Heat waves: a hot topic in climate change research

The coexistence of urban heat islands (UHIs) and heatwaves (HWs) enhances the stress on human society. However, how UHIs responds to the HWs and the interaction mechanisms between the UHIs and HWs in historical period and under different global warming levels (GWL) 1.5 ºC and 2 ºC scenarios are still elusive. In this study, the regional climate model ALARO-SURFEX (incorporated with LCZs) will be firstly used to estimate the historical interactions between the UHI and HW and the contribution of urbanization to the HWs in Beijing. The intensifying impact of three HWs (daytime HW, nighttime HWs, compound HW) on the UHI will be analyzed, and its energy mechanism will also be revealed. Secondly, the ensembled EAS-CORDEX data calibrated by adding hourly urban signatures will be used to force the offline SURFEX model to project the future urban climate in Beijing. The HW duration, frequency, and intensity will be estimated, and the UHI intensity and pattern will be investigated before, during, and after three kinds of HWs under different warming levels (GWL1.5 and GWL2). Understanding the interaction mechanisms of HWs and UHIs, as well as projecting for HWs and UHIs under the Paris Agreement, would be beneficial for stakeholders and city planners in developing future local adaptation policies.

<p>Climate change threatens to exacerbate existing problems in urban areas arising from the urban heat island. Furthermore, expansion of urban areas and rising urban populations will increase the numbers of people exposed to hazards in these vulnerable areas. We therefore urgently need study of these environments and in-depth assessment of potential climate adaptation measures.</p><p>We present a study of heat wave impacts across the urban landscape of Vienna for different future development pathways and for both present and future climatic conditions. We have created two different urban development scenarios that estimate potential urban sprawl and optimized development concerning future building construction in Vienna and have built a digital representation of each within the Town Energy Balance (TEB) urban surface model. In addition, we select two heat waves of similar frequency of return representative for present and future conditions (following the RCP8.5 scenario) of the mid 21<sup>st</sup> century and use the Weather Research and Forecasting Model (WRF) to simulate both heat wave events. We then couple the two representations urban Vienna in TEB with the WRF heat wave simulations to estimate air temperature, surface temperatures and human thermal comfort during the heat waves. We then identify and apply a set of adaptation measures within TEB to try to identify potential solutions to the problems associated with the urban heat island.</p><p>Global and regional climate change under the RCP8.5 scenario causes the future heat wave to be more severe showing an increase of daily maximum air temperature in Vienna by 7 K; the daily minimum air temperature will increase by 2-4 K. We find that changes caused by urban growth or densification mainly affect air temperature and human thermal comfort local to where new urbanisation takes place and does not occur significantly in the existing central districts.</p><p>Exploring adaptation solutions, we find that a combination of near zero-energy standards and increasing albedo of building materials on the city scale accomplishes a maximum reduction of urban canyon temperature of 0.9 K for the minima and 0.2 K for the maxima. Local scale changes of different adaption measures show that insulation of buildings alone increases the maximum wall surface temperatures by more than 10 K or the maximum mean radiant temperature (MRT) in the canyon by 5 K.  Therefore, additional adaptation to reduce MRT within the urban canyons like tree shade are needed to complement the proposed measures.</p><p>This study concludes that the rising air temperatures expected by climate change puts an unprecedented heat burden on Viennese inhabitants, which cannot easily be reduced by measures concerning buildings within the city itself. Additionally, measures such as planting trees to provide shade, regional water sensitive planning and global reduction of greenhouse gas emissions in order to reduce temperature extremes are required.</p><p>We are now actively seeking to apply this set of tools to a wider set of cases in order to try to find effective solutions to projected warming resulting from climate change in urban areas.</p>

<p>Both heat and cold waves cause extreme human thermal discomfort and a clear excess in mortality. This shows the importance of knowing the prevailing thermal comfort conditions and how thermal comfort conditions vary in various environments so measures can be taken. Microclimatic and outdoor human thermal comfort conditions are investigated in various built-up and green areas in the city of Ghent (Belgium) using meteorological measurements of six weather stations of the MOCCA (Monitoring the Cities Climate and Atmosphere) network in combination with calculations done by RayMan.</p><p>Normal to extreme summer heat wave periods show that dangerous strong heat stress prevails during the daytime periods at all locations. Comparison of thermal comfort during normal and extreme summer heat wave periods showed that heat stress is more extreme when a heat wave is more intense. Overall the urban park in Ghent was the most comfortable location during heat waves since it effectively mitigates heat stress in the city. These results should be taken into account in urban planning and design to keep mid-latitude cities livable.</p><p>Further, a one year data series revealed that outdoor cold stress was more apparent during 2017 in the mid-latitude city of Ghent that experiences a mild maritime climate. During spring and summer, both heat stress and cold stress occurred due to the larger diurnal temperature range compared to the other seasons. Even though high Physiological Equivalent Temperatures (PET) were obtained during a heat wave in summer, heat stress did not occur as intensely and as frequently compared to cold stress on annual level. It could thus be stated that outdoors, cold stress is a bigger threat than heat stress. However, one should keep in mind that the study was executed for outdoor thermal heat comfort and that people will take shelter or take measures when feeling uncomfortable. The question is thus rather, how are citizens protected against heat and cold stress? Currently, the Belgian society is better adapted to cold stress since most buildings contain central heating, while air conditioning is not standard. Future projections predict an increase in temperature, causing more occurrence of extreme heat stress, while extreme cold stress will be reduced. Additionally, the urban heat island effect currently has mainly a positive effect on the average annual outdoor thermal comfort conditions, while it will become a negative effect in the warmer future. Measures should thus be taken to reduce the threat of future heat stress.</p>

Urban Heat Island (UHI) effects adversely impact weather, air quality, and climate. Previous studies have attributed UHI effects to localized, surface processes. Based on an observational and modeling study of an extreme UHI (heat wave) episode in the Baltimore metropolitan region, we find that upstream urbanization exacerbates UHI effects and that meteorological consequences of extra‐urban development can cascade well downwind. Under southwesterly wind, Baltimore, MD, experienced higher peak surface temperatures and higher pollution concentrations than did the larger urban area of Washington, DC. Ultra‐high resolution numerical simulations with National Land Cover Data (NLCD) of 2001 show a nonlocal, dynamical contribution to UHI effects; when the upstream urban area is replaced by natural vegetation (in the model) the UHI effects could be reduced by more than 25%. These findings suggest that judicious land‐use and urban planning, especially in rapidly developing countries, could help alleviate UHI consequences including heat stress and smog.

The city of Chicago has experiences of Urban Heat Island (UHI) phenomena and a good example is its widely reported heat wave of 1995. This study conducted an ecological evaluation of Chicago’s UHI phenomena using LANDSAT TM of 2010, almost 15 years after the lethal heat wave and subsequently implemented various UHI mitigation efforts. The thermal characteristics of the city were assessed with a Land Surface Temperature (LST), a parameter retrieved from LANDSAT TM 6; a thermal-infrared (TIR) band (10.40 to 12.50μm), using mono-window algorithm. The ecological evaluation was made using an Urban Thermal Field Variance Index (UTFVI), which quantitatively evaluated the UHI effects on the quality of urban life. The accuracy of the model was assessed against theoretical relationships between the LST, the Normalized Difference Vegetation Index (NDVI), and Normalized Difference Built-up Index (NDBI). The LST of Chicago ranges from 18oC in Urban Cool Islands (UCI) to 44oC in UHI. In Chicago, although the cooling effects of Lake Michigan, downtown’s sky-high buildings, and urban parks and green spaces have worked together to suppress the concerns of UHI effects locally, still some areas experience a high UHI. In general, 25% of the city experiences ecologically bad or worse UHI effects, indicating a need for continued UHI mitigation efforts.

Under the background of global warming, interaction between heat waves (HWs) and urban heat island (UHI) has led to trends of increase in the intensity, frequency, and duration of extreme heat events in urban areas, seriously threatening the health of urban populations. Taking Guangzhou (a tropical megacity in China) as an example, this study used automatic weather station data and ERA5 reanalysis data to explore the interaction between HWs and UHI, and to elucidate the effects of wind speed and local climate zones (LCZs) on such interaction. Results revealed obvious HWs–UHI interaction in Guangzhou, whereby HWs induces an amplification effect on UHI intensity (UHII) that was most significant at night. In the main urban area, UHII and HWs both weakened with increasing wind speed, indicating that low wind speeds contribute to increased occurrence of HWs and enhancement of UHII. Differently, in some areas peripheral to the main urban area, the UHII at medium wind speeds was stronger than that at low wind speeds, which reflect the impact of heat advection from the urban center. For different LCZs in the main urban area, the strongest UHII, highest risk of HW occurrence, and most significant HWs–UHI interaction were found in the compact mid-rise buildings and compact low-rise buildings (LCZ2 and LCZ3, respectively), followed by the compact high-rise buildings (LCZ1), which was mainly affected by the shading effect of high-rise buildings. The weakest UHII and lowest risk of HW occurrence were found in open high-rise buildings and open mid-rise buildings (LCZ4 and LCZ5, respectively), which generally have good ventilation conditions. Our findings will help to understand urban warming and its association with UHI and HW events in tropical urban regions, which has implications for rational improvement of the urban thermal environment in other tropical urban regions globally.

Armenia is located in close neighbourhood to the Mediterranean region and Middle East, which have been identified as climate change ‘hot spots’ and water‐stressed regions. The continental climate and mountain topography of the country further increase the vulnerability to climate change. The trends in nine extreme temperature and five heat wave indices have been analysed over the period 1979–2023 using daily maximum and minimum temperature observations from 40 meteorological stations across Armenia. The results show that climate change induced a significant increase in the frequency and intensity of warm extreme temperature indices and heat waves in recent decades. The most prominent and statistically significant warming trends have been found for the summer season. Extreme temperature indices and heat waves that are based on daily maximum temperatures have shown stronger warming trends at most of the stations, especially in the summer season. These asymmetric warming trends of daytime temperatures have enlarged the daily temperature range. The asymmetric warming was partly attributed to an increase in sunshine duration and a decrease in precipitation and wet days observed in the summer season. The urban heat island (UHI) significantly impacted the frequency and severity of heat waves over Yerevan (the capital city). In contrast to the background climate change effect over the entire country, the asymmetric warming trends of nighttime temperatures were observed over Yerevan due to the UHI effect. The nighttime temperatures reached 25°C–28°C during the heat waves, while the maximum duration of heat wave events reached 10–12 days in recent decades.

Global warming has resulted in higher frequencies of climate extremes, such as drought periods or heat waves. Heat waves are intensified in urban areas due to the urban heat island effect. Studies are inconclusive as to whether the urban heat island effect is intensified during heat waves. Using the city of Hannover, Germany, as a case study, we analysed the intensity of the urban heat island under unprecedented summer heat conditions in the years 2018, 2019 and 2020, which were among the hottest in Germany since weather records began. We compared the intensity of the urban heat island across these years with the non-heat year of 2017. Differences were analysed for various inner-city urban locations and an urban park, while accounting for their distinct land use and land cover characteristics. We identified the urban heat island effect across all years investigated in the study and also found a significant intensified urban heat island effect during the years of unprecedented heat, when night-time temperature minima are considered. The urban heat island was identified on a lower level, however, with maximum daily temperatures when compared to the non-heat year. The lowest intensity of the urban heat island was found for the urban park site, highlighting the need for more city-wide greening strategies, including tree-covered and open green spaces, to provide all residents with the cooling services of green spaces.

Revealing the response of urban heat island effect to water body evaporation from main urban and suburb areas

[Figure: see text]

Objective To study the status of adaptive behavior during high temperature season and heat wave period among residents of Shenzhen city and to analyze the relationship between adaptive behaviors and heatstroke among the residents.Methods During November 2014,a household face-to-face questionnaire survey was conducted among 623 community residents selected with stratified random sampling in Shenzhen city to collect information on demography,awareness about heat wave,spontaneous adaptive behaviors during heat wave,and whether having heatstroke during previous one year.We used chi-square test to compare differences among various resident groups and unconditional logistic regression to analyze the relationship between adaptive behaviors and heatstroke among the residents.Results Among the residents,the rate of awareness about heat wave was 69.5% and the average score for heat wave risk assessment was 3.53±1.05.Almost all the residents (621) people adopted at least one adaptive practice during a heat wave period and drinking plenty of water was the most frequently used adaptive practice,which was reported by 93.1% of the residents.The results of logistic regression showed that taking air-conditioned buses for traffic,drinking plenty of water,wearing light color clothes,and limiting outdoor activities were protective factors for heatstroke during a heat wave period among the residents (P<0.05 for all).Conclusion Both the awareness about heat wave and the proportion of adopting adaptive practice spontaneously were high among the community residents in Shenzhen city during heat wave period and some of the adaptive practices may contribute to the prevention of heatstroke.

&amp;lt;p&amp;gt;Provided that the population living in cities is increasing, projected to reach 5.2 billion in 2030, and that heat waves are getting more intense and lasting as a consequence of the global warming, the urban heat island (UHI) phenomenon is leading increasingly to extremely high temperatures within cities. It is therefore important to find reliable and simple methods for estimating and characterizing at a high resolution the UHI.&amp;lt;/p&amp;gt; &amp;lt;p&amp;gt;In this work we characterize the urban heat island (UHI) of Rome, Italy, during summer, through a dense weather station network. Measurements were collected in summers 2019-2020. We calculate the UHI intensity using a method that relates the air temperature to imperviousness (IMP), which quantifies the presence of artificially sealed surface in a radius around each station using Copernicus Land Monitoring Service satellite data. To assess the reliability of this method we made a comparison with the LCZ-based approach, finding compatible daily trends of UHI intensity, with a fixed bias during night. Our method both simplifies the measurement area classification and allows to determine the UHI intensity even when measurements in totally urban and totally rural areas are not available. The correlation coefficient values between IMP and daily maximum, minimum and mean temperatures were 0.17, 0.81 and 0.82, respectively, evidencing the nighttime UHI peak observed in other cities. The UHI intensity diurnal cycle pattern showed, starting from its minimum of -0.1&amp;amp;#176;C at 10:00 (CET), a progressive increase which intensifies after sunset, reaching a maximum of 3.4&amp;amp;#176;C at midnight. During the night a slight decrease is observed, which exacerbates after sunrise. We did not find a relevant correlation between UHI and heat waves.&amp;lt;/p&amp;gt;

Energy-efficient mitigation measures for improving indoor thermal comfort during heat waves

BACKGROUND: Climate change has a significant impact on the population health including morbidity and mortality. The studies presented in this paper present characteristics of heat and cold waves on the Crimean Peninsula.&#x0D; AIM: To assess the heat and cold wave characteristics, identify differences in the wave characteristics depending on the coastal and mainland location of climatotherapeutic centers on the Crimean Peninsula.&#x0D; MATERIAL AND METHODS: In our research we used standard statistical data treatment methods. Moreover, data visualization techniques allowed identifying zones of maximum temperature values. Heat and cold waves were defined using the 3rd and the 97th percentiles of the average daily air temperatures. This approach allows considering the intensity and duration of the events under study. Measurements of the surface air temperature included eight observations per day at 19 stations on the Crimean Peninsula for the period from 2006 to 2021. Distribution of the permanently residing population in the Crimea and the city of Sevastopol by age and gender was analyzed using the Rosstat data.&#x0D; RESULTS: As many as 43.5% of the population of the Crimean Peninsula have increased risk of exposure to extreme air temperatures. Having applied the threshold values of the 3rd and the 97th percentiles of the temperature, we identified the periods of heat and cold waves at each station. Even though the Crimean Peninsula has a warm and humid climate, 1 to 4 cold and heat waves have been observed at each station per year. At the same time, the total intensity of cold waves was more than twice the total intensity of heat waves.&#x0D; CONCLUSION: The presented assessment of heat and cold wave characteristics on the Crimean Peninsula, using 3% and 97% percentiles of the distribution of the average daily air temperature over several years, provides the evidence for developing the measures to reduce the negative impact of extreme temperatures on human health on the Crimean Peninsula.

The urban heat island effect is often associated with large metropolises. However, in the Netherlands even small cities will be affected by the phenomenon in the future (Hove et al., 2011), due to the dispersed or mosaic urbanisation patterns in particularly the southern part of the country: the province of North Brabant. This study analyses the average night time land surface temperature (LST) of 21 North-Brabant urban areas through 22 satellite images retrieved by Modis 11A1 during the 2006 heat wave and uses Landsat 5 Thematic Mapper to map albedo and normalized difference temperature index (NDVI) values. Albedo, NDVI and imperviousness are found to play the most relevant role in the increase of night-time LST. The surface cover cluster analysis of these three parameters reveals that the 12 “urban living environment” categories used in the region of North Brabant can actually be reduced to 7 categories, which simplifies the design guidelines to improve the surface thermal behaviour of the different neighbourhoods thus reducing the Urban Heat Island (UHI) effect in existing medium size cities and future developments adjacent to those cities.