Despite many studies on climate change adaptation, implementation challenges are understudied. After screening over 39,000 articles, we used large language models on 892 articles to map global adaptation challenges in urban water systems. Publications have grown exponentially since 2011, but research is biased toward the Global North. Floods dominate literature, leaving droughts underrepresented despite their socio-economic impact. Our analysis of measure-challenge associations reveals: i) institutional and social challenges are pervasive in all adaptation types, ii) institutional challenges have moderate associations across measures where high frequencies often hinder CCA measures, iii) structural and technological CCA measures receive more research attention than behavioral, social, and anticipatory approaches. Our synthesis identifies research gaps and policy priorities: focus on socio-behavioral and anticipatory adaptation, research in understudied regions, and urgent institutional reform as an adaptation prerequisite.

While cities are facing increasing challenges of flood risk due to combined effects of climate change and socioeconomic development, understanding of the complexity of urban flood risk is still limited, hampering decision-making and urban adaptation planning. This study presents a qualitative system dynamics modelling framework to investigate urban flood risk and adaptation under climate change in a coupled socio-ecological system, the city of Hamburg. The developed integrated conceptual model provides a holistic understanding of key physical and socio-economic processes and the role of feedback loops underlying the urban system, and contributes to the understanding of vicious cycles of barriers that perpetuate and hinder adaptation processes within cities. The qualitative approach can help to break down silo-thinking in urban flood risk assessments. Decision-makers could use the framework to understand the complexity of interactions among multiple drivers of flood risk to overcome barriers and lock-in effects to adaptation in cities.

Cities are facing increasing challenges of flood risk due to combined effects of climate change and socioeconomic development. At the same time understanding of the complexity of urban flood risk is still limited, hampering decision-making and effective urban adaptation planning. A socio-ecological system (SES) perspective offers a promising approach to analyze risk as a non-isolated entity by recognizing human and natural systems as complex and coupled structures and considering their interactive dynamics (e.g., delays, feedbacks, and non-linearity). Qualitative system dynamics modeling tools, such as causal loop diagrams, are particularly useful for this, as they allow the inclusion of different kinds of system variables. This study applies a qualitative system dynamics modeling framework to holistically investigate urban flood risk under climate change and barriers to adaptation in a coupled SES using the city of Hamburg as a case study. The study deals with urban flood risk in the context of ‘water from 4 sides’ addressing questions in the growing research field of climate hazard interactions and compound risks. In a stepwise approach, a qualitative system dynamics model was developed based on an integrated interdisciplinary knowledge of researchers. Disciplinary mental maps were created by the researchers in various group interviews, followed by the development of an overall group causal loop diagram based on the disciplinary mental maps to form a holistic qualitative model. For the model analysis, causal chains of sub-processes and feedback loops were visually isolated and highlighted. Particular emphasis is placed on identifying and analyzing the reinforcing feedback loops underlying the complex urban system in order to understand the vicious circles of barriers that perpetuate and thus hinder the adaptation process. The findings on the system’s feedback loops help to understand why and how system behavior evolves in a specific direction. The integrated model shows that the main drivers of urban flood risk growth in the system are linked to socio-economic and institutional processes. Climate change mainly affects the city externally by increasing flood hazards, while the city itself contributes to flood risk through processes of exposure and social vulnerability. The results show that increasing flood risk and barriers to adaptation in the city are linked to the amplifying feedback loops of path dependency, river engineering measures, urban development, car dependency, the ‘levee effect’, poverty, urban health and silo-thinking. The case study demonstrates the usefulness of the qualitative system dynamics modelling approach in developing a shared understanding of the complex social, economic, environmental and political and institutional interactions among multiple drivers of flood risk. Causal loop diagrams can be successfully used to articulate the viscous circles of barriers and lock-in effects of unsustainable development in urban adaptation. However, it should be noted that the model reflects the state of knowledge of the researchers involved in the model-building process and therefore only represents a ‘dynamic hypothesis’ of the structure and dynamics of the system under consideration. Further work is in progress to place this qualitative system dynamics model in the broader context of decisions support and policy through stakeholder involvement.

In recent years, weather-related extreme events have shown the limits of technical approaches to urban water challenges and highlighted the urgent need to rethink the relationship between cities and water and to see water as a partner in shaping transformative, climate-safe and just urban futures. However, existing scientific studies depicting future trajectories of urban water management have struggled to make the intertwined social and ecological dynamics of (transformative) urban adaptation tangible and accessible. This study focuses on the potential of visual communication of scenarios to stimulate both learning among scientists (during the process of creating the scenarios) and social learning (as a next step using the developed “narrative images”) to motivate diverse societal actors to engage with the complexity of sustainable urban water management. Art can overcome barriers of scientific and technical concepts and touch peoples' inner motivation for preserving and sustainably transforming our cities in a way that written texts cannot. As sustainability challenges transcend disciplines, this study draws methodically on an interdisciplinary scenario approach. Three adaptation scenarios were developed in a participatory process and professionally visualized as “narrative images” using the city of Hamburg as a case study. The scenarios take place in 2050 depicting a gradient ranging from coping to incremental adaptation to transformative adaptation for managing the water-adaptation nexus: “Water defensive city,” “Water resilient city,” and “Water aware city.” The scenario study shows innovatively how to bring the humanities, natural and engineering sciences into a deliberative dialog, while at the same time promoting collective learning. It can serve as a model for successful future interdisciplinary research and scenario developing exercises.

In this study, we set out to develop a new social vulnerability index (SVI). In doing so, we suggest some conceptual improvements that can be made to existing methodical approaches to assessing social vulnerability. To make the entanglement of socio-spatial inequalities visible, we are conducting a small-scale study on heterogeneous urban development in the city of Hamburg, Germany. This kind of high-resolution analysis was not previously available, but is increasingly requested by political decision makers. We can thus show hot spots of social vulnerability (SV) in Hamburg, considering the effects of social welfare, education, and age. In doing so, we defined SV as a contextual concept that follows the recent shift in discourse in line with the Intergovernmental Panel on Climate Change’s (IPCC) concepts of risk and vulnerability. Our SVI consists of two subcomponents: sensitivity and coping capacity. Populated areas of Hamburg were identified using satellite information and merged with the social data units of the city. Areas with high SVI are distributed over the entire city, notably in the district of Harburg and the Reiherstieg quarter in Wilhelmsburg near the Elbe, as well as in the densely populated inner city areas of Eimsbüttel and St. Pauli. As a map at a detailed scale, our SVI can be a useful tool to identify areas where the population is most vulnerable to climate-related hazards. We conclude that an enhanced understanding of urban social vulnerability is a prerequisite for urban risk management and urban resilience planning.

Changing land surfaces can have a profound impact on local and regional climates. Cities in particular are associated with a comprehensive transformation of the local surface structure, which, in combination with anthropogenic emissions of radiative active gases and particles, leads to a significant modification of the energy balance of the urban boundary layer. One of the most well-known urban climate phenomena is the urban heat island effect. In addition to the thermodynamic effect, a clear urban influence on the wind field and the modification of precipitation above and around cities is documented by many studies. Observational and modeling studies provide convincing evidence that precipitation patterns over and/or around urban areas are altered, and convective precipitation and flash flood events may be enhanced or even triggered. The urban influence often is result of a combination of thermal effects in connection with the heat island, obstacle effects an aerosol influences on microphysical processes and on the heating profile over the city. Precipitation can not only be increased by cities, it can also be weakened or even averted, especially in connection with particle emissions. Inprinciple urban effects on precipitation are fairly well known, but the many published studies on them show a wide range in terms of the magnitude of precipitation changes and their location relative to the urban area. Simple statements concerning urban precipitation modification cannot currently be made, if this is at all possible given the complexity resulting from the overlaying influencing factors. In many cases, the lack of standardized reporting of study results makes it considerably more difficult to compile generalized statements. As part of the Cluster of Excellence "Climate, Climatic Change and Society" (CliCCS), a project is focussing on sustainable adaptation scenarios of cities with regard to hydrological pressures in connection with climate change. Within this project, we conducted a systematic literature review on precipitation modification by cities with a focus on the last 10 years. The reason for this was that the last comprehensive review on the subject was several years ago and the number of publications is constantly increasing. Here we report the first results of our systematic review broken down by spatial scale, dominant processes and possible relevance to climate change. In doing so, we highlight the influence of cities on heavy precipitation and point out the development of the research landscape and alleged research gaps. Selected results from recent publications are shown for illustrative purposes.

Die Veränderung von Landoberflächen kann einen profunden Einfluss auf das lokale und regionale Klima haben. Insbesondere Städte sind mit einer umfassenden Umgestaltung der lokalen Oberflächenstruktur verbunden, was im Zusammenspiel mit anthropogen Emissionen von strahlungsrelevanten Gasen und Partikeln zu einer deutlichen Modifikation der Energiebilanz der städtischen Grenzschicht führt. Viele der resultierenden Auswirkungen auf meteorologische Größen wurden in den letzten Dekaden umfassend untersucht. Eines der bekanntesten Stadtklimaphänomene ist die Überwärmung von Stadtgebieten im Vergleich zum Umland, die urbane Wärmeinsel. Neben dem Temperatureffekt ist ein klarer städtischer Einfluss auf das Windfeld und die Modifikation von Niederschlägen über und im Umfeld von Städten zu beobachten und durch viele Studien belegt. Beobachtungs- und Modellstudien liefern überzeugende Nachweise, dass Niederschlagsmuster über Stadtgebieten und/oder deren Umgebung modifiziert werden bzw. konvektive Niederschläge und Sturzflutereignisse verstärkt oder gar ausgelöst werden können. Der Einfluss der Stadt wird demnach über thermische Effekte in Verbindung mit der Wärmeinsel, Hinderniseffekten, Aerosoleinflüssen auf mikrophysikalische Prozesse und auf das Erwärmungsprofil über der Stadt sowie durch anthropogene Feuchtequellen (z.B. Kühltürme) bewirkt. Niederschläge können durch Städte nicht nur verstärkt, sondern unter Umständen insbesondere in Verbindung mit Partikelemissionen auch abgeschwächt oder gar abgewendet werden. Obwohl die Stadteffekte auf den Niederschlag ziemlich gut bekannt sind, weisen die vielen veröffentlichten Studien dazu eine große Bandbreite bezogen auf die Größe der Niederschlagsveränderungen und deren Lage relativ zum Stadtgebiet auf. Ein klareres, vollständigeres Bild mit deutlichen Aussagen zur urbanen Niederschlagsmodifikation kann derzeit noch nicht gezeichnet werden, wenn dieses bei der sich durch die überlagernden Einflussfaktoren ergebenden Komplexität überhaupt möglich ist. Vielfach wird durch eine fehlende standardisierte Berichterstattung von Studienergebnissen das Zusammenstellen von generalisierten Aussagen erheblich erschwert. Im Rahmen des Exzellenzclusters „Climate, Climatic Change and Society“ (CliCCS) fokussiert ein Projekt auf nachhaltige Anpassungsszenarien von Städten in Hinblick auf hydrologische Belastungen in Verbindung mit dem Klimawandel. Innerhalb dieses Projekts haben wir eine systematische Literaturrecherche zur Niederschlagsmodifikation durch Städte mit Schwerpunkt auf die letzten 10 Jahre durchgeführt. Anlass dafür war, dass der letzte umfassende Review zu dem Thema einige Jahre zurückliegt und die Anzahl der Veröffentlichungen stetig steigt. Zudem wurden in den letzten Jahren räumlich besser aufgelöste Modelle verwendet, die eine direkte Berechnung konvektiver Prozesse erlaubten. Dabei wurde häufig auch der Aerosoleinfluss auf die Niederschlagsbildung explizit betrachtet. Auch auf der Beobachtungsseite konnten durch den Einsatz von besser aufgelösten Fernmessverfahren (Radar, Satellit) kleinskaligere Prozesse aber auch regionale Muster in neueren Studien umfassender analysiert werden. Hier berichten wir über erste Ergebnisse unseres systematischen Reviews aufgeschlüsselt nach räumlicher Skala, dominanten Prozessen und möglichen Bezug zum Klimawandel. Dabei stellen wir den Einfluss von Städten auf Starkniederschläge heraus und weisen auf die Entwicklung der Forschungslandschaft und vermeintliche Forschungslücken hin. Zur Veranschaulichung werden ausgewählte Ergebnisse neuerer Veröffentlichungen gezeigt.

„Klima, Klimawandel und Gesellschaft“ (CLICCS) ist ein DFG-Exzellenzcluster an der Universität Hamburg, in dem gemeinsam mit Partner-Institutionen erforscht wird, wie sich das Klima ändert und mit ihm die Gesellschaft, die somit auf das Klima zurückwirkt. CLICCS umfasst sowohl Grundlagenforschung zur Klima- und Sozialdynamik als auch die transdisziplinäre Untersuchung von Mensch-Umwelt-Wechselwirkungen. Dabei orientiert sich CLICCS an der übergeordneten Frage: „Welche Klimazukünfte sind möglich und welche sind plausibel?“. Drei CLICCS-Projekte konzentrieren sich auf die Entwicklung und Bewertung von Szenarien zur nachhaltigen Anpassung an den Klimawandel auf regionaler Ebene, wo der Klimawandel für den Menschen sichtbar wird und eine nachhaltige Anpassung durch lokale Akteure realisiert werden kann. Eines von ihnen, CLICCS-C1 („Wasser von 4 Seiten“; https://www.cliccs.uni-hamburg.de/de/research/theme-c/c1.html), untersucht die gekoppelte Mensch-Umwelt-Dynamik auf städtischer Ebene und konzentriert sich auf durch den Klimawandel induzierte wasserbedingte Stressfaktoren. Das CLICCS-„Wasser von 4 Seiten“ Projekt zielt darauf ab, einen komplexen integrierten Modellierungsansatz für das städtische System zu entwickeln und anzuwenden, der (i) die wissenschaftliche Bewertung mehrerer wasserinduzierter Auswirkungen auf das städtische System und dessen Rückkopplungen ermöglicht, (ii) für die Entwicklung nachhaltiger Anpassungsszenarien geeignet ist und (iii) bei Entscheidungen unterstützt, in denen die Auswirkungen von Anpassungsszenarien auf das Erreichen der UN-Nachhaltigkeitsziele bewertet werden. Ein wesentliches Merkmal des Modellierungsansatzes ist die Integration der wasserbedingten Stressfaktoren, wie Grundwasseranstieg, Sturmfluten, Flussüberschwemmungen und durch Starkniederschläge ausgelöste Sturzfluten. Die Wechselwirkungen der Stressfaktoren sowohl untereinander als auch mit gesellschaftlichen Komponenten des Stadtsystems spielen dabei eine wichtige Rolle. Im Vortrag werden der Forschungsansatz sowie erste Schritte der Modellentwicklung vorgestellt. Danksagung CLICCS wird gefördert durch die Deutsche Forschungsgemeinschaft (DFG) im Rahmen der Exzellenzstrategie des Bundes und der Länder – EXC 2037 „Klima, Klimawandel und Gesellschaft“ – Projektnummer: 390683824, Dies ist ein Beitrag zum Centrum für Erdsystemforschung und Nachhaltigkeit (CEN) der Universität Hamburg.

In simulations of the boreal summer Asian monsoon, generations of climate models show a persistent climatological wet bias over the tropical western Indian Ocean and a dry bias over South Asia. Here, focusing on the monsoon developing stages (May–June), process-based diagnostics are first applied to a suite of NCAR models and reanalysis products. Two primary factors are identified for the initiation and maintenance of the wet bias over the northwestern Indian Ocean (NWIO; 5°–15°N, 52°–67°E): (i) excessive tropospheric moisture and (ii) restrained horizontal advection of the 1000–800-hPa levels cold–dry air couplet that originates offshore of Somalia. Second, guided by the diagnostics, we hypothesized that insufficient dilution of convective updrafts is one possible candidate for model bias and performed a series of enhanced entrainment sensitivity experiments with NCAR CAM4. Over the NWIO, the results suggest that globally increasing the maximum entrainment rateεmaxleads to a drier free troposphere, arrests the vertical extension of clouds, and weakens moisture–convection and cloud–radiation feedbacks; each factor contributes to a reduced wet bias. Moreover, a higherεmaxleads to a reduced dry bias over South Asia through changes in the local circulation features. In CAM4, improved precipitation climatology due to increasedεmaxsuggests that insufficient dilution is one factor, but not the only one, that contributes to systematic errors. Rather, realistic representation of boundary layer processes in climate models arising out of local ocean–atmosphere interaction processes off Somalia’s coast deserves attention in reducing the NWIO wet bias.

This study assesses the ability of a high-resolution downscaling simulation with the regional climate model (RCM) HIRHAM5 in capturing the monsoon basic state and boreal summer intraseasonal variability (BSISV) over South Asia with focus on moist and radiative processes during 1979-2012. A process-based vertically integrated moist static energy (MSE) budget is performed to understand the model's fidelity in representing leading processes that govern the monsoon breaks over continental India. In the climatology (June-September) HIRHAM5 simulates a dry bias over central India in association with descent throughout the free troposphere. Sources of dry bias are interpreted as (i) near-equatorial Rossby wave response forced by excess rainfall over the southern Bay of Bengal promotes anomalous descent to its northwest and (ii) excessive rainfall over near-equatorial Arabian Sea and Bay of Bengal anchor a local Hadley-type circulation with descent anomalies over continental India. Compared with observations HIRHAM5 captures the leading processes that account for breaks, although with generally reduced amplitudes over central India. In the model too, anomalous dry advection and net radiative cooling are responsible for the initiation and maintenance of breaks, respectively. However, weaker contributions of all adiabatic MSE budget terms, and an inconsistent relationship between negative rainfall anomalies and radiative cooling reveals shortcomings in HIRHAM5's moisture-radiation interaction. Our study directly implies that process-based budget diagnostics are necessary, apart from just checking the northward propagation feature to examine RCM's fidelity to simulate BSISV.

This study assesses the ability of a high-resolution downscaling simulation with the regional climate model (RCM) HIRHAM5 in capturing the monsoon basic state and boreal summer intraseasonal variability (BSISV) over South Asia with focus on moist and radiative processes during 1979–2012. A process-based vertically integrated moist static energy (MSE) budget is performed to understand the model’s fidelity in representing leading processes that govern the monsoon breaks over continental India. In the climatology (June–September) HIRHAM5 simulates a dry bias over central India in association with descent throughout the free troposphere. Sources of dry bias are interpreted as (i) near-equatorial Rossby wave response forced by excess rainfall over the southern Bay of Bengal promotes anomalous descent to its northwest and (ii) excessive rainfall over near-equatorial Arabian Sea and Bay of Bengal anchor a “local Hadley-type” circulation with descent anomalies over continental India. Compared with observations HIRHAM5 captures the leading processes that account for breaks, although with generally reduced amplitudes over central India. In the model too, anomalous dry advection and net radiative cooling are responsible for the initiation and maintenance of breaks, respectively. However, weaker contributions of all adiabatic MSE budget terms, and an inconsistent relationship between negative rainfall anomalies and radiative cooling reveals shortcomings in HIRHAM5’s moisture-radiation interaction. Our study directly implies that process-based budget diagnostics are necessary, apart from just checking the northward propagation feature to examine RCM’s fidelity to simulate BSISV.

This study investigates the future changes in the climate zones' distribution of the Earth's land area due to increasing atmospheric greenhouse gas concentrations in three IPCC SRES emissions scenarios (A1B, A2 and B1). The Koppen climate classification is applied to climate simulations of seven atmosphere-ocean general circulation models (AOGCMs) and their multi-model mean. The evaluation of the skill of the individual climate models compared to an observation-reanalysis-based climate classification provides a first order estimate of relevant model uncertainties and serves as assessment for the confidence in the scenario projections. Uncertainties related to differences in simulation pathways of the future projections are estimated by both, the multi-model ensemble spread of the climate change signals for a given scenario and differences between different scenarios. For the recent climate the individual models fail to capture the exact Koppen climate types in about 24-39 To of the global land area excluding Antarctica due to temperature and precipitation biases, while the multi-model ensemble mean simulates the present day observation-reanalysis-based distribution of the climate types more accurately. For the end of the 21st century compared to the present day climate the patterns of change are similar across the three scenarios, while the magnitude of change is largest for the highest emission scenario. Moreover, the temporal development of the climate shifts from the end of the 20st century and during the 21st century show that changes of the multi-model ensemble mean for the A2 and B I scenario are generally within the ensemble spread of the individual models for the A1B scenario, illustrating that for the given range of scenarios the model uncertainty is even larger than the spread given by the different GHG concentration pathways. The multi-model ensemble mean's projections show climate shifts to dryer climates in the subtropics (Australia, Mediterranean Basin, southern Africa). This is consistent with an increase of area classified as Tropical Savanna Climate as well as Dry Climates. Furthermore, there is a poleward extension of the warmer climate types in the northern hemisphere causing a retreat of regions with Cold Climate with Moist Winter and Tundra Climate. The European region shows largest changes comparing the shifts in the different continents (37.1 % of the European land area) as a result of a large extension of the Humid Temperate Climate across eastern and north-eastern Europe at the cost of the Cold Climate with Moist Winter.