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A new heatwave definition for the UK

2019; Wiley; Volume: 74; Issue: 11 Linguagem: Inglês

10.1002/wea.3629

1477-8696

Mark McCarthy, Lynne Armstrong, Neil Armstrong,

Climate Change and Sustainable Development

Heatwave is a widely understood term to describe sequences of unusually hot weather during the summer season. In the United Kingdom, these are typically associated with prolonged periods of settled weather during the summer season lasting several days to weeks, where temperatures will reach the high 20s or low 30s Celsius. Less frequently, temperatures in parts of the United Kingdom, notably the south east, may exceed 35°C (Met Office, 2018). These episodes are associated with numerous negative impacts such as the physiological stress from high temperatures that results in increased mortality and hospital admissions (e.g. Johnson et al., 2005) and heat illness (e.g. Smith et al., 2016), particularly affecting the most vulnerable. Other factors include heat stress on transport infrastructure such as rail lines (e.g. Ferranti and Chapman 2016) or roads, digital communication, water resources, agriculture and energy systems (Dawson et al., 2017). Some sectors of the UK economy do, however, benefit from heatwaves, such as tourism. Taylor et al. (2009) estimated that the 2003 heatwave in the United Kingdom had a positive impact on domestic tourism of between £14.8 and £30.3 million. In the United Kingdom, heatwaves can also be depicted as positive news stories, particularly during the early phase of heatwaves as many people might 'enjoy' the fine summer weather. In the communication of UK summer weather forecasts, it is therefore important to provide clear advice that can both engage and support UK society to benefit from the opportunities that summer weather can present but also recognises the need to mitigate associated high-temperature risks as an event develops. The climate of the United Kingdom is changing. At the time of writing, the average temperature for the most recent decade (2009–2018) has been 0.9 degC above the 1961–1990 reference climate (Kendon, 2019). All of the top 10 warmest years on record have occurred since 2002. Climate change is very likely to increase the frequency and intensity of heatwaves across Europe, with largely adverse impacts (Kovats et al., 2014). Stott et al., (2004) demonstrate that it is very likely that human influence on the climate has doubled the likelihood of a European heatwave exceeding that of 2003. The UK historical climate observations have shown strong variability in the occurrence of heatwaves over time, which are partly associated with large-scale modes of climate variability such as the Atlantic Multidecadal Oscillation (AMO, Sanderson et al., 2017). Climate change trends will therefore be superimposed on this natural variability in our climate. Despite the widespread usage, intuitive understanding of the term 'heatwave' and the associated impacts, there is no unified definition, either within the United Kingdom or internationally. As noted by Perkins (2015) in relation to the definition of heatwaves in climate studies: '…it seems that almost, if not every climatological study that looks at heatwaves uses a different metric.' This is largely due to the multiple factors that will affect the character of a heatwave event and that need to be clearly articulated. The World Meteorological Organization (WMO) has drafted guidelines on the definition and monitoring of extreme weather and climate events (WMO, 2018). The recommended definition of a heatwave is: 'A period of marked unusual hot weather over a region persisting for at least three consecutive days during the warm period of the year based on local climatological conditions, with thermal conditions recorded above given thresholds.' The WMO has also produced guidance on the development of heat health warning systems (WMO, 2015). The Met Office is the United Kingdom's national meteorological service and provides a range of weather and climate services under the banner of the Public Weather Service funded by the UK government. These services are provided to allow the UK public to make informed decisions about day-to-day activities and to warn people of extreme weather to mitigate its impacts – contributing to the protection of life, property and infrastructure. The Met Office also works with Public Health England on the Heat Health Watch Service for health professionals and contributes to the Heatwave plan for England (Public Health England, 2019). In addition to the provision of heat health planning, the Met Office needs to have an operational heatwave definition for the purpose of being better equipped to provide authoritative public communication during heatwave events. This definition must recognise that, from a public communication and media perspective, heatwaves in the United Kingdom can also be a positive news story, and heatwaves are part of public discourse much more frequently than would be triggered by current heat–health thresholds. The definition must be adaptable to the range of climates experienced across the United Kingdom and be scientifically justifiable but must also reflect common perception of heatwaves – that is, not be too stringent that heatwaves are limited to only a few exceptional events or too common as to become meaningless. The definition must be adaptable to future climate change. The most important requirement is that the definition must be easy to communicate to the general public through various media channels. A UK heatwave is met when a location records a period of at least three consecutive days with maximum temperatures meeting or exceeding a heatwave temperature threshold. The threshold varies by UK county in the range 25–28°C and is shown in Figure 1. Greater London is the only region in the United Kingdom for which the threshold is 28°C. Twenty-four counties in the southeast have a threshold of 27°C, and a belt of 12 counties has a threshold of 26°C. The rest of the United Kingdom, including most of Wales and all of Scotland and Northern Ireland, have a threshold of 25°C. Magnitude: Duration: Extent: Severity: The WMO definition quoted above was adapted for the United Kingdom by setting threshold levels appropriate to the UK climate. It was adapted for ease of communication by setting consistent thresholds for commonly recognised county geographies of the United Kingdom. The Met Office as a national service has not previously had a UK-specific application of a heatwave definition. Other definitions of heatwaves and hot spells will, and can still, be used in a range of applications such as climate research, health and epidemiology or sector-specific impact assessments, but this definition will act as the official meteorological heatwave definition of the Met Office. The thresholds were determined such that exceedance would be unusual given local climatological conditions. A widely used threshold in climate research is the 90th percentile of the climatological distribution of daily maximum temperature (e.g. Perkins and Alexander, 2013). The 90th percentile of daily maximum temperature estimates averaged across the metropolitan areas of four UK cities are shown in Figure 2. These are for a 1981–2010 climatological reference period using a 15-day window centred on each day of the year. The highest temperatures tend to occur through mid to late July and into early August, and the 90th percentile reaches 22.8°C in Edinburgh, 23.1°C in Belfast, 26.8°C in Cardiff and 28.9°C in London. It would be possible to have a time-varying threshold to consider climatologically unusual events at any time of year, to capture, for example, spring and autumn heatwaves. However, this would introduce a considerable degree of complexity into the application of the threshold for operations and for members of the public. Thresholds have therefore been set based on the 90th percentile for July days. An additional constraint was used to set a minimum threshold value of 25°C so that, to qualify as a heatwave, the temperatures do need to be in the upper 20s Celsius regardless of location. A 25°C threshold is used in the definition of a 'summer day' within international climate studies (e.g. Peterson et al., 2005), is a threshold for some heat exposure activity risks (WMO, 2015) and is adopted within heatwave definitions for some neighbouring countries such as Ireland (Met Eireann) and the Netherlands (KNMI). The resulting 90th percentile on a 1-km resolution dataset (Hollis et al., 2019) are shown in Figure 3. To achieve the thresholds presented in Figure 1, the percentiles were rounded to the nearest whole degree, and the median value was determined for each geographic county. Clearly, the adoption of counties results in some compromises of climatological detail, such as Lincolnshire, which is a reasonably large area and rather warmer in the south of the county. However, it significantly benefits the ease of communication. Elevated night-time temperatures are a particular health hazard during heatwave events as they can deny any recovery period from the heat of the day and maintain physiological stress. This can be a particular problem in major towns and cities where the urban heat island effect will further increase night-time temperatures, especially under the calm and clear sky conditions of a heatwave. Daily minimum temperature is therefore an important metric for understanding the potential magnitude and severity of a heatwave event. An evaluation of the evolution of temperatures during some notable UK heatwaves, however, shows that the daily minimum temperatures can often rise sometime after the daily maximum. For example, during summer 2018, the daily maximum temperature in Glasgow exceeded 25°C from 25 June to 28 June and on 1, 3 and 4 July, reaching 31.9°C on 28 June (the highest temperature on record for the city). During this period, only the nights of 28 and 29 June exceeded the 90th percentile of the daily minimum temperature. The Google Trends tool (Google, 2019) was used with a search term of 'heatwave' and with regions of Scotland (for Glasgow) and England (for London) as a proxy index for levels of public interest. The data are presented as purple lines in Figure 4 for summer 2018. The Google data are scaled so that a value of 100 is the peak popularity for the term over this period, and a value of 50 means that the term was half as popular on that date. The use of the term increases steadily approximately from 19 June as high pressure started to become established over the United Kingdom following on from a notable June storm (storm Hector, Met Office, 2018). The Google data reached a peak on 28 June, the hottest day of the year for Scotland. Interest in the term heatwave was renewed in late July, peaking around 24 and 25. These results and those from some other notable events (2013, 2006, not shown) suggest that the interest in 'heatwave' is more tightly associated with the daily maximum temperatures than minimum, particularly as elevated minimums can lag the maximum. Requiring both maximum and minimum thresholds to be met could limit or delay the detection of what are widely acknowledged heatwave periods. The use of daily maximum, rather than both max and min, also simplifies the definition. The occurrence of heatwaves is shown in Figure 5. The median value is 27% of years experiencing heatwaves. Shetland, the Western Isles and upland areas of Scotland and England, as well as some coastal areas, will very rarely meet or exceed the heatwave threshold. There are some regions that, by virtue of being the warmest part of the county within which they sit, meet the thresholds somewhat more frequently, such as the Vale of York and parts of East Anglia. The frequency and duration of heatwaves for major metropolitan regions since 1961 are summarised in Figure 6(a). For Tyne and Wear, Edinburgh and Belfast, heatwaves are relatively rare, affecting around 10% or fewer years over this period. Other regions had heatwaves in 30–50% of years. Most of these were for a duration of less than 7 days. Major summer heatwave years, such as 1976, 1995, 2006 and 2018, can have cumulative heatwave days exceeding 20 days, but these are not common, and counts may not all be adjacent days; for example, the heatwave of 2018 had two separate spells in late June and late July. The time series of summer heatwave duration for London and Glasgow are compared in Figure 6(b). The major heatwave years stand out. Over half (53%) of all heatwave days within the metropolitan areas fall in the month of July as shown in Figure 7, and 85% fall in the high summer season of July–August, with a further 12% in June. Heatwave events have also occurred in the months of May, September and October. A heatwave definition adapted for the UK climate and for ease of communication has been derived. The definition requires three consecutive days in which the daily maximum temperature exceeds a heatwave threshold defined for recognised counties of the United Kingdom ranging between 25 and 28°C. The thresholds have been determined based on the 90th percentile of the July daily maximum temperature for a 1981–2010 climatological reference period using a dataset of gridded observations (Hollis et al., 2019). Parts of the United Kingdom, particularly in northern Scotland and upland regions, rarely if ever exceed this threshold. For most regions of England and Wales between 1961 and 2018, 30–50% of years have experienced at least one heatwave period. A significant heatwave of a duration of 1–2 weeks has affected parts of the United Kingdom in 10–20% of years. The heatwave definition is intended to provide a useful tool for communicating to the government, public and private sectors the forecasts of warm spells, hot spells and heatwaves. The thresholds are sufficient in that they represent the upper range of current UK summer weather but are not restricted to the most severe heatwave events. Additional contextual information will always be required to place any event into an appropriate context. In this regard, it is just one component of national planning around hot weather and is complementary to, but does not replace, other initiatives such as the Public Health England heat health action planning.