By: Katty Huang and Andrew Charlton-Perez
Weather and climate can have great impacts on human health. One aspect of this is in relation to temperature exposure. In the UK, around 9% of deaths are associated with too warm or too cold outdoor temperatures. The majority (around 40,000) of this is related to cold weather, but as the climate warms, around 7000 additional deaths per year will be associated with heat exposure by the 2050s if the population does not adapt to the changes. Health consequences of heat and cold include increased risks of heart attacks, strokes, and respiratory diseases. Aging of the population compounds the problem, as the elderly are particularly at risk due to their increased vulnerability.
The Current System
The adverse health impacts of heat and cold events can be avoided by taking preventative measures to minimize exposure, especially for the vulnerable. Severe weather forecasting systems for health are now common around the world and form one key part of the global impact-based forecasting system. In the UK, Public Health England has a Heatwave Plan and a Cold Weather Plan. Each contains an alert system based on temperature threshold definitions of a “severe event”, and an increase in alert level is triggered when a severe event is forecasted 2 to 3 days in advance. Each level of the alert system is associated with advice and action plans for different sectors of society, particularly in health and social services.
With increasing health risks associated with non-optimal outdoor temperatures in the future, there is a growing incentive to develop a UK climate service for health. We have been funded as part of the Strategic Priorities Fund UK Climate Resilience Programme to help the Met Office to develop the technology to build this climate service, and since January, we have been working to bring some recent weather and climate research to bear on this important problem.
To ensure the development of a climate service benefiting the end users, it is key that we engage the stakeholders in conversation about their needs and to discuss the usability of our research in practical decision making. To this end, we have held a short workshop with Public Health England and are in active engagement with them and other public health stakeholders in Scotland, Wales, and Northern Ireland. We are keen to hear from all sectors and encourage anyone interested in a climate service for health to get in touch with us directly.
One field of weather and climate research that has seen a great deal of growth in recent years is the classification of atmospheric circulation patterns into groups, called weather regimes. These give an indication of the state of the large-scale atmospheric circulation, with particular consequences for temperature and weather and climate impacts, for example, in the UK. Understanding and quantifying how weather regimes and their local influences on temperature might change in the future is one way to develop a more sophisticated description of future climate.
There is some evidence to suggest that there is greater forecast skill in predicting future weather regimes than specific outputs such as temperature. For health risk prevention, this provides an opportunity to provide additional information to decision makers in advance of a potentially severe event, allowing for a longer response time for agencies and service workers.
We recently completed the first stage of our work looking at the relationship between weather regimes and mortality in the UK. We first use statistical modelling to establish the temperature-mortality relationship for 12 administrative regions in the UK (e.g. North East, North West). The statistical model allows us to calculate how many deaths on a given day could be attributed to non-optimal outdoor temperatures. By matching these attributed deaths with daily classification of weather regimes, we can see in detail which weather regimes lead to high mortality in the UK. Based on this analysis, we can already tell a lot about which weather conditions are most harmful to mortality in the UK.
Winter: Mostly about NAO-
In winter, the negative phase of the North Atlantic Oscillation (NAO-) is most likely to be associated with high mortality for all regions, confirming our previous work. The NAO- regime has a weakened jet stream across the UK, with winds more often coming from the east and north-east, bringing cold and dry air from the European continent in winter. For an idea of how the regime looks like on a weather map, see Figure 1a.
Figure 1:Average sea level pressure (in black lines) and its difference from the typical sea level pressure (in colours) during a particular weather regime which is (a) associated with high winter mortality, (b) associated with high summer mortality related to heat, and (c) associated with high summer mortality related to cold.
Summer: Both Cold and Warm Days
In summer, the picture is more complicated. We find that, in the current climate, many of the days in summer with large temperature related mortality have colder than average temperatures. In most regions, between 30 to 43% of the days with the highest 5% temperature-related mortality is associated with summer cold spells. The ratio is lower in London and East Midlands, but larger in Scotland and Northern Ireland.
This finding can be explained by the temperature-mortality relationship in the UK, which can be roughly described as U-shaped, with increased mortality risks at warm and cold temperature extremes (for an example, see Figure 2). Risks are also slightly elevated for moderately cold temperatures. This means that cold days in summer (where temperatures are around 10°C) can lead to similar numbers of additional deaths as mild warm days (with temperatures around 20°C).
Figure 2:The top panel is the mortality risk for each daily average outdoor temperature, here shown as an example for South West England. The mortality risk is expressed as a ratio relative to the regional optimal temperature where the overall risk is at its lowest (17°C in this case). Relative risk of 2 indicates that the mortality risk is twice the risk at the optimal temperature. The bottom panel shows how frequently each daily average temperature occurs in South West England on average. The vertical dashed lines indicate the maximum and minimum daily average temperatures observed between 1991 and 2018.
One major difference between the mortality associated with warm and cold temperatures is how long their impact on deaths lasts. While deaths related to heat mostly occur during or within the first days after high temperatures occur, cold-related deaths can be delayed by many days or weeks. This means that even though the total number of deaths associated with summer cold spells can be significant, they are spread out over more days and can be less noticeable than the rapid spike in deaths caused by a heatwave.
High mortality due to summer heatwaves is most likely to occur when there is a high pressure system over the North Sea and Scandinavia (shown in Figure 1b), which leads to clear sunny days in the UK. Cold-related mortality is most frequently associated with weather patterns with higher than usual pressure over the North Atlantic Ocean (see Figure 1c). The jet stream forms a ridge over the mid-Atlantic and recurves southeastward as it crosses the UK, bringing cool and humid air from the ocean.
During the next phase of our work, we are looking at applying our ideas to the UK Climate Projections produced by the Met Office, to help quantify and understand how climate-related mortality could change in the future. There are also important differences at even smaller, city-scales which can make climate-related mortality worse not least because of the socio-economic inequalities in some major cities in the UK. Urban modelling as part of the project will help us to map and understand some of these impacts.