The 2025 Hernandez Heat waves and social impact forecast updated

Heat waves represent a significant and growing threat to human health, well-being, productivity, and urban infrastructure worldwide. As city managers, elected officials, and property managers, understanding these phenomena and implementing effective strategies is paramount to building resilient urban communities. ### I. Understanding Heat Waves: A Growing Threat Heat waves are periods, usually lasting several days, with **temperatures significantly higher than average or maximum values observed in the past during the same dates**. They are characterized by abnormally warm conditions and prolonged exposure. Climate change is projected to increase their **frequency, intensity, duration, and spatial extent**. Some regions have already seen a remarkable growth in climate-related extreme events, including heat waves. For example, studies project that extreme heat events in European capitals will become a significant threat in the near and distant future, extending beyond traditionally exposed areas like the Mediterranean and Iberian Peninsula. The scientific definition of a heat wave can vary, but a common meteorological definition for a terrestrial heat wave is **three or more consecutive days where the maximum temperature is over the 90th percentile for a particular location at a particular time**. However, specific thresholds often depend on local factors like population acclimatization, age, health preconditions, and other meteorological variables such as humidity and wind speed. Different countries and cities have adopted their own criteria, often based on the relationship between temperature and excess mortality, such as France using the 98th or 99.5th percentile temperature values. The 2025 Hernandez Heat waves and social impact forecast examines the multifaceted impacts of heat waves, focusing on their climatic context, physiological effects, societal vulnerabilities, and adaptive strategies. They discuss how urban environments exacerbate heat through the urban heat island effect and explore the projected increase in heat wave frequency and intensity due to climate change. Furthermore, the texts analyze the direct and indirect health consequences of extreme heat, including mortality and morbidity, while also investigating the effectiveness of heat health warning systems and urban planning in mitigating these risks. The sources also touch upon the socioeconomic factors that increase vulnerability and the challenges in accurately attributing heat-related deaths. The 2025 Hernandez Heat waves and social impact forecast synthesizes information from the provided sources to offer a comprehensive understanding of heat waves, their characteristics, impacts, and the strategies employed to mitigate their effects. It also delves into the urban heat island (UHI) phenomenon and its interplay with extreme heat events. I. Introduction to Heat Waves and Their Growing Significance Extreme weather events, including heat waves, have significantly increased in number, leading to substantial human and material losses . The 2018 Global Risks Report of the World Economic Forum identifies the risk of extreme weather events as the most unfavorable combination. There is strong evidence that changes in many extreme weather and climate events have been observed since approximately 1950, with a notable decrease in cold temperature extremes and an increase in warm temperature extremes, some of which are linked to human influences. Heat waves are considered critical events and have already reduced global labor capacity to 90% in peak months, with a predicted further reduction to 80% by 2050 under high warming scenarios . Despite their growing significance, there is no single, formal, objective, or uniform definition of a heat wave in terms of magnitude, duration, and rapidity of onset, which makes comparing different studies challenging . Various studies employ different criteria, leading to variability in identifying heat wave events and their impacts. Some common methods involve statistical-meteorological criteria, where heat wave days are defined as those exceeding a given percentile of historical values. Other definitions use absolute temperature thresholds that must be exceeded for a specified duration, ranging from one to several consecutive days. For instance, a common meteorological definition for a terrestrial heat wave specifies at least three consecutive days where the daily maximum temperature exceeds the 90th percentile threshold of climatological daily maximum temperatures, centered on a 31-day window, for a base period like 1981–2010. Some studies define a heat wave as a period of at least two consecutive days when temperature exceeds a threshold limit, while others consider a single day exceeding the limit. The IPCC (Intergovernmental Panel on Climate Change) has also provided recommendations for defining heat waves. The lack of consensus on heat wave definitions and threshold values among the scientific community is a significant limitation, potentially becoming a life or death issue for vulnerable populations and regions . The 2025 Hernandez Heat waves and social impact forecast aims to consolidate the extensive knowledge presented in the provided sources to offer a clear and detailed understanding of this critical environmental and societal challenge. II. Characteristics and Trends of Heat Waves A. Observed Changes in Frequency, Intensity, and Duration Changes in the frequency, intensity, and duration of heat waves have been widely observed globally, with notable increases in many regions. Globally, there has been an increasing frequency, intensity, and duration of observed heat waves . Since the 1960s, the mean heat wave intensity, length, and number have increased by factors of 7.6 ± 1.3, 7.5 ± 1.3, and 6.2 ± 1.1, respectively, in the eastern Mediterranean basin, suggesting a higher occurrence than previously reported. In Ukraine, analysis from 1951 to 2011 shows a clear increase in the frequency of occurrence, duration, and intensity of heat waves. Similarly, the frequency of very hot days in central England has increased since the 1960s, with sustained hot periods becoming more frequent, particularly in May and July. Trends show that heat waves have been starting earlier and ending later in Slovenia, with increasing intensity . The IPCC’s Fifth Assessment Report confirms that changes in many extreme weather and climate events have been observed since about 1950, including an increase in warm temperature extremes . The IPCC special report on the risk of extreme events states that it is "very likely" that there has been an overall decrease in cold days and nights and an overall increase in warm days and nights. This trend is supported by sufficient evidence of significant increases in maximum day and night temperatures in North America, Europe, and Australia. Reports from various other world regions also indicate growing trends in the number and duration of heat waves. However, there remains a lack of information and analysis for large areas such as Central America, South America, and Africa . B. Drivers of Heat Waves Heat waves are a result of complex interactions between atmospheric, oceanic, and land surface processes . These interactions can lead to prolonged periods of stable, often clear weather with high inputs of solar radiation, resulting in hot, dry conditions. Alternatively, sequences of very warm and cloudy days can produce hot, humid conditions. The phenomenon of "locking action" of the westerly circulation, forming an "Omega-weather type," can cause high-pressure systems with cloudless skies, permitting extreme solar radiation and leading to repeatedly registered record temperatures. Climate change and human influences are identified as major drivers . C. Historical Heat Wave Events: Case Studies 1. The 1995 Chicago Heat Wave The Chicago heat wave of 1995 was a particularly severe event from July 12 to July 16, with temperatures exceeding 32 °C (90 °F) for seven consecutive days and surpassing 38 °C (100 °F) for two days at its peak . Critically, there was no nighttime relief, with minimum temperatures remaining over 27 °C (80 °F) during the hottest days. This event resulted in 739 "excess" deaths. Initially, 514 deaths were classified as heat-related, but a later reanalysis estimated 697 heat-related deaths. Even these figures might be underestimates, as some studies suggested higher excess mortality rates. While some deaths may have been "harvested" (i.e., anticipated by a few days to weeks), an estimated 26% of deaths were due to this displacement, leaving over 500 deaths directly attributable to the heat wave. The health impact of the 1995 Chicago heat wave was significantly magnified by failures in communication, and in physical and social infrastructures. For instance, some paramedics reported that their departments refused to release additional ambulances and staff to handle the increased workload. A study of 58 patients admitted with heat stroke during this heat wave found severe functional impairment in 33% of cases, with no improvement after one year in those who survived. Deaths from heat stroke may be underreported due to similarities with other causes of death like coronary or cerebral thrombosis. In contrast to the 2003 European heat wave where victims often died in institutional settings, most victims in Chicago died alone, in their apartments. Tight social networks and strong family bonds among the Latino population in Chicago contributed to their ability to cope, resulting in a relatively low number of heat-related deaths in that community . 2. The 2003 European Heat Wave (EHW) The European Heat Wave of 2003 was an "outstanding weather event" that impacted large regions of Western Europe, including France, Germany, Great Britain, Switzerland, Spain, Portugal, and Italy . The months of June and August were nearly the warmest since 1901 in Germany, with an average summer air temperature of 19.6 °C, 3.4 K higher than the mean value. Record maximum temperatures of 40.2 °C were measured in Karlsruhe and Freiburg, Germany. This extreme weather was caused by a stationary wave forming an Omega-weather type, leading to high-pressure systems and cloudless skies. The EHW of 2003 led to an extensive loss of life, with initial estimates of the death toll around 30,000. This estimate was later revised upward to over 40,000 deaths, and a more recent analysis identified a total of 70,000 excess deaths across 16 European countries during the summer of 2003. In France specifically, a deadly heat wave between August 1-20, 2003, resulted in approximately 14,800 excess deaths, a 55% increase. Paris was disproportionately affected, experiencing a 190% increase in mortality. While 65% of excess mortality in the rest of France affected institutionalized older people, in Paris, 74% of excess deaths were among those living alone at home or women aged 75 and over. The intensity and duration of the 2003 heat wave in France were "much greater than had ever been observed". Preliminary analysis of the 2003 heat wave in France estimated it caused significant attributable mortality. The August 2003 heat wave in London showed greater impacts than previous heat waves, with a 42% increase in all-age mortality compared to 15-16% in 1976 and 1995. The greatest increases in population-adjusted mortality occurred in London, despite higher temperatures recorded outside the city in the South East, suggesting the urban heat island effect contributed to the mortality rate. The 2003 heat wave highlighted significant failures in communication and data management within the French health system. Information was not adequately pooled due to compartmentalization between the health ministry, other government departments, and ground workers. The August leave system exacerbated the crisis by leaving many cabinet members and doctors on holiday. Failures in public health preparedness were noted, as health care and social systems were "ill prepared for thermal stresses" and lacked effective intervention plans. The event reinforced the need for societies to cope more effectively with heat waves . 3. The 2010 Russian Heat Wave The heat wave in Russia during the summer of 2010 is considered the most extreme heat wave of the present era . Its maximum magnitude exceeded a score of 60 on the Heat Wave Magnitude Index daily (HWMId), a value not previously recorded in other recent heat waves (1979-2015) in the datasets used. This heat wave extended far north, reaching the southern boundaries of the Arctic, and led to an estimated death toll of 55,000 persons and economic losses of US$15 billion. The longest heat wave duration in Ukraine was also observed from late July to the second 10-day period of August 2010, affecting almost all stations on the left bank of the Dnipro river, closer to Russia . 4. The 2020 Siberian Heat Wave The sources briefly mention a 2020 Siberian Heat Wave . No specific details about its characteristics or impacts are provided beyond its occurrence. 5. Other Notable Heat Waves ◦ Los Angeles, California, September 2009: A five-day heat wave event was analyzed to simulate city-scale building heat emission and energy use . During this heat wave, the average anthropogenic heat (AH) discharges from buildings increased by up to 20% compared to regular summer weather. The peak overall daily heat flux occurred at 3 pm each day . ◦ Athens, Greece, 1987: A 10-day heat wave resulted in 926 deaths classified as heat-related, but the attributable excess mortality was estimated to be more than 2000 . ◦ Portugal, 1981: Experienced significant attributable mortality from heat waves . ◦ Rome, Italy, 1983: Experienced significant attributable mortality from heat waves . ◦ London, England, 1976 and 1995: Showed rapid rises in mortality . The 1976 heat wave led to a nearly twofold increase in mortality rates for geriatric hospital inpatients. Increases in all-age mortality were 15% for 1976 and 16% for 1995, compared to 42% for 2003 . ◦ China, 2003: A heat wave hit China in the summer of 2003 . During the hottest summer recorded in Shanghai in 2003, high relative humidity played a significant role in exacerbating heat wave impact . ◦ India, 1960-2009: Summer mean temperatures have substantially increased, with a statistically significant upward trend. Accumulated heat wave intensity, annual heat wave count, mean heat wave duration, and heat wave days have all increased over this period, especially in northern, southern, and western India . ◦ United States: Studies report stronger positive associations between heat waves and mortality in northeastern and midwestern cities compared with southeastern cities, suggesting regional acclimatization may influence exposure-response relationships . ◦ Galicia, Spain: The main heat waves observed from 1987 to 2006 included one in July 1990, simultaneously detected across all four Galician provinces with varying intensities, and the August 2003 heat wave . The 1990 event had previously gone unrecognized as a major heat wave . D. Future Projections of Heat Waves Climate models project significant increases in the frequency, intensity, and duration of heat waves in the coming decades and century . Projections for the period 2071-2100 under A2 and B2 emissions scenarios reveal considerable warming across all seasons, ranging from 1°C to 5.5°C, with temperatures generally 1-2°C lower for low emissions scenarios. Maximum warming in summer is projected over Western and Southern Europe, with moderate changes over Southern England (1-2°C). For Europe, a simulation model for the 21st century projects a "highly significant increase" in the number of heat wave events per year, maximum average temperature, and frequency. From 2000 to 2100, the number of heat wave events per year is projected to increase from 1 to 2.5 in Western Russia, from 10 to 27 in Eastern Europe, and from 1 to almost 2.5 in Western Europe. Projections under the high-concentration RCP8.5 scenario for heat waves in the U.S. indicate that the projected increase in Cooling Degree Days (CDDs) for more extreme heat waves is roughly five times greater than historical mean values. This reinforces concerns about increased stress on energy systems due to extreme heat. Even seemingly "normal" summers in the coming century could experience mortality rates similar to or greater than those of the 1995 Chicago heat wave. Studies also project the yearly probability of apparent temperature peaks (ATpeak) exceeding dangerous thresholds. For instance, ATpeak higher than 40 °C (~105°F) and 55 °C (~130°F) are thresholds at which the US National Weather Service issues heat advisories due to dangerous and very dangerous health conditions, respectively. Humidity is an important amplifying factor for heat stress in projections. These projected changes highlight regions where adaptation measures will be increasingly necessary to cope with heat stress . III. Impacts of Heat Waves Heat waves exert significant and multi-faceted impacts across human health, economic output, social behavior, and critical infrastructure. A. Human Health Impacts Heat waves are well-known killers, often with severe losses of lives . Past decades have seen records of particularly dangerous heat wave events causing numerous deaths, even in developed countries with robust public health services . 1. Mortality and Morbidity A clear and immediate outcome of heat waves is a rapid rise in mortality . The 1995 Chicago heat wave, for example, resulted in 697-739 excess deaths. The 2003 European heat wave had an even more devastating impact, causing an estimated 70,000 excess deaths across 16 European countries, including 14,800 in France alone. Paris was disproportionately affected, with a 190% increase in mortality. The 2010 Russian heat wave caused 55,000 deaths. Deaths from heat-related causes are often underreported in official mortality statistics. For instance, the 1987 Athens heat wave recorded 926 heat-related deaths but estimated over 2000 attributable excess deaths. Excess mortality is typically calculated by subtracting "expected" mortality from observed mortality, but estimates are sensitive to the method used for "expected" mortality, making comparisons difficult across studies. Morbidity also increases significantly during heat waves. The 2003 EHW in Paris reported 2600 excess emergency room visits and 1900 excess hospital admissions. In France, between 2015-2019, heat waves resulted in significant excess emergency department (ED) visits and outpatient clinic visits, with extrapolated costs of €30.8 million and €1.96 million respectively. Hospitalization for respiratory causes was particularly correlated with extreme heat in some studies. Other adverse health outcomes include heat exhaustion, heat stroke, heat cramps, and even mental health issues. Studies also link heat with cardiovascular effects, pregnancy outcomes, and occupational health issues. Near-fatal heat stroke cases during the Chicago 1995 heat wave showed severe neurological damage in a significant proportion of survivors, with some dying even a year later . 2. Physiological Responses to Heat Stress Human beings are closely linked to the atmospheric environment through their heat budget . In hot environments, the body gains heat from the sun and loses heat primarily through sweat evaporation, with smaller contributions from convection, conduction, and respiration. For body temperature to remain stable, heat loss must balance heat production. Key effector mechanisms include sweat production (to lose heat from the skin) and skin blood flow (to transport heat from the body core to the skin). If these systems are overly stressed and cannot meet thermoregulatory demands, it leads to excessive strain on the body, potentially causing heat illness. Additional responses include an increase in certain hormones, respiratory rate, and heart rate. When muscle pump activity is high (e.g., during exercise), blood pressure can be maintained longer, leading to further body heating and high cardiovascular stress, which can result in heat exhaustion. If this high heat load is not removed, it can progress to heat stroke, characterized by extreme body temperatures (above 40.5 °C), causing damage to cellular structures and the thermoregulatory system, with a high mortality risk. Heat stroke often has a rapid onset and a high case-fatality ratio, particularly in fit young adults who continue exercising despite feeling unwell. Complications include acute respiratory distress syndrome, kidney failure, liver failure, and disseminated intravascular coagulation. Acclimatization plays a crucial role in adapting to heat. Short-term acclimatization involves a reduced core temperature threshold for skin vasodilation and increased venoconstrictor tone, but it gradually disappears over several weeks after heat stress ends . 3. Risk Factors and Vulnerable Populations Certain segments of the population are more at risk or vulnerable to extreme heat . The elderly, particularly those aged 65 and over, are overwhelmingly affected, and show the greatest sensitivity to extreme heat. Socially isolated elderly individuals are particularly susceptible. Living alone at home was a significant risk factor in Paris during the 2003 heat wave. Institutionalized elderly individuals, such as those in nursing homes or hospitals, may also face increased vulnerability due to high indoor temperatures. Other risk factors include pre-existing health conditions like psychiatric, pulmonary, or cardiovascular illnesses, and the use of medications that interfere with thermoregulation. Low-income groups in the U.S. are at higher risk of heat-related mortality, often due to a lack of access to or inability to maintain air conditioning during heat waves. Housing characteristics can also play a role in determining sensitivity. The location of a heat wave can also be a factor, with some studies showing stronger associations in northeastern and midwestern U.S. cities compared to southeastern ones, implying acclimatization effects. Urban areas in general show greater sensitivity to extreme heat. The 1995 Chicago heat wave showed that strong social networks and family bonds, such as those within the Latino population, can significantly enhance a community's ability to cope with heat and reduce heat-related deaths . B. Economic and Social Impacts 1. Economic Output and Productivity Loss Heat waves can lead to significant economic losses and reduced productivity. The 2010 Russian heat wave resulted in an estimated economic loss of US$15 billion . Studies indicate that heat accumulation, especially "intensity-based" accumulation (where both duration and intensity contribute), impacts productivity. Lost economic output from heat waves can be substantial, with the 2003 France heat wave estimated at -$31.9 billion in non-agricultural output and -$3.1 billion in agricultural output, and the 2010 Russia heat wave at -$18 billion (non-agricultural) and -$3.6 billion (agricultural). A conventional temperature-bin approach for estimation yielded significantly smaller total estimated damages ($13 billion vs $52 billion), emphasizing the importance of accounting for prolonged exposure. Occupational heat strain is a public health threat, directly influencing outdoor industries. Even in indoor settings, industrial heat production and building architecture become important factors. Industrial productivity has been observed to be affected in periods following heat waves rather than directly during the heat wave itself, with no direct correlations observed between outdoor/indoor temperatures and overall equipment effectiveness (OEE) for different shifts. However, significant impacts on labor capacity are projected, with environmental heat stress already reducing global labor capacity to 90% in peak months, and a further predicted reduction to 80% by 2050 under high warming scenarios. More research is needed to quantify the costs of productivity loss due to heat stress compared to the costs of implementing protective actions . 2. Energy System Strain Heat waves place heavy stress on energy systems, primarily due to the increased demand for electricity for air conditioning . This surge in demand can lead to shortages of resources, necessitating measures like rolling blackouts to prevent total system collapse. For example, New York City's power usage reached a record of 12,551 megawatts during the summer 2005 heat wave. The August 2003 European heat wave also saw impacts on power supply due to the shutdown of nuclear power plants because cooling water temperatures became too high. Increased air conditioning use due to intensified heat stress also contributes to greater carbon dioxide emissions, exacerbating the greenhouse effect and adding sensible heat directly to the urban atmosphere . 3. Infrastructure Vulnerability Cities rely on interdependent critical infrastructures and services, including energy, drinking water, sanitation, food distribution, health, communication (transport, telephone, IT networks), social control, and economic systems . A disruption in one system can cause cascading "domino effects" on others, multiplying the disaster. Extreme heat combined with drought can have severe impacts on communities, leading to increased demand for water and electricity, which may result in shortages of these resources. Food shortages can also occur if agricultural production is damaged by crop or livestock loss. Failures in communication and data management were identified as magnifying factors during the 2003 French heat wave, where information was not pooled among government departments and healthcare workers. Similarly, the 1995 Chicago heat wave experienced communication failures, with paramedics reporting difficulties in obtaining additional ambulances and staff . 4. Public Perception and News Media Influence Media reports play a role in connecting unusual weather patterns, such as heat waves, to the probability of global climate change . Individuals' exposure to news media, especially through the internet, can influence their understanding and response to heat waves. Those who frequently use the internet for news may be more sophisticated in obtaining information, as the internet facilitates news gathering from multiple sources and following links. This increased variety of sources may make more sophisticated news gatherers less affected by current weather conditions. Social media (SM) can be a valuable tool for "public crowdsensing" of heat waves, enabling data collection on public attention and reactions to unfolding events. By analyzing messages containing keywords related to heat, researchers can identify the volume of messages and spatio-temporal patterns of heat waves. For instance, a study in Italy during the summer of 2015 collected tweets filtered for semantically relevant criteria (e.g., "caldo," "afa," "sudo," "allarme," "emergenza") to compare tweet volumes with temperature patterns. SM activity metrics can provide quantitative and reliable feedback from large urban areas, serving as a responsive tool for emergency and disaster management . IV. Urban Climate and Heat Islands (UHI) A. UHI Definition and Characteristics The Urban Heat Island (UHI) is a phenomenon where urban areas experience higher temperatures than surrounding rural areas . It is relatively easy to measure. Studies have identified factors associated with the magnitude of the UHI effect. For instance, in London, the UHI effect has a minimal impact on maximum daily temperatures in the city, but sizeable differences (up to 3-4°C) occur at night (03:00). The intensity of London's nocturnal UHI decreases with distance from the city center, forming a marked plume of warmth oriented southwest-northeast, centered on the British Museum . B. Factors Influencing UHI Intensity Several factors contribute to the intensity of the urban heat island effect: • Size of the Town: The larger the urban area and the higher the population, the more pronounced the UHI becomes . Under ideal conditions, the maximum urban-rural temperature difference can increase from 2.5°C for towns of 1,000 inhabitants to 12°C for cities of 1 million. However, European cities typically have lower per capita energy use and anthropogenic heat production compared to North American cities, resulting in smaller rural-urban differences . • Geographical Latitude: The UHI intensity also depends on geographical latitude due to variations in anthropogenic heat production, radiation balance, and annual variability . Maximum urban-rural differences can be smaller in lower latitudes (e.g., 3°C in Parma) compared to higher latitudes (e.g., 8.7°C in Amsterdam). One study estimated the maximum UHI for Athens during daytime in summer to be 18°C, while another found 7.5°C . • Distribution of Urban Structures: The hottest zones within a city are characterized by the tallest and densest buildings, a lack of green spaces, and intense generation of anthropogenic heat . • Anthropogenic Heat Production: Heat generated by human activities, such as air conditioning units and vehicles, significantly contributes to the UHI, especially during heat waves . During a five-day heat wave in Los Angeles in September 2009, air conditioning heat rejection made the largest contribution (86.5%) to anthropogenic heat dispersion. The average heat rejection from AC systems was 77% more during heat wave days compared to non-heat wave days. The overall daily peak of anthropogenic heat discharge occurred from 3 pm to 6 pm. The spatial patterns of anthropogenic heat discharge are influenced by urban land use and building density data . C. Interaction with Heat Waves The "natural" heat hazard during heat waves can be exacerbated by the built form and land cover characteristics of cities, as well as emissions of anthropogenic heat . This leads to the urban heat island posing an additional risk to urban inhabitants during heat waves. The elevated nocturnal temperatures observed within London during the August 2003 heat wave support this argument. There is growing concern about how climate change may affect the intensity of London’s UHI. Studies in Dijon, France, show a phase lag between UHI maximas and temperature maximas. Seasonally, UHI maximas are observed from May to July, preceding temperature maximas (July to August). On a daily scale, analysis of two heat waves in July 2015 revealed a phase lag of a few days between UHI maximas and the heat wave peak. Hypotheses for these phase lags include changes in insolation and/or energy exchanges between the ground and atmosphere. There are synergistic interactions between urban heat islands and heat waves, where the impact in cities is greater than the sum of its parts . D. Mitigation Strategies for UHI and Heat Waves Urban planning, design, and architecture can play a crucial role in mitigating the UHI effect and its impacts during heat waves . Proposed measures include: • Increasing albedo and vegetation cover: Increasing a city's albedo (reflectivity) is nearly as effective as implementing both increased albedo and vegetation cover . However, increasing vegetation cover has a greater influence on radiant temperature than on air temperature. The mean radiant temperature is the meteorological parameter with the greatest effect on the sensation of thermal stress in humans. Studies have shown that greater vegetation cover and higher-albedo surface colors can reduce heat build-up, with vegetation also absorbing air pollution. Greening roof spaces is an alternative to air conditioning . • Reducing motor vehicles: Every motor vehicle is a source of anthropogenic heat, which exacerbates the urban heat island and urban climate . • Developing information systems on urban climate: This can help to better understand and manage urban thermal environments . • Maintaining natural heat acclimatization levels: This can be achieved through an active lifestyle and appropriately adjusted climatic exposure . V. Heat Health Warning Systems (HHWS) and Adaptation Strategies One primary strategy to reduce the mortality and morbidity burden from heat waves is the implementation of Heat Health Warning Systems (HHWS) . These systems utilize weather forecasts to predict heat-related health effects . A. Purpose and Components of HHWS The essential components of an HHWS include: • Identifying adverse weather situations: Pinpointing specific weather conditions that negatively affect human health . • Monitoring weather forecasts: Regularly tracking forecasts to anticipate heat events . • Issuing warnings: Implementing mechanisms to release warnings when health-adverse weather is forecast . • Promoting public health activities: Encouraging actions to prevent heat-related illness and death . The overall aim is to develop systems that can mitigate heat-related death and illness, often by developing city-specific air mass classification systems and statistical algorithms to predict health impacts based on daily mortality/morbidity and meteorological variables. These algorithms can also account for the interaction between population characteristics and "offensive" air mass types, as vulnerability to heat stress depends on this interaction . B. Methods and Heat Stress Indicators HHWS use various methods and indicators to define and predict heat stress: 1. Heat Wave Definition Criteria: ◦ Percentile-based thresholds: Defining heat wave days as those exceeding a given percentile (e.g., 85th, 90th, 98th, 99.5th) of historical temperature values for a specific location or period . For example, France's warning system initially used the 98th percentile temperature value, later updated to the 99.5th percentile for some cities. India's studies consider heat waves as three or more consecutive days above the 85th percentile of the hottest month for each location . ◦ Absolute temperature thresholds: Setting a specific maximum temperature that must be met or exceeded . In the UK, the Met Office National Severe Weather Service uses a regional system with thresholds for maximum day and night temperatures. Mexico City has proposed a threshold of maximum observed temperature ≥ 30°C for three or more consecutive days with an average temperature of 24°C. The ÍCARO Project in Portugal defines a heat wave with a temperature threshold of 32°C (90°F) combined with a minimum duration of 2 days . ◦ Duration: Most definitions specify a minimum duration, often two or three consecutive days, for a heat wave to be declared . 2. Heat Stress Indices: ◦ Apparent Temperature (AT) / Heat Index (HI): These indices combine temperature with other meteorological factors like humidity to estimate how hot it feels to the human body . The US National Weather Service uses the Heat Index, which is a nonlinear regression fit to apparent temperature values and considers surface temperature and relative humidity. Comparisons show strong linear correlations (≥0.98) between the Heat Index and simpler apparent temperature calculations. The Heat Index shows greater sensitivity to extreme temperature and humidity conditions than simpler approximations. The Apparent Heat Wave Index (AHWI) is a new index that builds on the Heat Wave Magnitude Index daily (HWMId) by replacing temperature with apparent temperature when AT > T, allowing for humidity amplification of heat wave magnitude . ◦ Physiological Equivalent Temperature (PET): Used to describe and quantify heat waves from a human-biometeorological perspective . ◦ Discomfort Index (DI): Defined as 0.5 * (dry bulb temperature + wet bulb temperature), with classifications for mild, moderate, and severe heat stress . ◦ Heat Wave Magnitude Index daily (HWMId): Depends only on temperature, defining a heat wave as at least three consecutive days with daily maximum temperature above the 90th percentile of a reference period . The value represents the sum of deviations of the maximum air temperature of all days in the heat wave . C. Implementation and Case Studies of HHWS 1. Rome, Italy: WHO, WMO, and UNEP collaborated to develop and implement a HHWS for Rome . Developed by the University of Delaware in cooperation with the Lazio Health Authority and the Italian Meteorological Service, it became active in summer 2002. The Italian Meteorological Service provides 72-hour forecasts for five meteorological variables. The system issues a bulletin targeting general practitioners, local health care agencies, hospitals, elderly homes, social institutions, mass media, and registered individuals. Guidelines for behavior during heat waves were developed in collaboration with the Italian Association of General Practitioners . 2. Philadelphia: The Philadelphia hot weather–health watch/warning system was developed and applied in summer 1995 . Based on linear regression analysis, issuing a warning during a heat-wave was found to save about 2.6 lives for each warning day and for 3 days after the warning ended . The estimated costs and benefits for Philadelphia between 1995-1998 showed that these systems save lives . 3. France: Following the 2003 heat wave, France defined a warning system with an index based on the maximum and minimum temperatures averaged over three days, applicable to 96 cities . Limits were determined by studying excess mortality caused by extreme heat, initially using the 98th percentile temperature value in 2004, and later updating to the 99.5th percentile for some cities in 2005. The System of Alert for Heat Waves and Health (SACS) in France was operational in 2004 . 4. Other European Countries: ◦ United Kingdom: The Met Office National Severe Weather Service uses a regional system with thresholds for maximum day and night temperatures to declare a heat wave . A Heat Wave Plan for England was put in place in recognition of significant health effects of recent heat waves . ◦ Portugal: The ÍCARO Project defines a heat wave using a 32°C temperature threshold and a minimum duration of 2 days, similar to a U.S. definition . ◦ Israel: Has a three-step warning procedure, with the first warning 2-3 days in advance, a second one day in advance, and a third 12 hours before the event . ◦ Europe-wide initiatives: The EuroHEAT project studied the impact of heat waves on mortality in 9 European cities . The 5-year European Heat-Shield project (2016–2020) aims to develop information portals and assess the effectiveness and benefits of strategies for worker health and productivity under heat . D. Public Health Responses and Recommendations Public health responses to heat warnings can be categorized as passive or active. • Passive response: Primarily involves issuing warnings of high temperatures through mass media (television, radio, public websites) . These warnings aim to inform the wider community to modify behavior and increase awareness of heat dangers . • Active response: Involves specific advice on recognizing heat-related problems and protective actions . Guidelines for reducing heat-related illness often include resting in shady areas, protecting oneself from the sun with hats and sunscreen, and staying hydrated. However, the advice on fan use might need revision, as fans can add to heat stress in severe, humid heat. Public health messages should be disseminated to all age and risk groups . • Intervention plans: In many countries, HHWS did not include interventions beyond passive warnings . However, components of planning for heat safety can include telephone helplines and cooling centers (air-conditioned public buildings open 24/7). Guidance should be issued for those caring for vulnerable people in homes, hospitals, prisons, and hostels. Warning systems should utilize informal communication methods (e.g., religious groups, charities, word-of-mouth) in addition to all forms of media, as vulnerable groups may lack regular media access . • Challenges in implementation: Despite efforts, significant gaps remain in understanding the effectiveness of early warnings and alerts . Implementing and evaluating more HHWS are crucial for gathering better insights. Heat health warning systems should be developed to fit local settings due to differing climates and cultures within Europe, requiring adjustment of information flow and intervention measures to local needs and infrastructure. Standardization across systems is beneficial for comparison and knowledge transfer, and regional coherence is needed for consistent warnings . E. Challenges and Evaluation of HHWS Evaluating HHWS effectiveness is complex . Key attributes for evaluation include transparency, integrity, acceptability, communication, effectiveness, sensitivity, specificity, timeliness, and sufficiency . • Sensitivity refers to how often a warning is issued and the forecast meteorological conditions actually occur. • Specificity refers to the accuracy of predicting heat-attributable mortality to avoid false positives that could undermine system credibility . • Timeliness ensures warnings are issued sufficiently in advance for response activities . • Cost-benefit analysis: Studies have attempted to quantify the costs of setting up and maintaining HHWS versus their benefits (e.g., lives saved) . For example, the estimated benefits per warning in Philadelphia in 2002 included saving lives . • Organizational structure: Ensuring consistent funding for all involved agencies and promoting regular meetings can prevent loss of knowledge and information flow interruptions . • Knowledge gaps: There is limited information on what public health interventions have actually been implemented as part of HHWS, and research is needed on the effectiveness of specific measures in reducing heat wave mortality or morbidity . VI. Research Gaps and Future Directions The sources collectively highlight several critical areas for further research and improvement: A. Need for Standardized Definitions A significant and recurring research gap is the lack of a formal, objective, and uniform definition of a heat wave . The varied definitions used across studies make it difficult to compare results, develop consistent public health warning systems, or conduct comparative analyses. Researchers recommend developing a standardized approach for defining heat waves to improve the comparability and reliability of findings . B. Further Research on Climate and Urban Environment More detailed regional and local climate models are needed, including downscaling to the urban level and integrating climate simulations into comprehensive assessment models . There is also a call for developing special urban scenarios to better understand how climate and global change will affect the thermal environment of cities . Specific areas for future study include: • Interaction of extreme heat with other hazards: More work is needed on how extreme heat interacts with other pressures on cities, such as drought, which can impact water and electricity demand and agricultural production . • Chronology of heat waves: Developing a comprehensive chronology of heat waves for regions like England, while acknowledging that such a chronology will be dependent on the chosen heat wave definition . • Relationship between warmth and social behavior: Exploring this relationship to identify socially relevant weather/climate thresholds . • Impacts in understudied regions: Emphasizing the need for more studies and resources in areas where climate variability is barely known or completely unknown, such as Central America, South America, and Africa . C. Evaluation of Intervention Strategies There are existing gaps in understanding the effectiveness of early warnings and alerts . It is crucial to implement and evaluate more HHWS to gain better insights into their setup and effective intervention strategies at both national and European levels. Research is specifically needed on the effectiveness of particular measures in reducing heat wave mortality or morbidity. While the benefits of heat watch/warning systems in saving lives have been estimated, such as in Philadelphia, more rigorous evaluation methods are needed . D. Data Availability and Quality Several sources point to challenges related to data availability and quality for robust analysis: • Historical temperature data: Gaps in time series, particularly in the 1940s, can limit the period for reliable heat wave analyses . While efforts are made to ensure completeness and homogeneity of data, access to data, such as for CMIP5 models, can be a limiting factor . • Reanalysis datasets: Significant differences between estimates computed from various reanalysis datasets highlight the need for careful consideration when using them as reference for model evaluations . • Epidemiological data: Lack of comprehensive epidemiological data, such as detailed information on hospitalization characteristics (e.g., patient age, severity of diagnosis, length of stay) and patient professional status for indirect cost estimation, introduces uncertainty in economic estimates of health impacts . Similarly, complete mortality databases in countries like France can take years to compile, requiring extrapolations from samples, which while reliable, still introduce some uncertainty . • Social media data: While useful for crowdsensing, limitations exist regarding data availability from platforms (e.g., Twitter APIs providing only a sample of public data) and the need for geographic references in tweets for correlation with temperatures . VII. Conclusion Heat waves represent a multifaceted and growing natural hazard, intensified by climate change and urban development. Their impacts span significant human health consequences, including increased mortality and morbidity, and considerable economic and social disruptions. Urban areas are particularly vulnerable due to the urban heat island effect and concentrated anthropogenic heat emissions. While critical heat health warning systems and adaptation strategies are being developed and implemented, significant knowledge gaps remain, particularly concerning standardized definitions, comprehensive impact evaluations, and the effectiveness of various interventions. Addressing these gaps through targeted research and improved data collection is crucial for developing more resilient communities capable of coping with increasingly frequent and intense heat waves in the future. ### II. Impacts of Heat Waves on Cities The impact of heat waves is particularly pronounced in urban areas due to several compounding factors: * **Urban Heat Island (UHI) Effect:** Cities are typically much warmer than their surrounding rural environments, a phenomenon known as the Urban Heat Island effect. This is exacerbated by the built environment's materials, lack of green spaces, and **anthropogenic heat emissions** from buildings (e.g., air conditioning units) and vehicles. This additional urban heat increases risks during heat waves. * **Health Impacts:** Heat waves cause elevated rates of illness and death due to heat stress. Specific health effects include: * **Direct Heat-Related Illnesses:** Heat cramps, heat exhaustion, and life-threatening heatstroke. * **Increased Mortality:** Severe heat waves have been responsible for significant excess deaths, such as nearly 800 deaths in Chicago in 1995, approximately 14,800 excess deaths in France in August 2003, and 1906 excess deaths in Lisbon during a 1981 heat wave. These deaths often occur in hospitals, homes, and nursing homes. * **Vulnerable Populations:** Certain segments of the population are more at risk, including the elderly, socially isolated individuals, those with pre-existing medical conditions (psychiatric, pulmonary, cardiovascular illnesses), young children, and people working outdoors. Poorly maintained housing stock and higher poverty rates also contribute to vulnerability. * **Morbidity:** Heat waves can increase hospital admissions for cardiovascular and respiratory diseases and adverse birth outcomes. Loss of well-being due to restrictions in daily activities and symptoms like fatigue, cramps, decreased alertness, and cognitive function are also reported. * **Economic and Infrastructure Impacts:** * **Energy and Utility Strain:** Heat waves significantly increase electricity demand due to increased use of air conditioning, leading to peak demand that can result in **rolling blackouts or even total blackouts**. This poses a serious threat to electrical grids. * **Productivity Loss:** High air temperatures impact human productivity, especially in cities and indoor occupational settings. * **Water Resources:** Heat waves, especially when combined with drought, can lead to **increased demand for water and electricity, resulting in shortages of resources** and potential food shortages if agricultural production is damaged. * **Emergency Services:** Emergency services (staff and vehicles), hospitals, and mortuaries can be overwhelmed during a crisis. Paramedics may even become prone to heat stress themselves due to overwork. * **Environmental Impacts:** Heat waves can affect the environment beyond human health, including **emissions of volatile organic compounds (VOC) from urban/suburban vegetation and corresponding ground-level ozone levels**. Plants themselves are vulnerable, with impacts ranging from sub-lethal to lethal effects, secondary ecosystem effects, and complex interactions with other disturbances like drought. ### III. Strategies for Mitigation and Adaptation Effective management of heat wave impacts requires a multi-faceted approach involving short-term responses and long-term planning. #### A. Heat Health Warning Systems (HHWS) These systems are crucial for immediate public health responses. An effective HHWS requires: * **Scientific Basis:** Development of city-specific air mass classification systems and statistical algorithms that describe the relationship between daily mortality/morbidity and meteorological data (temperature, humidity, wind speed) and heat stress indices. * **Clear Criteria:** Defined criteria for releasing and closing warnings, often based on temperature thresholds linked to excess mortality. * **Lead-Times:** Warnings transmitted to the public ideally **1 to 3 days in advance**. Some systems have multiple steps, with initial warnings given 2-3 days in advance, a second warning 1 day in advance, and a third 12 hours before the event. * **Targeted Communication:** Warnings should target the **whole population**, with special emphasis on vulnerable groups (families with small children, elderly, ill people, tourists, outdoor workers) and institutions responsible for their welfare (health service providers, care workers, sport event organizers). * **Multi-media Approach:** Utilize mass media (TV, radio, public websites), bulletins, web pages, and leaflets for dissemination. * **Localized Advice:** Advice should be **contextual to place and include cultural considerations**, providing more detailed guidance for populations not accustomed to heat. * **Integrated Warnings:** Link heat advice with warnings about other related hazards like **ultraviolet radiation and air pollutants (e.g., ozone)**, to avoid separate alerts. * **Intervention Plans (Examples from Philadelphia, Rome, and Lisbon):** * **Community Outreach:** Encourage friends, relatives, and neighbors to visit elderly people daily. * **Hotlines:** Operate telephone hotlines for information and counseling. * **Cooling Centers:** Utilize public air-conditioned buildings (e.g., senior centers, shopping centers, public swimming pools) as cooling centers, potentially with free transport. * **Home Visits:** Department of Public Health mobile field teams make home visits to high-risk individuals. * **Institutional Alerts:** Inform nursing homes, hospitals, and other facilities about high-risk situations and offer advice. * **Utility Coordination:** Utility companies and water departments halt service suspensions during warning periods. Warnings should also be communicated to electricity providers to avoid power failures. * **Emergency Services Staffing:** Increase staffing for fire departments and emergency medical services. * **Support for Homeless:** Provide daytime outreach for homeless people. * **Evaluation and Improvement:** HHWS effectiveness in reducing mortality needs formal evaluation. This includes assessing if all social groups have equal access and ability to use information, and considering factors like language barriers and access to broadcast media. #### B. Urban Planning and Building Design Long-term strategies focus on reducing the UHI effect and improving urban resilience: * **Climate-Related Urban Planning:** This includes maintaining and improving ventilation paths, restoring connection to ventilation paths, reducing pollution in sensitive areas, and reducing overall heat load. Since 1976, Germany's Federal Development Law has stipulated that climate, air pollution, and health be important factors in urban planning. * **Green Infrastructure:** Increasing green spaces (e.g., tree planting projects, urban parks, green roofs) can reduce urban temperatures and provide additional benefits like increasing a city's attractiveness for business and tourism. * **Building Regulations and Design:** Develop regulations that reduce thermal stress, air pollution, and increase quality of life. Incorporate these into new constructions and renovations. * **Favorable Indoor Climates:** Design buildings to create favorable indoor climates without excessive reliance on air-conditioning. Campaigns can educate the public on optimal indoor temperatures (e.g., cooling offices to no less than 25°C or 28°C) and proper use of windows and shading devices to minimize energy consumption and avoid power failures. * **Addressing Nighttime Temperatures:** Emphasize that high nighttime temperatures detrimentally affect health, and educate the public on strategies to mitigate this. * **Reduce Motor Vehicles:** Every motor vehicle is a source of anthropogenic heat, worsening the UHI effect. * **Education and Training for Professionals:** Train planners and architects in climate-relevant planning and building. Educate policymakers and local administrators to establish and enforce regulations. #### C. General Preparedness and Education * **Public Education Campaigns:** Develop long-term educational and health promotion strategies to raise awareness of heat hazards. Distribute guidelines for specific target groups like schools, residential care homes, and tourist resorts. * **Social Resilience:** Education and training are key to building social resilience to heat stress, empowering individuals to take risk-reducing actions. * **Identify Vulnerable Individuals:** Compile lists of elderly people who live alone or others who might need assistance, so workers can call or visit them. * **Coordination:** Preparedness plans should involve multiple partners, including city managers, public health and social services workers, emergency medical officers, and private organizations. A structure with funding and regular meetings is essential to ensure knowledge retention and uninterrupted information flow. ### IV. Key Challenges and Research Gaps Despite progress, several challenges and knowledge gaps remain: * **Definition Consensus:** A lack of consensus on heat wave definitions and thresholds among the scientific community poses a limitation for comparative analysis and can impact public health policies. * **Effectiveness Evaluation:** More research is needed on the effectiveness of specific interventions and early warning systems in reducing heat-related mortality and morbidity. * **Data Availability:** Mortality information may not always be available, or studies may be too costly for many communities. * **Interaction with Other Hazards:** Very little work has been carried out on the interaction of extreme heat with other hazards and pressures on the city, such as air quality or water security. * **Cultural Context:** Advice given during a heat wave must be adapted to the social and behavioral context of the target population, as climate and culture differ significantly across regions. * **Informing Decision-Makers:** The urgency for generating more reliable information and investing additional resources in this sensitive knowledge area is clear, to help decision-makers understand the impacts and prioritize actions. In essence, navigating the increasing frequency and intensity of heat waves is like steering a ship through a rapidly warming ocean. City managers, elected officials, and property managers are the captains, responsible for charting a course that not only avoids immediate icebergs (sudden health crises, power outages) through robust warning systems and immediate interventions, but also involves redesigning the vessel itself (urban planning, building codes) and training the crew (public education, professional development) to withstand the long-term, systemic changes of the climate. The success of this journey depends on continuous monitoring, collaborative action, and the courage to adapt to a new normal.