George Katavoutas

Climate projections indicate significant changes in temperature patterns and other meteorological parameters under different climate change scenarios, with temperature receiving special attention due to its influence on thermal conditions and human discomfort. This study examines the relationship between diurnal temperature range (DTR) and thermal discomfort in the five largest cities of Greece during summer. Thermal discomfort is assessed using Thom’s discomfort index (DI), where values ≥ 21 indicate the onset of thermal discomfort, focusing on thermal conditions at the upper (DI_h) and lower (DI_c) boundaries of daily variability. The analysis uses multiple CMIP6 projections for the reference period (1981–2010) and the near future (2031–2060) under the SSP2-4.5 and SSP5-8.5, representing intermediate and high greenhouse gas forcing pathways, respectively. The study aims to investigate associations between DTR and DI-based thermal discomfort. DTR is projected to increase in most cities in the near future relative to the reference period. This reflects a regional specific response that differs from the global tendency reported in the literature for minimum air temperatures (T_min) to increase faster than maximum air temperatures (T_max). Effect size analysis of DTR indicates generally small effects in Thessaloniki, medium to large effects in Larissa depending on the scenario, and large effects in Heraklion, Athens and Patra. Projected differences in DTR are consistent with the asymmetrical response of air temperature, specifically to the higher increase rate in T_max than in T_min in most cities. DI-based thermal discomfort shows a clear contrast between upper (DI_h) and lower (DI_c) boundaries of daily variability, reflected in higher discomfort classes for DI_h and lower classes for DI_c. Higher DTR values are associated with higher DI_h-based thermal discomfort, while the corresponding association between DTR and DI_c is weak or absent. The positive association observed for the DI_h-based conditions is largely governed by the shared contribution of T_max to both DTR and the discomfort index, whereas the absent or weak association for DI_c-based conditions may reflect the weaker association between DTR and T_min as well as the relatively smaller variability of T_min.

The study of hot extremes has traditionally been grounded in the analysis of maximum air temperature, often overlooking the effect of minimum air temperature but also the combined effect of daytime and nighttime thermal conditions (compound hot extremes). The present study examines climatic indices related to nighttime conditions like tropical nights (TRN), compound hot extreme days (CHD) but also three types of heat waves (HWs) (nighttime, daytime, compound) at large cities of the eastern Mediterranean, over the period 1960–2022. Three sub-periods (1960–1980, 1981–2001, 2002–2022) are also examined in the study. To this end, long-term observational data from one meteorological station with sufficiently long records in each city were used. The analysis reveals increasing trends in the number of TRN (+ 2.8 to + 6.2 tropical nights/decade) but also in their intensity (+ 0.11 to + 0.31 °C/decade), more prominent at the most highly urbanized cities. The frequency of CHD over the last two decades is almost twofold to threefold times the 1980s and 1990s levels, in all cities. Daytime HWs are more frequent than nighttime HWs in all cities except in the city with the largest urban area (Athens), where nighttime HWs dominate. The number of HWs during the last sub-period 2002–2002 is much higher compared to the previous one 1981–2001 (1.8–3.7 times higher for nighttime and 1.4–2.6 times higher for daytime HWs, depending on the city). The coupling between intensity and duration of daytime HWs is strong and apparent in all cities, indicating that the longer the duration of daytime HWs, the higher is their intensity on average, while the coupling for nighttime HWs seems to be weaker.

The study explores long-term changes in the maximum number of consecutive hours per day of heat-related discomfort in Athens over the period 1960–2024, using the Universal Thermal Climate Index (UTCI). This index includes a four-category scale to represent heat stress intensity, ranging from ‘moderate’ to ‘extreme’, as part of its broader multi-category classification system. The analysis indicated a clear increase in the frequency of days with a large number of consecutive discomfort hours over the past decades. Almost 70% of the total number of days with 11 consecutive hours under at least ‘strong heat stress’ and 7 consecutive hours under at least ‘very strong heat stress’ were detected after the year 2000.

An important indicator of climate change is the diurnal temperature range (DTR), defined as the difference between the daily maximum and daily minimum air temperature. This study aims to investigate the DTR distribution in European cities of different background climates in relation to the season of the year, climate class and latitude, as well as its response to exceptionally hot weather. The analysis is based on long-term observational records (1961–2019) coupled with Regional Climate Model (RCM) data in order to detect any projected DTR trends by the end of the 21st century under intermediate and high emission greenhouse gases (GHGs) scenarios. The analysis reveals marked variations in the magnitude of DTR values between the cities, on the one hand, and distinct patterns of the DTR distribution according to the climate class of each city, on the other. The results also indicate strong seasonal variability in most of the cities, except for the Mediterranean coastal ones. DTR is found to increase during hot days and heat wave (HW) days compared to summer normal days. High latitude cities experience higher increases (3.7 °C to 5.7 °C for hot days, 3.1 °C to 5.7 °C for HW days) compared to low latitude cities (1.3 °C to 3.6 °C for hot days, 0.5 °C to 3.4 °C for HW days). The DTR is projected to significantly decrease in northernmost cities (Helsinki, Stockholm, Oslo), while it is expected to significantly increase in Madrid by the end of the 21st century under both the intermediate- and high-emission scenarios, due to the asymmetric temperature change. The asymmetrical response of global warming is more pronounced under the high-emission scenario where more cities at higher latitudes (Warsaw, Berlin, Rotterdam) are added to those with a statistically significant decrease in DTR, while others (Bucharest, Nicosia, Zurich) are added to those with an increase in DTR.

Many regions across the globe have been witnessing changes in both mean climate and climatic extremes during the last decades. Cities in particular, where the concentration of urban population is high, have been in the spotlight of scientific research seeking to address climate-related issues. At the same time, climate change is expected to largely impacts cities until the end of 21st century. However, changes in air temperature levels, as an essential element of the climate, are probably not uniform nor of the same rate across different areas of the globe. An important indicator of the global climate change is the diurnal temperature range (DTR), defined as the difference between the daily maximum and daily minimum air temperature, thus reflecting the temperature variation within a day. This study analyses the distribution and long-term trends in DTR in seventeen European cities of different base climate. The response of DTR under exceptionally hot weather has also been investigated. The study uses observed and projected data of daily maximum and minimum air temperature over the periods 1961-2019 and 1971-2100, respectively. The projected data, over the studied cities for the closest land grid point to the stations’ location, were derived from the Regional Climate Model (RCM) RCA4 of the Swedish Meteorological and Hydrological Institute driven by the Max Planck Institute for Meteorology model MPI-ESM-LR, with the simulations carried out in the framework of the EURO-CORDEX modeling experiment. The projected data were bias adjusted applying the empirical quantile mapping technique. Future simulations were based on two climate scenarios, the Representative Concentration Pathways (RCPs) 4.5 and 8.5. The distribution of the DTR frequency based on observations shows a similar pattern in cities that share the same background climate, while is clearly differentiated at diverse climate types. The mean DTR for normal summer days (maximum air temperature lower than 95 percentile) ranges between 8.0 and 10.5 oC for all cities, with the exception of Nice that shows lower mean DTR and Nicosia, Madrid and Bucharest that present higher. The change of the mean DTR between summer normal days and hot days (maximum air temperature higher than 95 percentile) is greater for cities in higher latitudes, while it is smaller for cities in lower latitudes. According to the projected data for the period from 1971 to 2100 under the RCP4.5 scenario, a statistically significant decreasing trend in mean DTR/yr is projected for the cities in the highest latitudes (Oslo, Stockholm and Helsinki), suggesting higher increasing rates in the minimum air temperature compared to the maximum air temperature. At the same time, the opposite result is expected in Madrid (statistically significant increasing trend), while no statistically significant trends in mean DTR are projected for the rest of the cities.

In this study, a methodology that can reconstruct missing daily values of maximum and minimum temperatures over a long time period under the assumption of a sparse network of meteorological stations is described. To achieve this, a well-established software used for quality control, homogenization and the infilling of missing climatological series data, Climatol, is used to combine a mosaic of data, including daily observations from 15 European stations and daily data from two high-resolution reanalysis datasets, ERA5-Land and MESCAN-SURFEX; this is in order reconstruct daily values over the 2000–2018 period. By comparing frequently used indices, defined by the Expert Team on Climate Change Detection and Indices (ETCCDI) in studies of climate change assessment and goodness-of-fit measures, the reconstructed time series are evaluated against the observed ones. The analysis reveals that the ERA5-Land reconstructions outperform the MESCAN-SURFEX ones when compared to the observations in terms of biases, the various indices evaluated, and in terms of the goodness of fit for both the daily maximum and minimum temperatures. In addition, the magnitude and significance of the observed long-term temporal trends maintained in the reconstructions, in the majority of the stations examined, for both the daily maximum and daily minimum temperatures, is an issue of the greatest relevance in many climatic studies.

The thermal conditions that prevail in cities pose a number of challenges to urban residents and policy makers related to quality of life, health and welfare as well as to sustainable urban development. However, the changes in thermal stress due to climate change are probably not uniform among cities with different background climates. In this work, a comparative analysis of observed and projected thermal stress (cold stress, heat stress, no thermal stress) across four European cities (Helsinki, Rotterdam, Vienna, and Athens), which are representative of different geographical and climatic regions of the continent, for a recent period (1975 − 2004) and two future periods (2029 − 2058, 2069 − 2098) has been conducted. Applying a rational thermal index (Universal Thermal Climate Index) and considering two models of the EURO-CORDEX experiment (RCA4-MOHC, RCA4-MPI) under two Representative Concentration Pathways (RCP4.5, RCP8.5), the projected future changes in thermal conditions are inspected. The distribution of thermal stress in the current climate varies greatly between the cities, reflecting their climatic and urban heterogeneity. In the future climate, a reduction in the frequency of cold stress is expected across all cities, ranging between − 2.9% and − 16.2%. The projected increase in the frequency of optimal thermal conditions increases with increasing latitude, while the projected increase in the frequency of heat stress (ranging from + 0.2 to + 14.6%) decreases with increasing latitudes. Asymmetrical changes in cold- and heat-related stress between cities were found to affect the annual percentage of optimal (no thermal stress) conditions in future. Although future projections are expected to partly bridge the gap between the less-privileged cities (with respect to annual frequency of optimal thermal conditions) like Helsinki and Rotterdam and the more privileged ones like Athens, the former will still lag behind on an annual basis.

The Mediterranean has been identified as a ‘climate change hot spot’, already experiencing faster warming rates than the global average, along with an increased occurrence of heat waves (HWs), prolonged droughts, and forest fires. During summer 2021, the Mediterranean faced prolonged and severe HWs, triggering hundreds of wildfires across the region. Greece, in particular, was hit by one of the most intense HWs in its modern history, with national all-time record temperatures being observed from 28 July to 6 August 2021. The HW was associated with extreme wildfires in many parts of the country, with catastrophic environmental and societal consequences. The study accentuated the rarity and special characteristics of this HW (HW2021) through the analysis of the historical climate record of the National Observatory of Athens (NOA) on a centennial time scale and comparison with previous HWs. The findings showed that HW2021 was ranked first in terms of persistence (with a total duration of 10 days) and highest observed nighttime temperatures, as well as ‘cumulative heat’, accounting for both the duration and intensity of the event. Exceptionally hot conditions during nighttime were intensified by the urban heat island effect in the city of Athens. Human exposure to heat-related stress during the event was further assessed by the use of bioclimatic indices such as the Universal Thermal Climate Index (UTCI). The study points to the interconnected climate risks in the area and especially to the increased exposure of urban populations to conditions of heat stress, due to the additive urban effect.

The Mediterranean area is one of the most visited tourist destinations of the world, but it has also been recognized as one of the most vulnerable to climate change areas worldwide with respect to increased thermal risk. The study focuses on a top worldwide tourist destination of the Mediterranean, Santorini Island in Greece, and aims to assess the past, present and future thermal environment in the island based on the advanced Universal Thermal Climate Index (UTCI). The study utilizes historical observations capturing past (late 19th to early 20th century) and more recent (1982–2019) time periods, while future projections are realized based on four regional climate models (RCMs) under the weak mitigation scenario (RCP4.5) and the non-mitigation scenario with high emissions (RCP8.5). The frequency of cold stress conditions at midday decreases during winter and early spring months by up to 19.8% (January) in the recent period compared to the historical one, while heat stress conditions increase in summer by up to 22.4% (August). Future projections suggest progressive shifts of the UTCI towards higher values in the future and an increase in the exposure time under heat stress depending on the RCM and adopted scenario. The increase in moderate and strong heat stress conditions is mainly expected during the summer months (June, July, August); nevertheless, a noticeable increase is also foreseen in September and May. The highest occurrences of favorable (no thermal stress) conditions are also projected to shift by one month, from June to May and from September to October, in the future.