0. A Review of the Compounding Effects of Compound Extreme Heat Events on Urban Thermal Risk

1. Introduction: The Escalating Threat of Compound Extreme Heat Events to Urban Thermal Risk

This survey provides a comprehensive review of the escalating urban thermal risk posed by compound extreme heat events, which arise from the synergistic interaction between the Urban Heat Island (UHI) effect and heat waves (HWs). It critically analyzes existing definitions, methodologies, and findings to highlight current research deficiencies and propose future directions.

The subsequent sections will delve into specific facets of this multifaceted problem. "Defining the Urban Thermal Landscape and Heat Extremes" will establish foundational concepts, differentiating between the localized UHI effect, driven by urban morphology and anthropogenic activities, and regional heat waves, a consequence of broader climatic patterns . This section will articulate how these phenomena independently contribute to elevated temperatures and discuss the inherent warming characteristics of urban environments, such as reduced vegetation, prevalence of impervious surfaces, and heat entrapment by urban canyons .

Building upon this, "The Emergence of Compound Extreme Heat Events and Their Amplified Risk" will explicitly define compound extreme events as the co-occurrence of multiple extreme climate drivers, leading to impacts greater than the sum of their individual effects . This section will elucidate the critical synergistic interaction between UHIs and heat waves, demonstrating how the UHI effect superimposes additional warming onto regional heat waves, significantly intensifying urban thermal stress and health risks . It will highlight that this amplification is not merely additive but multiplicative, leading to more pronounced and dangerous heat-related outcomes, particularly for vulnerable urban populations .

A central theme throughout the survey will be the identification of deficiencies in current research and methodologies. Both introductory sections underscore a prevailing challenge: existing studies often fail to comprehensively frame the problem of compound urban heat events, either by treating UHI and heat waves in isolation or by not fully quantifying their synergistic impacts . This fragmentation leads to an underestimation of actual urban thermal risk and limits the development of holistic mitigation strategies. The complexity of modeling non-linear interactions, a historical emphasis on single-hazard analyses, and the lack of standardized definitions contribute to these research gaps . This survey aims to bridge these gaps by advocating for an integrated, interdisciplinary approach that considers the unique vulnerabilities arising from urban design, material choices, and anthropogenic heat sources during compound extreme heat events. The increasing frequency and intensity of these events globally necessitate a more robust understanding to inform effective adaptation and risk mitigation, especially given the high proportion of the global population residing in vulnerable urban settings .

1.1 Defining the Urban Thermal Landscape and Heat Extremes

Urban thermal risk is a critical concern, fundamentally shaped by the interplay between the Urban Heat Island (UHI) effect and heat waves (HWs).

The UHI effect describes the phenomenon where urban areas consistently exhibit higher temperatures than their surrounding rural counterparts .

Characteristic	Description	Impact on UHI Effect
Reduced Vegetation	Less green cover leads to reduced evapotranspiration.	Increases UHI
Impervious Surfaces	Paved surfaces (asphalt, concrete) absorb and re-emit solar radiation.	Increases UHI
Urban Canyons	Closely packed buildings trap heat and reduce air circulation.	Increases UHI
Anthropogenic Heat	Heat released from vehicles, industry, air conditioning, and heating systems.	Increases UHI
Building Materials	Dark, dense materials absorb and retain more heat.	Increases UHI
Reduced Nocturnal Cooling	Slow release of absorbed heat leads to higher nighttime temperatures.	Increases UHI

This temperature differential is a result of several anthropogenically altered urban characteristics. Specifically, reduced vegetation in urban settings leads to diminished evapotranspiration, a natural cooling process . The prevalence of dark, impervious surfaces like asphalt and concrete, which possess low albedo, absorb and re-emit solar radiation, contributing significantly to heat accumulation . These paved surfaces can reach temperatures 27-50°C hotter than the ambient air, and large cities can experience nighttime temperatures up to 12°C warmer than their surroundings due to the slow release of absorbed heat .

Furthermore, the architectural configuration of cities, characterized by closely packed buildings and "urban canyons," traps heat and impedes air circulation, exacerbating the UHI effect . Anthropogenic heat emissions from vehicles, industrial processes, air conditioning units, and furnaces also contribute substantially to urban warming . This cumulative effect means cities are not only inherently warmer but also experience reduced nighttime cooling, increasing continuous heat exposure for their dense populations .

In contrast, heat waves (HWs) are defined as sustained periods of excessively high temperatures that elevate ambient temperatures across both urban and rural landscapes . These events are becoming more frequent and intense globally, a direct consequence of climate change, leading to a rise in very hot days and nights . For instance, 2024 has been noted as potentially the hottest year on record, with global temperatures approaching the critical 1.5°C threshold, amplifying the severity of extreme heat events . Such extreme heat events (EHEs) are already responsible for more climate-related fatalities annually in the U.S. than any other extreme weather phenomenon .

The true amplification of urban thermal risk arises from the combination of the UHI effect and heat waves, forming a compound extreme event. While heat waves elevate temperatures across broad regions, the UHI effect superimposes an additional layer of warming specifically within urban environments . This synergistic interaction means that the impact in cities is greater than the sum of its individual parts . For example, an urban area experiencing a heat wave will face temperatures that are not only high due to the regional heat wave but are further elevated by the localized UHI effect. This significantly intensifies the heat stress on urban populations, many of whom, like those in Vancouver, are not accustomed to such extreme conditions and reside in buildings not designed for prolonged heat exposure . With over 30 million Canadians living in urban settings, their vulnerability to these amplified heat risks is particularly pronounced .

Despite the clear and increasing threat posed by compound urban heat events, existing works and current methodologies often exhibit deficiencies in comprehensively framing the problem. A primary critique is the limited consideration of urban-specific contributions to thermal risk. Many studies tend to focus on the broader atmospheric drivers of heat waves or the general characteristics of the UHI effect in isolation, without adequately integrating the complex, localized interactions that occur when both phenomena coincide. For example, while the increase in extreme weather and climate events in China, including heatwaves, is discussed, and definitions of extreme events using absolute and relative thresholds are provided, there is often a lack of detailed foundational concepts such as how urban heat islands specifically interact with these events to amplify risk within urban contexts .

The root causes of these deficiencies are multifaceted. One significant factor is the complexity of modeling and measuring the synergistic effects. Quantifying the precise additional temperature increment from UHI during a heat wave, and disentangling it from the broader atmospheric warming, requires sophisticated urban climate models and high-resolution observational data, which are not always readily available or integrated into standard analyses. Furthermore, the variability in urban forms—from sprawling cities to compact ones—introduces complexities, as different urban morphologies can exhibit varying vulnerabilities to climate change and extreme heat events . Current methods may also lack the interdisciplinary frameworks necessary to bridge the gap between meteorological definitions of heat waves and urban planning/design principles that influence UHI intensity. This leads to a fragmented understanding, where the unique vulnerabilities arising from urban design, material choices, and anthropogenic heat sources are not fully accounted for in thermal risk assessments during compound events. Consequently, there is an imperative to advance research that explicitly addresses this synergy, providing a more holistic and actionable understanding of urban thermal risk.

1.2 The Emergence of Compound Extreme Heat Events and Their Amplified Risk

The escalating threat of extreme heat in urban environments is significantly amplified by the emergence of "compound extreme events" . These events are defined as the simultaneous or consecutive occurrence of two or more extreme climate drivers or hazards, leading to consequences that are more severe than the sum of their individual impacts . This concept is crucial for understanding urban thermal risk, as the interplay of global warming trends and localized phenomena, such as the Urban Heat Island (UHI) effect, intensifies heat periods and poses severe hazards to urban populations and infrastructure .

The amplification of urban thermal risk due to compound extreme heat events is particularly evident in the synergistic interactions between UHIs and heat waves .

Phenomenon	Individual Impact	Combined Impact (Additive)	Combined Impact (Synergistic/Compound)	Amplification Factor
Heat Wave (HW)	Moderate	-	-	-
Urban Heat Island (UHI)	Moderate	-	-	-
HW + UHI	Sum of HW & UHI	HW + UHI	> HW + UHI	Significant
Overall Urban Thermal Risk	Lower	Higher	Much Higher	Amplified

Research indicates that the combined effect of these two phenomena in cities is greater than the mere summation of their individual impacts, a critical insight that underscores the heightened danger . This amplification stems from mechanisms that intensify the temperature difference between urban and rural areas specifically during heat waves, leading to more pronounced and dangerous heat-related health risks for urban residents . For instance, while the UHI effect generally raises urban temperatures, its interaction with a large-scale heat wave can lead to nighttime temperatures remaining exceptionally high, hindering the body's ability to recover from daytime heat stress and significantly increasing the risk of heat-related illness and mortality. This synergistic relationship mandates a more integrated understanding for effective adaptation and mitigation strategies .

Despite the growing recognition of compound extreme events as a frontier issue in climate change research, existing works and current methodologies often exhibit deficiencies in framing the problem, particularly concerning urban-specific compounding mechanisms . Many studies, while acknowledging the escalating threat of extreme heat and the UHI effect, do not explicitly define or discuss the synergistic interaction of multiple extreme heat factors to produce amplified impacts . For example, some resources focus broadly on urban heat risk management or the UHI effect's contribution to heat risks, without delving into the specific definition or complex interactions of "compound extreme heat events" as a distinct phenomenon . While they implicitly address the amplification of risk by discussing how UHI enhances heat during heat waves, they do not formalize the concept of "compound extreme heat events" with its unique synergistic properties . Similarly, the association between urban form, such as sprawling city patterns, and the frequency of Extreme Heat Events (EHEs) is acknowledged, implying an amplification of heat impacts, but the explicit compounding mechanisms are often not the primary focus .

The root causes of these deficiencies are multifaceted. Firstly, the complexity of capturing the non-linear interactions between various extreme events and localized urban factors makes comprehensive modeling and analysis challenging. Traditional climate models often focus on single extreme events or global climate trends, sometimes overlooking the finer-scale, urban-specific interactions that create compound risks. For instance, climate models that do not explicitly account for UHI effects may underestimate future heat risks in cities, which are projected to be significantly higher due to the combination of global warming and local urban heat intensification . Secondly, there is a historical emphasis on analyzing individual hazards rather than their co-occurrence or sequential impacts. This siloed approach limits the understanding of how multiple stressors combine to create heightened vulnerabilities. Thirdly, the lack of standardized definitions and methodologies for identifying and quantifying compound extreme heat events hinders comparative studies and the development of robust, universally applicable solutions. The nuanced mechanisms through which urban environments modulate these events, such as heat absorption by built surfaces, reduced ventilation in dense urban canyons, and anthropogenic heat emissions, require more detailed investigation.

The escalating threat posed by compound extreme heat events underscores the critical importance of a comprehensive survey. Anthropogenic climate change has already led to more pronounced weather and climate extremes globally, with a rising probability of these compound events that significantly amplify risk . Regions like Asia, which are already highly susceptible to weather, climate, and water-related hazards, urgently require an integrated understanding of how climate change influences compound events for effective forewarning and risk mitigation strategies . Given that a significant proportion of the global population resides in urban settings—for example, 4 out of 5 Canadians live in urban areas facing increased heat risks —the need to address urban thermal risk from a compound event perspective is paramount. This survey aims to synthesize existing knowledge, highlight critical gaps, and provide a framework for future research and policy development to mitigate the severe consequences of these increasingly frequent and intense phenomena.

2. Defining and Characterizing Compound Extreme Heat Events in Urban Environments

This section provides a comprehensive overview of compound extreme heat events within urban environments, essential for understanding and mitigating their complex impacts. It begins by establishing a precise conceptual framework, differentiating urban-specific compound heat events from broader climatic phenomena. The discussion then transitions to the methodologies employed for identifying and analyzing these events, critically examining current approaches, data sources, and analytical techniques. This dual focus ensures a thorough exploration of both the theoretical underpinnings and practical tools crucial for research in this domain.

The conceptual framework section delineates "compound extreme heat events" as those where the synergistic interaction between urban heat islands (UHI) and heat waves results in impacts disproportionately larger than the sum of their individual effects . This builds upon a general definition of "compound weather and climate extremes" that involves multiple drivers or hazards collectively increasing risk . Key physical drivers of this urban synergy include reduced moisture availability, low wind speeds, and distinct surface energy partitioning in urban areas, all of which amplify heat exposure during heat waves . These events are categorized into preconditioned, multivariate, temporally compounding, and spatially compounding types, adapting broader climate extreme typologies to the urban context . Despite these advancements, challenges remain in precise classification due to the inherent complexity and overlap of urban meteorological phenomena and human activities, suggesting a need for a more refined and integrated conceptualization .

Following the conceptual definitions, the section on methodologies for identifying and analyzing compound heat events details the diverse data sources, analytical techniques, and metrics utilized. Data sources range from ground-based meteorological stations and remote sensing data (e.g., MODIS, Landsat) to citizen weather stations and global climate model outputs like CMIP6 . Analytical techniques encompass statistical analysis and advanced climate modeling, particularly mesoscale models like the Weather Research and Forecasting (WRF) model coupled with urban canopy models (UCMs) for high-resolution simulations . Metrics commonly include extreme temperature indices, UHI-specific metrics (e.g., tropical nights), and heat stress indices like apparent temperature defined as $A = T - 0.55556 \times (6.11 \times e^{(5417.753 \times (1/273.16 - 1/(T+273.15)))} - 10)$ . Despite the efficacy of high-resolution models and extensive observational data, a significant limitation is the insufficient integration of UHI effects into broader climate projections and the challenge of representing complex urban morphology in large-scale assessments. Future research should prioritize enhancing data resolution, developing more sophisticated urban climate models that account for diverse urban surface materials and energy dynamics, refining compound event definitions specific to urban contexts, and coupling climate models with socioeconomic models to project comprehensive impacts and evaluate adaptation strategies . These improvements are vital for a more accurate understanding and effective management of urban thermal risk from compound extreme heat events.

2.1 Conceptual Framework of Compound Extreme Heat Events

Compound extreme heat events represent a critical area of research, necessitating a precise conceptual framework for effective analysis and mitigation. In a broader context, "compound weather and climate extremes" are characterized as the combination of multiple drivers and/or hazards that collectively contribute to societal or environmental risk, often yielding impacts far greater than the sum of individual extremes . This general definition encompasses four primary categories: preconditioned events, where a preceding condition exacerbates a subsequent hazard; multivariate events, involving multiple drivers or hazards simultaneously leading to an impact; temporally compounding events, marked by a succession of hazards; and spatially compounding events, where hazards affect multiple interconnected locations .

While the general definition provides a foundational understanding, the nuances of "synergistic interactions between urban heat islands and heat waves" are paramount in defining "compound extreme heat events" specifically . The Urban Heat Island (UHI) effect, where urban areas are inherently warmer than their rural surroundings due to modified energy balance from built materials, anthropogenic heat, reduced vegetation, and altered air circulation, serves as a persistent preconditioning factor . Heat waves, defined as extended periods of unusually high atmospheric heat stress, act as the primary hazard . The critical distinction lies in the synergistic interactions, where heat waves intensify the UHI effect, and conversely, the UHI amplifies the impact of heat waves, leading to an impact that is "larger than the sum of its parts" .

This synergistic interaction is driven by several key physical attributes. Firstly, reduced moisture availability in urban areas, a characteristic feature of built environments, exacerbates the UHI effect during heat waves by limiting evaporative cooling compared to vegetated rural areas . Secondly, the low wind speeds often associated with heat waves further intensify the UHI effect by reducing advective cooling within urban canyons . Finally, differences in surface energy partitioning between urban and rural areas contribute significantly; urban surfaces absorb and store more heat, releasing it slowly, particularly at night, which diminishes nighttime cooling and prolongs heat exposure during a heat wave . Researchers have developed analytical models to capture these intricate interactions, accounting for factors such as moisture availability, wind speed, and energy budget disparities between urban and rural landscapes .

Categorizing compound extreme heat events often aligns with the broader typologies of compound weather and climate extremes, adapted to the urban thermal context. Based on temporal and spatial co-occurrence, these events can be classified as:

1. Preconditioned Events: The persistent UHI effect acts as a precondition, elevating baseline urban temperatures, which then amplifies the severity of an incoming heat wave. While not explicitly defined as a distinct category of compound heat event, the UHI's role as a constant urban heat driver implies this preconditioning .
2. Multivariate Events: These involve the simultaneous occurrence of multiple factors contributing to urban heat risk, such as high air temperature, high humidity, and low wind speed, alongside the UHI effect, leading to significantly higher perceived temperatures and heat stress . An example noted is warm-humid events in the context of China .
3. Temporally Compounding Events: This category refers to a succession of heat-related hazards. For instance, multiple heat waves occurring in close succession with insufficient recovery periods between them, further exacerbated by the underlying UHI. While not explicitly detailed as a typology of compound heat events, the general concept of "succession of hazards" implies this application .
4. Spatially Compounding Events: These involve heat events affecting multiple connected urban areas or large urban sprawls, where the aggregate heat stress is amplified across a wider geographical extent .

Despite advancements, critiques and deficiencies in the current conceptualization of compound extreme heat events persist. A significant limitation is the ambiguity in classification, with many events exhibiting characteristics of multiple categories . This overlap can lead to incomplete typologies, making it challenging to precisely delineate and study distinct event types. The root cause often lies in the complex, interlinked nature of urban meteorological phenomena and human activities, which makes clear categorization difficult. For instance, the implicit acknowledgment of climate change exacerbating heat extremes and intensifying heat waves due to the UHI in cities highlights a concurrent and intensifying exposure that blurs simple distinctions .

Furthermore, while synergistic mechanisms between UHI and heat waves are increasingly recognized, a deeper understanding of these complex interactions is needed. The "synergistic interactions between urban heat islands and heat waves, the impact in cities is larger than the sum of its parts" study develops an analytical model for these interactions , but applying such models broadly and capturing all possible contributing factors (e.g., anthropogenic heat, specific urban morphologies, and local atmospheric dynamics) remains a challenge. Current methods may not fully capture the cascading and feedback loops between different heat drivers, potentially underestimating the total urban thermal risk. For example, the influence of urban sprawl on extreme heat event frequency and severity is acknowledged, but its precise role in compounding mechanisms requires further elucidation . The deficiency stems from the inherent complexity of urban systems, the multitude of interacting variables, and the limitations of observational data and modeling capabilities to fully resolve these processes across varying scales. More sophisticated modeling approaches that integrate atmospheric, land surface, and urban canopy models are essential to overcome these limitations and refine the conceptual framework of compound extreme heat events.

2.2 Methodologies for Identifying and Analyzing Compound Heat Events

The methodologies employed for identifying and analyzing compound heat events, particularly those involving urban heat island (UHI) effects and broader extreme heat phenomena, exhibit a diverse range of approaches in terms of data sources, analytical techniques, and metrics. A critical comparison reveals both robust frameworks and notable gaps in urban-specific applications.

Data sources for analyzing compound heat events are multifaceted. Ground-based meteorological stations provide essential observational records, as seen in the use of meteorological sensors for 2-m air temperature analysis in Baltimore and the broader mention of meteorological data in urban heat risk assessments . Remote sensing data, particularly from satellites like MODIS and Landsat, are crucial for capturing spatial patterns of heat, such as land surface temperature for model validation and for general urban climate analysis and heat mapping projects . Citizen weather stations (CWS) also contribute to dense urban observational networks . Furthermore, global climate model outputs, such as CMIP6, are frequently utilized for future projections and attribution studies of compound events . For specific urban analyses, urban morphology data is implicitly considered through urban design considerations and explicitly quantified through indices like the sprawl index, which measures land-use patterns based on centeredness, connectivity, density, and land-use mix .

Analytical techniques vary from statistical methods to sophisticated climate modeling. Statistical analysis is a common thread, applied in heat risk assessments , to investigate characteristics of compound events , and for correlating urban form with extreme heat event occurrences using partial correlation coefficients . Climate modeling, particularly mesoscale models like the Weather Research and Forecasting (WRF) model with urban canopy models (UCMs), is critical for high-resolution simulations of urban and rural temperatures . Models like MUKLIMO_3 are used for simulating urban climate processes, identifying hot spots, and evaluating adaptation measures . Attribution methods are also employed to understand the driving mechanisms of compound events, as seen in studies of compound heatwave-drought events . An analytical model, such as the one based on Yeh and Brutsaert (1971a,b), can be developed to unravel physical mechanisms, expressing the UHI index ($DT$) as: $DT = \left(1 - \frac{b\_u}{b\_r}\right)T\_{rs}^\* f\_1(x, z) + g(u\_{10})(Q\_u - Q\_r)f\_2(x, z)$ .

Common metrics used to quantify the intensity, duration, and spatial extent of these events include extreme temperature and precipitation indices developed by the ETCCDI . Specific UHI-related metrics include the mean annual number of summer days (Tmax ≥ 25°C) and tropical nights (Tmin ≥ 20°C) . Heat stress indices, such as apparent temperature, are also used, defined by the formula: $A = T - 0.55556 \times (6.11 \times e^{(5417.753 \times (1/273.16 - 1/(T+273.15)))} - 10)$, where $T$ is ambient air temperature (°C) and $e$ is water vapor pressure (kPa) . Projections often refer to increases in "very hot days" (e.g., +30°C days) .

Despite these advancements, significant strengths and limitations exist, particularly in capturing urban compounding effects. The strength of high-resolution climate models like WRF, coupled with urban canopy models, lies in their ability to simulate fine-scale urban atmospheric processes and assess urban and rural temperature differences with good agreement to observations (e.g., mean biases of -1.6 K and -1.2 K for air temperature at urban and suburban sites, respectively) . Observational networks and remote sensing data provide empirical evidence of UHI patterns and heat loads. The review of case studies and expert interviews provides a qualitative understanding of urban heat risk management .

However, several methodological deficiencies hinder a comprehensive understanding of urban compounding effects. A primary gap is the insufficient integration of UHI effects into broader climate projections. For instance, projections of increasing hot days often do not incorporate the UHI effect, implying that actual urban heat numbers will be higher . This highlights a lack of high-resolution urban data specifically tailored for climate models and an insufficient consideration of urban morphology in large-scale climate assessments. While some studies incorporate urban morphology via sprawl indices , a holistic, dynamic representation of urban design and land use within climate models is often absent.

The root causes of these deficiencies include the computational intensity required for high-resolution urban climate modeling across large domains, data coverage issues, and the inherent complexity of integrating diverse datasets (e.g., meteorological, remote sensing, socioeconomic, and urban planning data) . Furthermore, there is a knowledge gap in the complex spatial and temporal variability of urban atmospheric phenomena and their interaction with large-scale climate drivers .

Future research directions should focus on developing coupled urban climate-socioeconomic models. This would allow for a more comprehensive simulation of cascading impacts of compound heat events, moving beyond mere physical climate modeling to include human behavior, infrastructure vulnerability, and public health outcomes. Specifically, efforts should be directed towards:

1. Enhancing Data Resolution and Integration: Establishing denser urban meteorological observation networks, integrating CWS data more systematically, and improving the accessibility and fusion of high-resolution remote sensing data with detailed urban morphology and infrastructure datasets.
2. Advanced Urban Climate Modeling: Developing and validating urban canopy models that account for a wider range of urban surface materials, green infrastructure, and building energy dynamics. This includes improving the representation of anthropogenic heat release and its interaction with atmospheric processes.
3. Refining Compound Event Definitions and Metrics for Urban Contexts: Tailoring existing extreme event indices and developing new metrics that specifically capture the unique characteristics of urban heat compounding, such as combined intensity-duration-spatial extent metrics that account for UHI amplification.
4. Socio-economic-Climate Model Coupling: Creating interdisciplinary models that link urban climate simulations with socioeconomic models to project health impacts, energy demands, and economic losses under different compound heat scenarios. This would facilitate the assessment of adaptation strategies' effectiveness.
5. Attribution and Projection of Urban Compound Events: Focusing attribution studies on the specific contributions of urbanization to the frequency and intensity of compound heat events, and developing urban-inclusive climate projections that incorporate UHI effects for more realistic future scenarios.

By addressing these methodological gaps, researchers can significantly improve the understanding, prediction, and management of urban thermal risk from compound extreme heat events.

3. Amplified Urban Thermal Risk: Impact Pathways of Compound Extreme Heat Events

Compound extreme heat events, characterized by the synergistic interactions of phenomena such as urban heat islands (UHIs) and heat waves, pose a significantly amplified thermal risk to urban environments. Unlike isolated heat occurrences, these compound events create conditions where the cumulative impact is greater than the sum of its individual parts, leading to more severe and far-reaching consequences across various urban systems .

This section delineates the critical impact pathways through which these amplified thermal risks manifest, focusing on their profound effects on public health, the integrity and functionality of urban infrastructure and energy systems, and the exacerbation of existing socio-economic disparities. Furthermore, it explores the critical implications for urban planning and governance, highlighting both current challenges and future research imperatives for building resilient cities.

Impact Area	Manifestation	Notes on Compound Effects
Heat-Related Illness	Heat exhaustion, heatstroke, dehydration.	UHI + Heatwave amplifies ambient temperatures, increasing exposure duration and intensity.
Cardiovascular Strain	Aggravation of heart disease, increased risk of heart attacks.	Prolonged high temperatures and UHI lead to sustained physiological stress.
Respiratory Issues	Worsening of asthma, COPD due to increased ozone and air pollution.	Higher temperatures accelerate smog formation, exacerbated by urban emissions.
Allergies	Increased pollen and fungal spore production.	Heat can stimulate plant growth and spore release.
Vulnerable Populations	Disproportionate impact on elderly, children, chronically ill, low-income groups.	Limited access to cooling, poorer housing quality, and pre-existing conditions worsen outcomes.

The escalating thermal burden from compound extreme heat events directly translates into significant public health challenges. The synergistic effects of UHIs and heat waves intensify physiological stress, leading to a surge in direct heat-related illnesses and exacerbating pre-existing cardiovascular and respiratory conditions . Beyond direct physiological impacts, these events degrade air quality by accelerating ozone and smog formation and increasing allergen concentrations, further compromising public health . Critically, the vulnerability to these health impacts is not uniform, with demographic and socioeconomic factors creating marked disparities, disproportionately affecting the elderly, children, individuals with chronic illnesses, and those in lower-income brackets due to limited access to cooling and adequate housing . Existing research, while acknowledging these impacts, often lacks fine-grained data and comprehensive models to fully capture the complex compounding mechanisms and long-term health consequences, necessitating future studies that integrate epidemiological and microclimate data for more nuanced vulnerability assessments.

The amplified thermal risk also imposes severe stress on critical urban infrastructure and energy systems.

Infrastructure/System	Impact of Compound Heat Events	Mechanism	Consequences
Energy Systems	Escalating demand for cooling; strain on power grids.	Increased AC use due to amplified heat (UHI + Heatwave); thermal stress on equipment.	Vulnerability to blackouts, grid instability, increased reliance on fossil fuels for power generation.
Transportation	Potential for material degradation (e.g., asphalt softening), operational disruptions.	High surface temperatures; thermal expansion of materials.	Increased maintenance needs, reduced efficiency, potential for delays or accidents.
Water Systems	Increased demand for water (drinking, cooling centers); strain on distribution networks.	Higher evaporation rates; increased water consumption for personal cooling.	Potential for shortages, stress on pumping and treatment facilities.
Building Envelope	Increased heat gain; material degradation from prolonged high temperatures.	Absorption and retention of heat by building materials; thermal cycling stress.	Reduced comfort, higher energy costs for cooling, potential structural damage over time.
Communication	Potential disruption due to power outages affecting network infrastructure.	Reliance on electrical power for telecommunication equipment and data centers.	Loss of critical communication services during emergencies.

The most immediate and widely recognized impact is the escalating energy demand for cooling, which strains power grids and increases the vulnerability to outages and blackouts . This heightened demand, coupled with the direct thermal stress on equipment and aging infrastructure, exposes inherent systemic fragilities. While the energy sector's vulnerability is well-documented, the cascading failures across interconnected urban systems—such as transportation, water supply, and communication—remain less explored in detail within the current literature . Future research should prioritize developing sophisticated models for simulating cascading failures and conducting comprehensive analyses across all infrastructure types to bolster urban resilience.

Furthermore, the interplay between urban form and socio-economic disparities significantly shapes a city's vulnerability to compound extreme heat events. Sprawling urban areas, characterized by lower density and reduced land-use mix, have been shown to experience a higher rate of extreme heat events compared to compact cities . This underscores how urban planning decisions profoundly influence thermal risk. Marginalized communities, particularly those in poverty, women, children, and the elderly, disproportionately bear the brunt of these events due to factors like inadequate housing, limited access to green spaces and cooling resources, and systemic inequalities . Effective urban planning strategies, including increasing green infrastructure, promoting heat-resilient designs, and integrating disaster risk management, are crucial for mitigating these risks and fostering equity . However, current research often falls short in comprehensively integrating socio-economic data with urban morphology in spatially explicit vulnerability assessments and in analyzing the systemic roots of these disparities, highlighting critical areas for future investigation into equitable and effective heat adaptation strategies.

3.1 Health Impacts and Vulnerable Populations

The interaction between urban heat islands (UHIs) and heat waves significantly amplifies health risks for urban populations, creating compound extreme events that pose greater threats than individual phenomena . This compounding effect leads to more severe thermal conditions in cities, intensifying physiological responses to heat stress . Prolonged exposure to elevated temperatures, particularly above 35°C, can induce a range of direct health problems, from mild discomfort and heat exhaustion to severe heatstroke and even death . Beyond direct heat-related illnesses, the strain on the cardiovascular system is exacerbated, aggravating pre-existing conditions and increasing the risk of heart problems . For instance, epidemiological studies in the US indicate a 4.5% increase in mortality risk for every 1°C rise in heat wave intensity .

The synergistic interactions also worsen air quality, a critical indirect health impact. Higher temperatures accelerate the formation of ground-level ozone and smog from vehicle exhaust . Additionally, increased pollen and fungal spore production during heat events can trigger or exacerbate respiratory issues and allergies . These combined factors are estimated to contribute significantly to global heat wave-associated morbidity and mortality, as evidenced by fatalities during heat waves in British Columbia (2009) and Quebec (2010) . Research focusing on China further elucidates the health implications of compound extreme events, specifically continuous day-night hot extremes, temperature-humidity compound events, and high-temperature-ozone compound events, all of which contribute to increased mortality and morbidity, aggravation of cardiovascular and respiratory diseases, and sleep disruption . The intensified association between ozone and mortality under high temperatures highlights the complexity of these interactions .

Vulnerability to these compound heat events is not uniform across urban populations; rather, it is significantly shaped by demographic and socioeconomic factors, leading to marked disparities in health outcomes. Critically vulnerable groups consistently identified across studies include the elderly, infants, young children, individuals with chronic illnesses or disabilities, and pregnant women . Socioeconomic factors, such as living in poverty, contribute substantially to increased vulnerability . Poorly insulated housing, lack of access to air conditioning, and social isolation are critical exacerbating factors, particularly for those in lower-income brackets, as they limit effective relief from prolonged heat exposure . Outdoor workers and those engaged in high-activity professions are also at heightened risk due to occupational exposure . The higher mortality rates during heat waves in cities, largely attributable to the UHI effect, underscore the disproportionate impact on urban dwellers .

Case studies further illustrate these disparities. For example, in Amadora, Portugal, and Incheon, Republic of Korea, the elderly and homeless populations were particularly affected, while in Cape Town, South Africa, vulnerable residents required tailored interventions . The root causes of these disparities stem from a combination of physiological susceptibility and socioeconomic disadvantages that limit adaptive capacity. Age affects thermoregulation, chronic diseases compromise the body's ability to cope with heat stress, and poverty restricts access to cooling technologies or safe, climate-controlled environments. Increased urbanization and aging populations, combined with more frequent and intense heat waves, are projected to escalate heat-related deaths unless robust measures are implemented .

Despite advancements, existing research on the health impacts of compound heat events exhibits certain critiques and deficiencies. A primary limitation is often the availability of fine-grained data on specific vulnerable groups, which hinders a precise understanding of differential impacts and the efficacy of targeted interventions . Furthermore, current methods may not sufficiently consider the complex compounding mechanisms between UHIs and heat waves, which can lead to an underestimation of actual risks . While some studies highlight the synergistic interactions and increased mortality risk (e.g., 4.5% for every 1°C increase in heat wave intensity in the US and 0.38% per day increase in heat wave duration) , more comprehensive analysis of these compounding factors is needed to develop robust models for risk assessment.

Future research should prioritize integrating epidemiological data with fine-grained urban microclimate data. This would allow for a more nuanced understanding of how local urban morphology and materials influence heat exposure at the individual or neighborhood level, thereby refining vulnerability assessments. There is also a need for more research into the long-term health consequences of repeated exposure to compound heat events, beyond immediate mortality and morbidity. Investigating the effectiveness of different urban planning and public health interventions in mitigating these compound risks, particularly for the most vulnerable populations, is crucial. Moreover, research comparing physiological adaptation thresholds across different urban populations and climates could provide valuable insights, especially given suggestions that projected temperature increases may outpace current adaptive capacities .

3.2 Urban Infrastructure and Energy Systems Stress

Sustained and intense heat, particularly in the context of compound extreme heat events, imposes significant stress on urban infrastructure and energy systems, often leading to impacts far greater than those resulting from individual heat occurrences. A primary concern is the escalating energy demand for cooling, driven by amplified temperatures in urban areas. Studies indicate that increased reliance on air conditioning during prolonged heat waves leads to higher electricity demand, particularly during peak hours, thereby straining power grids . This elevated demand significantly increases the vulnerability of urban energy grids to outages or blackouts . The compounded effect of urban heat island (UHI) phenomena intensifying heat exposure further exacerbates this energy demand, creating a positive feedback loop where increased cooling efforts, if reliant on high-carbon fossil fuels, can contribute to global warming and subsequent higher temperatures .

The compounding nature of heat events, such as the synergistic interactions between urban heat islands and heat waves, means that the overall impact on energy systems is larger than the sum of its parts . Intensified and prolonged heat waves increase the risk of stressing electric grids, directly correlating with the increased reliance on cooling systems to mitigate heat-related health risks like mortality and morbidity . This vulnerability is not limited to electricity supply; for instance, the implicit threat of disrupted cooling equipment during compound flood-heat events highlights a broader vulnerability of infrastructure components beyond just the electrical grid .

While the direct impact of sustained heat on transportation networks and water systems is less explicitly detailed in some of the reviewed literature, the implications are evident. For example, the "Analysis of Heat Waves and Urban Heat Island Effects in Central European Cities" report implies potential impacts on various infrastructure types through increased accident risks and productivity losses during heat events, though specific details on transportation or water supply were not provided . Similarly, other studies allude to general property damage and economic losses from compound events , and the necessity for cooling centers and heat-resistant materials implicitly suggests stress on broader urban systems . However, a detailed analysis of how compound heat events specifically affect road integrity, rail systems, or water distribution and waste management remains largely underexplored in the current digests.

Cascading failures, while a critical aspect of infrastructure vulnerability under extreme heat, are not extensively documented with specific examples in the provided literature. The general concern about blackouts due to overloaded electrical systems during peak cooling demand suggests a potential for cascading failures within the energy grid itself . For instance, an initial power outage can disrupt essential services, leading to failures in communication systems, water pumping stations, or even traffic signals, subsequently impacting emergency response capabilities. The underlying causes of such vulnerabilities often stem from an aging infrastructure, insufficient capacity planning for extreme conditions, and a lack of integrated resilience strategies that consider interdependencies between different urban systems. The compounded effects of heat waves and UHIs further expose these weaknesses by creating unprecedented stress levels that individual systems are not designed to withstand .

A significant critique of existing work, as evidenced by the scope of the provided digests, is the limited focus on the direct and detailed analysis of cascading failures within urban infrastructure systems resulting from compound heat events. While some papers acknowledge general impacts on infrastructure or implicitly touch upon vulnerabilities, specific examples of how failures in one system propagate to others are not explicitly provided . Furthermore, there is a deficiency in comprehensively addressing the impacts across all critical urban infrastructure types. The emphasis tends to be heavily placed on energy grids, with less detailed examination of transportation networks, water supply systems, and waste management infrastructure under extreme heat conditions . While the health impacts of heat are well-documented, the systematic linkage between these health risks and specific infrastructure failures requires more in-depth exploration.

Future research directions should therefore prioritize the development of integrated resilience assessments for critical urban infrastructure under compound heat events. This would involve:

1. Modeling Cascading Failures: Developing sophisticated models that can simulate the propagation of failures across interconnected urban systems (e.g., energy to water, energy to transportation) under various compound heat scenarios. This would allow for the identification of critical nodes and pathways for vulnerability.
2. Comprehensive Infrastructure Analysis: Expanding research to systematically evaluate the impacts of compound heat events on all facets of urban infrastructure, including transportation networks (roads, railways, public transit), water supply and wastewater treatment, and waste management systems. This should involve detailed studies on material degradation, operational disruptions, and maintenance challenges.
3. Socio-Technical System Integration: Investigating the interplay between technological infrastructure failures and their socio-economic consequences, including how disruptions affect vulnerable populations and essential services. This would include assessing the role of policy, governance, and community preparedness in mitigating infrastructure-related risks.
4. Cost-Benefit Analysis of Adaptation Measures: Quantifying the economic costs of infrastructure damage and disruption from compound heat events, alongside the benefits of implementing various adaptation strategies, such as smart grid technologies, resilient materials, and demand-side management.

Comparing the reported impacts, it is clear that increased energy demand for cooling represents a universally recognized and immediate stressor on urban energy systems, directly linked to the compounded effect of heat waves and urban heat islands . The root causes of system vulnerabilities can be attributed to: (1) Increased Demand: The physiological need for cooling drives up electricity consumption to unprecedented levels. (2) Thermal Stress on Equipment: High temperatures can directly affect the efficiency and longevity of power generation and transmission equipment. (3) Aging Infrastructure: Many urban power grids were not designed to handle the sustained loads and extreme temperatures now being experienced. (4) Interdependencies: The reliance of other critical services (e.g., water, communication) on a stable power supply creates systemic fragility. While the immediate stress on energy systems is well-documented, the downstream impacts on other infrastructure types remain less explored, signifying a critical gap in understanding the holistic implications of compound extreme heat events on urban resilience.

3.3 Socio-Economic Disparities and Urban Planning Implications

Urban form significantly influences a city's vulnerability to extreme heat events, with discernible differences between sprawling and compact urban structures. Sprawling cities, characterized by low-density land use and reduced land-use mix, exhibit a heightened susceptibility to extreme heat events compared to their compact counterparts . Research indicates that the rate of increase in annual extreme heat events (EHEs) can be more than double in the most sprawling metropolitan regions compared to the most compact ones . This suggests that urban planning decisions regarding density, land use, and green space preservation, often negatively impacted by sprawl, critically determine heat vulnerability . The urban heat island (UHI) effect, a phenomenon of built environments, further exacerbates thermal stress in dense urban areas, necessitating careful urban planning and architectural considerations to mitigate its impacts .

Compound heat events disproportionately affect marginalized communities, exacerbating existing social inequalities . Vulnerable populations, including individuals in poverty, women, children, and the elderly, are particularly susceptible . Case studies from cities like Amadora, Cape Town, and Nairobi reveal that low-income neighborhoods and informal settlements, often lacking adequate green spaces and cooling resources, experience higher heat vulnerability . This disparity is reinforced by the fact that many low-income individuals lack access to air conditioning, a critical cooling mechanism during heat waves .

Insights from public health and sociology underscore the multifaceted nature of these disparities. Sprawl, for instance, is linked to broader adverse health outcomes beyond heat exposure, such as reduced physical activity and poor air quality . The lack of access to public amenities, such as air-conditioned facilities, public pools, or splash pads, further compounds the vulnerability of low-income families and the homeless during heat waves . Developing countries, particularly in regions like South Asia, face greater risks from climate extremes due to inherent higher exposure and vulnerability . This heightened vulnerability is rooted in systemic socio-economic inequalities, including limited infrastructure, inadequate access to healthcare, and a higher prevalence of informal settlements that lack resilient designs.

Urban planning strategies are crucial for mitigating these risks and fostering equity . Equitable solutions in urban planning include increasing green spaces, integrating heat-resilient designs in disadvantaged areas, and promoting passive cooling elements in building architecture, such as strategic window placement and green roofs . Cities like Toronto have implemented large-scale measures, such as providing access to air-conditioned facilities during heat waves, which can serve as critical immediate relief measures . The UHI effect can expand areas with excessive heat load due to urbanization, as observed in Zagreb, highlighting the need for sustainable urban development and climate-sensitive urban planning to address these risks . Understanding urban climate processes and UHI effects is paramount for future development plans and adaptation strategies, enabling cost-effective, evidence-based solutions . Integrating disaster risk management approaches into urban planning is essential for building urban resilience to heat .

Despite existing efforts, current methods regarding socio-economic disparities and urban planning often suffer from limitations, such as incomplete consideration of equity or the exclusion of certain urban planning aspects. For instance, while some studies highlight vulnerable populations, the comprehensive integration of socio-economic data with urban morphology in spatially explicit vulnerability assessments remains a critical gap. There is often a deficiency in analyzing the root causes of socio-economic disparities in the context of compound heat events, moving beyond mere identification of vulnerable groups to understanding the systemic factors that create and perpetuate these vulnerabilities.

Future research should focus on developing spatially explicit vulnerability assessments that rigorously integrate socioeconomic data with urban morphology. This would involve mapping heat exposure alongside indicators of poverty, access to green infrastructure, housing quality, and healthcare access at fine spatial resolutions. Such assessments could reveal hyper-local hotspots of vulnerability, guiding targeted interventions. Furthermore, comparative studies on the effectiveness of different urban planning approaches in addressing socio-economic disparities in heat risk are needed. This includes evaluating the long-term impact of green infrastructure investments, zoning reforms that promote mixed-income and dense developments, and policy incentives for passive cooling designs in affordable housing. Research should also delve into the governance structures and decision-making processes that either perpetuate or mitigate heat-related inequities, exploring how community engagement and participatory planning can lead to more equitable and effective heat adaptation strategies.

4. Mitigation and Adaptation Strategies for Urban Thermal Risk

Mitigation and adaptation strategies for urban thermal risk are critical responses to the intensifying challenges posed by compound extreme heat events, particularly the synergistic interactions between heatwaves and urban heat island (UHI) effects .

This section provides a comprehensive overview of existing and emerging strategies, categorized into urban planning and green infrastructure solutions, technological innovations and early warning systems, and policy frameworks and governance for heat risk management. Each subsection will delineate the primary mechanisms, highlight successful applications, and critically assess current deficiencies, emphasizing the need for approaches explicitly designed for compound heat events rather than isolated UHI or heatwave mitigation.

The first subsection, "Urban Planning and Green Infrastructure Solutions," explores how intentional urban design and the integration of natural elements can significantly reduce urban temperatures. This includes examining the roles of "green city" approaches, such as parks, green roofs, and increased tree canopy, which leverage shading and evapotranspiration to cool urban environments . It will also cover "blue city" strategies, which utilize water bodies for evaporative cooling, and "white city" approaches, focusing on reflective materials to reduce solar absorption . A critical analysis of urban form, particularly the benefits of compact development over sprawl, in mitigating heat vulnerability will also be presented . The discussion will address the limitations of current strategies in effectively addressing compound extreme heat events and identify gaps in comprehensive assessment frameworks, paving the way for future research directions that prioritize integrated modeling and long-term empirical studies.

"Technological Innovations and Early Warning Systems" constitutes the second subsection, focusing on engineered solutions and information dissemination crucial for immediate and proactive responses to heat risks. This part will detail advancements in cool materials and smart urban infrastructure designed to reduce heat absorption and enhance cooling, such as cool roofs, reflective pavements, and smart grid technologies . A significant portion will be dedicated to early warning systems (EWS), including Heat Health Warning Systems (HHWSs) and Heat Health Action Plans (HHAPs), outlining their components and communication strategies . The subsection will critically evaluate current EWS deficiencies, particularly the limited integration of multi-hazard data and challenges in reaching vulnerable populations . Future research will emphasize the integration of AI and ML for advanced forecasting and personalized alerts, along with citizen science initiatives to enhance data granularity and user-centric design .

Finally, "Policy Frameworks and Governance for Heat Risk Management" will examine the overarching structures necessary for effective and equitable heat adaptation. This subsection will review existing policy frameworks, such as dedicated Heat Action Plans, and their integration into broader urban planning and disaster resilience initiatives . It will highlight the critical need for policies that specifically address the compounding effects of heatwaves and UHI, emphasizing land-use planning decisions that promote compact development and heat resilience . The discussion will also delve into the importance of multi-level governance, inter-departmental collaboration, and multi-stakeholder engagement to foster resilient and adaptive cities . A critical assessment of policy deficiencies, rooted in fragmented responsibilities and insufficient integration across sectors, will underscore the necessity for adaptive governance models and equitable implementation strategies. This will lead to recommendations for future research focusing on integrating scientific understanding into policy development and prioritizing interventions for the most vulnerable populations .

4.1 Urban Planning and Green Infrastructure Solutions

Urban planning and design interventions are pivotal in mitigating urban heat, particularly in addressing the compounding effects of Urban Heat Islands (UHIs) and heat waves. Comprehensive strategies encompass green infrastructure, cool surfaces, and considerations of urban form .

Green infrastructure solutions, often referred to as "green city" approaches, involve increasing vegetation cover through various modalities such as parks, unsealed surfaces, green roofs, and vertical gardens . The primary mechanisms for their cooling effect are shading and evapotranspiration, which contribute to restoring moisture availability in urban areas and reactivating the negative feedback of evaporation on urban temperatures . For instance, simulations in Graz demonstrated significant cooling benefits from reducing pavement, increasing tree canopy, and implementing green roofs . Case studies from cities like Amadora, Quito, Cape Town, and Nairobi illustrate the practical application and implied effectiveness of urban greening and tree planting initiatives . Preserving existing regional green spaces and increasing street trees are also crucial, as deforestation rates have been directly linked to increased extreme heat events .

In parallel, "blue city" approaches leverage water bodies like ponds, lakes, and fountains to cool urban environments through evaporation and enhanced ventilation . The integration of green-blue infrastructure, as seen in Quito, represents a holistic strategy for urban thermal risk management . These natural cooling mechanisms offer substantial co-benefits, including recreational opportunities, enhanced biodiversity, and energy savings .

Complementary to green and blue infrastructure are "white city" approaches, which involve the use of cool, reflective materials for roofs, walls, and pavements . These materials reduce solar heat absorption and re-emission, thereby alleviating the UHI effect . Cities like Incheon have successfully implemented cool roofs and reflective surfaces . The combined application of green and white city measures, as shown in Klagenfurt am Wörthersee, resulted in a substantial reduction in heat load and daily temperature extremes through afforestation and increased roof albedo .

Beyond specific material and vegetation choices, urban form plays a critical role in heat vulnerability. Strategies promoting more compact urban designs, which often integrate features like preserved regional green spaces, increased street trees, reflective surfaces, and green roofs, are indicated as effective in reducing heat exposure . This contrasts with sprawling urban forms, which may be more vulnerable to climate change-related heat impacts . Additionally, optimizing building orientation and materials, along with incorporating passive cooling design elements like strategic window placement and improved insulation, can further reduce indoor heat gain and the demand for air conditioning, indirectly easing pressure on electrical grids during heat waves .

Despite the demonstrated effectiveness of these strategies, current approaches exhibit deficiencies, particularly concerning compound extreme heat events. Many existing solutions are not explicitly designed or rigorously tested for their efficacy under the combined stresses of UHI and heat waves, often focusing on UHI mitigation in isolation. There is limited empirical evidence quantifying their performance in specific urban settings during actual compound heat events, suggesting a gap in comprehensive assessment frameworks. For instance, while urban climate models quantify the effectiveness of individual measures in Graz and Klagenfurt am Wörthersee , these models often simplify the complex interactions inherent in compound events.

A key critique is the lack of integrated assessment frameworks that can evaluate the holistic performance of green and blue infrastructure under compound heat events. Current research predominantly focuses on the benefits of individual strategies or their cumulative impact on average temperatures, rather than their resilience and sustained cooling capacity during prolonged, intense heat waves amplified by urban heat island effects. This deficiency stems from the complexity of modeling and measuring the synergistic interactions between these phenomena and the varied efficacy of interventions across diverse microclimates within urban areas. For example, while vegetation restores moisture and activates evaporative cooling , its effectiveness can be limited by water availability during droughts that often accompany heat waves. Similarly, while cool roofs reduce absorbed energy , their efficacy might be diminished in areas with complex urban geometries that impede radiative cooling.

Future research should focus on developing robust, integrated assessment frameworks that evaluate the performance of green and blue infrastructure specifically under compound heat events. This would involve utilizing advanced urban climate models coupled with hydrological and energy balance models to simulate the coupled effects of UHI and heat waves on infrastructure performance. Longitudinal studies are needed to gather empirical data on the long-term effectiveness, feasibility, and scalability of these interventions in diverse urban contexts, considering varying climates, urban densities, and socioeconomic conditions. Furthermore, research should explore potential unintended consequences, such as increased humidity from extensive blue infrastructure in certain climates or the water demands of green infrastructure during drought periods. The root cause of these observed deficiencies lies in the historical fragmentation of research and policy efforts, which often address UHI and heat waves as separate entities rather than interacting phenomena. Overcoming this requires a more interdisciplinary approach that integrates urban planning, climatology, hydrology, and public health perspectives to design solutions that are truly resilient to the compounding risks of urban heat.

4.2 Technological Innovations and Early Warning Systems

Effective urban heat mitigation strategies increasingly rely on a suite of technological innovations designed to reduce heat absorption and enhance cooling, alongside sophisticated early warning systems (EWS) that provide timely information to vulnerable populations. Emerging technological solutions include the widespread adoption of cool roofs and reflective pavements, which significantly reduce surface temperatures by increasing albedo . Beyond these foundational materials, innovations such as shade screens, cooling fog systems, and smart bus shelters are being implemented to create more comfortable urban microclimates . Furthermore, addressing the strain on electrical grids caused by widespread air conditioning use during heat waves, particularly noted in regions like Canada , necessitates the integration of energy-efficient building designs and insulation, alongside smart grid technologies to manage fluctuating energy demand more effectively .

Parallel to these technological advancements, the development and implementation of robust early warning systems are critical for mitigating the amplified health risks associated with compound heat events . Heat Health Warning Systems (HHWSs) and Heat Health Action Plans (HHAPs) serve as vital components for preparedness and response. HHWSs typically encompass weather forecasts, methodologies for assessing heat-health relationships, clearly defined threshold values for issuing warnings, graded alert levels, and targeted communication strategies . For instance, Incheon employs a five-level heatwave EWS, while Amadora utilizes SMS-based alerts, and Santiago has implemented a tiered alert system . Communication channels are diverse, leveraging SMS, social media, loudspeakers, and partnerships with community organizations to ensure alerts reach vulnerable groups effectively . The effectiveness of these systems lies in their ability to inform the public and enable timely preparedness and response actions .

Despite these advancements, existing technological solutions and early warning systems exhibit several deficiencies. A primary critique is the limited data integration, particularly in EWS. While various European countries utilize diverse variables and thresholds for warnings, ranging from maximum or mean temperature to minimum temperature or humidity, and incorporate thermal comfort indicators like the Universal Thermal Climate Index (UTCI), PET, and perceived temperature , there remains a challenge in comprehensively integrating multi-hazard data. The current reliance on single-hazard forecasting models often overlooks the compounding nature of extreme events, leading to warnings that may not fully capture the complex risks posed by simultaneous or cascading heat-related hazards . This deficiency stems from the historical development of EWS, which often focused on individual hazards rather than synergistic interactions.

Another significant weakness lies in insufficient public engagement and the challenge of reaching all vulnerable populations, despite efforts through various communication channels . While Toronto and Vancouver have implemented "extreme hot weather strategies" implying public health interventions , the actual mechanisms for ensuring community understanding and behavioral change are not always robust. The root cause of this deficiency often involves a lack of user-centric design in warning dissemination and a failure to account for diverse socio-economic and cultural contexts. Additionally, the broader integration of "compound thinking" into impact-based forecasting models is still evolving .

Future research directions must address these limitations to enhance the efficacy of both technological solutions and early warning systems. A crucial area is enhancing the integration of diverse data sources for more accurate compound event prediction. This includes leveraging advanced technologies such as satellite and mobile networks for real-time hazard data dissemination, alongside the transformative potential of Artificial Intelligence (AI) and Machine Learning (ML) methods for creating more accurate forecasting models and sophisticated EWS . AI and ML can process vast datasets from various sources (weather, health, urban infrastructure, social media) to identify complex interdependencies and predict compound events with greater precision.

Furthermore, developing user-centric early warning systems is paramount. This involves not only improving communication strategies but also actively integrating citizen science data into EWS models. Citizen science initiatives can provide hyper-local heat data and ground-truthing for satellite observations, offering a more nuanced understanding of urban heat pockets and their impact on specific communities. AI can also be utilized for personalized risk alerts, tailoring warnings based on an individual's location, vulnerability profile, and exposure to compound heat events, thereby moving beyond generic, broad-area alerts. This personalized approach would enhance the relevance and actionability of warnings, fostering greater public engagement and more effective responses. Strengthening multi-hazard EWS at national and regional levels, enhancing cross-border data sharing, and promoting impact-based forecasting are also essential steps to provide more actionable warnings that consider the full spectrum of risks posed by compound extreme events .

4.3 Policy Frameworks and Governance for Heat Risk Management

Effective urban heat risk management necessitates robust policy frameworks and adaptive governance structures, particularly when addressing the amplified challenges of compound extreme heat events. Existing frameworks, such as dedicated Heat Action Plans, serve as foundational elements, integrating heat risk into broader urban planning and disaster resilience initiatives . For instance, the City of Toronto provides access to air-conditioned facilities during heat waves, and Vancouver has developed an "extreme hot weather strategy" to protect vulnerable populations, shifting focus from their traditional cold-weather risks . These municipal responses exemplify proactive measures within existing policy capacities.

However, the complexity of compound extreme heat events, where heatwaves interact synergistically with urban heat island (UHI) effects , demands a more integrated and adaptable policy approach. A significant policy gap lies in the insufficient consideration of land-use planning decisions that contribute to urban sprawl, which can exacerbate extreme temperatures and increase vulnerability . Promoting compact development, through specific land-use policies, is crucial for heat resilience and necessitates its incorporation into climate change impact models .

Strengthening policies to address compound heat events requires a multifaceted approach. National governments are advised to conduct heat risk mapping, develop national strategies, provide supportive legal frameworks, and revise building codes and land use planning . Concurrently, local governments should establish clear planning goals, dedicate focal points for heat management, enhance coordination across city systems, and foster public awareness . Case studies from cities like Amadora, Incheon, and Cape Town illustrate varied governance structures, with civil protection, natural disaster divisions, and risk and resilience departments serving as focal points for heat management, highlighting the potential for diverse institutional arrangements .

The importance of multi-level governance and stakeholder engagement is paramount in developing and implementing effective heat resilience plans. Drawing insights from sociology and political science, effective governance requires fostering regional collaborations, building interdisciplinary and transdisciplinary capacity, and strengthening multi-hazard early warning systems . Engaging policymakers, sectoral representatives, and the public in co-designing research and actionable knowledge is essential to ensure policies are contextually relevant and widely accepted . This co-design approach facilitates the integration of policy and development decisions that influence vulnerability and may otherwise aggravate hazards . Furthermore, national efforts, such as China's commitment to carbon neutrality, underscore the importance of coordinated efforts in climate change mitigation and air pollution control, emphasizing the integration of public health into policy design . This highlights the critical need for inter-departmental collaboration and improved information sharing at all governance levels .

Critiques of existing policy frameworks often stem from a lack of integration across sectors or insufficient mechanisms for equitable implementation. For instance, while some municipalities address UHI causes through reflective surfaces and expanding green spaces , a more fundamental "reinvention of the city" with expanded green spaces and less disruption of natural water cycles is required . This points to a deficiency in comprehensive urban planning that fully accounts for the synergistic impacts of heat waves and UHI effects on urban populations . The overarching policy recommendation to curb global climate change to mitigate future heat-related problems often lacks granular, actionable strategies for urban adaptation, particularly for vulnerable populations.

The root causes of these policy and governance deficiencies include fragmented institutional responsibilities, limited cross-sectoral communication, and insufficient recognition of the compounded nature of heat risks. Often, heat risk management is siloed within emergency response departments rather than integrated into long-term urban development and health planning . This leads to reactive rather than proactive measures. Moreover, a lack of consistent risk and impact information, combined with inadequate understanding of urban development and climate change projections, hinders the selection of effective adaptation measures such as green, blue, and white city approaches .

To address these systemic issues, specific, actionable future research directions and solutions are needed. Developing adaptive governance models for urban heat risk management that incorporate feedback loops from climate impacts and community engagement is crucial. Such models should facilitate continuous learning and adjustment of policies based on real-time data and community experiences. This includes strengthening the resilience and adaptability of vulnerable communities and improving urban infrastructure through climate-adaptive health systems . Furthermore, solutions must prioritize equitable implementation, ensuring that adaptation measures reach and benefit the most vulnerable populations. This requires not only improved preparedness and response through defined arrangements and investments for heat health warning systems (HHWSs) and heat health action plans (HHAPs) but also targeted interventions for vulnerable groups and effective communication strategies . The feasibility of such solutions hinges on strong political leadership and a sustained commitment to integrate scientific understanding into policy development, ensuring that urban planning decisions foster compactness and reduce vulnerability to extreme heat .

5. Challenges, Gaps, and Future Research Directions

The current understanding and management of compound extreme heat events (CEEs) in urban environments face significant challenges, necessitating a comprehensive approach to address existing knowledge gaps and propel future research. These challenges span data limitations, methodological deficiencies in climate modeling, and complex issues in interdisciplinary integration for effective urban thermal risk management.

A primary challenge lies in the fragmentation and scarcity of high-quality, high-resolution data . There is a notable lack of historical and real-time data for crucial parameters like soil moisture and precipitation, especially in data-sparse regions. Urban environments further suffer from fragmented monitoring systems and insufficient granular data, hindering real-time heat risk management . While satellite data offers some utility for urban climate analysis, it often suffers from limitations such as cloud cover and inadequate spatial/temporal resolution, distinguishing surface from ambient air temperatures . Furthermore, limited data sharing across international borders impedes regional collaboration in understanding CEEs .

Methodological limitations in current climate models present another critical hurdle. These models struggle to accurately simulate extreme precipitation events, particularly convective systems, and often oversimplify complex land-atmosphere feedbacks crucial for Compound Heat and Drought Extremes (CHDEs) . A significant knowledge gap persists regarding the intricate interactions between marine heatwaves and tropical cyclones, impacting forecasting accuracy . More fundamentally, the underlying mechanisms of CEEs, including land-atmosphere and teleconnected atmospheric feedbacks, are not fully comprehended, emphasizing the need to move beyond pairwise interactions to understand complex risk chains . The synergistic interaction between Urban Heat Islands (UHIs) and heat waves (HWs) remains insufficiently understood, and the generalizability of findings is limited by reliance on a restricted number of observational sites . Analytical models require further enhancements to represent heat storage, release, and anthropogenic emissions explicitly . Attribution of CEE drivers is also nascent, with historical research focusing primarily on single-variable events .

Challenges in interdisciplinary integration and effective management strategies are multifaceted. Many municipalities lack expertise in climate modeling, data analysis, and sustainable urban design, coupled with a nascent awareness of urban heat prioritization . Financial barriers, including insufficient budgets and high upfront costs for cooling solutions, impede implementation . Over-reliance on air conditioning, while protective, strains energy grids and highlights systemic energy infrastructure challenges . The effectiveness of adaptation measures is highly location-dependent, requiring meticulous planning considering technical, institutional, social, and financial aspects . Critical issues include the differential impact of heat on vulnerable populations, which often lack adequate targeted measures , and the exacerbating effect of sprawling urban development on CEE frequency, with differential health impacts between sprawling and compact cities remaining understudied . A lack of coordination across sectors, fragmented governance, and gaps in technical capacities further hinder comprehensive urban heat strategies . Inconsistent heat wave definitions and warning thresholds across regions complicate response coordination . Furthermore, local adaptation measures alone may be insufficient under high-emission scenarios, underscoring the dual necessity of both adaptation and global emissions reduction .

Addressing these challenges necessitates a multi-faceted approach. Future research must prioritize enhancing modeling capabilities to explicitly represent synergistic interactions between phenomena like UHIs and heat waves, refining analytical models to incorporate heat storage, release, and anthropogenic emissions, and developing generalized UHI models . This includes advancing theoretical understanding of mechanisms driving compound events through a hierarchy of models and improved air-sea flux representations .

The application of Artificial Intelligence (AI) and Machine Learning (ML) offers transformative potential for pattern identification, accurate forecasting, data integration, and sophisticated early warning systems for CEEs . Investment in high-resolution data collection is crucial to support robust AI/ML training . However, potential weaknesses such as black-box models, data quality, and bias management must be addressed.

Specific research directions include the need for more nuanced modeling of synergistic interactions, requiring extensive observational datasets and broader heatwave event consideration . The development of integrated urban planning tools drawing on interdisciplinary perspectives from public health, social sciences, and engineering is paramount .

A crucial aspect is the emphasis on localized studies and data collection to inform context-specific solutions, including enhancing observational networks with improved Earth system observations and leveraging crowdsourced data . Integrating local knowledge with top-down scientific methodologies can provide invaluable insights .

Proposed solutions, such as passive cooling design, green roofs, increased green space, and reflective surfaces, offer tangible benefits for urban thermal regulation . These interventions offer co-benefits but require careful consideration of scalability and unintended consequences, such as water demand for green infrastructure or glare from reflective surfaces.

Ultimately, holistic solutions must encompass strengthening Early Warning Systems (EWS) through impact-based and people-centered forecasting, fostering community-based preparedness, expanding technology use, and promoting open data policies . Strengthening risk governance and promoting interdisciplinary capacity-building are essential for co-creating knowledge and solutions . It is recognized that preventing global warming remains the most crucial step, providing the fundamental backdrop for all localized adaptation and mitigation efforts .

5.1 Identified Challenges and Knowledge Gaps

The current understanding and management of compound extreme events (CEEs) in urban contexts face multifaceted challenges spanning data availability, methodological limitations, and the complexities inherent in interdisciplinary integration for effective management strategies. A fundamental limitation identified across multiple studies is the inadequate design of urban infrastructure and buildings to withstand extreme heat events, exemplified by Vancouver's historical focus on cold-weather strategies . This lack of preparedness is compounded by climate projections that frequently omit the Urban Heat Island (UHI) effect, leading to underestimations of dangerously hot days and nights in cities .

Data Deficiencies and Fragmentation: A pervasive challenge highlighted is the scarcity of high-quality, high-resolution historical and real-time data crucial for accurate assessment, model validation, and attribution of CEEs . This includes vital parameters such as soil moisture and precipitation, particularly in data-sparse regions like remote mountainous areas, where inadequate station networks hinder comprehensive understanding . The issue extends to urban environments, where fragmented data systems, a lack of granular, real-time information, and poor integration across various monitoring systems impede effective heat risk management . While satellite data offers valuable insights for urban climate analysis, it presents limitations such as cloudiness and insufficient spatial/temporal resolution, further distinguishing surface temperatures from ambient air temperatures . Furthermore, limited data sharing across borders represents a significant barrier to regional collaboration in understanding CEEs .

Methodological Limitations and Understanding Complex Interactions: Climate models currently exhibit significant limitations in accurately simulating the magnitude, location, and timing of extreme precipitation events, particularly convective systems . These models also struggle to represent complex land-atmosphere feedbacks that are crucial for Compound Heat and Drought Extremes (CHDEs), often oversimplifying vegetation responses to drought . A critical knowledge gap concerns the intricate interactions between marine heatwaves and tropical cyclones in the Indian Ocean, with cyclone intensity forecasting remaining a persistent challenge . More broadly, the underlying mechanisms of CEEs, including the role of land-atmosphere feedbacks and teleconnected atmospheric feedbacks, are not fully comprehended . There is a particular need to move beyond pairwise interactions to understand complex risk chains involving a broader spectrum of future extreme events . The synergistic interaction between Urban Heat Islands (UHIs) and heat waves (HWs) is also not fully understood, and the generalizability of findings beyond specific case studies remains limited due to reliance on a restricted number of observational sites and events . Analytical models, while valuable for generalizing physical mechanisms, require further enhancements to explicitly represent heat storage and release, and to incorporate anthropogenic heat emissions . The attribution and understanding of the drivers of CEEs are also nascent, with most research historically focusing on single-variable events .

Challenges in Interdisciplinary Integration and Management Strategies: Effective management strategies for CEEs necessitate robust interdisciplinary integration, which faces several hurdles. There is a lack of expertise in climate modeling, data analysis, and sustainable urban design within many municipalities, alongside a nascent awareness regarding urban heat prioritization . Financial barriers, including insufficient municipal budgets, high upfront costs for cooling solutions, and difficulties in accessing diverse financing mechanisms, further impede the implementation of effective heat mitigation measures . The reliance on air conditioning, while a protective factor, strains energy grids and can be counterproductive if powered by fossil fuels, highlighting a systemic challenge in energy infrastructure and sustainability . Furthermore, the effectiveness of various adaptation measures is highly dependent on local conditions, and the optimal combination of measures requires meticulous planning considering technical, institutional, social, and financial aspects . A critical issue is the differential impact of heat on vulnerable populations, necessitating targeted measures that are often not adequately in place . Sprawling urban development patterns are identified as exacerbating CEE frequency, yet the differential health impacts between sprawling and compact cities remain understudied . The relative contributions of various UHI factors (e.g., vegetation loss, albedo, heat sources) to CEE frequency also require more detailed assessment . Finally, a lack of coordination across sectors, fragmented governance, and critical gaps in technical capacities and data collection, as seen in Nairobi, underscore the systemic barriers to developing comprehensive urban heat strategies . The diversity in heat wave definitions and warning system thresholds across different regions, such as Europe, presents challenges for consistent assessment and response coordination . Moreover, local adaptation measures alone may be insufficient to counteract large-scale warming under high-emission scenarios (e.g., RCP8.5), emphasizing the dual necessity of both adaptation and global emissions reduction strategies .

Addressing these challenges requires a concerted effort to improve data infrastructure, enhance modeling capabilities to capture complex CEE interactions, and foster robust interdisciplinary and cross-sectoral collaboration. This includes developing more sophisticated early warning systems, increasing public awareness, and overcoming financial and technical barriers to implement sustainable urban design and heat mitigation solutions.

5.2 Future Research Directions and Innovative Solutions

Addressing the escalating urban thermal risk from compound extreme heat events necessitates a multi-faceted approach, with future research focusing on enhancing modeling capabilities, leveraging advanced computational techniques, and fostering interdisciplinary collaborations. A critical deficiency identified in current understanding is the inadequate representation of synergistic interactions between phenomena such as urban heat islands (UHIs) and heat waves . Future work should prioritize the explicit modeling of these interactions, refining existing analytical models to incorporate crucial elements like heat storage and release, and anthropogenic heat emissions, thereby developing more generalized UHI models applicable across varied temporal scales .

To overcome observational and modeling constraints, researchers must advance theoretical understanding of the mechanisms driving compound events, including tropical-extratropical interactions, soil moisture-temperature feedbacks, and atmosphere-cryosphere interactions. This calls for developing a hierarchy of models of varying complexities, alongside researching wave effects on air-sea fluxes and their role in tropical cyclone intensification . Specifically, improving modeling capabilities involves utilizing high-resolution climate models, global storm-resolving models, and next-generation tropical cyclone models with enhanced air-sea flux representations. Coupling atmospheric, oceanic, surface wave, and land surface processes is essential, as is increasing the use of large ensemble simulations and accelerating Earth System Reanalysis datasets for improved subseasonal-to-seasonal forecasting . The integration of land-use patterns into urban climate change impact models is also crucial, particularly for understanding how urban form, such as sprawl versus compactness, influences vulnerability to extreme heat events .

The application of Artificial Intelligence (AI) and Machine Learning (ML) is posited as a transformative avenue for addressing current deficiencies. AI/ML can facilitate pattern identification, create accurate forecasting models, integrate diverse datasets, and develop sophisticated early warning systems for compound events . Policymakers are encouraged to invest in high-resolution data collection systems to support the robust training of these AI/ML models . Furthermore, AI and big data can be instrumental in developing integrated strategies for carbon reduction and air quality improvement, thereby potentially lowering the frequency of compound extreme events . However, a weakness of over-reliance on AI/ML is the potential for black-box models, which may lack transparency in their decision-making processes, hindering interpretability and trust, especially in critical public health applications. Ensuring data quality and managing biases in large datasets for AI/ML training remain significant challenges.

Specific and actionable future research directions must address identified gaps. The need for more nuanced modeling of synergistic interactions, as highlighted by studies on UHI and heatwave compounding, requires more extensive observational datasets and the consideration of a broader range of heatwave events to confirm synergistic effects . The development of integrated urban planning tools is paramount, drawing on interdisciplinary perspectives from fields such as public health, social sciences, and engineering . This interdisciplinary approach is essential for identifying complex risk chains of compound events and developing models that assess their impacts, particularly on vulnerable populations .

A crucial aspect of future research is the emphasis on localized studies and data collection to inform context-specific solutions . This includes enhancing observational networks with improved earth system observations and leveraging crowdsourced data from citizen weather stations to achieve higher spatial resolution and complement traditional monitoring networks . Integrating local knowledge and grassroots approaches with top-down scientific methodologies can provide invaluable insights into community-specific vulnerabilities and adaptation strategies . The feasibility of localized data collection hinges on accessible and sustainable financing mechanisms and enhanced public-private partnerships .

Proposed solutions, such as passive cooling design elements, green roofs, increased green space, tree canopy, blue infrastructure (water features), and reflective surfaces, offer tangible benefits for urban thermal regulation . These interventions promote density, green space, and public transit, which are characteristic of compact cities, offering co-benefits such as reduced air pollution, fewer injuries, and increased physical activity . However, their scalability and potential unintended consequences require careful consideration. For instance, extensive green infrastructure demands significant water resources, which could be problematic in water-scare regions. Reflective surfaces, while effective at reducing heat absorption, can increase glare and potentially redirect heat to adjacent areas.

Holistic solutions that address the interconnected nature of urban thermal risk and compound heat events must encompass strengthening Early Warning Systems (EWS) through impact-based forecasting and people-centered approaches, fostering community-based preparedness, and expanding the use of advanced technology and data sharing . Promoting open data policies for high-resolution datasets across borders and developing regional collaborative platforms are crucial for data accessibility and cross-sectoral coordination . Furthermore, strengthening risk governance mechanisms and promoting interdisciplinary and transdisciplinary capacity-building through training programs and engagement with policymakers and the public are essential for co-creating knowledge and solutions . Ultimately, preventing global warming remains the most crucial step in mitigating the threat of very high temperatures, forming the fundamental backdrop for all localized adaptation and mitigation efforts .

6. Conclusion

The comprehensive review of current literature unequivocally establishes that urban thermal risk is significantly amplified by the synergistic interactions between urban heat islands (UHIs) and heat waves . This amplification results in urban temperatures that are considerably higher than the sum of individual effects, leading to intensified heat stress and severe concerns for urban populations . The escalating frequency, intensity, and duration of such compound extreme events, driven by accelerating climate change and rapid urbanization, pose profound and multifaceted risks to human health and urban infrastructure . Notably, existing climate projections often underestimate urban heat risks by failing to fully account for the UHI effect, thus highlighting the urgency of more precise predictive models and integrated assessments .

Imperative	Description	Supporting Strategies
Acknowledge Amplification	Recognize that UHI + Heatwaves create risks greater than individual impacts.	Integrate UHI effects into climate projections and risk assessments; focus on synergistic interactions.
Comprehensive Management	Implement integrated strategies for adaptation and resilience.	Combine physical interventions, urban planning, early warning systems, and policy frameworks.
Physical Interventions	Mitigate UHI effects and provide cooling benefits.	Green infrastructure (parks, trees, green roofs), blue infrastructure (water bodies), white infrastructure (reflective surfaces), compact urban forms.
Preparedness & Response	Establish robust mechanisms to protect urban populations.	Heat health warning systems, actionable plans, enhanced observational networks, improved modeling capabilities, leveraging advanced technologies (AI/ML).
Collaboration & Data	Foster interdisciplinary, transdisciplinary, and cross-sectoral approaches, alongside data sharing.	Regional collaborations, open data policies, integrated urban planning tools, public-private partnerships.
Address Root Causes	Mitigate global warming and implement sustainable urban development.	Global emissions reduction; land-use planning promoting compactness; policies prioritizing vulnerable communities.
Research & Innovation	Continuously refine understanding, explore novel strategies, and develop equitable solutions.	Study synergistic interactions, develop context-specific models, explore AI/ML applications, assess adaptation measure scalability and equity.

The imperative for comprehensive urban heat risk management is consistently underscored across the literature . Effective strategies necessitate a multi-faceted approach, integrating adaptation measures with proactive urban planning for enhanced resilience. Green infrastructure, such as parks, urban forests, and green roofs, and blue infrastructure, like water bodies and permeable surfaces, are critical for mitigating UHI effects and providing cooling benefits . Furthermore, the implementation of white city strategies, involving reflective surfaces and passive cooling designs, can significantly reduce heat absorption in urban environments . The spatial planning of urban areas also plays a crucial role; compact urban forms have demonstrated a lower rate of increase in extreme heat events compared to sprawling cities, partly due to better preservation of vegetative cover . This emphasizes the strategic importance of land-use planning in mitigating heat-related health impacts and building urban resilience.

Beyond physical interventions, robust preparedness and response mechanisms are essential. The development and strengthening of heat health warning systems and actionable plans are vital for protecting vulnerable populations . This requires enhancing observational networks, theoretical understanding, and modeling capabilities, particularly for compound weather and climate extremes . The integration of advanced technologies, such as Artificial Intelligence and Machine Learning (AI/ML), can significantly improve forecasting accuracy and early warning systems, thereby enabling more timely and effective interventions . Data sharing, regional collaborations, and interdisciplinary and transdisciplinary approaches are also paramount for fostering a holistic understanding and management of these complex risks .

Ultimately, building climate-resilient urban environments in the face of escalating compound extreme heat events necessitates a sustained commitment to research, policy development, and implementation. Local governments play a pivotal role in enacting targeted solutions, supported by multi-sectoral collaboration and equitable approaches that prioritize the most vulnerable communities . Integrating heat risk management into long-term urban planning and addressing the root cause of global warming through global emissions reduction remain foundational to mitigating urban heat risks . Future research must continue to refine our understanding of these synergistic interactions, explore novel adaptation strategies, and develop innovative solutions that are both technologically advanced and socially equitable. The proactive pursuit of science-informed recommendations, coupled with collaborative efforts across scientific, governmental, and community sectors, will be indispensable in safeguarding urban populations and ensuring the sustainability of cities in a rapidly warming world.

References

Heat Waves and Urban Heat Island Effects - GFDRR https://www.gfdrr.org/sites/default/files/publication/wb\_broschuere\_urbmod\_20200603\_reduced.pdf

Urban Heat Island Effect | Climate Atlas of Canada https://climateatlas.ca/urban-heat-island-effect

Urban Form and Extreme Heat Events: Are Sprawling Cities More Vulnerable to Climate Change Than Compact Cities? - PubMed Central https://pmc.ncbi.nlm.nih.gov/articles/PMC2957923/

URBAN HEAT RISK MANAGEMENT - IIHL https://www.onlinelibrary.iihl.org/wp-content/uploads/2025/02/2025-Making-Cities-Resilient-2030-Urban-Extreme-Heat-Risk-Management-Resource-Package.pdf

Synergistic Interactions between Urban Heat Islands and Heat Waves: The Impact in Cities Is Larger than the Sum of Its Parts* https://sites.bu.edu/efm/files/2016/02/final\_version.pdf

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