Reducing urban heat wave risk in the 21st century ScienceDirect

Global warming increases the frequency, intensity and duration of heat waves, particularly endangering urban populations. However, the health risks of heat waves are distributed unequally between people because of intrinsic person-specific characteristics and extrinsic factors. The confluence of forecasted urbanisation and projected heat wave increase necessitates the identification of strategies that both lower the overall health impact and narrow the gap in risk distribution within urban populations. Here, we review the literature on vulnerability to heat, highlighting the factors that affect such distribution. As a key lesson we find that the literature strands on public health, risk reduction and urban planning all contribute to the identification of alleviation options for urban heat wave health impacts, but that they are rarely jointly evaluated. On the basis of the literature review, we suggest a common framework. We also evaluate response measures in addressing total and distributed risks. We find that personspecific risk is effectively addressed by public health and risk reduction intervention, while intra-urban variations of extrinsic factors can be efficiently tackled with urban planning, both in scale and scope. Introduction Projected changes in heat waves and ongoing urbanisation provide the setting for a dramatic increase in heatrelated morbidity and mortality in urban settings. Recent forecasts suggest that a robust, several-fold increase in the frequency, intensity and duration of observed global heat waves and warm spells will occur irrespective of the emission scenario Methodology The objective of this paper is to critically review and compare current theoretical and practical approaches to heat-related risk reduction in cities. On this basis we attempt to answer the following research questions: a. Which factors contribute to health impact and the existing differences in risk distribution from extreme heat events in cities (Section 'UHW: health risk factors')? b. Which interventions effectively reduce the different risk factors (Section 'Evaluation of responses to heatwaves')? In order to address these questions we review the literature on urban heat risk, concentrating on peer-reviewed papers that have been published over the last five to ten years. We uncover health studies and research on disaster risk reduction, climate change, adaptability and resilience at the city level as well as reviews of current responses to heat, including public health and urban planning perspectives. In the following paragraphs we introduce concepts from public policy and health literature and combine them with the body of literature on urban resilience in order to develop a framework for the evaluation of interventions. We will use this framework in Section 'Evaluation of responses to heatwaves' to discuss the contributions and limitations of current intervention approaches and potential improvements for those to come. Risk can be addressed at multiple levels: by avoiding and reducing exposure to hazards, lessening susceptibility and improving preparedness through response and recovery mechanisms [18, Current Opinion in Environmental Sustainability 2015, 14:221-231 www.sciencedirect.com revenue and expenditure the government generates overall by implementing this policy UHW: health risk factors Risk from urban heat waves differs with so-called intrinsic and extrinsic factors. Intrinsic factors refer broadly to the physical condition of individuals (typically named susceptibility factors or sensitivity in public health literature), whereas extrinsic factors point to stratified levels of risk across socio-economic and spatial urban settings. We briefly review the literature on both sets of factors to clarify their importance. Intrinsic factors Among the susceptibility explanatory variables of risk unequal distribution, age is at the forefront Extrinsic factors Additional uneven distribution of risk among populations in suburban population groups can be explained through the interaction of social, economic, environmental and political characteristics. These characteristics influence heat exposure directly or individual susceptibility indirectly On a macro scale, regional climates and geography modify heat wave hazards among cities in terms of intensity, frequency and duration The awareness of the role of environmental factors (also known as urban climate) and their interaction with the urban fabric in explaining different levels of risk within cities is explored by an increasing body of research on urban structures and their link to public health. The landscape of the urban periphery shapes the decreases in temperature as the airflow enters the leeward rural area Finally, urban governance may also change risks and risk distribution among populations; this refers to how response and recovery mechanisms in place interact with all the previously described factors Evaluation of responses to heatwaves Research on strategies for coping with urban heat waves has grown considerably in the last decade, but interdisciplinary differences limit the overall understanding of how cities can respond to this ever increasing climate hazard. Health literature explores how and why particular populations are more at risk than others -intrinsic factors; climate change, risk reduction and urban planning research addresses extrinsic factors [18, Although the population is continually adapting to heat [55], the efficiency of HHWS has not yet been proven for future scenarios, where increasing heat-wave frequency, severity and duration is coupled with a rapidly growing atrisk population [48,55, Generally, public health interventions address those with higher intrinsic risks under a distributive justice perspective (i.e. prioritising the assistance of individuals at higher risk), but lack adequate measures to also intervene with commutative justice considerations Urban planning interventions become increasingly relevant but are rarely deployed Urban heat has a strong interaction with built structures, ultimately shaping the frequency, intensity and duration of heat waves and further shaping exposure levels. Information on location specific thermal radiative power helps define the impact of urban structures at the community level Well-known strategies include the increase of albedo though the modification of urban building materials and colours and optimising the urban canopy layer [107 ,120]. These alternatives are of particular importance from a cost-efficiency perspective at the household level; they are particularly recommended in housing for middle and low-income occupants, which also makes them very inclusive from an equity point of view [121 ,122,92 ,123]. In general, strategies that move away from electricity requirements perform better from a commutative justice perspective; effectiveness is not reduced when loss of power or electric system failures occur At the street and neighbourhood level, one could also reduce exposure times through shadowing and enhancing urban connectivity Recent case studies of Seoul and Berlin, lead by the department of landscape planning at the Technical University of Berlin suggest specific urban planning strategies All these measures perform very well for commutative justice; the effects are in principle equally distributed among the whole population because they help to decrease the overall UHI effect. Research tells us that most of these strategies reduce overall anthropogenic heat intensity, thus providing benefits to all Achievements, challenges and the way forward While public health measures focus on individual susceptibilities and levels of exposure, urban planning succeeds in lessening UHW hazard severity as well as intracity exposures. Public health responses perform very effectively under a distributive-justice approach, yet it is difficult to up-scale intervention actions to the city level from a cost-efficiency perspective. This fact makes prioritisation a design criteria, and health care interventions do not directly address options aimed at the total population and tend to be based on mainly short term interventions [21,88 ,99,143]. They cannot alter urban climates Currently, the majority of responses are based on susceptibility or individual exposure factors, but these do not reduce the population's exposure either on a larger scale or in the long term. Little has been done in regards to Person-specific risk is well addressed by public health interventions, while extrinsic factors can be tackled through urban planning considerations. Current Opinion in Environmental Sustainability 2015, 14:221-231 www.sciencedirect.com urban planning apart from isolated initiatives focussing on albedo modification The combination of actions addressing intrinsic and extrinsic factors at the same time could fill the gaps each approach has on its own. Conclusion This review identifies intrinsic and extrinsic factors that contribute to stratified vulnerability to urban heat waves. We show that public health interventions are highly effective and can be used to address intrinsic risk factors. Urban planning measures show their efficiency on longer time scales but are rarely deployed. Crucially, urban planning measures would reduce exposure and through this help to mitigate extrinsic risk factors. We claim that this reduction of extrinsic risk factors becomes increasingly relevant due to the confluence of urbanisation and rising levels of urban heat hazards. We conclude that a coordinated effort between public health and urban planning departments would most effectively counter the threat of future heat waves worldwide, and specifically would help to address both individual susceptibility (intrinsic factors) and overall exposure (extrinsic factors). Acknowledgemen