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

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

Planning Multifunctional Green Infrastructure in Urban Areas - Advanced Approaches Based on Case Studies from Denmark, Germany and the UK

Green infrastructure (GI) is considered to be a planning concept that has potential to improve green space planning in urban areas by offering a holistic, integrated approach (e.g., Pauleit et al., 2011; Davies et al., 2015). In this paper we focus on multifunctionality as an important principle of GI planning. By scrutinizing case studies in Germany (Berlin), the UK (Edinburgh), and Denmark (Aarhus), we examine how multifunctionality is acknowledged by urban green space practitioners and provide recommendations on how to consider multifunctionality more proactively and comprehensively

A single tree model to consistently simulate cooling, shading, and pollution uptake of urban trees

Extremely high temperatures, which negatively affect the human health and plant performances, are becoming more frequent in cities. Urban green infrastructure, particularly trees, can mitigate this issue through cooling due to transpiration, and shading. Temperature regulation by trees depends on feedbacks among the climate, water supply, and plant physiology. However, in contrast to forest or general ecosystem models, most current urban tree models still lack basic processes, such as the consideration of soil water limitation, or have not been evaluated sufficiently. In this study, we present a new model that couples the soil water balance with energy calculations to assess the physiological responses and microclimate effects of a common urban street-tree species (Tilia cordata Mill.) on temperature regulation. We contrast two urban sites in Munich, Germany, with different degree of surface sealing at which microclimate and transpiration had been measured. Simulations indicate that differences in wind speed and soil water supply can be made responsible for the differences in transpiration. Nevertheless, the calculation of the overall energy balance showed that the shading effect, which depends on the leaf area index and canopy cover, contributes the most to the temperature reduction at midday. Finally, we demonstrate that the consideration of soil water availability for stomatal conductance has realistic impacts on the calculation of gaseous pollutant uptake (e.g., ozone). In conclusion, the presented model has demonstrated its ability to quantify two major ecosystem services (temperature mitigation and air pollution removal) consistently in dependence on meteorological and site conditions

Peri-urban agriculture: lessons learnt from Jakarta and Addis Ababa

The 21st century has been called the urban century, as most of the human population is now living in urban areas. Current and future urbanization is mostly taking place at great speed in the Global South. The challenges are enormous and request different models of urbanization as developed in the Global North. High levels of informality and poverty will not go away in the decades to come, while capacity will remain limited to adequately provide urban dwellers with basic infrastructure and economic opportunities. Grave economic shocks and environmental disasters such as experienced in both the Jakarta and Addis Ababa urban regions are likely to remain features of these regions. Therefore, it is of utmost importance to strengthen the resilience of these cities in the best possible ways. The continued existence of agriculture in and near urban areas is an important means for this purpose that, however, is still poorly, or at best intermittently, acknowledged by politicians and urban planners, who are inspired by visions of “modern” cities where agriculture is rather a negligible activity

Smart environment for smart cities: Assessing urban fabric types and microclimate responses for improved urban living conditions

Urban areas are particularly vulnerable to the impacts of climate change; they are also the chosen living environment of a significant majority of Europe‘s population. Global warming increasingly influences the urban climate and affects the future health and well-being of the urban population. The urban climate is mainly influenced by the urban form and the open space structure, which significantly modify the regional climatic conditions, and thereby directly affect the (thermal) comfort of the citizens. At the same time, urban open spaces are generally becoming more important as a result of their role in helping to support sustainable urban development from an ecological, social and economic point of view. Thus the future quality of life within cities is highly dependant on the “smart” treatment of its open space structure. The objective of the present study within the ACRP 3rd call was to better understand the way in which the small scale structure of the urban fabric contributes differentially to heat island effects and other urban climate phenomena, and to use this information to develop specific strategies for counter-acting and mitigating these effects on a local basis. A major focus has been laid on the urban morphology and in particular the urban landscape, and on understanding its interaction with urban microclimate. The aim was to identify climate sensitive urban patterns – using the example of Vienna - and to suggest concrete open space design measures to counteract the overheating effect during hot summer days. On the basis of a grid used by Statistik Austria (quadrants of 500 m x 500 m) an urban fabric typology for the city of Vienna has been generated taking into account aspects of urban climate and urban structure with regard to terrain, open space and built structure, which influence the microclimatic conditions and parameters. The derived “urban fabric types” have been analysed, characterised, and a sample of the most critical types formed the basis for further investigation of potential open space design measures aimed at counteracting the overheating. This was undertaken using the microclimate simulation programme ENVI-met 4.0. The evaluation of the data generated has focused on thermal comfort and on its most relevant climate factors and has taken the form of maps, mean values and diurnal variations. Based on the evaluation of the simulation results and with regard to results of a previous project, a general catalogue of open space design measures has been compiled. Representative packages of measures have been defined for each sample quadrant, highlighting their specific conditions based on their open space patterns and climate sensitivity, and focused at obtaining the optimal influence on thermal comfort amelioration

Understanding the whole city as landscape. A multivariate approach to urban landscape morphology

The European Landscape Convention implies a requirement for signatory states to identify their urban landscapes which goes beyond the traditional focus on individual parks and green spaces and the links between them. Landscape ecological approaches can provide a useful model for identifying urban landscape types across a whole territory, but the variables relevant for urban landscapes are very different to those usually addressing rural areas. This paper presents an approach to classifying the urban landscape of Vienna that was developed in a research project funded by the Austrian Ministry for Transport, Innovation and Technology: ‘Urban Fabric and Microclimate Response’. Nine landscape types and a number of sub-types were defined, using a multivariate statistical approach which takes account of both morphological and urban climate related variables. Although the variables were selected to objectively reflect the factors that could best represent the urban climatic characteristics of the urban landscape, the results also provided a widely plausible representation of the structure of the city’s landscapes. Selected examples of the landscape types that were defined in this way were used both to simulate current microclimatic conditions and also to model the effects of possible climatic amelioration measures. Finally the paper looks forward to developing a more general-purpose urban landscape typology that allows investigating a much broader complex of urban landscape functions

Understanding the whole city as landscape. A multivariate approach to urban landscape morphology

The European Landscape Convention implies a requirement for signatory states to identify their urban landscapes which goes beyond the traditional focus on individual parks and green spaces and the links between them. Landscape ecological approaches can provide a useful model for identifying urban landscape types across a whole territory, but the variables relevant for urban landscapes are very different to those usually addressing rural areas. This paper presents an approach to classifying the urban landscape of Vienna that was developed in a research project funded by the Austrian Ministry for Transport, Innovation and Technology: ‘Urban Fabric and Microclimate Response’. Nine landscape types and a number of sub-types were defined, using a multivariate statistical approach which takes account of both morphological and urban climate related variables. Although the variables were selected to objectively reflect the factors that could best represent the urban climatic characteristics of the urban landscape, the results also provided a widely plausible representation of the structure of the city’s landscapes. Selected examples of the landscape types that were defined in this way were used both to simulate current microclimatic conditions and also to model the effects of possible climatic amelioration measures. Finally the paper looks forward to developing a more general-purpose urban landscape typology that allows investigating a much broader complex of urban landscape functions

Review article : Potential of nature-based solutions to mitigate hydro-meteorological risks in sub-Saharan Africa

Sub-Saharan Africa (SSA) is the region most vulnerable to climate change and related hydro-meteorological risks. These risks are exacerbated in rapidly expanding urban areas due to the loss and degradation of green and blue spaces with their regulating ecosystem services. The potential of nature-based solutions (NBSs) to mitigate hydro-meteorological risks such as floods is increasingly recognised in Europe. However, its application in urban areas of SSA still needs to be systematically explored to inform and promote its uptake in this region. We conducted a multidisciplinary systematic review following the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) protocol to establish the general patterns in the literature on NBSs and hydro-meteorological risk mitigation in SSA. We searched scientific journal databases, websites of 12 key institutions and 11 NBS databases and identified 45 papers for analysis. We found at least 1 reported NBS in 71ĝ€¯% of urban areas of SSA across 83 locations. Of the papers, 62ĝ€¯% were clustered in South Africa, Kenya, Tanzania and Nigeria only, while the most studied cities were Dar es Salaam and Kampala. Moreover, 66ĝ€¯NBS practices were identified, most of which (nCombining double low line44) were for flood mitigation. With only Mozambique (nCombining double low line2) among the most at-risk countries reporting NBSs, we found that NBSs are implemented where risks occur but not where they are most severe. Mangrove restoration (nCombining double low line10) and wetland restoration (nCombining double low line7), reforestation (nCombining double low line10) and urban forests (nCombining double low line8), and agroforestry (nCombining double low line3) and conservation agriculture (nCombining double low line2) were the most common NBS practices identified for floods, extreme-heat and drought mitigation, respectively. Traditional practices that fit the definition of NBSs, such as grass strips and stone bunds, and practices that are more popular in the Global North, such as green roofs and green façades, were also identified. These NBSs also provided ecosystem services, including 15 regulatory, 5 provisioning and 4 cultural ecosystem services, while 4 out of every 5 NBSs created livelihood opportunities. We conclude that the reported uptake of NBSs for hydro-meteorological risks in SSA is low. However, there could be more NBSs, especially at the local level, that are unreported. NBSs can help SSA address major development challenges such as water and food insecurity and unemployment and help the sub-region progress towards climate-resilient development. Therefore, we recommend that NBSs be mainstreamed into urban planning and knowledge exchange opportunities between SSA and Europe and that other regions be explored to promote uptake

Urban morphological determinants of temperature regulating ecosystem services in two African cities

Urban green infrastructure provides important regulating ecosystem services, such as temperature and flood regulation, and thus, has the potential to increase the resilience of African cities to climate change. Differing characteristics of urban areas can be conceptualised and subsequently mapped through the idea of urban morphology types (UMTs) – classifications which combine facets of urban form and function. When mapped, UMT units provide biophysically relevant meso-scale geographical zones which can be used as the basis for understanding climate-related impacts and adaptations. For example, they support the assessment of urban temperature patterns and the temperature regulation services provided by urban green structures. UMTs have been used for assessing regulating ecosystem services in European cities but little similar knowledge is available in an African context. This paper outlines the concept of UMTs and how they were applied to two African case study cities: Addis Ababa, Ethiopia and Dar es Salaam, Tanzania. It then presents the data and methods used to understand provision of temperature regulation services across the two cities. In total, 35 detailed UMT classes were identified for Addis Ababa and 43 for Dar es Salaam. Modelled land surface temperature profiles for each of these UMTs are presented. The results demonstrate that urban morphological characteristics of UMTs, such as land surface cover proportions and associated built mass, have a much larger potential to alter neighbourhood level surface temperatures compared to projected climate changes. Land surface cover differences drive land surface temperature ranges over 25 ◦C compared to climate change projections being associated with changes of less than 1.5 ◦C. Residential UMTs account for the largest surface area of the cities, which are rapidly expanding due to population increase. Within the Residential UMTs, informal settlements and traditional housing areas are associated with the lowest land surface temperatures in Addis Ababa. These have higher proportions and better composition of green structures than other residential areas. The results have implications for planning policies in the cities. In Addis Ababa, the current urban renewal strategy to convert high density informal unplanned settlements into formal planned housing needs to explicitly account for green structure provision to avoid adverse effects on future supply of temperature regulation services. In Dar es Salaam, condominium UMTs have some of the largest proportions of green structures, and the best provision of temperature regulation services. In this case the challenge will be to maintain these into the future