Research and analysis

Mortality Insights from GAD - April 2023

Published 6 April 2023

How changing temperatures are affecting mortality in the UK

Welcome to Mortality Insights. In this edition, we explore the effect of outside temperature on mortality in the UK. In particular, we focus on the consequences of periods of extreme heat, following the record summer temperatures recorded in the UK during 2022.

We also consider whether it is the same demographic group that tend to be affected by colder winters or hotter summers in terms of mortality.

Rising temperatures

On 19 July 2022, a temperature of 40.3°C was recorded by the Met Office in Coningsby, Lincolnshire. This set a new record high temperature for the UK, and occurred during the summer heatwaves of 2022. This new record meant that 3 of the top 5 hottest UK days on record have occurred since the start of this decade.

This highlights the ongoing trend of rising temperatures in the UK, both in the summer and the winter, as illustrated on the graph below.

According to the Met Office, average temperatures have increased by around 1°C since the pre-industrial period (1850-1900). The Met Office also believes the likelihood of heatwaves is increasing, bringing about more heat events as experienced in summer 2022.

Source: Met Office National Climate Information Centre

In addition to the more frequent periods of extreme heat during recent summers, colder days during the winter are becoming milder and less frequent. As an example, the Met Office reports the average coldest day of the year became 1.7°C warmer between 1961 and 2017. Both these changes appear to influence mortality in the UK in different ways.

Temperature and mortality

The number of deaths occurring in the UK varies over the year. However, if these are tracked over several years, the number of deaths in each season follow a clear pattern. The risk of death in the UK during periods of cold temperature is greater than during warmer periods. As a result, there have been more deaths in the winter than in non-winter periods historically. This phenomenon is known as ‘excess winter mortality’.

Excess winter mortality: A statistical measure of the difference of the increase in mortality during winter months (December to March) compared with non-winter months (the preceding August to November and following April to July).

Source: Office for National Statistics

Influence of warmer weather on UK mortality

Joint research by the Office for National Statistics (ONS) and UK Health Security Agency (UKHSA) states that in the UK between June and August 2022 there were 5 ‘heat-periods’.

These represent days where the mean central England temperature was greater than 20°C and a Level 3 Heat Health Alert was issued.

The ONS and UKHSA research identified that during these 5 ‘heat-periods’, around 56,000 deaths occurred in England and Wales. This represents more than 3,200 excess deaths above the 5-year average for the equivalent days.

Excess deaths: Deaths during a particular period above the usual, expected number of deaths under normal conditions. In the last couple of years, the term ‘excess deaths’ has been widely talked about in respect to deaths related to COVID-19.

There also was variation between different areas, for example the West Midlands had the highest number of excess deaths, with over 600 during the ‘heat-periods’. Conversely, London recorded only 25 excess deaths.

Periods of extreme heat can put upwards pressure on mortality rates for several reasons. These include exposure to direct heat, disrupted services and increased mortality risk from outdoor activities, increased violence, and mental health problems.

ONS undertook research on excess deaths caused by climate change in England and Wales between 2001 to 2020. It found the leading causes of death in the 4 warmest months of the year relate to respiratory and cardiovascular issues.

Deaths from extreme heat in the UK have been very rare but this could change over time if the temperature continues to rise.

Excess mortality in 2021 to 2022

The UK is experiencing a trend of warmer winter temperatures, with rarer and less severe extreme cold weather events. This has been reducing recent excess winter mortality.

ONS research on mortality in England and Wales stated that during the winter months of 2021 to 2022, there were over 13,000 excess winter deaths.

Winter mortality in 2021 to 2022 showed the second-lowest proportion of winter deaths to non-winter deaths since 1950 to 1951.

While the mild winter of 2021 to 2022 played a part in the low winter excess mortality, there was an unusually high number of deaths across the rest of 2022. It should be noted that COVID-19 complicates comparisons between winter and non-winter mortality. COVID-19 was the leading cause of winter mortality in 2021 to 2022 in England, with over a quarter more deaths occurring in the winter than the rest of the year.

Overall effect of temperature change on mortality

Both extreme cold and warm temperatures influence mortality in the UK. However, as shown across several studies, it is the low winter temperatures that have a greater effect on the number of deaths.

Winter months

There has been a net decrease in the number of deaths in the winter months since the start of the millennium. This is likely to have been driven in part by the UK’s trend of milder winters and rarer extreme cold weather events.

A decline in deaths from cold temperature periods has more than offset any increase in the number of deaths associated with warmer temperature over the same period.

Between 2001 and 2020, the ONS estimates there was a net decrease in deaths associated with warm or cold temperature of around half a million.

Hotter spells

However, periods of extreme heat may play a more important role in mortality trends in the future, particularly if temperatures continue to rise. More heatwaves, such as those experienced during the summer of 2022, are expected as temperatures rise, with the length of these heatwaves likely to increase.

Longer heat periods pose a higher risk as more secondary impacts come into play. These can include long spells of increased pollution and breakdowns in infrastructure that don’t usually happen over short periods of excessive heat.

Risk for older people

In terms of risk, both extreme cold and warm temperatures have the greatest effect on older people. A study in The Lancet and reported on by the London School of Hygiene and Tropical Medicine show for people over 85, the relative risk of mortality is highest at both extreme hot and cold temperatures.

For people below aged 85 the relative risk is higher at extreme cold temperatures than extreme hot. At all ages the risk increases sharply at extremely hot temperatures, whereas the risk increases more gradually as temperatures approach extreme cold.

Final thoughts

Changing temperatures in the UK are one factor relating to climate change that are likely to have an effect on future mortality rates.

Other climate-related impacts such as flooding, air quality and food availability are also likely to affect mortality.

With climate change increasingly being factored into policy making, mitigating factors are also likely to influence climate-based mortality risk. This could mean that mortality trends will be even more difficult to predict.