Urban and rural mortality rates during heat waves in Berlin and Brandenburg, Germany
Highlights
► Periods of heat stress enhance mortality rates in Berlin and Brandenburg. ► Heat-related mortality is an urban as well as a rural problem. ► During extreme events highest mortality rates can be found in the city centre. ► Mortality rates correlate well with the distribution of sealed surfaces. ► Health risks are higher for older than for younger people.
Introduction
Of all natural disasters heat waves often claim the largest number of fatalities. In the United States several hundred people lost their lives during the heat waves of 1980 (Smoyer, 1998) and 1995 (Klinenberg, 2002, Semenza et al., 1996). This is a far greater loss of life than occurs during blizzards, floods, and cyclones combined (National Weather Service, 2007). Large numbers of fatalities also occurred in southern and western Europe during 2003, when a prolonged and exceptionally intense heat wave resulted in 70,000 heat-related deaths (Robine et al., 2007). There is also evidence that morbidity increases together with mortality rates during extreme heat events (Dolney and Sheridan, 2006, Golden et al., 2008, Mastrangelo et al., 2007).
Simulations of the future climate indicate that the frequency of extreme weather events is very likely to increase (Solomon et al., 2007). Heat phenomena such as that of the summer in 2003 are thus expected to become more common in the near future (Schär et al., 2004, Meehl and Tebaldi, 2004, Kalkstein and Greene, 1997). Beniston (2004) suggested that the extreme thermal situation experienced in Europe during the summer of 2003 could be quite normal by the end of this century. The number of hot days per year is therefore likely to increase over nearly all land areas.
The most important human impact on the local climate of cities is known as the urban heat island (UHI) effect, reflecting the temperature difference between an urban area and the rural surroundings. Since the first evidence presented by Luke Howard (1833) in London, many investigations have shown the importance of this phenomenon, which is largely due to heat storage in buildings and sealed roads. The UHI effect is especially important during the summer months and is usually more evident at night. The intensity of a city’s heat island is dependant on the size of the city and the building density. Maximum differences of about 10 K or more between city centres and rural areas have been recorded on clear summer evenings (Oke, 1973). During heat waves the local effect of an UHI is superimposed on the regional temperature, producing an even more extreme event.
According to United Nations projections, urban populations will continue to grow over the next decades (United Nations (UN), 2008, United Nations (UN), 2010). The combined effect of global warming and worldwide increases in urban populations means that thermal stress in cities is likely to become an increasingly important issue, even in the more temperate climate of central Europe (50–55°N). Further investigation was therefore considered to be warranted into whether or not thermal stress is enhanced in large cities, especially during extreme weather events, and thus leads to higher mortality rates in urban areas than in the rural surroundings. It was also considered important to establish whether variations in UHI effects due to differences in building density within large cities (i.e. Urban Heat Archipelagos) result in similar variations in human mortality rates.
The high proportion of vulnerable people assembled in these urban areas has resulted in the observation that “Urban heat waves are among the deadliest of all weather emergencies” (Stéphan et al., 2005, p. 39). It is therefore important to investigate the specific patterns of heat stress and associated health risks for urban populations.
This study has been centred on Berlin, which is Germany’s largest city with 3.5 million inhabitants. The objectives were to investigate whether an urban–rural differentiation of heat wave mortality rates can be seen between the city of Berlin and the rural surroundings of Brandenburg, and also whether an intra-urban differentiation exists between the various districts of Berlin. The hypothesis to be tested was that rural districts with no enhancement of thermal stress, and urban districts with relatively low population densities and correspondingly moderate thermal discomfort due to weak UHI development, would record lower heat-related mortality rates than urban districts with high population densities and high levels of thermal discomfort.
Section snippets
Area of investigation, data, and methods
The area investigated covers the two German Federal States of Berlin – which is the capital of Germany – and Brandenburg, in the north-eastern part of the country. The entire area covers 30,370 km2 with maximum dimensions of 291 km N–S and 244 km E–W, centred approximately at 52° 31′ N and 13° 24′ E. The urban agglomeration of Berlin is encircled by the rural State of Brandenburg (Fig. 1). Areas of settlement and those used for transport (roads, railroads, airfields) cover 70% of Berlin, but
Climatic conditions
The hottest 3-week period within the 17 years investigated occurred in 1994 (576 ‘stress points’ between July 22nd and August 8th), followed by a period in 2006 (538 ‘stress points’ between July 10th and July 30th) – see the x-axis of Fig. 2. During both of these periods the average daily maximum temperature was above 30.0 °C (Table 1), with absolute maxima of 38.2 °C in 1994 and of 36.6 °C in 2006. Although the average daily minimum temperature was well below 20.0 °C for both periods, some
Discussion
Our research has revealed a positive correlation between thermal stress and mortality rates within the study area. The observed increase in mortality rate corresponds well with results from other European (e.g. Huynen et al., 2001, Koppe et al., 2004, Sartor et al., 1997) and non-European (e.g. Nitschke et al., 2007, Smoyer, 1998) studies of heat-related mortality.
Although the investigation area is situated in north-eastern Germany, which has a temperate climate, episodes of heat-related
Conclusion
In view of the increasing summer temperatures and more frequent hot periods predicted for most land areas by the IPCC (Solomon et al., 2007), mitigation of heat wave effects will need to take into account not only the thermal comfort inside buildings, but also the structures and land cover in urban areas. Although rural areas are affected in a similar way to urban agglomerations the need to reduce risk factors by urban planning is greater in cities, not only because heat waves are intensified
Acknowledgements
The authors would like to thank the German Research Foundation and the Federal State of Berlin for funding this research project.
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