribution”.

    The other strikingly significant factor is shown on page 15 of the article ( see http://postimg.org/image/ylber6u5n/ ) which shows for a range of cities how much a small change in temperature affects both the death rates due to cold and due to heat. A small change in temperature makes only a small reduction in the deaths due to the cold while it causes a much greater increase in deaths due to heat. The relatively flat blue curves compared to the steeply rising red curves illustrates this.

    Most of the data presented was assembled from a period which ranged from 10 to 20 years prior to about 2006 to 2012. If the study was to use the latest data or was repeated in a few years the picture would be likely to be very different, even for just the urban communities that were studied.

  6. kenskingdom Says:

    Mike, once again you assume that I attribute all decrease in mortality to rising temperature, which is completely wrong. My point is that deaths from all causes are more likely in winter than summer, and warmer winters can be expected to have lower mortality rates. Despite the great improvement in CV mortality, if you have CV disease you are still more likely to die of it in winter than summer. In Victoria in 2011, in spite of all improvements in medicine, 1,408 more people died in winter (June-September) than in summer (December-March). The Daily Death Rate per 1000 peaked in September (following an anomalously warm August which *may* have had something to do with the fewer than expected deaths in August) and was 17.8% greater than February. August was still 14.1% higher than February.

  7. MikeR Says:


    If I gave the impression that I was accusing you of attributing the mortality decrease solely to temperature changes it might have been a misunderstanding due to figures 6,7, 11 12, 13 and 14 above which have temperature on the horizontal axis .I overlooked your statement ‘Improved public health measures, influenza vaccines, and improved public awareness – plus warmer winters- have led to a decrease in the Victorian mortality rate in the period 2002-2011’. For this I humbly apologize.

    However I have also not disputed, in any of my comments above, the increase in deaths in Victoria during winter months compared to the summer months and the decrease in the mortality rate over the decade 2002-2011.

    The point that is much more relevant is the change in mortality rates as temperatures increase. Yes winter in 2011 had less deaths than in 2002, but can we attribute this to being partly due to the temperature increases in this period? Perhaps, but whether it is significant or insignificant depends on considering all the other possible causes for this decrease.

    As I pointed out above there are numerous other reasons ( I could think of at least seven from the top of my head) of varying significance for the decrease in mortality due to cardiovascular events alone . I think it would be stretching the limits of incredulity to think that a less than 0.3 degree increase in temperatures is likely to be one of the more significant reasons for the decrease in mortality.

    Accordingly the multifactorial nature of mortality rates makes figures 11 to 14 particularly meaningless. It would have been no more misleading to have plotted the mortality against ice cream consumption, along the lines of -http://www.slate.com/blogs/crime/2013/07/09/warm_weather_homicide_rates_when_ice_cream_sales_rise_homicides_rise_coincidence.html .

    In summary, you state ‘warmer winters can be expected to have lower mortality rates’.

    Yes I agree with this , as long as medicine progresses and all the other factors that impact mortality continue to improve at the same time as the weather improves.

    With regard to your earlier comment regarding death rates during the most recent Indian heat wave. The exercise you suggested comparing actual to expected death numbers would be particularly difficult to do due to the underreporting of deaths during heat waves in India.

  8. kenskingdom Says:

    Apology accepted Mike. I’m also pleased that (in a roundabout way) you agree with the general thrust of my post. Nowhere have I said that lower death rates are *caused* by warmer temperatures, but indisputably are *associated*, and the reverse is demonstrably not true, except in rare and extreme heatwaves, which still have lower death rates than normal winters. All else being equal, a warmer winter will mean those vulnerable will live a few months longer. Re: India. Cause of death is not accurately reported in developed countries either. Health authorities thus use the concept of Excess Deaths.

  9. ngard2016 Says:

    Here is a 2014 study from Adelaide Uni that found death rates from hypothermia are higher in South OZ than the Nth European country of Sweden. Most of the deaths in SA are elderly women living alone and Swedish deaths are mostly drunken middle aged men who die outside from exposure. Here is their summary and the link.


    “Higher rate of hypothermia deaths in SA than in Sweden

    Wednesday, 12 February 2014

    South Australia has a higher rate of deaths from extreme cold compared with the northern European nation of Sweden, according to new research from the University of Adelaide.

    The study, by a team from the University’s School of Medical Sciences, analysed forensic cases of hypothermia deaths from 2006-2011 in both South Australia and Sweden.

    The results show that South Australia had a rate of 3.9 deaths for every 100,000 people, compared with Sweden’s 3.3 deaths per 100,000. In total, there were 62 fatal cases of hypothermia in South Australia and 296 cases in Sweden over the six-year period.

    “Despite considerable demographic, geographic and climate differences, the death rate from hypothermia was slightly higher in South Australia than in Sweden, which is a very surprising result,” says the leader of the project, the University’s Professor Roger Byard AO.

    Hypothermia is defined as a decrease in core body temperature below 35°C, with fatal hypothermia occurring at body temperatures of 26°C to 29°C.

    “Most of the deaths from hypothermia in South Australia involved elderly women indoors who were living alone, often with multiple underlying illnesses and limited contact with the outside world. Many of them had been dead for at least a day before they were discovered,” Professor Byard says.

    “This is in contrast with the majority of hypothermia deaths in Sweden, which usually occur outdoors and involve middle-aged males, commonly under the influence of alcohol. These bodies are often uncovered from snow drifts.

    “The fact that South Australia has a much warmer climate than Sweden, with higher average temperatures and milder winters, does not stop people from being at risk of death from hypothermia. Elderly, socially isolated people are at greatest risk in this State,” Professor Byard says.

    Medical Sciences PhD student Fiona Bright says descriptions of the houses were not available in the South Australian cases, so the reasons for the higher rates of indoor deaths in SA can only be speculated on.

    “In addition to the many underlying medical conditions involved in these cases, it’s likely that poor heating and insulation, and lack of energy efficiency, are playing a role here. For example, only 2.6% of Australian homes have double-glazed windows compared with 100% of homes in Finland and Sweden,” Ms Bright says.

    The results of this research will be published in the Journal of Forensic Sciences later this year.”

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