Trade-Offs between Urban Green Space and Densification: Balancing Outdoor Thermal Comfort, Mobility, and Housing Demand

Sabrina Erlwein,Stephan Pauleit

doi:10.17645/up.v6i1.3481

Abstract

Urban green spaces reduce elevated urban temperature through evaporative cooling and shading and are thus promoted as nature-based solutions to enhance urban climates. However, in growing cities, the supply of urban green space often conflicts with increasing housing demand. This study investigates the interplay of densification and the availability of green space and its impact on human heat stress in summer. For the case of an open-midrise (local climate zone 5) urban redevelopment site in Munich, eight densification scenarios were elaborated with city planners and evaluated by microscale simulations in ENVI-met. The chosen scenarios consider varying building heights, different types of densification, amount of vegetation and parking space regulations. The preservation of existing trees has the greatest impact on the physical equivalent temperature (PET). Construction of underground car parking results in the removal of the tree population. Loss of all the existing trees due to parking space consumption leads to an average daytime PET increase of 5°C compared to the current situation. If the parking space requirement is halved, the increase in PET can be reduced to 1.3°C–1.7°C in all scenarios. The addition of buildings leads to a higher gain in living space than the addition of floors, but night-time thermal comfort is affected by poor ventilation if fresh air circulation is blocked. The protection of mature trees in urban redevelopment strategies will become more relevant in the changing climate. Alternative mobility strategies could help to reduce trade-offs between densification and urban greening.<p>Urban green spaces reduce outdoor temperatures through evaporative cooling and shading and are thus promoted as nature based solutions to enhance urban climates. However, in growing cities, supply of urban green space often conflicts with increasing housing demand. This study investigates the interplay of densification and availability of green and its impact on outdoor human thermal comfort. For the case of an open-midrise (LCZ 5) urban redevelopment site in Munich, eight densification scenarios were elaborated with city planners and evaluated by microclimate modelling in ENVI-met. The chosen scenarios consider varying building heights, different types of densification, vegetation amount and parking space regulations. The greatest impact on physical equivalent temperature (PET) has the preservation of existing trees. Construction of underground car parking results in the removal of the tree population. Loss of all existing trees due to parking space consumption leads to an average daytime increase of 5 °C PET compared to the current situation. If the parking space requirement is halved, the increase in PET can be reduced to 1.3 to 1.7 °C in all scenarios. Adding buildings leads to a higher gain in living space than adding floors, yet night time thermal comfort is affected by poor ventilation if fresh air circulation is blocked. Protecting mature trees in urban redevelopment strategies will gain more relevance in changing climate. Alternative mobility strategies can help to reduce trade-offs between mobility, densification and microclimate.</p>

Highlights

In the past decade, green and blue spaces in cities have been promoted as no or low regret adaptation measures to climate change (European Commission, 2016)
This study investigated the impact of densification on urban green space availability and outdoor thermal comfort for an open midrise development site in Munich
Densification scenarios for a typical housing area of freestanding multistorey blocks in Munich were developed alongside planners and were considered to be realistic

Summary

Introduction

Green and blue spaces in cities have been promoted as no or low regret adaptation measures to climate change (European Commission, 2016). Trees are the most effective in reducing incoming shortwave radiation (Erell, 2017; Zölch, Maderspacher, Wamsler, & Pauleit, 2016). Dense, foliated tree crowns reduce the transmissivity of direct solar radiation to 1%–5% (Konarska, Lindberg, Larsson, Thorsson, & Holmer, 2014), reducing daytime Ta by up to 3°C, the mean radiant temperature by up to 37°C and the physical equivalent temperature (PET) directly beneath the tree crown by up to 16°C (Lee, Mayer, & Kuttler, 2020). Urban areas undergoing densification by the addition of buildings or the increase in the size of existing buildings often exhibit a lack of urban green space (Haaland & van den Bosch, 2015)

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