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Viability of public spaces in cities under increasing heat: A
transdisciplinary approach
Kathrin Foshag, Nicole Aeschbach, Bernhard H¨ofle, Raino Winkler,
Alexander Siegmund, Werner Aeschbach
PII: S2210-6707(20)30202-X
DOI: https://doi.org/10.1016/j.scs.2020.102215
Reference: SCS 102215
To appear in: Sustainable Cities and Society
Received Date: 21 October 2019
Revised Date: 9 March 2020
Accepted Date: 17 April 2020
Please cite this article as: Foshag K, Aeschbach N, H¨ofle B, Winkler R, Siegmund A,
Aeschbach W, Viability of public spaces in cities under increasing heat: A transdisciplinary
approach, Sustainable Cities and Society (2020),
doi: https://doi.org/10.1016/j.scs.2020.102215
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Viability of public spaces in cities under increasing heat: a transdisciplinary ap-
proach
Authors:
Kathrin Foshaga,b,c (Corresponding author)
kathrin.leutz@iup.uni-heidelberg.de
a TdLab Geography, Institute of Geography, Heidelberg University, Im Neuenheimer
Feld 368, 69120 Heidelberg, Germany
b Institute of Environmental Physics, Heidelberg University, Im Neuenheimer Feld 229,
69120 Heidelberg, Germany
c Heidelberg School of Education, Heidelberg University, Voßstraße 2, Building 4330,
69115 Heidelberg, Germany
Nicole Aeschbacha,g
nicole.aeschbach@uni-heidelberg.de
a TdLab Geography, Institute of Geography, Heidelberg University, Im Neuenheimer
Feld 368, 69120 Heidelberg, Germany
g Heidelberg Center for the Environment (HCE), Heidelberg University, Im Neuenheimer
Feld 229, 69120 Heidelberg, Germany
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Bernhard Höfled,g
hoefle@uni-heidelberg.de
d 3D Geospatial Data Processing (3DGeo) Research Group, Institute of Geography,
Heidelberg University, Im Neuenheimer Feld 368, 69120 Heidelberg, Germany
g Heidelberg Center for the Environment (HCE), Heidelberg University, Im Neuenheimer
Feld 229, 69120 Heidelberg, Germany
Raino Winklere
raino.winkler@heidelberg.de
e City of Heidelberg, Office of Environmental Protection, Trade Supervision and Energy,
Prinz-Carl, Kornmarkt 1, 69117 Heidelberg, Germany
Alexander Siegmundf,g,h
siegmund@ph-heidelberg.de
f Department of Geography – Research Group for Earth Observation (rgeo) & UNESCO
Chair on World Heritage and Biosphere Reserve Observation and Education, Heidel-
berg University of Education, Czernyring 22/10-12, 69115 Heidelberg, Germany
g Heidelberg Center for the Environment (HCE), Heidelberg University, Im Neuenheimer
Feld 229, 69120 Heidelberg, Germany
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h Institute of Geography, Heidelberg University, Im Neuenheimer Feld 348, 69120 Hei-
delberg, Germany
Werner Aeschbachb,g
aeschbach@iup.uni-heidelberg.de
b Institute of Environmental Physics, Heidelberg University, Im Neuenheimer Feld 229,
69120 Heidelberg, Germany
g Heidelberg Center for the Environment (HCE), Heidelberg University, Im Neuenheimer
Feld 229, 69120 Heidelberg, Germany
Highlights
Integrated transdisciplinary set of methods to co-design climate change adapta-
tion.
Heat adaptation measures for public squares in Heidelberg, Germany.
Measures have a regulating effect on the microclimate and increase well-being.
Local actors are indispensable partners in transdisciplinary projects.
Abstract
Cities are particularly sensitive to the effects of climate change, causing an increasing
incidence of heat waves. Extreme temperatures can impair the use of public spaces in
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cities, as heat stress endangers human well-being and health. Identifying suitable adap-
tation measures to maintain the full functionality of public spaces requires a multidimen-
sional approach, accounting for interrelated scientific, social, and practical aspects. As
one result we introduce an inter- and transdisciplinary concept that addresses the chal-
lenge of adapting public spaces to climate change. Additionally we present a pilot study
from Heidelberg, Germany, where a new, sustainable urban quarter experienced more
pronounced heat stress than the historic city centre in the hot and dry summer of 2018.
The study shows the suitability of our approach to identify appropriate heat adaptation
measures. Solar potential modelling and mental map surveys proved to be particularly
effective methods. We find that adaptation measures generate synergy effects by improv-
ing both climatic and social conditions.
Keywords: Climate change adaptation; Urban climate; Urban environment; Trans-
disciplinary sustainability research; Heat stress; Public places; Health improve-
ment
1. Introduction
Climate change and adaptation to climate change are among the major challenges of our
time (Intergovernmental Panel on Climate Change, 2014; Rüegg, 2019; United Nations,
2015). The increase in the number and severity of heat stress events in cities requires
the urgent development of adaptation measures in order to protect the health and well-
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being of inhabitants as well as the social function of public places in future sustainable
cities (cf. Sustainable Development Goals 3 and 11, (United Nations, 2015). There is
extensive evidence that anthropogenic climate change, both observed and modelled,
continues to raise the frequency and intensity of extreme heat events, especially in urban
regions and their public areas (Bastin et al., 2019; Christidis, Jones, & Stott, 2014; Russo,
Sillmann, & Fischer, 2015; Schär, 2015; Tomczyk & Bednorz, 2016; Wang, Jiang, & Lang,
2017; Wouters et al., 2017; Zhao et al., 2018). Heat extremes can be detrimental to hu-
man health, including dehydration, discomfort or exhaustion (de’ Donato et al., 2015;
Keeler et al., 2019; Lafortezza, Carrus, Sanesi, & Davies, 2009; Nikolopoulou & Lykoudis,
2006; Ragettli, Vicedo-Cabrera, Schindler, & Roosli, 2017; Schuster, Honold, Lauf, &
Lakes, 2017; Vogel, Zscheischler, Wartenburger, Dee, & Seneviratne, 2019; Zhao et al.,
2018) and can increase heat-related mortality (de’ Donato et al., 2015; Lafortezza et al.,
2009; Muthers, Laschewski, & Matzaraki, 2017; Ragettli et al., 2017; Rüegg, 2019;
Schuster et al., 2017).
Public open spaces and squares in cities will become increasingly unusable in the future
without successful adaptation measures to the changing climatic conditions (Keeler et al.,
2019; Nikolopoulou & Lykoudis, 2006). Open spaces are important not only for their reg-
ulating effect on the urban climate, but especially for their social aspects and multifunc-
tionality, which is reflected in the diversity of users, styles of design and demands (Castan
Broto & Bulkeley, 2013; Lafortezza et al., 2009; Nikolopoulou & Lykoudis, 2006). Main-
taining the usability of public spaces is crucial for regulating the microclimate in urban
areas and for recreational and leisure activities of city dwellers and visitors (Gehl, 2011;
Nikolopoulou & Lykoudis, 2006; Nikolopoulou & Steemers, 2003). This adaptation is part
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of sustainable urban development to transform cities into attractive and climate-resistant
areas (European Environment Agency, 2016; Nikolopoulou & Lykoudis, 2006). We have
developed a novel integrated transdisciplinary set of methods to co-design climate
change adaptation measures for public places of cities. We found that a successful de-
velopment of measures could only occur with the involvement of the relevant stakehold-
ers: citizens, city planners, experts from research and local experts such as interest
groups, initiatives and city government (Hirsch Hadorn & Pohl, 2007; Lemos et al., 2018;
Pohl, 2008). Developing effective and feasible measures means incorporating physical
parameters, human perception and practical requirements. Measures that have a regu-
lating effect on the microclimate of public squares align with the citizens' suggestions for
changes that would increase their perceived well-being and contribute to sustainable ur-
ban development (Capstick, Whitmarsh, Poortinga, Pidgeon, & Upham, 2015; Lafortezza
et al., 2009; Rosenzweig, Solecki, Hammer, & Mehrotra, 2010). Furthermore, with appro-
priate measures, focused building and planning regulations can enhance the resilience of
cities and spaces to heat stress (Hatvani-Kovacs, Belusko, Skinner, Pockett, & Boland,
2016).
Climate data relevant to (local) decisions, interdisciplinary and transdisciplinary research
approaches, solution-oriented concepts, the identification of co-benefits and closer coop-
eration between the natural and social sciences are some of the requirements for closing
the gap between knowledge and action (Knutti, 2019). With our pilot study, we provide
these impulses and potential solutions for Heidelberg, Germany. Our dataset consists of
climatic and meteorological measurements, modelling of solar potential of current and
future situations at public squares, surveys and mental maps, from which we distinguish
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the effectiveness of the individual methods. Solar potential modelling yielded quantitative
insights on the substantial climatic benefits of specific measures. Standardised question-
naire surveys proved to be significantly less effective and meaningful than mental map
surveys, which allowed a deeper and more interrelated insight into the perception of citi-
zens (Capstick et al., 2015; Lafortezza et al., 2009).
Our methods set is transferable to comparable cities and can be the basis for a sustain-
able (ecological, social and economic) adaptation of public areas to current and future
climate change and the needs of city dwellers. The approach therefore serves as both a
methodological development and as a scientific contribution to addressing challenges in
the context of climate change (Carter et al., 2015). Cities are particularly relevant in this
context as they are at the forefront of climate adaptation with the potential to radiate sus-
tainable development trends to other regions (WBGU, 2016).
1.1 Cities facing climate change
Cities occupy a unique position in anthropogenic climate change (Intergovernmental
Panel on Climate Change, 2014; Wouters et al., 2017)in light of their substantial contri-
bution to CO2 emissions (Moran et al., 2018). Due to their structure and large number of
inhabitants, they are more vulnerable than other regions (McCarthy, Best, & Betts, 2010;
Rosenthal et al., 2007; Wouters et al., 2017). Global climate change and its effects are
expected to intensify in the coming decades, affecting numerous social and biophysical
systems such as population health, urban infrastructure, energy demand and water sup-
ply (Bambrick, Capon, Barnett, Beaty, & Burton, 2011; Bulkeley, 2010; Intergovernmental
Panel on Climate Change, 2014; McCarthy et al., 2010). At the same time, the strong
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global trend towards urbanisation poses increasing challenges to local administrations to
maintain and protect the viability of growing cities and the well-being of their inhabitants
(Rosenthal et al., 2007).
Many factors determine the microclimate in cities and their public spaces (Nikolopoulou
& Lykoudis, 2006). For the well-being of the urban population the bio climate is particularly
important (Mayer, Holst, Dostal, Imbery, & Schindler, 2008; Nastos & Matzarakis, 2008;
Nikolopoulou & Lykoudis, 2006). It describes the totality of atmospheric influences on the
human organism, in particular, thermal influencing factors that comprise short-wave and
long-wave radiation, wind, humidity and air temperature and that have a decisive influ-
ence on human comfort, thermal stress and health (Mayer et al., 2008; Nastos &
Matzarakis, 2008). Increased temperatures and direct sunlight due to a lack of shade are
increasingly causing heat stress conditions in European medium-sized cities such as Hei-
delberg, Germany as climate change progresses (de’ Donato et al., 2015; Muthers et al.,
2017). In general in the mid-latitudes, there is greater thermal stress in summer heat than
in winter cold. This goes hand in hand with the fact that people are generally more ex-
posed to thermal conditions in the summer months than in winter (Mayer et al., 2008;
Müller, Kuttler, & Barlag, 2013; Nastos & Matzarakis, 2008; Nikolopoulou & Lykoudis,
2006).
When adapting cities and public spaces to climate change, solutions must be found that
simultaneously meet the requirements of microclimate, ecology, design, sociology and
economy. These solutions can only be developed on the basis of inter- and transdiscipli-
nary cooperation and by incorporating local knowledge (Adler, Hirsch Hadorn, Breu,
Wiesmann, & Pohl, 2018; Lemos et al., 2018; Leutz, 2018). Through our research we
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have found that it is imperative to overcome disciplinary and institutional boundaries to
make comprehensive use of these approaches.
2. Transdisciplinary research concept
Adaptation to climate change can be viewed as an opportunity to improve the quality of
life in cities. We designed an integrated transdisciplinary set of methods to co-design
adaptation measures and facilitate their implementation (Fig. 1). This design attempts to
grasp the complexity of problems and accounts for the diversity of scientific and social
perspectives. In combining abstract science and case-specific knowledge, concepts that
orientate towards the common good and produces feasible solutions are developed. Co-
operation (co-design) in the development of possible solutions to major societal chal-
lenges at the interface between science and society must integrate the expertise, per-
spectives and knowledge of interest groups (civil society, politics, administration, busi-
ness) into the research process (Lang et al., 2012; Pohl, 2008).
We aim to find a technically effective and “publicly accepted” solution to keep urban
spaces viable under increasing heat stress. The local stakeholders are indispensable
partners and must be perceived as experts in order to successfully develop and imple-
ment adaptation measures (Adler et al., 2018; Hirsch Hadorn & Pohl, 2007; Pohl, 2011;
Pohl, Krütli, & Stauffacher, 2017).
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The challenges of transdisciplinary research include strengthening the role of non-aca-
demic participants and bridging the gap between knowledge and behaviour at the individ-
ual, social, political and economic levels (Pohl, 2008; Pohl et al., 2017; Strohschneider,
2014).
2.1 Study area
The city of Heidelberg, Germany, is located at 49°25´N / 8°43´E at 114 m a.s.l. at the exit
of the Neckar river valley into the Upper Rhine Graben (Fig. 2). The study area is part of
the Rhine-Neckar Metropolitan Area, a highly urbanised and industrialised area with a
polycentric structure of cities, and with associated climate-ecological stress factors. The
Upper Rhine Graben is traditionally considered a climatically favoured area that is turning
into a particularly vulnerable region in terms of heat stress under global warming. Due to
its latitude protected location within the Upper Rhine Graben, and low maximum altitude
(100-200 m a.s.l.), it regularly reaches higher temperatures than the surrounding areas
(City of Heidelberg, 2015; Fezer, 1977; Leutz, 2018).
The densely built-up areas of the city of Heidelberg are particularly prone to urban over-
heating and a worsening of the climatic situation caused by the current warming (Fezer,
1977; Leutz, 2018). For the comparative microclimatic analysis, the public places “Uni-
versity Square” in the historic (city) center and “Schwetzinger Terrace” in the Bahnstadt,
a modern city quarter, were selected (Fig. 2). Public spaces fulfil a multitude of functions,
including traffic junctions and areas for events, leisure and recreation. They must also be
drained, cleaned and artificially illuminated and should also meet design requirements
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wherever possible. Each function has its own legal and technical framework. The func-
tions can also compete with each other, with competition increasing the pressure of urban
growth and rising land prices. The quality of stay in the area, which is decisive for ac-
ceptance and determined by human bioclimatic factors, is one criterion among many.
The University Square in the centre of the old town has existed since the 18th Century.
The surrounding building complexes and the structure of the old town have also existed
in this constellation for many centuries. In comparison, the Schwetzinger Terrace is a very
young urban development project, which was completed in 2013 as part of the urban
expansion and development of the Bahnstadt district on former brownfield sites. This dis-
trict is characterised by a perforated perimeter block development of passive houses with
loosened inner courtyards and green spaces. The inner courtyards play an important role
in this high density area, opening up the possibility of generating an inner climate that can
create an improvement on a local scale. In addition, individual measures such as green
roofs or water elements (fountains, ponds etc.) have a positive effect on the microclimate
(Leutz, 2018).
3. Methods
Transdisciplinarity (td) denotes a concept of research, organisation and work rather than
as a method or theory in itself. The TdLab of the Department of Environmental Systems
Sciences at ETH Zurich defines transdisciplinary research as “an interdisciplinary ap-
proach to scientific inquiry that deals with complex, real-world problems and which places
an emphasis on joint problem framing between people inside and outside of academia
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with the aim of developing possible solutions. Building reflexive collaboration processes,
where researchers can react adaptively to changes in the real-world while working with
project partners, is central to the td approach.” (USYS TdLab, 2019).
The process of a transdisciplinary project comprises several phases: Definition and struc-
turing of the object of research, problem solving and knowledge generation, and value
creation, which can trigger transformations in both areas (science and life). The basis of
a transdisciplinary working method and the implementation of results on site is the ex-
change of knowledge and experience between the involved specialist departments and
responsible planning departments within a project (Adler et al., 2018; Cabrera & Cabrera,
2018; Hirsch Hadorn & Pohl, 2007; Lang et al., 2012; Leutz, 2018; Pohl, 2008).
Inter- and transdisciplinary projects typically involve several research groups from differ-
ent fields. Interdisciplinary work requires communication across disciplinary boundaries,
which is often impeded by the diversity of concepts and approaches of the individual sci-
entific cultures. In this project, expertise from physical geography, environmental physics,
human geography and digital geography is represented in the core team with the individ-
ual methods covered by the core team itself. The cooperation with the Environmental
Office of the City of Heidelberg as a stakeholder also builds on a long-standing coopera-
tion. This close integration of all work steps and project participants has made the pre-
sented transdisciplinary approach possible.
The research approach is described in detail below, focusing on the individual methods
drawn upon in this case study.
3.1 Climate science
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Weather stations were installed at two locations for long-term monitoring ambient air tem-
perature and humidity. Standardised weather shelters were used for the measurements
which are ventilated and provide shade for the measuring device inside. At both locations,
these were placed over grass or vegetation at a distance from surrounding buildings (> 5
m) sufficient to mitigate any effects from the buildings themselves. The measuring height
was ~2 m above ground. Mini thermal hygrographs with internal sensors for air tempera-
ture and relative humidity were used. The measuring range for the temperature sensor is
between -20 °C and 50 °C with an accuracy of ± 0.3 °C in the range from 0 °C to 40 °C
(display accuracy 1 digit) and a resolution of 0.1 °C. For relative humidity, the measuring
range is 10 % to 95 % with an accuracy of ± 2 % (display accuracy 1 digit) and a resolution
of 0.5 %. The ambient temperature is measured via a semiconductor or temperature-
dependent resistor with negative temperature coefficient (NTC). In addition, the following
parameters were recorded and evaluated on selected summer days as part of the mete-
orological analyses representative of the urban microclimate at different locations in the
city: air temperature and relative humidity, wind speed and surface temperatures (infrared
radiation) of various materials. Additional digital interface data loggers from Vernier Soft-
ware & Technology (LabQuest 2) and infrared thermometers were used. In order to en-
sure comparability, the measurements were carried out simultaneously under the same
conditions at the different locations. All climate data collected and used for comparison
were evaluated using common calculation and graphics software.
3.2 3D solar modelling
The open source software VOSTOK (Voxel Octree Solar Toolkit) (GIScience Heidelberg
University, 2016) was used for the detailed modelling of the solar irradiation distribution
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in 3D space and over time. VOSTOK simulates the incident solar radiation in 3D point
clouds (i.e. any XYZ coordinates) with a configurable voxel resolution. The tool is based
on the use of the open-source database SOLPOS.H (Solar Position and Intensity) of the
U.S. Department of Energy National Renewable Energy Laboratory to calculate the sun's
position for a specific location during the day and year. CityGML files (City Geography
Markup Language) of the public spaces in Heidelberg serve as 3D scene input for the 3D
solar potential calculation. The first step was to convert the 3D city model into regularly
rasterized 3D point clouds for use in VOSTOK. The shadow cast by the surrounding ob-
jects is modelled using 3D cubes (voxels), which define the environment of a 3D point.
For each location and time step, VOSTOK calculates the solar potential by accounting for
direct and diffuse components under clear-sky conditions, which depend on shadowing
effects of surrounding objects (GIScience Heidelberg University, 2016; Jochem, Höfle, &
Rutzinger, 2011; Liang, Gong, Li, & Ibrahim, 2014; Lin et al., 2017). The resulting file
contains the calculated solar potential for the given locations and time period in watt hours
per square meter (W h/m2), summed for each grid point. In a second step, artificial objects
such as trees can be placed in the 3D scene to quantify the reduction by installing different
design elements. In this way, future development of average shading situations of the
surfaces can be modelled and displayed. Increasing the proportion of green or simulating
a complete state of development of the vegetation reduces the solar irradiation and the
associated heating of the surfaces due to the shading caused. Based on the modelling,
different design and development scenarios of the selected areas can be presented and
measures can be derived (GIScience Heidelberg University, 2016; Jochem et al., 2011;
Liang et al., 2014; Lin et al., 2017).
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3.3 Questionnaire survey
A standardized questionnaire was developed to determine the perception of climate
change and the perception or presentation of public spaces in Heidelberg. This enabled
an identical interview situation to be created for all respondents. The operationalization
and conceptual foundation of the questionnaire is based on a topic-related mind map for
structuring the characteristics and variables in the run-up to the questionnaire conception.
In addition, a targeted literature search was conducted to identify relevant studies, theo-
ries and appropriate scales. For the survey, a pre-test of the questionnaire was conducted
via an online application and discussed in different settings. Based on the pre-test, the
questionnaire modules were modified and finalised, containing both open and closed
questions. The first part of the questionnaire deals with the specific perception of the place
where the survey was conducted. The following data are collected with reference to the
respective location: the way of use, the attractiveness, the positive and negative charac-
teristics, and the assessment and perception of weather conditions. In the second section,
questions are asked about the change in the general urban climate and the importance
of environmental protection. The third section centers on factors that can increase the
attractiveness and quality of stay at a square. The quality of stay refers to the amenities
that public places can provide for the well-being of their users during their stay there. The
questionnaire concludes with the collection of socio-demographic data on the interviewee
(Aschemann-Pilshofer, 2001; Atteslander, 2003; Krosnick, 2018; Nardi, 2018; Rattray &
Jones, 2007). The questionnaire used is included in the supplementary material (A) in a
translated version.
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The survey was conducted as a face-to-face survey with passers-by on several summer
days in 2017 as part of a student’s course. The days of the week, the time of day and the
locations of surveys were varied in order to generate a distribution that corresponds as
closely as possible to a random selection. The sample contains 91 cases, which are
evenly distributed in terms of age and gender. The data were evaluated using descriptive
statistics and SPSS statistics (Supplementary Material; Fig. B1 and B2).
3.4 Mental maps
Perception research deals with the subjective perception of individuals. While the real
environment refers to an assumed materialistic spatial construct that proceeds from an
existing system of order, the relative concept of space refers to a constructivist system of
order that includes elements of communication between individuals and their actions in
space. Each person therefore develops subjective images of reality. This includes not
only urban space, but also its structures and problems. The perceived image of the envi-
ronment is achieved not only through daily indirect or direct contact spaces of an individ-
ual, but above all through personal evaluations, needs and motivations and varies with
age, social status or group membership (Downs & Stea, 1977; Gould & White, 1987).
Individuals typically make subconscious decisions that affect the space or the orientation
in it. These decisions are influenced by how each individual perceives its environment or
how our environment is cognitively represented. In the following, cognitive representation
of space is understood to mean cognitive maps or mental maps. The cognitive maps in
the minds of individuals not only influence their behaviour, but are also subject to constant
change processes due to their own behaviour. Thus, subjective cognitive mapping also
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results in partially distorted, individual inner spatial images of sections and individual as-
pects of the environment (Downs & Stea, 1977; Gould & White, 1987).
Work by Lynch (1960) underpins the application of mental maps used here. A general
formulation of the objectives, planning of the procedure and clarification of the distribution
of tasks in the field should be the starting point of every survey. The interviews took place
in the old town and in the Bahnstadt in Heidelberg. In the first step, open questions were
chosen in order to find access to the method and to obtain an initial assessment of the
respondents' reaction. Paper (DIN A4, white), pens in different colours and clipboards
were provided for the sketches. In addition, the sketchers were allowed to record im-
portant elements for orientation. The survey was carried out in teamwork: while one per-
son spoke with the respondent, a second recorded the course of the conversation and
comments (participant observation). A total of 55 sketches of various public places in
Heidelberg were produced during the survey days. In addition to the resulting sketches,
the dialogues with the interviewees were included in the evaluation. Compared to the
standardized questionnaire survey, the method represents an increase in information and
meets the demand for more realistic models of human behaviour and action and allows
geographical questions to be dealt with at the micro level. The maps and dialogues were
viewed and categorized. The evaluation of the elements presented and the statements
made was carried out by coding and counting. Given that the place of residence can
influence the accuracy of the images, personal information such as the place of residence
and the purpose of the stay in the city were noted (Downs & Stea, 1977; Gould & White,
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1987; Lynch, 1960). In-person questioning allowed spontaneous answers and inter-
viewee thoughts to be recorded. This is especially important when using the mental maps
method. In this way we obtained direct, qualitative data for evaluation.
4. Results and discussion
An interdisciplinary data set was created and the effectiveness of individual methods was
assessed. Coordinated planning recommendations were developed on the basis of the
following findings.
4.1 Microclimatic comparative data of the extreme summer 2018
Mean annual air temperature (MAAT) values show that Heidelberg is among the warmest
areas of Germany, alternating with regions such as Lake Constance or Kai-serstuhl. Both
the number of summer days (maximum temperature ≥ 25 °C) and hot days (maximum
temperature ≥ 30 °C) are predicted to increase, representing an in-tensification of ex-
tremes. The urban heat island effect can be discerned in the central districts with a greater
frequency of hot days per year in the order of 24 days while in the surrounding forest
areas only three to four hot days are documented (average values related to the 30-year
reference period 1971-2000). In the long-term climate projection (RCP 8.5) for the period
2071-2100, the trend of global climate change becomes clearer (GEO-NET
Umweltconsulting GmbH & ÖKOPLANA, 2017; Intergovernmental Panel on Climate
Change, 2014). In the core city areas such as the Old Town, up to 41 hot days per year
are expected, while the average annual frequency of hot days in the nearby Odenwald
forest area will be around nine to twelve days. The agricultural areas to the west of the
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city also show up to 40 hot days per year during this period (GEO-NET Umweltconsulting
GmbH & ÖKOPLANA, 2017).
The summer of 2018 was marked by severe heat and drought in many parts of Europe,
North America and Asia (Rüegg, 2019; Vogel et al., 2019). Our two monitoring stations
installed at the two selected squares in Heidelberg recorded the highest average summer
temperature during June to August (JJA) of 22.7 °C in the Bahnstadt. The old town and a
comparison station of official state measuring stations in the city were each 0.2 K lower
(Fig. 3). In comparison, the two extreme summers 2015 and 2003 showed an average
temperature of 21.9 °C (2015) and 22.6 °C (2003) in Heidelberg between June and Au-
gust (Fig. 3) (Leutz, 2018).
The number of hot days for 2018 exceeded the average for 1981-2010 and reached the
range projected for the late 21st century (City of Heidelberg, 2015; GEO-NET
Umweltconsulting GmbH & ÖKOPLANA, 2017). In the historic city centre and the Bahn-
stadt, the number of hot days in 2018 was 44 and 46, respectively, higher than the 32 hot
days at the reference monitoring station outside the city centre. About half of the days in
the observation period June-August 2018 can therefore be classified as hot days. With
15 tropical nights, one more night with a minimum temperature of ≥ 20 °C was docu-
mented in the Bahnstadt than at the reference stations (Leutz, 2018).
The thermal isopleth diagram (Fig. 4) of the daily mean temperatures in 2018 compared
to the years since 2001, which were documented at an official state measuring station in
Heidelberg, shows the constantly high values of summer 2018. Daily mean temperatures
> 22 °C were attained almost continuously at all the stations as compared to previous
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years, where the daily mean temperatures in June, July and August frequently fell below
20 °C. In 2018, short temperature changes with daily averages below 20 °C only occur
on a few consecutive days, further illustrating the extremely consistent temperature con-
ditions with lower precipitation as compared to previous years.
Temperatures of different building materials and surfaces were measured during summer
of 2018. In the historic city centre a concrete bench under direct sunlight and the leaf
surface of a shrub in the sun were monitored during one day. The highest temperature
was reached by the concrete bench, starting at 34.5 °C and rising to a maximum of
47.2 °C in the afternoon. The average surface temperature was 42.6 °C. The tree leaf
surfaces heated up to 38.7 °C under direct sunlight (Leutz, 2018), representing a tem-
perature difference between a green element and a sealed design element of ~ -10 K,
despite prolonged dryness. The extreme heat and prolonged drought in summer 2018
(Åström, Bjelkmar, & Forsberg, 2019; Buras, Rammig, & Zang, 2019; Hartick, Furusho,
Goergen, & Kollet, 2019) also affected the vitality of the vegetation in Heidelberg, Ger-
many (Leutz, 2018). While vital green spaces on summer days 2017 showed values be-
tween 25-30 °C, the same but dried-up surfaces heated up to over 50 °C in 2018. Con-
sequently, the green areas show severe limitations in terms of their regulating effect under
prolonged drought and heat (Leutz, 2018).
4.2 Modelling of the solar potential
Given the high variability in urban solar radiation that arises from complex building struc-
tures and shadowing (Liang et al., 2014), a high resolution (sub-meter accuracy) solar
radiation simulation was undertaken. This analysis serves to model the effects of different
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22
degrees of greening and shading measures in public places on the average solar radia-
tion, taking into account the formation of shade (Liang et al., 2014). We computed the
solar potential in full 3D space for the period June to August 2018, i.e. how much solar
irradiation is theoretically received at a location over a given time period under clear-sky
conditions. Two scenarios were calculated, a current state based on a detailed 3D-build-
ing model without vegetation and a future scenario including fully developed vegetation
and artificial adaptation measures (Fig. 4). The scenarios’ differences in solar potential
provide a theoretical basis of the magnitude of reduction of direct solar irradiation as one
of the main drivers for local effects on thermal comfort.
The average solar potential at the Schwetzinger Terrace (white rectangle in Fig. 5),
summed up for the period June, July and August 2018, is 655 kWh/m², the adapted future
state is reduced to 274 kWh/m² (Fig. 5). At University Square, the average amounts to
450 kWh/m², the mean value of the solar potential for the adapted future state is 228
kWh/m² (Leutz, 2018).
As shown here, adjustments that enhance shading and minimize irradiation and heating
can reduce the solar potential of selected areas by more than 50 % (Schwetzinger Ter-
race) (Fig. 5). As described above, the current tree population of the squares was not
taken into account for the current state, due to a lack of data. This leads to an overesti-
mate of the effect of shading measures at University Square, where some trees are cur-
rently present. However, the large potential reduction calculated for the Schwetzinger
Terrace can be considered as a good approximation, because there the tree population
as of 2018 has made little progress in its development with very little shadow casting
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effect that can therefore be neglected for the current state (Fig. 5) (Leutz, 2018). Reduc-
tion of heat generation due to shaded areas increases the quality of life in public places
during the hot summer season (Lafortezza et al., 2009). Thus the spatial model aims to
emphasise the effect of solar radiation shading measures and quantifies the added value
of the adaptation measures (Leutz, 2018; Liang et al., 2014).
4.3 Approaches from social sciences
Beyond physical adaptation to the climatic conditions, social aspects play an important
role in the design of public spaces (Menny, Palgan, & McCormick, 2018). In urban areas,
in contrast to rural areas, every built element has an assigned function. Open spaces,
principally squares and parks, have a unique selling point in the built-up city in this respect
and do not follow this pattern. In public space one encounters a diversified simultaneity
of cultures, uses, generations, active and passive being. They are dynamic, both in the
course of the seasons and in the interaction with the users, and are mirrors of change in
the more static, sealed space of the city (Castan Broto & Bulkeley, 2013; Lafortezza et
al., 2009; Nikolopoulou & Lykoudis, 2006; Petrow, 2011).
The questionnaire survey showed a clear perception of more extreme temperatures and
weather events, as well as a positive opinion regarding the squares. In most cases, how-
ever, these squares are not used for recreational purposes. The majority of the respond-
ents cited their inadequate design as the reason for this. An increase in the proportion of
green areas and the integration of natural elements were represent the most important
factors increasing the quality of stay (Leutz, 2018) (Fig. B1 in Supp. Mat.)
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The mental maps highlight possible improvements for public spaces. In general, the par-
ticipants assessed the design and usability of public spaces more critically than respond-
ents to the questionnaire survey. Citizens identified the lack of shading, the low proportion
of green spaces and the social structure of the users of public spaces as the most nega-
tive aspects. In addition to these factors, the problematic traffic situation and heat stress
during the summer months also play a central role at the University Square (Fig. 6). The
traffic situation was described by one respondent as life threatening. In addition, partici-
pants responded negatively (“absurd” and “ugly”) to the architecture of the surrounding
buildings and their facades (Leutz, 2018). The homogeneity of the Schwetzinger Terrace,
both visually and socially, was also criticised. The "grey" architecture of the Schwetzinger
Terrace and the surrounding buildings were often criticized. Participants instead called
for "other materials, more green, [and] natural buildings" (Leutz, 2018).
In many cases specific and practical suggestions with regard to more diversity, greening
and shading in public places in Heidelberg were expressed (Leutz, 2018). Nature and
vegetation bear positively on well-being, reduce stress and facilitate recovery (Fig. 6).
Free spaces are therefore important for biodiversity, health promoting life conditions and
the competitiveness of cities and regions (Lafortezza et al., 2009; Nikolopoulou &
Lykoudis, 2006; Schuster et al., 2017).
4.4 Evaluation of the effectiveness of the methods used
The case study has shown differences in the effectiveness of the various methods de-
pending on the objective. In general, the first part of the study focussed on quantitative
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methods, which can only represent partial aspects of reality. The collection of meteoro-
logical and climatic data is necessary to assess the need for adaptation and to make
places comparable. However, its application was limited in time and space compared to
the variability in temperatures from climate change. Modelling the effects of shading
measures on the microclimate of public spaces can be used for various purposes and
serves to evaluate the reduction of the direct solar irradiance through adaptation
measures. The effectiveness of solar modelling relates mainly to the properties of the
underlying data, with the accuracy and validity of our results limited by the fact that the
official 3D city models do not contain any information about vegetation objects. In addi-
tion, the results only show the reduction of solar potential, but they cannot be interpreted
as a measure for quality of life. However, the data could be used as valuable input in
models for thermal comfort or biometeorology (Ketterer & Matzarakis, 2014; Matzarakis
& Endler, 2010).
Methods from the social sciences allow insights into how citizens and users of the squares
perceive the climate and how this affects their well-being. Mental maps provide even
deeper personal insights into the perception of places and the users' desire for a more
pleasant design and adaptation to the increasing generation of heat. The questionnaire
survey is a standardised test procedure whose results can be subject to many factors
(e.g. social desirability, interview situation, motivation of participants, number of cases
and objectivity). The method allows general conclusions to be drawn only in the case of
a high number of interviews. Furthermore, no causes of the current conditions and opin-
ions are included. Mental maps represent two-dimensional, subjective images. They are
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26
essential in order to obtain suggestions for concrete measures from the users of the lo-
cations, which are based on people's desire for quality of stay and attractive design. In
addition, the involvement of citizens can create bond and acceptance for places and ad-
aptation measures. However, these maps and images are simplified, showing only ex-
cerpts. Nevertheless, the multi-perspective approach is emphasized by balancing the lim-
itations of the individual methods. Finally, the active involvement of non-scientific actors
and perspectives can strengthen mutual trust in the cooperation and thus create ac-
ceptance and increase feasibility (Adler et al., 2018; European Environment Agency,
2016; Hirsch Hadorn & Pohl, 2007; Lemos et al., 2018; Strohschneider, 2014).
4.5 Co-design of coordinated planning recommendations
Whether a public area is actually accepted and how the quality of stay is evaluated can
be determined by observation and questioning. A particular focus is on the quality of open
spaces in the summer months, when the bioclimatic recreational function makes an im-
portant contribution to health protection. In the context of climate change adaptation, rec-
ords of the summer quality of stay in a public square form the basis of a combination of
objective, scientifically ascertained and subjective criteria. These social science methods,
providing valuable information and recommendations for sustainable open space plan-
ning and forms the basis for standardisation and evaluation of design elements adapted
to climate change. The data provide important arguments for a climate change-adapted
open space design and can help to ensure that this aspect is better accounted for in the
planning process. In this way, the needs of future users can be better assessed in ad-
vance, especially for new open spaces to be planned, and planning conflicts, which could
lead to a rejection of planning, may be avoided.
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Based on the individual results, a co-benefit of the wishes and ideas of the respondents
with the climate-regulating effect of design elements can be generated. Multifunctionality
and diversity in terms of design (colour, elements, greenery, etc.) and use (working, re-
laxing, viewing, playing, communicating, consuming, etc.) are the most important aspects.
Adjustments such as creating seating in shaded areas, unsealing, separating traffic and
recreation areas, and integrating temporary solutions all take equally account of both ob-
jectives. Shady trees in particular can have a positive effect on the temperature and mi-
croclimate of open spaces with sparse vegetation and reduce heat stress (Lindberg,
Thorsson, Rayner, & Lau, 2016). In addition, green areas generally have a positive effect
on the mental health of city dwellers (Tost et al., 2019). Furthermore, identity-creating
measures play a role (Lafortezza et al., 2009; Leutz, 2018; Nikolopoulou & Lykoudis,
2006). Based on the data collected, no conflicts of use are to be expected during imple-
mentation.
Our approach could be further enhanced, e.g. by various methods of observation and
quantification that can provide further insight into the interactions between public life and
space. These include techniques of counting, recording, locating, photographing, tracing,
geotracking, routing, mapping and some other methods like documentation of dwell times
under the key questions Who? When? Where? and What?. Detailed and comparative
studies on urban life can be the starting point for improving urban quality of life. Moreover,
in some cases, adaptation to future developments can be based on traditional knowledge.
The positioning, choice of materials or orientation of buildings or urban structures to nat-
ural occurrences are well known, but are gaining importance again in the trend towards
climate change and adaptation.
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5. Conclusion
The pilot study demonstrates how transdisciplinary research can be structured and de-
scribes a holistic approach that is supported by local stakeholders. The diversity of per-
spectives creates an empirical basis for what is desirable in sustainable urban planning
and the adaptation of public spaces to climate change. Furthermore, the measurement of
climatic conditions provides arguments for additional measuring stations to capture com-
plementary parameters and to achieve a higher temporal and spatial resolution. The 3D
solar analysis is also transferable to other locations and makes adaptation measures
quantifiable.
The involvement of the public (citizen science) formed an essential component of this
work. The transdisciplinary approach also makes the study more significant and mean-
ingful with regard to multidimensional adaptation strategies of urban open spaces to cli-
mate change. The involvement of stakeholders and citizens was viewed positively by all
project partners in terms of the results and the generated added value. The data on the
perception of climate change and the ideas about the design of public spaces reflect
grievances and perceived problems.
The combination of physical measurements, solar modelling, and surveys of public per-
ceptions represent an integrated set of methods whose results give rise to adaptations
and improvements. Taking into account the perspective of the relevant stakeholders and
users in the sectoral planning, the overall data set serves to develop practical solutions
for improved designs of open spaces in the city of Heidelberg under the aspect of climate
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change adaptation and methodological enhancements in basic research (Fig. 1, 5) (Leutz,
2018).
Author contributions
K.F. conducted the measurements and surveys and analysed the data. The concept of
the study was mainly co-designed by K.F., N.A. and R.W. The focus of B.H.'s contribution
was to provide enriching impulses through solar potential analysis. W.A. and N.A. pro-
vided guidance and support for the different parts of the project. A.S. contributed expertise
in the area of physical geography and provided the necessary technical measuring equip-
ment. R.W. supported the project as a stakeholder with local practical knowledge. All
authors worked on the manuscript.
Competing interests
The authors declare no competing interests.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
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Additional information
Correspondence should be addressed to K.F.
Acknowledgements
We thank Vivien Zahs for support in the application of solar modelling. Sincere thanks are
given to the City of Heidelberg, especially Hubert Zimmerer, Sabine Lachenicht, Chris-
toph Czolbe, and colleagues for supporting different parts of the project. We thank Timo
Mifka from Heidelberg University as well as Martin Geißler and Tim Szpalecki from the
City of Heidelberg for providing access to the measuring areas, Helmut Scheu-Hachtel
and Zarko Peranic from the LUBW for support in data provisioning. Further we would like
to thank Viktoria Reith and the staff of the Department of Geography of Heidelberg Uni-
versity of Education for their support in the measurements, equipment provision and sur-
veys. Many thanks are given to Jack G. Williams for advice on the initial draft and proof-
reading of the manuscript. We gratefully acknowledge fruitful discussions and inspiration
provided by Michael Stauffacher (USYS TdLab ETH Zurich). This work was partly funded
by the Federal Ministry of Education and Research (BMBF) of Germany in the framework
of the project ER3DS (FKZ: 01DO19001). Funding for the project (position of K.F.) was
provided by the Heidelberg School of Education (Heidelberg University and University of
Education Heidelberg, Germany).
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Fig. 1 │ Transdisciplinary concept of methods to co-design adaptation measures.
The transdisciplinary aspect of this case study is achieved by drawing on the expertise of
public institutions such as the Office of Environmental Protection, Trade Supervision and
Energy Heidelberg, Germany (Heidelberg Environmental Office) and the Heidelberg City
Planning Office, by involving citizens through surveys, and by communicating scientific
results to the public.
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Fig. 2 │ Location of the city of Heidelberg, Germany. In addition, the positions and
aerial photographs of the two public squares of the city considered in the pilot study are
displayed (© OpenStreetMap contributors 2019).
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Fig. 3 │ Summer air temperatures in Heidelberg since 2001. Mean values of the air
temperature for the period June to August (squares), with the standard deviation of the
respective distribution of daily mean temperatures (shading). For 2001 to 2018, values
from one official weather station were averaged, for 2018 data of the stations installed for
this study are shown in addition (lighter red points).
Fig. 4 │ Thermal isopleth diagram of the daily mean temperatures at three measur-
ing stations in Heidelberg, Germany. The upper three lines represent the 3 curves for
2018 at the three measuring stations Bahnstadt (b), old town (c) and the official state (a)
measuring station of Heidelberg. In addition, the measurements of the previous years at
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the official state measuring station are shown for comparison (a). The measurements
were taken in the summer months June, July and August. Compared to the previous
years, the year 2018 appears extremely warm and hot.
Fig. 5 │ Actual and possible future situation and solar potential for Schwetzinger
Terrace. Modelling of solar potential at the ground surface and heating of surfaces was
performed using the tool VOSTOK (GIScience Heidelberg University, 2016). Values are
solar potential summed up over the entire summer (June through August 2018) for
Schwetzinger Terrace in the sustainable district Bahnstadt, Heidelberg. A Modelled solar
potential for the current state, B Aerial photograph of the Schwetzinger Terrace, summer
2018, showing the barely existing shade, C Visualization of the adapted future state with
fully developed vegetation and artificial shading measures, D Modelled solar potential for
the assumed adapted future state.
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Fig. 6 │ Two different results from the mental maps survey in Heidelberg, Germany
in summer 2018. On the left a representation of the university square in the old town of
Heidelberg. The square is presented rather negatively, with a danger symbol concerning
the traffic. Green elements like trees are not shown. Furthermore, the use of the square
for demonstrations is discussed. On the right side a positive example of a public square
in Heidelberg is shown. The market place in the district of Neuenheim is used as a meet-
ing place and for staying. Cafés and shops are located there, which are positively per-
ceived. In addition, the large trees on the square are characteristic positive features.
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