18. Seasonal Pattern of Human Mortality
S Subak

Percentage of annual all-cause deaths in England and Wales occurring in January shown in relation to mean January temperature in Central England

Specific Indicator
Regardless of the cause, on average, more people die in the UK in winter than in summer. Warming may lessen this seasonal fluctuation. The relationship between number of deaths and temperature is particularly close in January in England and Wales. The specific indicator chosen is therefore the proportion of annual all-cause deaths in England and Wales occurring in January.
[Office of National Statistics, 1 Drummond Gate, London SW1v 2QQ]

Relevance
In the UK, over the past two decades, an average of about 250 more people died each day during January than during August. A small number of people die from direct exposure to cold (hypothermia) during the winter months but most 'excess' deaths are related to lowered resistance to illnesses and so appear as an increase in the all-cause death rate. During milder winters, resistance to illness is higher and the mortality rate is lower. Therefore, higher winter temperatures result in fewer winter deaths.
Sensitivity to climatic and other factors

Mild weather in the month of January tends to have a greater effect on lowering the death rate than does warm weather during other months of the year. In addition to the effect of temperature, the number of deaths that occur in January can be quite variable depending upon the incidence and intensity of influenza outbreaks then and in other months of the year.

Change over time
An inverse relationship between January temperature and the proportion of annual deaths occurring in January is clear at the beginning of the record and through the 1990s, but the relationship is weaker during the 1980s and in the last few years. In 1983 and 1990 the proportion of deaths occurring in January was high although January temperatures were among the highest in the record. One explanation is that, in both these years, there were influenza epidemics. This is known to be an explanation for the unusually high proportion of deaths in January 1997, when there was a severe influenza outbreak. Since 1996, the series has become more variable, with an exceptional high percentage of January deaths in 1997, 1999 and 2000. The 1997 peak can be related to low January temperatures, but the 1999 and 2000 peaks cannot. Low percentage values were recorded in 1998 and 2001. Although January CET in 1998 was well above average, this was not the case in 2001.

The proportion of annual deaths occurring in January would be expected to decline in a warmer climate. Models suggest that, for a 1oC increase in temperature, about 30 fewer deaths would be expected to occur each day in January and that this would hold true for temperatures up to 3oC higher than the January norm.

Note that that this does not mean that we shall live longer in a warmer climate, rather that we may be more likely to end our lives in summer than is the case at present.