The Rising Health Burden in a Warming World

Introduction

Global surface temperatures have risen substantially since the pre-industrial baseline (1850-1900), primarily because of anthropogenic greenhouse gas emissions (IPCC, 2021, 2023). As shown in Figure 1, observational records from several major scientific institutions, including the National Centers for Environmental Information (NOAA), NASA, and the Japan Meteorological Agency’s JRA-55 reanalysis, display close agreement on the trajectory of global warming (Copernicus Climate Change Service, 2025; Hersbach et al., 2020; GISTEMP Team, 2025; Lenssen et al., 2019; Morice et al., 2021; NOAA NCEI, 2025; Vose et al., 2012).

Figure 1. Global average surface temperature anomalies (°C) relative to the 1850–1900 pre-industrial baseline (Copernicus Climate Change Service, 2025; Hersbach et al., 2020; GISTEMP Team, 2025; Lenssen et al., 2019; Morice et al., 2021; NOAA, 2025; Vose et al., 2012).

Despite year-to-year variability, the datasets indicate a persistent warming trend, with global temperatures surpassing approximately +1.5 °C above pre-industrial levels by 2024 (Copernicus Climate Change Service, 2025; Hersbach et al., 2020; GISTEMP Team, 2025; Lenssen et al., 2019; Morice et al., 2021; NOAA, 2025; Vose et al., 2012). The steepest warming has occurred over the past two decades, underscoring the accelerating pace of climate change (Copernicus Climate Change, 2025; GISTEMP Team, 2025). The IPCC projects that the mean global temperature will likely approach 2.0 °C under the intermediate SSP2-4.5 emissions pathway by 2041 (IPCC, 2021, 2023).

This intensification of extreme heat is not spatially uniform. IPCC assessments indicate that the hottest days in some mid-latitude and semi-arid regions are projected to warm at approximately 1.5 to 2 times the rate of mean global temperature rise (IPCC, 2021, 2023). Each additional degree of global warming, therefore, produces disproportionate increases in the annual hottest-day temperatures, especially in mid-latitude, semi-arid, and densely populated urban areas (as Figure 2 indicates below). These regions are particularly vulnerable because climatic exposures converge with human factors such as high population density, urban heat-island effects, and limited access to cooling (IPCC, 2023).

Figure 2: Projected changes in the annual hottest-day temperature (°C) under different global warming levels relative to the 1850–1900 baseline (IPCC, 2023).

Heat exposure is not only uneven across space but also stratified across generations. The baseline climate into which people are born is progressively warmer, shaping the number of days each cohort experiences above health-relevant thresholds. Under high-emissions scenarios, infants born today are expected to spend much of their adult lives in substantially hotter conditions, with more frequent and intense extreme heat events. 

As illustrated in Figure 3, cohorts born in 1950, 1980, and 2020 encounter progressively higher temperature baselines over their life course, with total exposure to extreme heat heavily contingent on future emissions trajectories (IPCC, 2021, 2023). This progressive intensification of thermal conditions across birth cohorts translates directly into a magnified burden on public health throughout the 21st century.

Figure 3: Generational exposure to extreme heat under alternative global warming scenarios (Calvin et al., 2023).

Physiological and Clinical Heat Burden

Prolonged exposure to elevated ambient temperatures activates an integrated thermoregulatory response involving sweating and cutaneous vasodilation, which increases skin blood flow and enhances heat dissipation. As depicted in Figure 4, these responses reflect a continuum of physiological adjustments to heat stress (Baker, 2019; Crandall & Wilson, 2015; Kaissassou et al., 2023).

Increased skin perfusion and profuse sweating redistribute fluids toward the periphery and promote substantial loss of water and electrolytes. This leads to dehydration and reduced circulating blood volume, which in turn decreases cardiac preload, stroke volume, and arterial pressure. Compensatory mechanisms, tachycardia, and increased myocardial contractility attempt to maintain cardiac output and blood pressure. However, persistent vasodilation and blood pooling in cutaneous vessels can exacerbate hypotension and impair perfusion of vital organs (Crandall & Wilson, 2015). As fluid deficits worsen, sweat production declines, limiting evaporative cooling and allowing core body temperature to rise further, potentially culminating in heat exhaustion or heat stroke.

Clinically, these processes manifest as thirst, dizziness, headache or syncope, nausea, vomiting, muscle cramps, fatigue, and warm or clammy skin, symptoms highlighted in Figure 4 (Kaissassou et al., 2023). 

Figure 4: The body’s physiological responses to heat stress (Kaissassou et al., 2023)

Mental Health, Cognition, and Social Harm

Extreme heat has profound implications for mental health. Epidemiological studies consistently demonstrate associations between elevated temperatures and psychiatric morbidity. Large multi-city analyses report 7–11% increases in emergency department visits for mental health conditions, including anxiety, mood disorders, and self-harm, on the hottest days (Nori-Sarma et al., 2022; Wellenius et al., 2022). Cumulative exposure during heatwaves has been linked to increased psychotropic medication use and behavioural disturbances among people with dementia (Cornali et al., 2004; Watts et al., 2023).

Heat stress is also associated with aggression and interpersonal violence. Meta-analyses and crime data suggest 4–6% increases in violent incidents during heatwaves, reflecting impaired emotional regulation, heightened irritability, and intensifying social tensions under thermal stress (Burke et al., 2015; Hsiang et al., 2013).

Cognitive performance declines significantly under sustained heat exposure. Educational research shows that exam scores can fall by up to 15% on days exceeding 32 °C, with the largest effects observed in schools lacking air conditioning or other cooling measures (Park et al., 2020). Occupational epidemiology similarly documents elevated error rates and workplace accidents at higher temperatures, with an estimated 28,000 heat-related injuries annually in the United States alone (Spector et al., 2019).

Extreme heat is therefore not only a physical hazard, but also a pervasive threat to mental health, cognitive functioning, and social stability, impacts that are likely to intensify as global temperatures rise (as shown in Figure 5).

Figure 5: Schematic representation of the impacts of extreme heat on mental health, cognition, and social harm (Modified from Hussain, 2022)

Populations Most Vulnerable to Extreme Heat

Individuals at greatest risk during extreme heat events are those in whom biological susceptibility intersects with limited physiological reserve. Older adults, infants, and people with chronic cardiometabolic, respiratory, or renal disease have reduced sweating capacity, impaired vasodilation, and diminished cardiovascular reserve. These factors constrain their ability to dissipate heat and maintain adequate perfusion during thermal stress (Baker, 2019; Crandall & Wilson, 2015).

Clinical guidelines highlight that these groups—together with pregnant individuals and those experiencing acute illness or dehydration—are at elevated risk of rapid progression from mild heat stress to heat exhaustion and heat stroke. This risk is particularly high when early symptoms, such as dizziness or confusion, are not recognised and managed promptly (Eifling et al., 2024; Groot et al., 2014).

Vulnerability is further increased among individuals with dementia or severe mental illness, who may have limited capacity to adapt their behaviour in response to rising temperatures. Heat-associated behavioural disturbances in dementia, as well as increased psychotropic medication use during heatwaves, illustrate this risk (Cornali et al., 2004; Watts et al., 2023). Population-level analyses of emergency department presentations and hospitalisations indicate that people with pre-existing mental health conditions experience disproportionately large increases in service use during hot weather, underscoring their heightened sensitivity to thermal stress (Nori-Sarma et al., 2022; R. Thompson et al., 2023; Wellenius et al., 2022).

Figure 6: Populations most vulnerable to extreme heat (Bwire, 2025)

Heat vulnerability is also strongly patterned by occupation, geography, and socio-economic position (Figure 6). Outdoor and manual workers in agriculture, construction, transportation, and related sectors experience prolonged exposure to high radiant and ambient heat. This exposure is often combined with intense physical exertion, inadequate shade, insufficient rest breaks, and limited access to water. These conditions markedly increase the risks of both heat-related illness and occupational injury (Sankar et al., 2024; Spector et al., 2019; Sokas & Senay, 2023).

Residents of mid-latitude, semi-arid, and tropical regions already experiencing high thermal loads, including parts of Central Africa and South Asia, face additional risk where rapid warming, urban heat-island effects, and substandard housing or cooling infrastructure compound exposure (Calvin et al., 2023; IPCC, 2021, 2023; Kaissassou et al., 2023). Within these settings, low-income communities, informal settlement residents, and people lacking reliable access to air conditioning or health care bear a disproportionate burden of heat-related morbidity and mortality, a pattern highlighted in both global assessments and local public health messaging (Collins, 2025; Watts et al., 2023).

The Areas Most At Risk For Extreme Heat Impacts

Geographical regions most at risk from extreme heat impacts are those where physical hazard, population exposure, and limited adaptive capacity converge. Extreme-value analyses of annual maximum temperatures suggest that Afghanistan, the Central American Integration System countries (Guatemala, El Salvador, Honduras, Nicaragua, Costa Rica, and Panama), far eastern Russia (Khabarovsk), the Beijing–Hebei–Tianjin region of northern China, and densely populated parts of western Europe (including Germany, the Netherlands, and Belgium) have substantial “headroom” for record-breaking heatwaves relative to their historical records (V. Thompson et al., 2023). These areas also have large or rapidly growing populations and, in several cases, constrained health and energy systems, making them priority settings for anticipatory heat adaptation.

Such adaptation includes robust heat-health action plans, upgraded housing and energy infrastructure, and well-functioning early warning and response systems. Figure 7 shows the global distribution of the probability of record-breaking heatwaves, with colours denoting low, medium, and high risk. Understanding these risks informs where adaptation is most urgently needed. Extensive high-risk zones occur across western and central North America, southern Europe and the Middle East, northern China, portions of Central and South America, and selected regions of Africa and Australia (V. Thompson et al., 2023).

Figure 7: Areas at high risk of record breaking heatwaves (Childs, 2023)

Preventing and Responding to Heat Stress

Given the increasing frequency, severity, and duration of extreme heat events, preventive strategies and early clinical responses are critical components of public health adaptation. Effective personal heat management involves behavioural and environmental strategies to support physiological cooling and mitigate cardiovascular strain and thermoregulatory failure. These measures are particularly important during prolonged heatwaves or in high-exposure settings such as outdoor labour, athletic events, and crowded urban environments (Collins, 2025; Eifling et al., 2024; Sankar et al., 2024).

As summarised in Figure 8 below, key preventive actions include scheduling strenuous activities for the coolest parts of the day, moderating physical exertion, and prioritising regular hydration, ideally before the onset of thirst. Fluid intake should be maintained consistently throughout the day, with electrolyte replacement considered during episodes of heavy sweating. Clothing choices are also crucial: lightweight, loose-fitting, light-coloured garments made from breathable fabrics facilitate evaporative cooling, while wide-brimmed hats and umbrellas reduce direct solar exposure (Collins, 2025; Sokas & Senay, 2023).

Figure 8: Behavioural and environmental strategies for preventing and responding to heat stress (Collins, 2025).

Environmental adaptations, such as seeking shade, using fans or ventilation systems, and accessing air-conditioned spaces, are especially important for vulnerable populations. Simple cooling strategies, including applying cool, damp cloths to the neck and wrists, using misting sprays, or taking cool showers, can lower core temperature and restore thermal comfort. Indoor environments should be kept as cool as possible by closing curtains or blinds during the hottest hours and minimising the use of heat-generating appliances (Collins, 2025; Groot et al., 2014).

Prompt recognition of early warning signs, including headache, dizziness, nausea, muscle cramps, and excessive sweating, is essential to prevent progression to more severe heat-related illness. Individuals experiencing these symptoms should stop exertion, move to a shaded or air-conditioned area, and begin rehydrating. If symptoms persist or worsen, particularly if confusion, loss of consciousness, or hot, dry skin develops, urgent medical attention is warranted, as these features may indicate heat stroke, a potentially fatal emergency (Collins, 2025; Crandall & Wilson, 2015; Eifling et al., 2024).

Preventive efforts should also extend to community-level vigilance. Checking on older adults, young children, people with chronic illness, and those taking medications that impair thermoregulation can help identify early signs of distress. During outdoor work or exercise, a buddy system is advisable so that early symptoms are noticed and acted upon. It is also essential to stress that no person or animal should be left in a parked vehicle, as cabin temperatures can reach life-threatening levels within minutes, even in moderately warm weather (Collins, 2025).

Conclusion

As global temperatures rise toward and beyond 1.5°C, extreme heat is becoming one of the most immediate and life-threatening impacts of climate change. The evidence reviewed shows that high temperatures strain cardiovascular and thermoregulatory systems, worsen mental health, impair cognition and learning, and increase occupational injuries and social violence. These harms fall most heavily on people with limited physiological reserve or pre-existing physical and mental illness. Older adults, infants, outdoor workers, low-income communities, and residents of rapidly warming urban and semi-arid regions are particularly at risk due to their constrained ability to avoid exposure.

Limiting further warming through rapid emissions reduction is essential but is insufficient on its own. Heat must be treated as a core public health and equity issue, requiring early warning systems, heat-health action plans, safer working conditions, and urban and housing policies that secure access to shade, cooling, water, and care. By coupling ambitious climate mitigation with targeted, health-centred adaptation, societies can substantially reduce the growing burden of heat-related illness and protect the most exposed populations in a warmer world.

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Figure 6: Developed by the author, based on information from Baker (2019); Calvin et al. (2023); Collins (2025); Cornali et al. (2004); Crandall and Wilson (2015); Eifling et al. (2024); Groot et al. (2014); IPCC (2021, 2023); Kaissassou et al. (2023); Nori-Sarma et al. (2022); Sankar et al. (2024); Sokas and Senay (2023); Spector et al. (2019); R. Thompson et al. (2023); Watts et al. (2023); and Wellenius et al. (2022).

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Childs, J. W. (2023, April 27). The areas most at risk for extreme heat impacts. The Weather Channel. https://weather.com/news/climate/news/2023-04-26-impacts-of-extreme-heat-waves

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Collins, T. (2025, June 24). CCHD reminds the public to stay safe as heatwave continues. WTYE. https://www.wtyefm.com/2025/06/24/cchd-reminds-the-public-to-stay-safe-as-heatwave-continues/

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