By Glenn McGregor, Durham University
In early August 2003, a wave of devastating heat swept across Europe lasting around 10 days. Temperature records were broken and wide-ranging impacts in the form of wildfires, water shortages, power outages and a large heat-related death toll, resulted. For the UK, close to 2000 heat related deaths occurred during this period with deaths across Europe in excess of 70,000. Fast-forward seventeen years to the second week of August 2020 and there is an uncanny resemblance of the temperature extremes and associated anomalies to those of 2003 (Figure 1).
In 2020 and 2003, the extreme heat arose because of a persistent strong ridge of high pressure – a ‘blocking high’ – over western Europe (Figure 2). Under such a situation, clear skies with large inputs of solar radiation to the surface, subsiding warm air under stable atmospheric conditions and advection of warm air from the sub-tropics, drove up surface temperatures.
Periods of Extreme Heat as Heatwaves
Heatwave (or ‘heat wave’) is a codification for a period of extreme heat. The term is widely used in the popular press without much thought and in the academic literature with some care. While much remains to be learned about these so-called ‘silent killers’– silent because the arrivals of heatwaves are unheralded, unlike hurricanes and floods that figuratively ‘make some noise’ in the public realm little space is given over to considering what a heatwave is.
There are probably as many heatwave definitions as there have been heatwaves over the last couple of decades – one of the holy grails of heatwave science is settling on a global definition. That aside, heatwaves are periods of unusually hot weather lasting for several days. Embodied in this loose definition are three uncertainties – what constitutes hot, unusual and several.
In relation to hot, most heatwave definitions consider temperature and in many cases assume it represents heat stress, despite the latter being the combined effect of temperature, humidity, ventilation (wind) and radiative heat.
Used interchangeably with rare or extreme, unusual refers to exceedance of a climatological or sector-defined (e.g. health) absolute or relative threshold value for one of a range of temperature (e.g. mean, maximum or minimum) or heat related metrics (e.g. apparent temperature or humidex). Relative thresholds, taking the form of a percentile (e.g. 95th percentile of maximum temperature) are preferred because they account for the local context – an absolute threshold applied globally (e.g. 35oC) would mean the absence of heatwaves for many regions of the world.
Lastly, several days usually refers to at least three days because discernible societal impacts are associated with this minimum duration criteria. Annoyingly the popular press has, in a strict definition sense, a penchant for wrongly referring to one day of unusual heat as a heatwave!
Heatwaves and Research Challenges for Geographers
Heatwaves pose a number of research challenges, pertinent to the recent August 2020 event and relevant to the discipline of Geography as a whole.
Currently there appears to be no end in sight to solving the heatwave definition problem, apart from broad agreement that a climatological definition should be based on percentile criteria (e.g. 90th or 95th) for maximum temperature and a minimum duration of three days. Sector- or indeed location-specific heatwave definitions are required because of geographical and societal variations of heat sensitivity – a ‘one size fits all’ definition approach would be overly blunt. By applying research methodologies from across their discipline to climate sensitivity studies, Geographers could effectively contribute to the identification of societal impact thresholds and inform sector-specific heatwave definitions.
Other fertile areas for investigation abound. Forecasting the onset and cessation of heatwaves and thus the transition from a blocking to non-blocking situation and vice-versa, remains a challenge. Much work remains to be done on measuring surface-atmosphere interactions and determining the nature of positive feedback mechanisms associated with land and marine based ‘mega’ heatwaves. For urban climatologists the problem of solving how urban surfaces may intensify and sustain heatwaves in cities endures – the six-day stretch of 34oC plus temperatures in London and high nocturnal temperatures during the August 2020 heatwave begs these sort of questions. Although there is some progress, the question of whether reliable and timely sub-seasonal to seasonal prediction of heatwaves is possible largely remains an open one, inviting further research on the role of modes of climate variability (e.g. North Atlantic Oscillation) in increasing the likelihood of heatwave occurrence.
In the absence of serious Greenhouse Gas mitigation efforts and adaptation, an increase in heatwave frequency due anthropogenic climate change is likely. Establishing how the geography of heatwaves and associated impacts might alter under climate change is essential as is ascertaining the degree of attribution of specific heatwave events (e.g. August 2020) to climate change. Both these issues call for more geographical data than currently exists on the nexus of climate and social systems.
Similarly, the economic, political and cultural dimensions of heatwaves remain largely unexplored with a range of questions begging the attention of Geographers, such as:
- At what speed do heatwave effects propagate through social and economic systems?
- Are there strong spatial and temporal variations of the social construction of heatwave related impacts and risk?
- What are the climate justice dimensions of heatwaves?
- What are the non-physical determinants of the efficacy of heatwave warning systems and associated risk management interventions?
- What are the economic costs of heatwave events for specific sectors of the economy and within particular communities?
- Are there urban and rural (non-urban) contrasts in the societal manifestations of heatwaves?
- Does heat as a risk problem have a cultural dimension?
- What are the politics of heatwaves?
- Particularly timely, are questions of what compound effects may have arisen from the co-occurrence of heatwaves and the COVID-19 pandemic – have human behaviours changed, are heat-related deaths lower than what might be expected due to COVID-19 related mortality displacement, have social distancing and other measures rendered some heat risk management strategies ineffective ?
The August 2020 heatwave event may well break some climatological records but more importantly, it may act as a catalyst for turning the heads of more Geographers in the direction of intriguing heatwave related questions.
About the author: Glenn McGregor is Professor of Climatology in the Department of Geography, and Principal of Ustinov College, Durham University. As a geographer climatologist, he is interested in the geographies of extreme climate events, that is, analysing the nexus of climate and social systems, and applying that knowledge to the development of climate related risk management policy.
Suggested further reading
McGregor, G.R., Bone, A. and Pappenberger, F., 2017. Meteorological risk: extreme temperatures. In: K. Poljanšek (Editor), Science for disaster risk management 2017: knowing better and losing less. European Union, Luxembourg, pp. 257-270. https://ec.europa.eu/jrc/en/publication/science-disaster-risk-management-2017-knowing-better-and-losing-less
McGregor, G.R. and Vanos, J.K., 2018. Heat: a primer for public health researchers. Public Health, 161: 138-146.
Grimmond, S. (2007), Urbanization and global environmental change: local effects of urban warming. Geographical Journal, 173: 83-88. doi:10.1111/j.1475-4959.2007.232_3.x
Cole L. (2017). City heat: preparing for urban heat waves. Geographical Magazine https://geographical.co.uk/places/cities/item/2453-city-heat