Research PaperPlanning for cooler cities: A framework to prioritise green infrastructure to mitigate high temperatures in urban landscapes
Introduction
Globally, extreme heat events (EHE) have led to particularly high rates of mortality and morbidity in cities as urban populations are pushed beyond their adaptive capacities. Recent EHE examples include: Chicago, USA (1995; 31% mortality increase) (Whitman et al., 1997), Paris, France (2003; 130% mortality increase) (Dhainaut, Claessens, Ginsburg, & Riou, 2003), Moscow, Russia (2010; 60% mortality increase) (Revich, 2011) and Melbourne, Australia (2009; 62% mortality increase) (Department of Human Services, 2009). Many cities expect catastrophic EHEs more often, as the frequency, intensity and duration of EHEs are projected to increase with climate change (Alexander & Arblaster, 2009).
There is evidence that increased mortality and morbidity from EHE are exacerbated in urban populations by the urban heat island (UHI) effect (e.g. Gabriel & Endlicher, 2011). Modified land surfaces from urbanisation lead to the formation of distinct urban climates (Coutts, Beringer, & Tapper, 2007). Natural surfaces and vegetation are replaced with a complex, three-dimensional impervious surface that absorbs large amounts of solar radiation during the day and this energy is then slowly released at night, keeping urban areas warmer than the surrounding rural countryside and leading to the UHI (Oke, 1982). Rainfall is rapidly drained via stormwater pipes leaving little moisture in the urban landscape, which reduces evapotranspiration and increases sensible heating of the local atmosphere (Coutts et al., 2007). Several studies have shown that higher night time temperatures limit people's recovery from daytime heat stress (Clarke & Bach, 1971). Consequently, many urban populations must adapt to the compounding effects of the UHI, climate change and EHE (Bi et al., 2011).
Many governments are now strategically planning for EHE (O’Neill et al., 2009), often with a focus on short-term preparation and prevention, for example warning systems, promoting behavioural change and preparing emergency services (Kovats and Hajat, 2008, Queensland, 2010). Increasing the amount of vegetation, or green infrastructure, in a city is one way to help address the root cause of the problem, by reducing urban air and surface temperature maxima and variation (Bowler, Buyung-Ali, Knight, & Pullin, 2010). However, to substantially reduce the UHI, widespread implementation of green infrastructure is required. For example, measurements during an EHE in Melbourne, Australia, suggested a 10% increase in vegetation cover could reduce daytime urban surface temperatures by approximately 1 °C (Coutts & Harris, 2013).
Urban green infrastructure (UGI) can be defined as the network of planned and unplanned green spaces, spanning both the public and private realms, and managed as an integrated system to provide a range of benefits (Lovell and Taylor, 2013, Tzoulas et al., 2007). UGI can include remnant native vegetation, parks, private gardens, golf courses, street trees and more engineered options such as green roofs, green walls, biofilters and raingardens (Table 1). This paper focuses on the integration of UGI into the public realm to mitigate high urban temperatures and considers the various UGI types and possible locations.
UGI research is not well integrated with urban design and planning, which contributes to the lack of guidance on how best to implement UGI (Bowler et al., 2010, Erell, 2008). UGI is a particularly good option for temperature mitigation in Mediterranean or warm temperate climates due to the greater relative cooling benefits in hot, dry climates (Ottelé, Perini, Fraaij, Haas, & Raiteri, 2011), particularly if water is available to maintain vegetation health and evapotranspiration. Yet, there is a dearth of empirical evidence regarding the benefits of UGI in cities experiencing a Mediterranean climate, nor information on successful and cost effective UGI implementation strategies (Williams, Rayner, & Raynor, 2010). Clearly a cross-disciplinary approach is required.
We present a framework, supported by relevant literature, for green space managers, planners and designers to most effectively integrate UGI into existing urban areas for the primary goal of improved urban climate. With the aid of thermal mapping, a decision framework was developed for local government authorities in Melbourne, Australia. A step by step case-study implementing the framework is provided, drawing on high resolution, airborne thermal mapping as a tool within this framework. Melbourne (37°49′ S; 144°58′ E), on the southern coast of south eastern Australia, has a warm Maritime Temperate climate (Peel, Finlayson, & McMahon, 2007), but has long periods of summer drought and extreme heat. This framework can be applied to cities with classic Mediterranean climates (e.g. Perth, San Francisco, Seville, Beirut and Athens) and those that experience extended summer periods of hot, dry conditions, such as Adelaide and Melbourne. Cities in colder or more humid climates may have different considerations, for example in humid areas there can be a greater emphasis on maximising air flow (Emmanuel, 2005).
Section snippets
A framework for using UGI to mitigate excess urban heat
We propose a hierarchical, five step framework to prioritise urban public open space for microclimate cooling (Steps 1–4) using the most appropriate ‘fit for place’ UGI (Step 5) (Fig. 1). The same principles will apply to privately-owned outdoor space, although this may be complicated by issues of multiple ownership (Pandit, Polyakov, Tapsuwan, & Moran, 2013).
The framework operates firstly at the ‘neighbourhood’ scale, then the ‘street’ scale and finally the ‘microscale’ (Fig. 1). While the
Case study—city of Port Phillip, Melbourne, Australia
The City of Port Phillip comprises 20.62 km2 of predominantly pre-1900 suburbs on the north shore of Port Phillip Bay in inner city Melbourne, Australia (City of Port Phillip, 2014) and is home to over 91,000 people (Australian Bureau of Statistics, 2011b). The City of Port Philip was a key partner in this research and keenly aware of the impacts of heat on communities especially from the 2009 extreme heat event in Melbourne which contributed to 374 excess deaths (Department of Human Services,
Discussion
We have reviewed the potential of urban green infrastructure to mitigate high temperatures and integrated this information with census data and remotely sensed thermal data to provide a decision framework that prioritises effective implementation of UGI. Although we make recommendations on what types of UGI will be most suitable in different circumstances, the selection of appropriate UGI will always depend on the local climate, soils, water availability as well as community norms and cultural
Conclusions
Mitigating extreme heat in urban climates will become increasingly important as climate change progresses and urban populations expand. UGI should be an important component of any urban climate change adaptation strategy because of the multiple benefits it provides to the community and local ecosystems. However, any UGI initiative should determine what the key objective(s) is at the outset. This study assumes the key objective is temperature mitigation. As such, in a situation where a decision
Acknowledgements
This paper arose from a project funded by the Victorian Centre for Climate Change Adaptation Research (VCCCAR). Sincere thanks to the City of Port Phillip for making available the thermal imagery data and supporting GIS layers. The manuscript has benefited from input during the project from Brod Street, all workshop participants, and Karyn Bosomworth and Alexei Trundle from RMIT University as well as from two anonymous reviewers. Andrew M. Coutts is funded by the Cooperative Research Centre for
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Present address: Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK.