Social vulnerability assessment of the Cologne urban area (Germany) to heat waves: Links to ecosystem services

Abstract

More than three-quarters of the European population live in urban areas and this proportion is increasing, leading, in some cases, to increased vulnerability of cities to environmental hazards. The health impacts of heat waves are aggravated in cities due to the high density of buildings, the fragmentation of green areas and the higher concentrations of air pollutants. Ecosystems can provide important benefits that mitigate the impacts of heat waves but at the same time can themselves be affected by the hazard, thus limiting their services. The objective of this study was to assess the vulnerability of the Cologne urban population to heat waves, taking into consideration a range of social and ecological variables. Based on the MOVE framework, indicators were developed and GIS applications were used to spatially assess the relative vulnerability of the 85 districts of Cologne to heat waves. The insights gained were integrated and corroborated with the outcomes of stakeholders' interviews. As environmental factors play a major role in this assessment, it is suggested that ecosystem management in Cologne and its surroundings be improved. In addition, though vulnerability is higher in central districts, attention needs to be paid to the periphery where the most susceptible groups reside.

Full text

Social vulnerability assessment of the Cologne urban area
(Germany) to heat waves: links to ecosystem services
Yaella Depietri
a,b,
n
, Torsten Welle
a
, Fabrice G. Renaud
a
a
United Nations University, Institute for Environment and Human Security (UNU-EHS), UN Campus, Hermann-Ehlers-Str. 10,
53113 Bonn, Germany
b
Institut de Ciència i Tecnologia Ambientals (ICTA), Universitat Autònoma de Barcelona (UAB), ETSE, QC/3103, 08193 Bellatera,
Barcelona, Spain
article info
Article history:
Received 7 May 2013
Received in revised form
26 September 2013
Accepted 2 October 2013
Available online 9 October 2013
Keywords:
Vulnerability assessment
Heat waves
Ecosystem services
Urban areas
GIS
abstract
More than three-quarters of the European population live in urban areas and this
proportion is increasing, leading, in some cases, to increased vulnerability of cities to
environmental hazards. The health impacts of heat waves are aggravated in cities due to
the high density of buildings, the fragmentation of green areas and the higher concentra-
tions of air pollutants. Ecosystems can provide important benefits that mitigate the
impacts of heat waves but at the same time can themselves be affected by the hazard, thus
limiting their services. The objective of this study was to assess the vulnerability of the
Cologne urban population to heat waves, taking into consideration a range of social and
ecological variables. Based on the MOVE framework, indicators were developed and GIS
applications were used to spatially assess the relative vulnerability of the 85 districts of
Cologne to heat waves. The insights gained were integrated and corroborated with the
outcomes of stakeholders' interviews. As environmental factors play a major role in this
assessment, it is suggested that ecosystem management in Cologne and its surroundings
be improved. In addition, though vulnerability is higher in central districts, attention
needs to be paid to the periphery where the most susceptible groups reside.
&2013 Elsevier Ltd. All rights reserved.
1. Introduction
In Europe, 75% of the population live in urban areas and
the proportion is increasing [1]. However, absolute popu-
lation growth is not the major contributor to the increase
of the disaster potential in cities [2]. Rather, the shift in the
location of industries and homes, driven by economic
factors and lifestyles plays a significant role in the conver-
sion of rural lands near European cities [3] which in turn
alters ecosystems, affecting their services through e.g. the
compaction of soil and the impairment of its functions.
These changes increase the loss of water permeability (soil
sealing), compromise the availability of water supply in
terms of groundwater recharge and fragment green cover
which is accompanied by an increase in resources and
energy consumption [1]. In addition, under scenarios of
climate change, geographical areas that were less affected
by heat spells are likely to become at higher risk of
extreme hydro-meteorological events [4]. The vulnerabil-
ity of urban populations to hazards is in this way further
exacerbated.
The impacts of heat waves on the ageing segment of
the population that lives in the highly modified ecosystem
of urban areas are of increasing concern for European
cities. In fact, urbanization affects climate locally, as cities
tend to be warmer than their surroundings, producing the
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/ijdrr
International Journal of Disaster Risk Reduction
2212-4209/$- see front matter &2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.ijdrr.2013.10.001
n
Corresponding author at: United Nations University, Institute for
Environment and Human Security (UNU-EHS), UN Campus, Hermann-
Ehlers-Str. 10, 53113 Bonn, Germany. Tel.: þ39 340 49 59 230.
E-mail addresses: yaella.depietri@e-campus.uab.cat (Y. Depietri),
welle@ehs.unu.edu (T. Welle),renaud@ehs.unu.edu (F.G. Renaud).
International Journal of Disaster Risk Reduction 6 (2013) 98–117

so-called Urban Heat Island (UHI) effect. This manifests
itself especially at night and principally as a consequence
of the properties and density of built infrastructures, low
albedo, low green cover and low moisture availability in
cities. Air quality is also degraded in cities, mainly due to
the higher concentration of road traffic and industrial
activities which require fuel combustion leading to the
emission of air pollutants dangerous for human health. As
a result, during heat waves, the rates of heat-related
morbidity and mortality are often higher in cities than in
their surroundings [5]. This is especially true for densely
populated areas, where heat-retaining buildings, few and
fragmented green areas with a lower cooling capacity, and
higher levels of air pollution due to higher road traffic,
amplify the impacts of the hazard [6–11]. It is therefore
suggested that excess deaths occurring in urban areas
during periods of extreme heat can be significantly
reduced through appropriate urban land cover planning
[12]. Land use patterns are in fact related to the capacity of
urban ecosystems to provide regulating services which can
be assessed through landscape functions (i.e. the capacities
of a landscape to provide goods and services to the society)
[13,14]. It should however be emphasized that social and
institutional considerations (e.g. early warning systems,
the adoption of appropriate behaviours, facilitating tighter
social networks) remain paramount while dealing with
this type of vulnerability.
The objective of this study is to assess the social
vulnerability of the 85 districts of the Cologne urban area
as part of the Methods for the improvement of Vulner-
ability Assessment in Europe (MOVE) project funded by
the European Commission. In the following introductory
sections, vulnerability assessment, social vulnerability to
heat waves, the role of ecosystem services in mitigating
the impacts of heat waves, as well as the assessment of
ecosystem services as landscape functions are briefly
reviewed and defined. Section 2 presents the methodology
developed, Section 3 presents the results, which are then
discussed in Section 4.
1.1. Vulnerability assessment
An extensive review of the vulnerability terminology
was carried out by Thywissen [15] and includes a compre-
hensive list of definitions that primarily differ according to
the school of thought in which these are developed and in
use. According to these different schools, which can mainly
be clustered into “political economy”,“social-ecology”,
“holistic vulnerability and disaster reduction assessment”
and “climate change science”[16], various approaches and
frameworks have been developed to assess vulnerability.
The MOVE project was intended to overcome these differ-
ences by producing a generic Framework (Fig. 1) that
would bring together and be applicable both in the
domain of disaster risk reduction and climate change
adaptation. The MOVE framework is intended to be a
guiding tool more than a close representation of reality.
By assembling the main elements of the vulnerable social–
ecological system (i.e. a coupled system of biophysical and
social components that interact and evolve according to
complex dynamics) at multiple scales, and representing
the risk factors, the framework closes the loop through the
adaptation section. Adaptation is actually considered as a
central element in shaping vulnerability in the long term
Fig. 1. The MOVE Generic Framework.
Y. Depietri et al. / International Journal of Disaster Risk Reduction 6 (2013) 98–117 99

within the risk governance frame. The framework is also
intended to facilitate the integration of different indices
and methodologies to contribute to a more integrated
assessment. To respond to the need of holistic approaches,
using quantitative, qualitative and participatory methods
at different scales, it is in effect a challenge to combine
different methodologies [17].
The present study also applies the multidimensional
concepts of vulnerability assessment developed within the
MOVE project. Vulnerability is defined as “the propensity
of exposed elements such as physical or capital assets,
human beings and their livelihoods to experience harm
and suffer damage and loss when impacted by single or
compound hazard events”[16]. In the Framework, vulner-
ability is considered to be the result of the contributions
and interactions of three components: exposure, suscept-
ibility (or fragility) and lack of resilience. Exposure defines
“the extent to which a unit of assessment falls within the
geographical range of a hazard event”; susceptibility
“describes the predisposition of elements at risk to suffer
harm”; while lack of resilience is defined by the “limitation
in access to and mobilization of the resources of a com-
munity or a social–ecological system in responding to an
identified hazard”, compromising the capacity to antici-
pate, to cope and to recover of the system [16]. Six
dimensions of vulnerability are considered within MOVE
to characterize the susceptibility and the lack of resilience
of the system: physical, ecological, social, economic, cul-
tural and institutional. While socio-economic and institu-
tional aspects of vulnerability are well explored in the
literature, other dimensions of vulnerability, such as the
ecological or the cultural ones, are currently not well
integrated in vulnerability assessments.
The present study, through the application of the MOVE
generic framework and definitions, aims at illustrating the
operationalization of these theoretical concepts for prac-
tical use in the decision-making context. The assessment
presented in this work focuses on the core part of the
framework, which considers the main interactions
between the three components of vulnerability, and this
from the social as well as the ecological dimensions.
1.2. Heat waves and social vulnerability
A heat wave is considered to be a continued and
intensive period of heat stress, which may be accompanied
by high humidity, and that directly affects human health
[18]. Various morbidity and mortality impacts on the
human population have been assessed for past events
[19–24]. It is estimated that during the heat wave which
affected Europe in 2003, more than 70,000 people died
[25]. Similarly, the 2010 heat wave that affected Russia
claimed the lives of ca. 55,000 people [26]. Also, extreme
heat events are expected to increase in number, length and
intensity in the future [4].
Koppe et al. [18] note that “skin eruptions, heat fatigue,
heat cramps, heat syncope, heat exhaustion and heat
stroke are traditionally considered as heat related ill-
nesses”. Those most likely to die or be affected by heat
are the elderly people, the chronically ill and the isolated
[27]. In Chicago, people older than 65 accounted for 72% of
the heat-related deaths due to the mid-July 1995 heat
wave [28] and in the 1999 Chicago heat wave, the
strongest risk factor for heat-related death was living
in isolation [29]. In the elderly people, physiological
responses to changes in the environment are less acute
and medications may interact with thermoregulation and
risk perception, further increasing their vulnerability to
heat [30]. Fouillet et al. [21] found excess mortality during
the 2003 heat wave in France to be higher for people living
at home and in retirement institutions than for those in
hospitals, and that the mortality of widowed, single and
divorced individuals was greater than that of married
people. In a review article, Bouchama et al. [31] found
that being confined to bed, not leaving home on a daily
basis, and being unable to care for oneself were associated
with the highest risk of death during heat waves. Addi-
tionally, pre-existing psychiatric illness tripled the risk of
death, followed by cardiovascular and pulmonary illness,
while using home air-conditioning, visiting cool environ-
ments, and increasing social contact were strongly asso-
ciated with reduced mortality [31]. A further increase in
losses in central European regions was due to a higher
vulnerability of the population as this was located where
hot spells are relatively infrequent [32]. The presence of
strong social networks may increase the resilience of the
system when the hazard strikes. Although, direct positive
relationship between tight social networks and resilience
for elderly people exposed to heat waves in two UK cities
was not found [33], Yardley et al. [5], reviewing both
environmental and socio-economic factors that may deter-
mine the health and mortality impacts of heat waves,
found in several studies that lack of social contact is a
major risk factor, as people might only become aware of
their condition and seek help when it is already too late
[5,34]. Ethnicity is also considered in studies, especially in
the US where non-white
1
people had a higher death rate
(sometime double) than white people. This was linked
to poverty rates, showing that socio-economic factors
(i.e. income, education), more than ethnicity, play an
important role in heat related mortality [5]. The research-
ers concluded that a socio-ecological approach, able to
take into account the multiple factors that play a role in
heat mortality risk and the different local circumstances, is
required.
1.3. Heat waves and ecosystem services
Mortality risk increases by between 0.2% and 5.5% for
every 1 1C rise in temperature above a location-specific
threshold [35], though it is unlikely to be a truly linear
trend. Therefore, zones of the city where the UHI effect is
stronger are those where the risk of illness or death during
a heat wave is generally higher.
Green cover and trees in streets make important con-
tributions to the improvement of urban climate, especially
during summer months and periods of heat stress [36–42].
Various studies have analyzed how vegetation influences
1
“White”is one of the race categories used in the US census, which
includes both racial and national-origin groups.
Y. Depietri et al. / International Journal of Disaster Risk Reduction 6 (2013) 98–117100

the thermal microclimate of urban areas and mitigates the
UHI effect. Depietri et al. [43] reviewed some of these
studies, reporting data taken from measurements in dif-
ferent cities and showing how the cooling potential of
green areas, while being considerable, varies from one
urban area to another depending on the local conditions.
On average, an urban green area is 1–21C cooler if
compared with a non-vegetated zone [40,44] and a
0.8 1C reduction in ambient air temperature should follow
a 10% increase in the ratio green/built up area in a city [39].
Studies also stress the importance of placing vegetation
(e.g. street trees) within the urban fabric and consider
visits to green areas as a good coping strategy in case of
heat stress [43]. Vegetation structure for improving the
microclimate is a significant factor: maximum benefits
seem to be obtained by planting two or three rows of
trees with a relatively high density and adequate ventila-
tion [45,46].
Higher concentrations of air pollutants during heat
waves can lead to an increase of excess death [7,8,47,48].
There is increasing evidence for a synergic effect on
mortality between high temperatures and ozone (O
3
)
concentrations [49]. Similar, but less pronounced differ-
ences, have been found for other pollutants such as
particulate matter less than 10 mm (PM
10
), black smoke,
nitrogen dioxide (NO
2
), and sulphur dioxide (SO
2
)[49].
The mortality increase due to the combined effect of heat
and air pollution can be reduced by decreasing exposure to
O
3
and PM
10
on hot days [35]. Interestingly, ground level
O
3
and PM
10
are the pollutants whose concentrations
declined the least in Europe between 1990 and 2009 and
that directly affect human health [50]. European ecosys-
tems are also most affected by substances that cause
acidification, eutrophication and vegetation damage (i.e.
resulting from O
3
exposure) [50].
As reviewed in Depietri et al. [43], urban trees can
improve air quality in different ways such as by intercept-
ing atmospheric particles and absorbing various gaseous
pollutants. Additional information is provided by Nowak
et al. [51] who estimate, for instance, that the total annual
air pollution removal by US urban trees amounts to
711,000 metric tons
2
($3.8 billion value). To contribute to
better air quality in cities it is important to plant appro-
priately selected tree species which are more tolerant to
air pollutants and more effective in their removal.
On the other hand, heat waves may have a series of
impacts on cities' ecosystems and services that would
amplify the vulnerability of the urban population. For
instance, sources of water supply (i.e. surface and ground-
water) for agriculture, drinking water, water treatment
plants and cooling of hydropower plants may experience
shortages or may fail due to a sudden increase in demand
[52]. Food production and distribution, as well as forestry,
may be affected when peri-urban agricultural land sees its
productivity diminished. This in turn can have implica-
tions for employment in the rural sector [52]. More
broadly, the well-being of the urban population may
diminish when recreational services are affected by heat
stress: vegetation in parks may be damaged by heat and
the higher concentration of air pollutants; the use of small
watercourses, ponds and lakes may be interrupted when
high levels of eutrophication leads to algal blooms and
hypoxia. At the wildland-urban interface, the risk of forest
fires may also increase. Some of these impacts have been
described in detail in Depietri et al. [43]. However, most of
the information available from the literature concerns the
role of ecosystems in mitigating the impacts of the hazard,
and much less refers to indirect impacts, namely those
that, affecting the urban and peri-urban ecosystems, could
increase the magnitude of the impact on the human
population.
1.4. Measuring landscape functions
Flows of ecosystem services remain poorly character-
ized at local-to-regional scales mainly because there is no
direct relation between land cover and functionality of
ecosystems [53,54]. Verburg et al. [55] state that, “a proper
representation of land function will always require addi-
tional data beyond land cover observations”: information
on the spatial distribution of landscape functions generally
needs additional intensive field observations or carto-
graphic work [54]. Also, according to Burkhard et al. [56],
using quantitative and qualitative assessment data in
combination with Land Use (LU) and Land Cover (LC)
information originating from remote sensing and GIS, the
state of ecosystem services can be evaluated.
Some studies have assessed the state of ecosystem
services through land use and land cover features at
different scales, from the global to the national/regional
scales and to the local e.g. [57–62]. Most of them make use
of additional quantitative biophysical information to build
proxies for landscape functions for a wide range of services
at different scales. The advantage of this approach is that,
besides providing an estimate of the ecosystem services
considered, it conveys information which has a spatial
component and facilitates the presentation of the results
to policy and decision makers. In this study, we contribute
to this field focusing on the local level to assess the
potential use of land cover and land use as a proxy for
regulating services.
2. Methods
2.1. Case study: the Cologne urban area
The case study is the Cologne urban area (50157′N and
6158′E), situated within a floodplain along the river Rhine,
in central-western Germany and in the Federal State of
North Rhine Westphalia (NRW). Cologne is the fourth
largest city in Germany and the largest one both in NRW
and within the Rhine-Ruhr Metropolitan Area, with
around 1 million inhabitants. It is also considered to be
the warmest city in the country with a sub-Atlantic
climate with traits of mild oceanic to mild continental
climate due to the surrounding relief and its position in
the landscape [63]. On a daily basis, the winds that enter
2
The pollutants considered in the study are carbon monoxide (CO),
NO
2
,O
3
,PM
10
and SO
2
.
Y. Depietri et al. / International Journal of Disaster Risk Reduction 6 (2013) 98–117 101

the city at night prior to the onset of the Rhine Valley
Wind (from 01:00 CET), namely the country breeze and
the down-slope wind, are insufficient to adequately venti-
late the city centre [64]. Heat wave events have affected
the region in the past and elevated temperatures have
characterized the recent summers. The highest tempera-
ture ever reached in the Cologne-Bonn Metropolitan Area
since 1957 was recorded on the 12th August 2003 and was
about 38.8 1C.
3
The health department of Cologne reported
an increase of 16.5% of deaths in the month of August 2003
compared to the mean values in August for the previous
three years (i.e. 775 deaths per month) [65].
From a geographical point of view, the urban area
includes industrial sites, inner harbours, historic city
centres and residential areas with different vegetation
portions and fallow land [66]. In the environs of Cologne
and along the river Rhine, the land use pattern is com-
posed mainly of agricultural land with maize, sugar beet
and forests as well as several artificial lakes [66]. Hills to
the northeast and southwest comprise mixed cultivations
of both meadows and maize fields. Historically, the spatial
structure of Cologne was significantly influenced by per-
pendicular alignments dating back to the Roman occupa-
tion of the area [67]. It developed radially from the centre
around rings arranged in a semicircle along the Roman city
wall. At the beginning of the 20th century, while the city
was expanding outwards, certain spaces were protected to
ensure environmental quality [67]. In fact, during the term
of office of Konrad Adenauer (Cologne City Mayor between
1917 and 1933), the city's former fortifications were con-
verted into a greenbelt. The initial plan of the outer
greenbelt by the German urbanist Fritz Schumacher
(who collaborated with Konrad Adenauer) was put in
place in the early 1920s and was further developed
throughout the century. The external green ring was
planned to act as a buffer between the urbanized city
and its peripheral industrial areas [68]. At present, the
main green areas of the city are composed of an outer ring
and inner ring connected radially by green axes. A plan to
develop open green spaces along the River Rhine was
initiated in 1978. Overall, Cologne has abundant natural
land: some 230 km
2
covering 57% of the urban area [69].
From a socio-economic point of view, the vast majority
of the lowest wealth neighbourhoods are clustered in two
parts of the city: the larger one is located east of the river
Rhine and the second one is located in the north-western
part of the city [70]. Compact suburbs with subsidized
housing settlements and public transport systems for low-
income groups were built during the baby boom in the
1960s after 60% of the city was destroyed during the
Second World War [71]. The city districts with the highest
social status are more dispersed, though most of them are
located at the border of the city [70]. In the surrounding
“green peripheral areas”, wealthier families developed
larger, individual plots during the 1980s [71].
2.2. Data used
For the quantitative part of the study, four main
datasets were used: socio-economic data regarding the
population of the city of Cologne (i.e. statistical values),
remote sensing data in the form of thermal infrared
imagery, land use (LU) and land cover (LC) classification
maps, and a map of the forest cover. All of the socio-
economic data for each of the city districts were obtained
from the Statistical Department of Cologne for the year
2001. Data about elderly people living alone per district
were available only for the year 2005. The Environmental
Department of Cologne provided the thermal imagery. The
dataset was captured using a thermal infrared camera
airborne at 3000 m altitude which delivered an image
with a resolution of 7.5 m. The thermal scans were con-
ducted on 30th June 1993 at 9 p.m. (reflecting the heat
accumulated during the day; Fig. 2) and on 1st July 1995 at
4 a.m. (reflecting night temperature; Fig. 3). The LU and LC
data were obtained from the Centre for Remote Sensing of
Land Surfaces (ZFL) of the University of Bonn (Germany).
Based on the Landsat TM satellite image of 2001, the LU
and LC class informationwere categorized with a resolution
of 30 m. The classes defined include both sealed (i.e. low,
middle, high and other areas) and unsealed (i.e. grassland,
coniferous forest, deciduous forest, mixed forest, agricul-
tural land and water bodies) surfaces (Fig. 4). The shape file
of the Cologne urban forest measured in 2003 and provided
by the Office of Landscaping and Green Spaces of Cologne
was used. For the administrative subdivision of Cologne the
85 districts were used, see Fig. 5.
We considered as negligible the chronological mis-
match between the datasets used. Based on the LU and
LC maps for the years 1984, 2001 and 2005, also provided
by ZFL, most of the major land use changes in Cologne that
took place between 1993 and 2005 involved the marked
decrease in grassland and a symmetrical increase of
urbanized areas characterized by low fractions of imper-
vious surfaces (o40%) (ZFL, Univ. Bonn). The maximal Leaf
Area Indexes (LAI) (providing information on evapotran-
spiration capacity) for the two CORINE classes, discontin-
uous urban fabric and natural grasslands, do not differ
significantly [72]. In addition, city areas occupied by high
impervious surfaces and forests, which most influence the
intensity of the UHI, have changed little during the same
period. Therefore, the thermal images of 1993 and 1995
can be considered as representative of the environmental
conditions in which the people of Cologne lived between
2001 and 2005.
Stakeholders' interviews were carried out between
December 2011 and January 2012 to investigate the
perception of relevant local authorities regarding the
capacity of a range of ecosystems to mitigate the impacts
of heat waves, and to gather their opinion on past or
potential indirect impacts of the hazard on the urban and
peri-urban ecosystem. Interviewees were identified at the
city level amongst those institutions in charge or contri-
buting to the planning and management of urban ecosys-
tems and those responsible or active in the sector of
human health at the city level. In particular, a list
of 25 institutions and organization working in a field
3
〈http://www.dwd.de/bvbw/generator/DWDWWW/Content/Oeffen
tlichkeit/KU/KUPK/Wetterrekorde/absolute__hoechsttemperaturen__brd
__en,templateId=raw,property=publicationFile.pdf/absolute_hoechsttem
peraturen_brd_en.pdf〉.
Y. Depietri et al. / International Journal of Disaster Risk Reduction 6 (2013) 98–117102

relevant to our focus at the city level was drawn up. The
list was discussed internally, benefiting from our
previous experience of working on disaster risk in Cologne.
We then contacted each one of the institutions via
mail and/or phone to find out about their interest in
participating in an interview. Some of them refused as
they did not consider their daily work to be strictly
relevant for our study, but, nevertheless, some of these
pointed out other local authorities that they thought
would be more appropriate to be involved in our
analysis. We finally ended up with a list of seven institu-
tions willing to be interviewed which covered exactly
the set of dimensions we considered in our study (see
Table 1).
The interviews were carried out based on a questionnaire
composed of open questions. Following the above-mentioned
aim of the qualitative assessment, the questionnaires were
divided into two sections: the first set of questions focused on
the perceptions of the interviewees with respect to the role of
the Cologne urban and peri-urban green and blue areas in
regulating city climate and air quality; while the second
focused on past and potential impacts of heat waves on the
local ecosystem and its services (e.g., water supply, recrea-
tional activities and peri-urban agriculture and gardening)
and thus, indirectly on the urban population. Each interview
lasted from 40 to 60 min, was recorded and then transcribed.
The transcribed text was analyzed making use of the Atlas.ti
(ATLAS.ti Scientific Software Development GmbH, Berlin)
software which supports qualitative data analysis. By allow-
ing coding of the text and insertion of quotations, the
software facilitates the cross comparison of the information
contained in the transcripts and the generation of knowl-
edge, while avoiding reducing the complexity contained in
the data.
2.3. Indicators selection and development
Based on the MOVE framework and the definitions
presented in Section 1.1, an extensive literature review,
stakeholders' involvement and data availability, relevant
indicators were identified to characterize the three com-
ponents of vulnerability (i.e. exposure, susceptibility and
lack of resilience). The indicators used and the composite
indicators developed were then assessed and spatially
represented through the application of the GIS ArcMap
10 software (ESRI, Redlands, CA).
The exposure component (E) was defined in time and
space as the social and material context, represented by
persons, resources, infrastructure, goods, services and eco-
systems that may be affected by the hazard. In this study, it
was measured as the number of people per city district
differently exposed to heat waves due to the UHI effect
which aggravates the intensity of the hazard. For Cologne it
was not possible to investigate the effect of the spatial
Fig. 3. Morning thermal scan of Cologne (July 1st 1995 at 4 a.m.).
Source: Environmental Department of Cologne.
Fig. 2. Evening thermal scan of Cologne (June 30th 1993 at 9 p.m.).
Source: Environmental Department of Cologne.
Y. Depietri et al. / International Journal of Disaster Risk Reduction 6 (2013) 98–117 103

distribution of air pollutants and the role of air purification
capacity of urban green areas on the exposure: while data
on emissions for road traffic, households and industries are
abundant and spatially detailed, few measurement points
were available for the concentration of pollutants in the air.
With the objective of exploring different methods and
assessing the opportunity to use each one of them
according to the data availability, exposure is calculated
in two ways named respectively E
1
and E
2
:E
1
is obtained
by multiplying the number of inhabitants per city district
(I) with the normalized mean surface temperature (nor-
malized using the Min–Max Normalization method) per
city district (T) derived from the thermal infrared satellite
images (see Eq. (1)); while E
2
is obtained by multiplying I
Fig. 4. Land use and land cover map of Cologne.
Source: Centre for Remote Sensing of Land Surfaces (ZFL), University of Bonn (Germany).
Y. Depietri et al. / International Journal of Disaster Risk Reduction 6 (2013) 98–117104

with one minus the percentages of different LU/LC types
per city district (L
n
where ngoes from 1 to 8 as listed in
Table 2) weighted by specific coefficients (c
n
) (see Table 2)
which indicate the capacity of the LU/LC cover types to
provide climate regulating services (Eq. (2)). Following the
literature presented in Section 1.4,c
n
were calculated as
the average between the values presented in Burkhard
et al. [56], who assign, through expert judgment,
coefficients from 0 to 5 to the capacity of CORINE land
cover types to provide ecosystem services (0¼no relevant
capacity, 1¼low relevant capacity, 2¼relevant capacity,
3¼medium relevant capacity, 4¼high relevant capacity
and 5¼very high relevant capacity), and the values
obtained through the stakeholder interviews of the local
authorities (see Table 2). The CORINE land cover classes
were homogenized and translated into the classes used by
Fig. 5. Districts of Cologne.
Y. Depietri et al. / International Journal of Disaster Risk Reduction 6 (2013) 98–117 105

ZFL, Univ. Bonn.
E
1
¼IT ð1Þ
E
2
¼I1∑
8
n¼1
L
n
c
n
 ð2Þ
Indicators measuring susceptibility translate the pre-
disposition of a society (and ecosystems) to suffer harm
resulting from the levels of fragilities of settlements,
disadvantageous conditions and relative weaknesses. As
shown in the literature reviewed in Section 1.2, which
mainly reflects on the extreme heat events in Chicago of
1995 and in Europe of 2003 (thus on the features of
vulnerability to heat waves in richer countries as relevant
to the location of our case study), the age and health
conditions of the population, followed by socio-economic
and socio-cultural factors, are the main drivers that shape
susceptibility to heat waves. The elderly, the unemployed
and the immigrant are considered to be the most suscep-
tible groups to suffer harm in the case of extreme heat
events. Based on these findings and on the discussions
held during expert workshops, the following indicators
were chosen as representative: the percentage of the
population per city district older than 65 years (El); and
the percentage of unemployed per city district (U)asa
proxy for low income. Initially, the number of immigrants
per city district was also considered as a proxy for low
income and of disadvantageous conditions due to difficul-
ties in understanding warning messages. However, it
proved to be highly correlated with the number of unem-
ployed per city district (r¼0.979) and Uwas kept as it
facilitates the comparison with relevant and related stu-
dies. El and Uare both derived from census data on the
population of Cologne and percentages are calculated with
respect to the total population of each city district. The
composite indicator of susceptibility (S) is obtained,
according to Eq. (3), by normalizing and by equally
weighting the two single indicators.
S¼1
2Elþ1
2Uð3Þ
Lack of resilience (LoR) describes the limitations in
access to and mobilization of resources of the social–
ecological system and its incapacity to respond by
absorbing the impact. This component of vulnerability
includes the capacity to anticipate, cope and recover in
the short term. In our assessment, it is calculated as a
composite indicator of two single indicators. During a
heat wave, the majority of deaths generally occur
amongst the elderly who live alone as they are less
able to promptly recognize, seek help and be assisted in
case of malaise (see literature presented in Section 1.2).
It is therefore assumed that the percentage of elderly
living alone per city district (El
a
) is a proxy for the lack
of coping capacity of the population. Second, the
vicinity of a household to urban parks and forests
encourages and facilitates visits. The most susceptible
groupscangainreliefincaseofextremeheat,benefit-
ing from the cooler microclimate and cleaner air. The
percentage of the surface of Cologne covered by urban
forest per city district (Cf)isusedasaproxy,thuscity
districts with a low percentage of or no urban forest
indicate a lack of coping capacity. The composite indi-
cator for LoR is then calculated by normalizing, giving
equal weights, and aggregating these two indicators
according to Eq. (4).
LoR ¼1
2El
a
þ1
2ð1CfÞð4Þ
For all three components equal weightings were given
to provide, as a first step, a more generalized approach that
can be applicable in a wide range of situations. On another
occasion, the weightings can be allocated with local
experts to tailor the analysis to specific conditions.
Table 1
Characterization of the stakeholders interviewed.
Interviewee Male (M)/Female (F) Position Type of institution Sector
1M Project manager Municipal Environment
2M Head of department Municipal Landscape and urban green areas
3M Head of department Municipal Public health
4F Head of department Municipal Urban planning
5M Head of department Municipal Water management
6M Professor University Institute Public health
7M Project manager Local branch of a national NGO Forest management
Table 2
Matrix of the coefficients (c
n
) which estimate the capacities of different
LU/LC types to provide climate regulation services, as derived in Burkhard
et al. [56] and local stakeholders (SHs) interviews.
ES coefficients c
n
LU/LC type
nName Burkhard et al.
[56]
SHs
interviews
Average
1Continuous urban
fabric
0.00 0.00 0.00
2Discontinuous urban
fabric
0.00 1.60 0.80
3Agricultural land 2.00 3.10 2.55
4Deciduous forest 5.00 4.40 4.70
5Coniferous forest 5.00 4.00 4.50
6Mixed forest 5.00 4.40 4.70
7Natural grassland 2.00 3.70 2.85
8Water bodies 2.00 4.30 3.15
Y. Depietri et al. / International Journal of Disaster Risk Reduction 6 (2013) 98–117106

Finally, the vulnerability (V)ofthecityofColognetoheat
waves is calculated by normalizing and aggregating the
composite indicators of the three components through
Eq. (5).
V¼E½ðSþLoRÞ=2ð5Þ
This formulation takes into account that with no ele-
ments exposed, there would be no vulnerability.
For the spatial representation and mapping of the
single and composite indicators shown in Figs. 6–19 (see
Section 3), the values obtained were grouped into
five classes using the quantile method, which is a pre-
defined function of the ArcGIS 10 software (ESRI, Redlands,
CA). With this method, each class contains an equal
number of features, thus all classes differ in their values
ranges. To facilitate comparison, the qualitative labels
“very high, high, medium, low, very low”are used in the
legends.
3. Results
3.1. Vulnerability assessment to heat waves
In this section, we present and describe the results
obtained through the spatial analysis for all single and
composite indicators used for the calculation of the final
map of vulnerability.
First, the spatial distribution of the UHI effect on
exposure is calculated through two methods (E
1
and E
2
)
and presented in Figs. 6 and 7.Fig. 6 is based on the
thermal infrared images and shows the mean surface
temperature per city district while Fig. 7 is based on the
capacity of different land cover types to cool the environ-
ment. The first map clearly shows that the stronger effect
of the UHI is concentrated in the central districts, where
building density is higher and it then diminishes departing
from the city centre towards less densely populated and
more green/rural areas. Fig. 7 shows a less clear pattern in
the UHI effect. While reflecting the fact that more densely
built districts are affected by higher temperatures, the
presence of forest areas and parks appears to counter-
balance this effect and takes an excessively high weight in
the equation. In fact, the almost central districts on the
western side of Cologne, although crossed by the green
belt, actually present high average surface temperatures
possibly due to their vicinity to the dense centre and the
still significantly elevated percentage of high sealed areas.
On the other hand, in the southeast, the local and
peripheral concentration of sealed surfaces does not seem
to significantly affect the surface temperature (Fig. 6). The
discrepancies in the results between the two methods are
shown in Fig. 8 which compares Figs. 6 and 7. These
differences appear to be significant throughout all the
urban area, with relatively higher values of UHI measured
Fig. 6. Degree to which each district is exposed to heat waves, based on
mean surface temperatures derived from thermal infrared satellite data.
Fig. 7. Degree to which Cologne districts are exposed to heat waves
based on the capacity of different land covers to regulate the urban
microclimate.
Y. Depietri et al. / International Journal of Disaster Risk Reduction 6 (2013) 98–117 107

through the thermal scans compared with the land cover
classification, the opposite being true for the more per-
ipheral districts.
The one million inhabitants of Cologne are principally
distributed in the western-central districts and along the
river Rhine (see Fig. 9). Multiplying separately the spatial
values obtained with the two methods to calculate the UHI
with the spatial distribution of the population we obtain
respectively E
1
(Fig. 10) and E
2
(Fig. 11). Both show a higher
degree of exposure for the districts at the city core which
hosts most of the population and for which the UHI is
higher. The differences between the results obtained with
the two methods are also mitigated when calculating the
composite indicators (Fig. 12) but still remain high for
highly populated districts such as Lindental and Sülz.
Some additional considerations and quantitative data
are needed to derive an adequate estimation of the UHI
based on the land cover types. The vicinity to and
the concentration of districts with high or low sealed
surfaces need, for instance, to be integrated into the
calculation. Consequently, and given the reliability of the
data input for the two methods, E
1
is used in the final
calculation of vulnerability.
For the calculation of susceptibility, two indicators,
elderly per city district and unemployed, were measured
and spatially represented. The elderly are sparsely distrib-
uted in often isolated districts, all around the city centre
Fig. 9. Population per city district.
Fig. 8. Difference between the UHI effect calculated through the mean
surface temperatures per city district (Fig. 6) and through the land cover
capacity to cool the environment (Fig. 7).
Fig. 10. Exposure of the Cologne population to heat waves based on
surface temperatures distribution (E
1
).
Y. Depietri et al. / International Journal of Disaster Risk Reduction 6 (2013) 98–117108

(Fig. 13). The unemployed are mainly concentrated in two
zones of the city: one towards the north-west around
Ossendorf and one close to centre but on the eastern side
of the river Rhine (Fig. 14), as also described in Wolf [70].
As a result of the combination of these two indicators,
Cologne presents hotspots of susceptibility in districts
situated all around the city centre (e.g. Zollstock and
Raderthal, Bocklemünd/Mengenich and Vogelsang, Roden-
kirchen, Longerich, Flittard) with a concentration of sus-
ceptible areas on the eastern side of the river Rhine,
around Ostheim (Fig. 15). The most exposed central dis-
tricts are thus not densely inhabited by the most suscep-
tible groups.
For the calculation of the lack of resilience we used two
indicators: elderly people living alone and green cover per
city district. Elderly people living alone, who form the
group with least capacity to be promptly assisted in case
the hazard becomes an event, are principally distributed in
the central areas of the city on the western side of the
Rhine (Fig.16). High percentages of elderly people living
alone are also located north to the city centre, on both
sides of the river. Green cover per city district links the
lack of resilience to environmental components as it gives
a measure of the capacity to cope with the event by having
access to nearby cooler areas. As mentioned, urban parks
are in fact at least 1–21C cooler then surrounding built up
areas [40,44], thus this might not significantly affect the
thermal condition of the surrounding areas [43]. Interest-
ingly, some of the districts with very high percentages of
elderly people living alone are crossed by the outer green
ring of the Cologne urban forest which provides an
opportunity to benefit from cooler places and a healthier
environment (Fig. 17). As a result, the lack of resilience is
higher in very central districts of Cologne and with high
values for some single districts such as Lövenich in the
west, Porz, Ensen and Libur in the south (Fig. 18). The
distribution of the Cologne forest seems to play an
important role in this component.
As a result of the combination of the three composite
indicators, the highest vulnerability of Cologne to heat
waves affects the central districts on the western side of
the river. In the assessment, exposure has a strong influ-
ence as illustrated by the sensitivity analysis below. This
explains the high degrees of vulnerability which affect also
the wealthy districts crossed by the external green belt.
The vulnerability map is presented in Fig. 19.
3.2. Validation of the results using a sensitivity analysis
A sensitivity analysis was performed with “IBM SPSS
Statistics”(IBM, New York) in order to assess the impact
of each indicator on the model output. This analysis
examines the sources of variation in a model and can
therefore be used to determine input variables largely
Fig. 11. Exposure of the Cologne population to heat waves based on the
capacity of different land cover types to regulate microclimate (E
2
).
Fig. 12. Difference between the exposure based on temperature distribu-
tion (E
1
) and exposure based on the capacity of different land covers to
regulate microclimate (E
2
).
Y. Depietri et al. / International Journal of Disaster Risk Reduction 6 (2013) 98–117 109

Fig. 13. Percentage of elderly (older than 65 years) per city district.
Fig. 14. Percentage of unemployed per city district.
Fig. 15. Susceptibility of the population of Cologne to heat waves.
Fig. 16. Percentage of elderly living alone per city district.
Y. Depietri et al. / International Journal of Disaster Risk Reduction 6 (2013) 98–117110

contributing to the variation and those with low influ-
ence on the outputs [73]. The results of the analysis are
shown in Fig. 20. The figure consists of three parts (a, b
and c). Fig. 20(a) shows the effect of each indicator as a
curve. On the x-axis is the original input data for each
indicator scaled between 0.5 and þ0.5, and the y-axis
shows the variance of these indicators scaled between 0
and 1 in terms of overall response on the final index of
vulnerability. The stronger the influence, the steeper is
the curve. Fig. 20(b) shows a boxplot with the different
indicators on the x-axis and the sensitivity on the y-axis.
On the x-axis, the indicator names are in the same order
as they are ranked in Fig. 20(a). The size of the box
explains the degree of dispersion of the value of each
indicator in influencing the index. The smaller the box,
the more distinct is the influence on the index. The bold
line in each box describes the median, whereas high
values on the y-axis explain the strength of the influence
of each indicator to the overall index. Fig. 20(c) shows the
influence and interaction of each indicator with the other
in the case of changes of one indicator. This could lead to
the following effects e.g. the total sensitivity index of one
indicator would be y¼0 meaning that this indicator has
no influence on the model output and thus could be
neglected whereas a high median represents a non-
substitutable and meaningful indicator.
Overall Fig. 20 shows that the exposure has the highest
impact on the model output, followed by the percentage of
Fig. 17. Percentage of forest cover per city district.
Fig. 18. Spatial distribution of the lack of resilience of the population of
Cologne to heat waves per city district.
Fig. 19. Map of the vulnerability of the population of Cologne to
heat waves.
Y. Depietri et al. / International Journal of Disaster Risk Reduction 6 (2013) 98–117 111

forest cover as a measure of the coping capacity of the
population.
3.3. Experts'interviews
The results of the stakeholders' interviews are summar-
ized in Tables 3 and 4. The first table presents the gathered
opinions on the role urban and peri-urban ecosystems
may have in mitigating the impacts of heat waves in
Cologne. Most of the respondents agreed on the need to
conserve and have well managed green and blue areas in
the city as these can contribute to mitigating the impacts
of extreme heat. In particular, this is due to the cooling
functions performed by the urban vegetation of Cologne. A
mix of indigenous species, resistant to droughts but also
presenting adequate levels of evapotranspiration is
thought to be suitable for this purpose. A smaller emphasis
was put on the effectiveness of green areas to remove
pollutants and thus reduce the impact on health of
extreme heat. To mitigate this impact a reduction in
emissions is primarily necessary. It is recommended that
the full range of services provided by urban ecosystems is
taken into account in planning and management. Deci-
sions on the allocation of green areas and street trees
should not be based only on their aesthetic value. An
additional expectation and suggestion made was to have
ecosystems as much connected as possible through the
urban fabric to form corridors that would bring fresh air
into the city centre from the surrounding cooler areas. In
this way, obstacles encountered in the process of reconver-
sion of built-up areas into green and blue spaces can be
partially overcome.
The second table, Table 4, presents the perceptions and
previous experience of stakeholders regarding the impacts
of heat waves on urban/peri-urban ecosystems and their
services in and around Cologne which would indirectly
further increase the vulnerability of the city. Negative
impacts of heat waves would mainly hit agricultural land
and gardening, diminishing their productivity. In this
regard, strategies to adapt to increasing warm conditions
(such as the selection of more resistant crops) are already
being taken by farmers. Recreational activities can also be
affected by the hazard due to the deterioration of small
water bodies and of the vegetation in urban parks and
forests distributed in the surrounding areas. The risk of
forest fire should be monitored, especially in periods of
extreme heat. Local mixed forests have shown to be the
most appropriate to cope with these impacts. All respon-
dents agreed that the provision of drinking water is not
affected at present by the impacts of extreme heat, thanks
to the high quantity of groundwater available in the
region.
4. Discussion and conclusions
The assessment of the vulnerability of Cologne to heat
waves presented in this paper is based on the MOVE
project generic framework and integrates both quantita-
tive and qualitative data as well as the social and ecologi-
cal dimensions of vulnerability. This allowed consideration
of a broader set of drivers and elements that come into
play in determining morbidity and mortality from heat
waves in the urban environment. In fact, though the
vulnerability of Cologne to heat waves is expected to be
low compared to that of other municipalities in NRW due
to the relatively low percentage of elderly population [74],
it represents a relevant case to investigate the role of the
extended set of variables that shape urban vulnerability to
heat waves, particularly the environmental ones.
From the spatial quantitative analysis, vulnerability was
higher overall in the central and western districts where
most of the population resides and where the percentage
of sealed surfaces is high and contributes to the UHI. This
is further accentuated by the fact that most of the elderly
people living alone live in these central districts. On the
other hand, the measurement and representation of each
Fig. 20. (a), (b) and (c) Sensitivity analysis.
Y. Depietri et al. / International Journal of Disaster Risk Reduction 6 (2013) 98–117112

Table 3
Stakeholders perceptions on the capacity of the Cologne ecosystem to regulate climate and air quality and thus mitigate the impacts of heat waves.
Ecosystem Negative opinion Positive opinion What cannot/should not be done What could/should be done
Green areas Important for their cooling capacity,
especially for the high
evapotranspiration rates of plants.
Broad woodlands are the most
effective in this sense.
To broaden urban green areas is thought to be
unrealistic as it would imply the reconversion of
buildings and streets over large areas and would
be in contrast with the spreading concept of
compact city.
Appropriate management of existing green
spaces and increase of the number of trees
along streets.
Green roofing. To date there are no shared
guidelines, only ad-hoc projects.
The cooling function of peri-urban grasslands
and agricultural areas should also be taken into
account
Opening wind corridors (both green corridors
and large streets) to build connections between
green areas. This would bring cooler air into the
city centre from the surrounding woodand
agricultural lands.
It is often problematic to reserve space for street
trees as these compete for space mainly with
parking for cars.
Street tress should be considered in urban
planning for their cooling function in addition to
their aesthetic value. This is valid also for parks
and urban forests.
While planting Mediterranean species has become
a trend in the last years because they consume less
water, these also evapotranspirate less reducing
their cooling functions.
Selected species should be able to cope with
2–31C increase in temperature. Indigenous
beech forests and oak trees are considered
to be the appropriate species to cope with the
condition of projected climates.
Visits to parks are generally not considered as a
relevant coping strategy in periods of heat waves.
Parks should however be well distributed
amongst the city to allow easy access of the most
vulnerable (i.e. elderly, poor, ill).
Green areas have minor positive effects on the
reduction of air pollutants. Urban parks and street
trees are thought to be the less effective in
delivering this service and can themselves be
affected by high concentrations of air
contaminants in inner parts of the city.
Urban and peri-urban forests are the
most effective in purifying the air.
Trees positioned along streets with narrow
spacing can form a dense canopy that traps
pollutants paradoxically worsening air quality.
A reduction of air pollutants at the source is
needed. Zones at the centre of the city restricted
to traffic should be delineated.
Blue areas The cooling effect of water bodies is noticeable on
a seasonal rather than on a daily basis. This
constrains their effect in mitigating the impacts of
heat waves in urban areas.
Most of urban water bodies have been buried
under streets and their conversion is complicated.
Small water bodies should be restored
throughout the city. Fountains should also
be reactivated through the city.
Y. Depietri et al. / International Journal of Disaster Risk Reduction 6 (2013) 98–117 113

Table 4
Stakeholders' perceptions on the potential impacts of heat waves on the urban ecosystem and its services to the inhabitants of Cologne.
Ecosystem/system Direct impacts Indirect impacts Potential/undertaken actions Notes
Surrounding agricultural
land/gardening
Negative impacts on crops growth and gardening. Decrease in local
agricultural
productivity.
Farmers are already aware of these
impacts and are starting to select and
grow alternative, more resistant crops.
Dry periods occurring in spring times preceding a heat
wave are the main responsible for the reduction in
productivity rather than the heat waves itself.
It can become an even more important issue in the
future due to climate warming.
Sources of drinking water Generally not relevant. In some cases, when the water
table is low, the flow of water can change and get exposed
to chemical compounds.
It is mainly due to the high rate of infiltration in the
region which allows the city to rely on sufficient
groundwater resources. In future climates drinking
water might also be affected due to an increase in
surface runoff which leads to the decrease in
groundwater recharge.
Small water bodies The quality can be deteriorated due to eutrophication and
consequent decrease in the concentration of oxygen.
Some smaller ponds may even dry-up.
On recreational
activities around
the city.
Water treatment plants Not relevant. In case of low levels of the river Rhine
due to dry spells the concentration of
effluents should be kept under control.
Power plants Not relevant. These are not much present along the river
Rhine so there is no risk of increase in temperature of the
cooling water.
Vegetation Negative. These have also occurred in the past: trees
lose their leaves, photosynthesis is reduced which leads
to a reduction in the production of oxygen. Grasslands are
the most affected while mixed forest and indigenous
vegetation have shown to be the most resistant to
droughts. There might also be a risk of forest fires,
especially in the future with the increase of heat
extremes.
On recreational
activities in and
around the city.
A good mixture of native plants,
avoiding monoculture or grassland,
increases the resilience of the forest
system to fires and should be preferred.
The risk of forest fires also depends on the type of trees.
Beech, oak, ash and lime trees are thought to be quite
resistant while coniferous forests are considered to be
more at risk.
Y. Depietri et al. / International Journal of Disaster Risk Reduction 6 (2013) 98–117114

single indicator allowed highlighting a different geography
in which the most susceptible groups are sparsely dis-
tributed towards the periphery of Cologne and on the
eastern side of the Rhine. The urban forest also plays a
relevant role in our assessment. Distributed along circular
green belts, it contributes to reducing the vulnerability of
susceptible groups (mainly the elderly people) in some of
the more peripheral areas.
These results are directly linked to the historical
dimension of the vulnerability of Cologne. Its present
distribution clearly appears to be the result of processes
that occurred through the centuries but that culminated in
the last century when planning decisions more strongly
influenced the assessed patterns of vulnerability. Two of
these urban developments should be underscored: the
allocation of space for the greenbelts in and around the
city at the beginning of the century, along which the
wealthiest segments of the population are located; and
the intense sub-urbanization which took place during the
reconstruction period following World War II and asso-
ciated with the baby boom of the 60s. In these latter parts
of the city, low income, more susceptible groups are still
concentrated (see Section 2.1).
The results of the quantitative assessment, suggest that,
to effectively tackle the vulnerability of Cologne to heat
waves, local city authorities in charge of urban planning,
environmental management and health should collaborate
to implement strategies which improve the social–envir-
onmental conditions of the city centre due to the higher
levels of exposure and lack of resilience, but also need to
consider the fate of susceptible groups which are located
in its surrounding areas. The development of social ser-
vices should be prioritized to cope with heat waves in
Cologne, but, as shown in the sensitivity analysis, the
environmental factors have a strong influence on the
assessment and are integrated in exposure (i.e. the UHI)
and in the lack of resilience (i.e. the distribution of the
forest cover) in our analysis. This result indicates that the
ecological dimension is an important factor and needs to
be taken into account in tackling vulnerability to hydro-
meteorological hazards.
The qualitative assessment provided additional and
complementary information to the quantitative analysis
especially regarding the links between ecosystem services
from the surrounding areas and the vulnerability of
Cologne to heat waves. It stressed how in Cologne it is
necessary to acknowledge the cooling functions of urban
trees and green areas in urban planning as well as those of
grasslands and peri-urban agricultural land, and draw
better links between the city core and its surrounding
areas, as it is also suggested for Stuttgart [75] or Freiburg
[76]. This consideration supports the more sustainable
model of the compact city [77]. Well designed green
corridors could improve microclimate in the inner parts
of the city bringing fresh air from the outskirts and
bettering the living condition of the high density city
centre. Furthermore, even if the urban forest of Cologne
covers a relatively high area compared to other German
cities, the type of species planted should also be carefully
thought through, preferring, a mix of indigenous species,
according to our respondents. In this regard, a debate is
ongoing as to whether climate change might favour
invasive species, thus caution needs to be taken when
selecting species for adequate green areas management.
Furthermore, the qualitative data gathered provided
insights into additional sources of vulnerability that
could originate by the failure of certain ecosystems to
provide services to the urban population. It emerged that
most of the impacts of extreme heat affect peri-urban
ecosystems such as forests and small water bodies while
compromising agricultural production and gardening. The
benefits derived from recreational activities in these per-
iods can thus be hindered, especially around the city, while
water supply seems not to be at risk. This adds to the
quantitative assessment and further prompts considera-
tion of the wider urban/rural interface dynamics, moving
the focus partly outside the urban core.
Within the set of actions taken from the local govern-
ment to adapt to climate change, plans and strategies are
developed in collaboration with the regional or federal
authorities and awareness is rising in Cologne with respect
to the impact of weather-related hazards such as heat
waves.
4
At the Federal level, guiding documents are pre-
pared by the Ministry in charge of the environment and
nature conservation which set the strategic framework for
cities situated in the region to plan and adapt to climate
change, including to the increase of heat related stress.
5
A
broad list of measures is here suggested but some seem to
be specifically relevant for the Cologne urban area. Accord-
ing to our research, a reduction of the exposure, especially
through green infrastructure and an increase of the con-
nectedness between the city and its green and surround-
ing areas, are urgent measures to be implemented. This
overall calls for a broader collaboration between sectors in
charge of health, environment as well of urban green and
urban planning at the city level. This field based study
therefore demonstrates the context specificity of the
choice of strategies to cope or adapt to increasing fre-
quency and impacts of heat waves and that resources
should be allocated at the local level to conduct such
studies to select and prioritize strategies.
Additional guidance and frameworks come from the
European level. Several projects, such as the EuroHEAT
project, contributed to the implementation of the EC
Environment and Health Action Plan. The final report of
this project concluded that, although coordination
between institutions for timely and appropriate response
actions and early warning systems should be priority
actions of the health plan to mitigate vulnerability to heat
waves, the reduction of exposure through improved urban
planning should also be prioritized [78].
In summary, our analysis showed that, while the higher
vulnerability of the population of Cologne to heat waves is
concentrated in the city centre, policies that aim to
tackling it should also take into account the connections
and interactions between the city centre, the surrounding
districts and its hinterland, reducing the susceptibility of
4
〈http://www.stadt-koeln.de/3/umwelt/klima/klimawandel/07145/〉.
5
〈http://www.umwelt.nrw.de/klima/klimawandel/anpassungspoli
tik/anpassungsstrategie/index.php〉.
Y. Depietri et al. / International Journal of Disaster Risk Reduction 6 (2013) 98–117 115

lower status social groups and enhancing ecosystem
management.
Acknowledgements
The work presented in this paper was carried out as
part of the EU funded Seventh Framework project Meth-
ods for the Improvement of Vulnerability Assessment in
Europe (MOVE, Project no. 211590, www.move-fp7.eu). We
thank Tobias Raphael Blätgen, Divya Rajeswari and Philipp
Koch for their contributions and support as well as two
reviewers for their helpful feedback and suggestions.
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