Characterizing Physical and Social Compositions of Cities to Inform Climate Adaptation: Case Studies in Germany

Cities are key to climate change mitigation and adaptation in an increasingly urbanized world. As climate, socio‐economic, and physical compositions of cities are constantly changing, these need to be considered in their urban climate adaptation. To identify these changes, urban systems can be characterized by physical, functional, and social indicators. Multi‐dimensional approaches are needed to capture changes of city form and function, including patterns of mobility, land use, land cover, economic activities


Introduction
In the context of climate change, cities play a dual role: They accelerate climate change through increased greenhouse gas emissions and are places particularly affected by climate change (e.g., heat stress and flash floods; Rößler et al., 2014).To ensure a high quality of living for urban residents in a changing climate, adaptation measures have to be implemented at different spatial scales (Rößler et al., 2014).Urban planning can contribute to climate change mitigation through, for example, compact settlement forms, infrastructure that supports sustainable mobility and lifestyles, or resilience measures (e.g., nature-based solutions; Somarakis et al., 2019;Wende et al., 2010).Adaptation to the effects of climate change is an important challenge for spatial and urban planning.Future planning approaches must assess and consider a range of impacts on environment, society, and economy.Adverse societal impacts of climate change are significantly impacted by exposure and vulnerability of different population groups or settlement types and infrastructures (see Intergovernmental Panel on Climate Change [IPCC], 2012).Temperature rise, the apparent shift of the distribution of precipitation from summer to winter, and the increase in frequency of extreme events (e.g., heavy rain and heat waves) are expected to intensify further, based on current projections (IPCC, 2018).Socioeconomic development and physical changes of cities and regions modify exposure and vulnerability patterns (Birkmann et al., 2013).To assess vulnerability of urban residents to different impacts of climate change, multiple physical and socio-economic indicators that are also relevant for planning strategies need to be considered (Kappes et al., 2012;Li et al., 2016).
Cities are often subdivided into spatial units with similar physical conditions (e.g., building types, transport access, types of open space, and functionality).In some cases, socio-economic structures also are considered, such as residents' ages, income, and access to services.Because of their specific characteristics, each urban structure type may be affected differently by climate change and weather events (Beermann et al., 2013).Building typologies can influence exposure and vulnerability to different weather and climate related events, such as heat stress (IPCC, 2014).For example, in single-family houses with large gardens, air conditioning, and vegetation that provides shade, people may be less exposed to heat compared to multi-family homes in densely populated inner-city areas without these cooling potentials.Consequently, considering current and future characteristics of physical and social structures in cities, including different urban typologies within cities and their dynamics, is important for adaptation planning.Present adaptation strategies in cities in Germany consider different urban structure typologies for their formulation of adaptation needs and adaptation goals.
In this article, we investigate existing climate change adaptation plans for urban development and planning in two German cities-Berlin and Karlsruhe.These case study cities are used to examine (a) how different urban areas are characterized for adaptation in urban planning, and (b) how typologies within these strategies and plans differentiate physical and socio-economic structures that are relevant to identifying adaptation needs and climate resilient development.

Climate Vulnerability and Adaptation
It is widely acknowledged, in addition to mitigation strategies, that adaptation strategies are essential to proactively manage future risks and to reduce or even prevent adverse consequences of climate change for societies and cities (Birkmann, 2013;IPCC, 2012IPCC, , 2014;;Mertz et al., 2009).There is emerging consensus that next to hazard or climate information, the differential vulnerability of people and infrastructure exposure needs to be assessed to develop a more comprehensive information basis for adaptation (Birkmann, 2013;Ford et al., 2018;IPCC, 2014).The strong interest from different disciplines on concepts of vulnerability and the multi-dimensional nature of vulnerability (e.g., physical, social, and economic) has led to different definitions, approaches, and methods across disciplines.Taking into account the existing variety of approaches to assess vulnerability (e.g., Bogardi & Birkmann, 2004;Carreño et al., 2007;Füssel & Klein, 2006;IPCC, 2012;Turner et al., 2003;Wisner et al., 2004), we build on this literature and use the definition of vulnerability developed within the context of recent IPCC (2012,2014,2018) reports by researchers from both climate change research and disaster risk reduction.
In this context, vulnerability is defined as "the propensity or predisposition to be adversely affected" (IPCC, 2018, p. 560).Climate vulnerability may incorporate components including sensitivity or susceptibility to harm, but also response capacities, such as the lack of capacity to cope and adapt (Birkmann, 2013;IPCC, 2012).Considered this way, vulnerability does not solely focus on the fragility or susceptibility of a community, population group, or infrastructure, but also considers capacities to deal with and to adapt to shocks and hazards.Other approaches examine the vulnerability of ecosystems or capture and assess the vulnerability of coupled social-ecological systems (Bennett et al., 2016;Burton et al., 2002;Ford et al., 2018;O'Brien et al., 2007).
Therefore, the operationalization of vulnerability is a challenge, since it needs to measure and reflect social structures and societal development processes as well as material outcomes within systems that appear highly complex and are characterized by interdependencies that are difficult to capture (Adger, 2006).However, assessing vulnerability is an essential element to understand risks and to highlight the importance of social factors and societal structures in the construction of risk and the identification of adaptation options (Birkmann, 2013).Information about climatic hazards and physical structures within cities needs to be complemented with information about human vulnerability and respective socio-economic drivers of vulnerability to support urban adaptation planning.Hence, the analysis of urban typologies used within present adaptation strategies is an important research task to better understand what type of factors are, or are not, currently considered.
The term "urban structure types," Stadtstrukturtypen in German, was established in the 1990s to categorize different urban settlements.Since then, this concept has been used in planning and monitoring of cities and settlements (Novack & Stilla, 2014).Mapping urban structure types allows differentiation of the urban system into distinct areas that include various configurations of built, open spaces, green spaces, and infrastructure (Heiden et al., 2012).To ensure these typologies can effectively inform future climate adaptation policies, an integrated approach that incorporates the physical and socio-economic characteristics of a city is needed.In this article, we examine whether and how such socioeconomic aspects and profiles are linked to settlement types/structures used in present approaches.In addition, we explore how these typologies could be strengthened to also include aspects of human vulnerability.
To aid development of a more integrated approach encompassing multiple dimensions of urban development, indicator-based methods provide a useful tool as they reduce complexity and allow a systematic operationalization and monitoring of the various aspects through time (Chrysoulakis et al., 2021).Indicators in ecology and environmental planning are used to depict and evaluate environmental conditions or changes (Heink & Kowarik, 2010).To assess climate vulnerability and adaptive capacities, different sets of indicators have been developed (e.g., Birkmann, 2013;Chrysoulakis et al., 2014;He et al., 2019;Parsons et al., 2016;Wolf & McGregor, 2013).However, it is important to note that indicator-based approaches regarding vulnerability also have limitations and are criticized because of uncertainties and data limitations (see, e.g., Turner et al., 2003).In many studies, aspects of economic vulnerability are represented with "conventional" economic indicators; at the same time, social vulnerability also often encompasses intangible factors that are difficult to quantify and validate (Sorg et al., 2018).There are numerous other indicator systems that are used in closely cognate urban disciplines and applications (e.g., consideration of green infrastructure cost and benefits in cities for various environmental services; Grimmond & Souch, 1994;McPherson et al., 1997).However, in this article we focus particularly on the indicators used within existing urban adaptation concepts in two cities.Furthermore, we highlight the importance of other indicators that could provide further information about societal vulnerability at the household or settlement structure scale, such as household composition, age, education, income, and employment.

Typologies of Urban Structure
In general, typologies are both analytical and descriptive tools for developing and refining ideas, creating categorical classification, and sorting various case studies (Collier et al., 2012).In urban planning and architecture, recent examples include typologies that aim to provide historical narratives, reflect the urbanization process, categorize development trends, classify economic activities, and examine a wide range of environmental issues (Fragkias & Seto, 2009;Kloosterman & Lambregts, 2007;Li et al., 2020;Nijman, 2007;Zhou et al., 2017).
One key-use is to identify parameters that allow variability across a city to be assessed.In climate change research, urban typologies can be broadly categorized into those that consider physical and socio-economic aspects (Solecki et al., 2015).Typologies of urban structure have also been used globally in an attempt to categorize cities in the context of climate adaptation (Hrabovszky-Horváth et al., 2013;Salas & Yepes, 2018;Storch & Schmidt, 2008).
There are several ways through which planners, sociologists, geographers, economists, and environmentalists have attempted to define the physical and social structure of cities (including the economic, political, cultural, and institutional characteristics of the society).According to Wilson (2010, p. 201), "social structure refers to the way social positions, social roles, and networks of social relationships are arranged in our institutions, such as the economy, polity, education, and the organization of the households."There is increasing evidence that socio-economic urban structure is a centraldriving consideration in global environmental research and climate change studies (Banzhaf & Hofer, 2008;Crenshaw & Jenkins, 1996).
Physical urban structure mostly corresponds to spatial configuration of various structural elements of the built environment (Roca Cladera et al., 2009).Implicitly, physical/spatial structure includes "the characteristics of urban form and structure, as well as spatial configurations of structural elements, which can influence ecological functioning and human well-being in cities" (Larondelle et al., 2014, p. 427).
Overall, urban structure types are an important method and entry point for the analysis of intra-urban variations, both in terms of physical as well as social structures and dynamics.Urban structure types can be categorized with a variety of indicators which are used to quantify and measure different societal structures and specific dynamics.Table 1 provides an overview of indicators used to assess physical, socio-physical, and socio-economic structures and typologies in cities.These indicators range from capturing urban form, to featuring spatial configurations of different societal groups, integrating physical infrastructure to social infrastructure, and combining building typologies together with the household characteristics within an urban area.

Methodology
For this study we have undertaken an extensive literature review, assessing publications from multiple disciplines including urban planning, environmental planning, and social science, and examining physical and socio-economic indicators used to define and characterize "urban structure types" in adaptation research and applied in urban adaptation concepts.Both case study cities, Berlin and Karlsruhe, have published urban adaptation plans that use urban structure typologies.For these case studies, adaptation plans, project articles/reports,

Case Studies: Berlin and Karlsruhe
Large and medium-sized cities in Germany play a key role in climate adaptation.German cities are often characterized by a polycentric structure that provides important economic, social, and cultural functions for the residents and surrounding areas.However, these structures may be vulnerable to climate change due to: (a) concentration of vulnerable groups, (b) climate change combining both urban (e.g., urban heat island) and regional influences (e.g., heat waves; Founda & Santamouris, 2017), (c) damage potential, (d) high dependency on infrastructure services that might collapse in extreme events, and (e) adaptive capacity.The city of Berlin has experienced significant heat stress from increasing regional temperatures exacerbated by the urban heat island effect (Behrens & Grätz, 2010).The German Weather Service and the Senate for Urban Development's analysis of mean annual air temperature found that it increased by 1°C between 1971 and 2000 and that the number of "tropical nights" (nocturnal air temperature above 20°C) also increased, particularly in the inner city (Behrens & Grätz, 2010).By 2050, the number of very hot days (maximum daytime temperature above 30°C) in the dense inner-city areas will increase to 25 days per year (Senate Department for Urban Development and Housing, 2011).Given Berlin's continental location, the summertime heat is often associated with challenges posed by water scarcity (Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, 2008).
The city of Karlsruhe has large impervious cover and summer air temperature in the city than can be 10°C warmer than the surroundings (Beermann et al., 2014).Located within the Upper Rhine Rift Valley, this is the warmest region in Germany.Karlsruhe is consid-ered to be a city with heat-related health risks, which are a key concern (Beermann et al., 2014).Considerable small-scale temperature differences occur within the various built-up areas, thus underlining the relevance of urban structure types, including the degree of sealing and green spaces (Hackenbruch, 2018).

City of Berlin
Climate adaptation and mitigation pose new challenges to sustainable urban development for the city of Berlin (Senate Department for Urban Development and Housing, 2011).On the one hand, these issues are relevant for the modification of existing urban structures (e.g., optimization of existing buildings, infrastructure, and green/open spaces).On the other hand, mitigation and adaptation issues need to be considered from the beginning when planning and implementing new urban areas.To improve the consideration of climate change adaptation and mitigation, Berlin has developed and approved a city climate development plan, which is updated occasionally (Senate Department for Urban Development and the Environment, 2016b).It examines spatially differentiated impacts of climate change in Berlin and identifies action for urban development.A core question addressed within the urban climate development plan is the following: How can Berlin strengthen its urban sustainability and resilience within a changing climate, focusing on citizens and infrastructure?(Senate Department for Urban Development and Housing, 2011).Special emphasis is given to heat stress and heavy precipitation events as these are hazards that may be critical in future climates.In the next section, we examine the settlement and building typologies used within this urban development plan for climate adaptation.

Typologies
The Senate Department for Urban Development and Environment developed an environmental atlas that classifies the city based on urban structure types.These are defined by their building structure and density, open spaces, and representative land use and building use typologies (e.g., industrial versus residential use).Grimmond (2007) and Hertwig et al. (2021) underscore that neighbourhood structure, built volume, and people's behaviour significantly modify the local urban climate (e.g., local air temperature, wind speed).Different neighbourhood compositions or archetypes will influence the adaptation measures that are needed or feasible (Ward & Grimmond, 2017).Against this background it is interesting to note that the environmental atlas for Berlin encompasses 52 area-types grouped into 16 settlement-structure types and six main groups (Senate Department for Urban Development and the Environment, 2016a).The main groups are: These classifications are used to distinguish adaptation needs, since these settlement typologies refer to areas that are: (a) already exposed to climatic stress today and/or particularly high stress is expected in the future, (b) undergoing (or expected to undergo) extensive changes, such as new construction and densification, (c) relatively homogeneous and therefore suggested measures are transferrable (no special cases), and (d) relevant for the entire city and cover a high proportion of the urban landscape as a whole (Senate Department for Urban Development and the Environment, 2016b).
The climate adaptation plan of Berlin primarily focuses on residential areas.The first two structure types (i.e., perimeter block development and town houses) are indicative of the proposed adaptation planning strategies of re-densification.Schools and technology parks are infrastructures with important functions extending beyond their specific ward (Senate Department for Urban Development and the Environment, 2016b).
To examine how the structure types are used to frame adaptation strategies and measures within Berlin, we selected the compact perimeter block development (Figure 1) as an example.About 15% of the residential housing is from the Wilhelminian Period (1890−1918), covering around 8% of the total area of Berlin (approximately 3,880 ha).This structure type is more common in inner-city districts (e.g., 65% of Mitte, 73% of Friedrichshain-Kreuzberg, 37% of Charlottenburg-Wilmersdorf districts; Senate Department for Urban Development and Housing, 2011).About 36% of Berlin's population (more than 1.2 million people) resides in perimeter blocks (Reusswig et al., 2014).These are largely dense areas, with limited access to green space and heterogeneous ownership patterns.With more frequent heat stress, these areas are likely to be affected, due to the relatively high density (exposure) and the limited adaptive capacity (e.g., access to green spaces).The urban adaptation plan suggests measures (Figure 1) focused on improving green infrastructure (e.g., greening façades, development of small parks) and increasing roof albedo to decrease short-wave radiation absorption (Senate Department for Urban Development and the Environment, 2016b).The proposed adaptation measures and goals in the urban adaptation plan for Berlin are closely coupled with selected settlement typologies used to characterize adaptation needs.

City of Karlsruhe
The development of an urban adaptation plan for Karlsruhe was triggered by the lack of a city-wide overview of where the city quarters most affected by heat stress were located (Beermann et al., 2013).The urban climate adaptation plan had two main phases of development and the formulation of adaptation measures.First, the plan identified and defined specific urban structure types according to their physical structure and aspects of stability and dynamics.Second, climate change "hot spots" were identified by assessing the structure types and their susceptibility to weather extremes and other important factors (e.g., demographic composition and access to green space).Using urban structure types aids transferability of adaptation measures across the city when the physical and social structures are brought together.This should help identification of locations of concern not yet exposed to heat stress (Beermann et al., 2013).

Typologies
Karlsruhe classified all of its 556 neighbourhoods (in German, Stadtviertel) into one of the 12 identified urban structure types (Figure 2).A multi-criteria analysis combined the structural characteristics of the neighbourhood with human and societal characteristics, with the latter giving some hints on aspects of human vulnerability (Section 4.2.2).The urban adaptation strategy anticipates that nine of the 12 structure types will require adaptation measures by 2050 because of their relatively high vulnerability metrics (Beermann et al., 2014).The structure types are grouped into three classes (Table 2): medium to high climatic stress, low to no climatic stress, and low exposure to heat stress.These refer to different levels of concern and demonstrate differential adaptation needs to climatic hazards.

Climate Adaptation in Urban Planning
The identification of adaptation needs for specific neighbourhoods and settlement typologies in Karlsruhe also consider future changes, particularly: (a) high level of local climatic stress (e.g., heat stress) at present or in the near future (2046-2055); (b) sensitive land and building use in the area; (c) high population density (more than 250 inhabitants/km²); (d) high proportion of young children (less than four years of age), elderly people (65 or more years of age), and people living in one-person households; (e) no green space within walking distance; and (f) low energy efficiency of buildings in the area (Beermann et al., 2013(Beermann et al., , 2014)).
The criteria include physical characteristics of the urban structure, but also aspects of human exposure and human vulnerability.Identification of hot spots and their adaptation measures within the urban structure adaptation measures are proposed for row development (Figure 3), which have these characteristics (Beermann et al., 2013)   • Proportion of senior citizens of 14.6%; • Proportion of young children of 5.7%.
It is assumed that these areas already are significantly affected by heat stress (about 50 days per year) and expected to increase (58 days of heat stress and 10 days of tropical nights per year) by 2050 (Beermann et al., 2014).Urban structure types are identified as being a hot spot if neighbourhood vulnerability is classified as high due to its population structure and composition (Beermann et al., 2014).Adaptation measures to improve the resilience in a specific neighbourhood (Figure 3) of this type include adding pocket parks, improving or providing blue (water) infrastructure in public spaces, and reducing impervious areas.In addition to these physical measures, the adaptation concept also focuses on reducing human vulnerability (e.g., reducing isolation of elderly residents).Lastly, the adaptation plan suggests improving the energy efficiency of buildings and modification of building thermal properties (e.g., greening walls) to improve both adaptation and mitigation.The proposed adaptation strategies and actions need to: (a) consider the specific conditions in each neighbourhood, (b) promote vulnerability reduction of residents exposed, and (c) support adaptation measures appropriate for transfer to other areas with the same structure type (Beermann et al., 2013).
Overall, the city of Karlsruhe proposed 19 adaptation measures for different structure types focusing on three spatial scales of intervention: city, neighbourhood, and building (Beermann et al., 2014).

Discussion
The comparative analysis of the urban adaptation strategies linked to urban development and planning in Berlin and Karlsruhe shows that urban structure typologies are an important entry point for these cities to identify adaptation needs and measures.The two cities use various indicators (Table 3) within their definition of urban structure types.While the physical indicators used to characterize urban form are similar, the characterization of social aspects and use of social indicators differ.Although both use social indicators to identify hot spots and, in part, adaptation measures, they differ in the criteria they use for identifying specific adaptation measures (Table 3).Both cities use land cover characteristics, building typologies, population density, and the availability of green spaces to identify adaptation needs along different settlement typologies.While Berlin uses more detailed criteria (e.g., related to impervious area), Karlsruhe focuses more on the access to pervious (green) spaces.
In both cities, socio-economic indicators such as household income or unemployment are not explicitly integrated into the formulation and assessment of structure types.However, in Karlsruhe the proportion of young children (less than four years of age) and seniors living alone in specific neighbourhoods is part of the adaptation measures.Hence, Karlsruhe takes a more integrative approach to urban adaptation to climate change in terms of renewal and new urban development which includes goals and measures to safeguard a larger mix of different age groups within a ward and to avoid isolation of the elderly who are likely to be most vulnerable.
In Berlin, additional strategies and tools exist that account for differential human vulnerability, such as those related to health and civil protection (Reusswig et al., 2016).However, the urban typologies used do not sufficiently address these issues.Both cities use urban structure types to help identify intra-city variations and adaptation needs using similar procedures and definition of adaptation measures (Figure 4).First, urban structures are classified using physical indicators (Tables 1 and 3).Second, present and future climatic stress is assessed by structure type and adaptation needs formulated with some quantitative analyses.Third, adaptation measures are formulated for specific structure types and hot spots.Societal and social indicators are sometimes used (Figure 4) to further specify adaptation needs and measures.Societal indicators capture mostly aspects of demography and population density, but sometimes they also capture social isolation (e.g., elderly in Karlsruhe).A broader integration of socio-economic indicators covering the functions within these neighbourhoods is still missing (Table 1).In this regard, Capel-Timms et al. ( 2020) and Grimmond et al. (1996) underscore that socio-economic indicators are critical proxies of how neighbourhoods function and how humans interact with and affect the physical environment characteristics and dynamic behavioural patterns in each area (see also Grimmond & Oke, 1986;Kokkonen et al., 2018;Quattrone & Zannou, 1998;Ward & Grimmond, 2017).Therefore, there is a co-dependence between the physical and the socio-economic environment (Banzhaf & Hofer, 2008;Grimmond et al., 1996).Moreover, urban socio-economic structure is an essential driving force of urban climate change (Banzhaf & Hofer, 2008;Crenshaw & Jenkins, 1996;Grimmond, 2007;Krellenberg et al., 2011).However, the urban adaptation plans in Berlin and Karlsruhe capture these broader aspects and neighbourhood functions only partially, if at all.

Conclusion
Analysis of urban adaptation strategies used in urban planning reveals that urban structure types play an important role in assessing climate risks and formulating adaptation needs and actions.The review of strategies and planning documents for Berlin and Karlsruhe underscores that within the definition of urban structure types, physical indicators play a key role, while less attention is given to social indicators, particularly socio-economic aspects.However, some social indicators are included in the adaptation measures developed for both cities.While most attention is paid to the physical structure of the respective urban typology, socio-demographic aspects also receive attention, but significantly less or at a later stage within the assessment.While Berlin emphasises improving the physical structures to better adapt to climate change (e.g., improving green spaces or reducing impervious area), Karlsruhe gives greater importance to societal indicators in its formulation of adaptation goals.
We see an urgent need to further strengthen urban adaptation concepts and link these to formal and informal tools of urban development that allow those responsible for climate adaptation to address both physical and social structures within the city.In addition, interactions between different neighbourhoods and functions, for example in terms of mobility and commuting patterns, are at present not sufficiently captured and should receive more attention in future urban adaptation plans.
Overall, an integrated approach considering both urban physical and social structures can better support and inform urban planning, urban development, and climate adaptation.More research is needed on how to enhance an integrative assessment that can link physical and social characteristics of urban areas.In this regard, constraints and limitations of linking physical and social structures need to be better understood.In addition, a more dynamic understanding of cities and their exposure and vulnerability to climatic hazards is needed.This requires, among other issues, new data and new methods for the identification and development of urban archetypes (societal driven settlement structure typology) that also capture the dynamics of behaviour (work, travel, recreation, etc.) and dynamics of urban development (migration, densification vs. urban sprawl, economic development trends, etc.).programme under grant agreement no.855005.The authors gratefully acknowledge helpful discussions with Jörn Welsch of Berlin's environmental atlas team and Elke Plate from the Senate Department for Urban Development and Housing of the City of Berlin.

Figure 1 .
Figure 1.Potential adaptation measure for a compact perimeter block development proposed in the urban development plan.Source: Authors' work adapted from Senate Department for Urban Development and the Environment (2016b).

Figure 3 .
Figure 3. Proposed adaptive modifications for a neighbourhood with the row development (Figure 2) structure type in Karlsruhe.Source: Authors' work adapted from Beermann et al. (2014).

Figure 4 .
Figure 4. Planning sequence that is common in both case studies.Terms are defined in various places in the text.

Table 1 .
Examples of indicators used to characterize social and physical structures of cities. are analysed (e.g., environmental atlas in Berlin; Senate Department for Urban Development and the Environment, 2016a).To verify core findings, expert interviews, for example with representatives of the Senate Department for Urban Development and Housing of the City of Berlin, were con- Malakar and Mishra (2016)s adapted fromLowry and Lowry (2014)andMalakar and Mishra (2016).Urban Planning, 2021, Volume 6, Issue 4, Pages 321-337 and cartographic information

Table 2 .
Structure type classes (Figure 2) in Karlsruhe are distributed across 556 neighbourhoods (Stadtviertel) with different levels of concern with respect to climatic stress now (2010) and in 2050.
Note: Indicators used are identified with a Y (yes) or comments.Urban Planning, 2021, Volume 6, Issue 4, Pages 321-337