Heat stress and heatwave trends across Türkiye (1975–2024) using the Extended Heat Index

Abstract

Heat stress is an increasingly important climate-related hazard in regions experiencing rapid warming and changing temperature–humidity interactions. This study examines heat stress and heatwave trends across Türkiye during 1975–2024 using ERA5 reanalysis data aggregated to the provincial scale. Heat stress was quantified using the Extended Heat Index (EHI), which integrates air temperature and relative humidity to represent physiologically meaningful thermal exposure. EHI-based heat stress events were defined as periods with EHI ≥ 27 °C lasting at least three consecutive days, representing sustained health-relevant thermal stress rather than local extremes. Trends were quantified using Sen's slope estimator and the Mann–Kendall test. Results show a pronounced and spatially coherent intensification of EHI-based heat stress across Türkiye. Heatwave days increased by 6.51 days/decade and season length by 7.28 days/decade. Events became more frequent(+0.24 events/decade), longer(+1.49 days/decade), and more intense(+0.28 EHI units/decade), while the heatwave season shifted toward earlier onset(−2.85 days/decade) and later cessation(+2.50 days/decade). Although relative humidity declined across much of Türkiye, dew-point temperature and actual vapour pressure generally increased, indicating that this decline did not reflect widespread atmospheric drying. Instead, rapid warming increased saturation vapour pressure and vapour-pressure deficit. Temperature remained the dominant upward influence on EHI, while regional humidity conditions modified the magnitude of thermal stress. Overall, warming and increasing atmospheric evaporative demand have substantially reshaped heat-stress conditions across Türkiye. By translating ERA5 reanalysis into province-level EHI indicators and warning categories, this study provides climate-service information that can support heat-health adaptation planning, early-warning systems, public health preparedness and decision-making. Practical implications This study shows how long-term climate information can be translated into practical indicators for heat-related climate services and public-health preparedness in Türkiye. Using ERA5 reanalysis, province-level Extended Heat Index (EHI) indicators, heatwave characteristics, and warning categories were developed to track changes in health-relevant thermal stress in a nationally consistent way. The results show that heatwaves have become more frequent, longer, and more intense, while the heatwave season has expanded through earlier onset and later cessation. These changes suggest that heat preparedness should increasingly be considered as a seasonal and long-term planning issue, rather than only as a short-term response to isolated summer extremes. Heat-health action plans and early-warning systems The province-level EHI indicators can support the development of Heat-Health Action Plans and heat early-warning systems by providing information on sustained thermal stress that reflects both temperature and humidity. Because EHI accounts for combined temperature–humidity conditions, it can complement temperature-only warning products and provide a broader representation of heat exposure. A common EHI framework also allows long-term changes to be compared consistently across provinces. This can help national and provincial authorities identify areas where heat stress is increasing most rapidly and where preparedness may need to be strengthened. However, the absolute EHI threshold used in this study is not intended to replace locally calibrated warning thresholds. Where local climate, health-outcome, acclimatization, and population-vulnerability data are available, these can be used alongside the national framework to refine operational warning systems. Preparedness across the expanding heat season A key operational finding is the expansion of the heatwave season. At the national level, season length increased by approximately 7.3 days per decade, with both earlier onset and later cessation. This suggests that preparedness activities may need to begin earlier and continue beyond the conventional peak-summer period. Public-health surveillance, risk communication, emergency preparedness, and health-service planning could therefore be guided not only by fixed seasonal calendars, but also by indicators of heatwave onset, persistence, and cessation. Province-level EHI trends may help identify where earlier activation of preparedness measures is warranted and where monitoring should continue later into the warm season. Regional differences are also important for planning. In parts of the Marmara and Western Black Sea regions, the main change was a shift from Safe to Caution conditions, indicating more frequent moderate heat stress. In Mediterranean provinces, including Adana, Mersin, Osmaniye, and Hatay, increases were more pronounced in higher EHI warning categories. These patterns suggest that preparedness priorities should reflect the type and intensity of heat-stress change observed in each region rather than relying on a single national response pattern. Planning and adaptation implications across sectors for climate governance The implications of increasing heat stress extend beyond the health sector. Occupational health, urban planning, emergency management, agriculture, transport, and other sectors involving outdoor or high-temperature exposure may also benefit from this information. Province-level data on declining Safe days, increasing higher-risk EHI categories, and seasonal expansion can support strategic prioritization within national adaptation planning and provincial climate-risk assessments. For occupational health, these indicators may help identify regions and periods in which outdoor workers are increasingly exposed to sustained heat stress. For urban planning, the results provide a broad climatic baseline that can help identify areas where more detailed local assessment is needed. In rapidly urbanizing locations, province-scale warming may interact with localized urban heat-island effects, potentially increasing heat exposure beyond that represented by regional-scale reanalysis. High-resolution observations and city-level validation are therefore important before specific neighbourhood-scale or urban-design interventions are considered. Persistently warm nights are another area requiring greater attention. Nighttime heat can prolong thermal exposure beyond daytime peaks and reduce opportunities for physiological recovery. Future climate-service development would therefore benefit from research on nighttime-specific and other sub-daily EHI metrics, particularly where these can be linked with local health, exposure, and vulnerability data. Such work could complement daytime-oriented indicators and provide a more complete basis for heat-health planning. The province-scale framework should therefore be viewed primarily as a tool for strategic screening and prioritization. It can indicate where heat stress is changing and where further local assessment is most needed, but it should not replace local meteorological observations, neighbourhood-scale exposure studies, or health-based vulnerability assessments. Climate service integration and risk communication The EHI-based products developed in this study can help strengthen links between climate information providers and public-health institutions. Potential users include the Turkish State Meteorological Service, the Ministry of Health, provincial and municipal authorities, emergency management agencies, occupational health institutions, and regional adaptation planners. For operational climate services, useful products may include province-level heatwave trends, changes in EHI warning categories, seasonal timing indicators, and maps showing where heat stress is increasing most rapidly. These products can make complex reanalysis data easier to use in routine planning, preparedness, and communication. Future systems could build on this framework by combining the long-term ERA5 baseline with near-real-time meteorological station observations. This would help represent local temperature and humidity conditions more accurately, particularly in complex terrain and densely urbanized areas. Linking these climate indicators with health-surveillance data could also support the future development of locally calibrated health-response thresholds and more targeted warning services. Effective communication is also essential for translating EHI-based information into practical public-health action. EHI warning categories can provide a consistent way to communicate combined temperature and humidity stress, but they should be accompanied by simple public-health guidance explaining what the categories mean and what protective actions may be appropriate. Used in this way, the EHI framework can complement existing meteorological warning systems and support more integrated, health-oriented climate services for Türkiye.