Urban Heat Island

IMPACT OF URBAN HEAT ISLAND: CITY LIFE IS GETTING INTO OVEN

Effects of Urban Heat Island on the City Environment

Urbanization, the process of transforming an area into an urbanized one [1, 2], is occurring at a rapid rate in both developing and developed nations worldwide. Population projections indicate that by 2100, the world's population will have increased to 10.9 billion from the current 8 billion [2]. With the rapid increase in population, urban areas are undergoing a rapid expansion process, converting vegetation, desolate land, and bodies of water into urban areas [3] [4], which accelerates rural-urban migration [5]. The process of rapid urbanization increases impervious surfaces [6]. [7] altering the physical landscape through deforestation and biodiversity loss [8] [9] [10]. Due to an increase in impervious surfaces (e.g., roads, buildings, and infrastructures) [11], the world is experiencing a variety of effects, such as urban heat islands as a result of a massive increase in land surface temperature over urban areas [12], which have negative effects on the urban environment by increasing airborne pollution and mortality [13] [14] [15]. With the passage of time, remote sensing technology becomes more sophisticated for the in-depth study of spatio-temporal LST estimation and monitoring over time [16], thus the UHI effect [17] [18]. Satellite data has some defects, such as data continuity, which makes long-term and continuous data monitoring problematic [19], but data aggregation and spatiotemporal interpolation could be viable solutions [20]. UHI is now a major concern that is continuously putting pressure on scientific communities to mitigate its effects [21]. Consequentially, numerous studies have been conducted on mitigation strategies for ameliorating the current scenario, the exhausting warming temperatures experienced in both urban and rural regions [22]. Numerous communities are attempting to reduce the impact by employing various strategies, such as promoting greenery, implementing cool and green roofs, constructing cool pavement, and balancing natural resources and urban areas in city planning [23]. This study's objective was to conduct a literature review in order to comprehend the underlying causes of urban heat islands, their effects on city life, and consequently mitigation strategies to ameliorate the existing scenarios.

Relationship between Urbanization and Urban Heat Island

Several studies have been conducted to determine the effect of urbanization on urban heat island in order to establish the relationship between urbanization and land surface temperature, but only a small number of studies have examined this relationship in ocean-surrounded regions. Moazzam, Doh, and Lee. 2022 conducted a study on Jeju island, South Korea, with its diverse environmental settings and strong monthly and annual temperature variation, to determine the spatio-temporal changes in landuse landcover (LULC) and consequently the effect of urbanization on land surface temperature using landsat data. Maximum likelihood classification algorithm was performed on satellite imagery (spatial resolution 30m) of 2002, 2011 and 2021 based on classification scheme, i.e. urban, forest, water, barren and clouds, to obtain comparative accurate results in LULC using ArcGIS software. Accuracy was evaluated using 400 random points from each class.

LULC in 2002 (a) LULC in 2011 (b) LULC in 2021(c); Source: [2]

The study revealed that with the decline of water bodies, vegetation, and barren land, urban areas were expanded during the study period, reaching 8.69% and 20.81% of total area in 2002 and 2021, respectively. Most barren land was transformed into urban areas over the years.

LST calculation using simplified urban extent algorithm; Source: [2]

Thermal bands were used for land surface temperature (LST) calculation using the simplified urban extent algorithm i.e., Band-6 for the Landsat 7 image and Band-10 for Landsat 8/9 image. The study demonstrated that the average temperature increased significantly over the study period as it was 17.130C in 2002 and 21.650C in 2021 but due to different environmental settings, the highest temperature recorded (340C) was lower than inland areas. The authors found urban land with a higher temperature than other LULC classes and with the rapid urbanization process, LST was in increasing trend.

LST in 2002 (a) LST in 2011 (b) LST in 2021(c); Source: [2]

Comparison of LULC & LST, Relation Between LST & Urbanization, Relation Between LST & Wind Speed; Source: [2]

The authors also investigated the correlation between LST and wind speed, and they found it negative, as with the increase in LST, the wind speed decreased over time. They concluded with the statement that the expansion of urban areas was strongly influenced by the ocean, and LST had been increasing due to urban expansion.

Impact of Urban Heat Island on Urban Settlement

Ramsay et al. (2023) conducted a study on Makassar City, Indonesia, to investigate local-scale urban heat island (UHI) for finding out the trend of LST in formal and non-formal settlements area between 1993 and 2019 quantifying spatio-temporal patterns. The classification was performed on QGIS by semi-automatic classification plugin using Landsat 5 and Landsat 8 satellite imagery. The study demonstrated that urban land cover increased by 175% from 1993 to 2019. The authors delineated whole area manually into four regions based on lulc change i.e., urban core area, an area where urban change was taken place, informal settlements, non-urban area after verificiation from ground truthing to investigate the variance of LST over these specific regions . After delineating the whole area into four specific regions, LST data was clipped into the regions. The study revealed that the highest annual average temperature recorded in urban core area where massive transformation was taken place due to less vegetation and waterbody. On the other hand, lowest annual average temperature recorded in informal settlement area where urbanization growth was less due to less transformation of other lulc classes into urban in comparison to urban core area. Moreover, this study also found that there were less variation in LST in urban core area annually in comparison to informal settlement area. This study was perfomed only few selected informal settlements area and the study dictated that informal settlements were still vulnerable to health hazards due to poor housing because of small magnitude in LST in infomal settlements areas. Green space and proximity to coastal area made some differences in LST in informal settlement area and made their life little bit comfortable in comparison to areas which was adjacent to urban core area and having distal distance from coast with less green space.

LULC Change in Urban area between 1993 and 2019 in Makassar, Indonesia, Seasonal Trends in LST from 2017 to 2020 (Upper Image), Urban Heat Islands in Urban core and informal settlements ; Source: [3]

Strategies for cooling down cities

Due to population growth and rural-urban migration, urbanization is ubiquitous and inevitable. Wang, Li, and Sodoudi. 2022 conducted a study to mitigate the impact of urban heat island to ensure sustainable living environment. They disccused two strategies regarding this issue i.e., cool and green roofs. The authors investigated on five scenarios to see the effectiveness of these two strategies in improvement of human thermal comfort for city of Berlin using Weather Rechearch and Forecasting model, Urban Canopy model and RayMan model. Weather rechearch and forecasting model couple with urban canopy model classified the whole areas into three regions i.e., low intensity residential area, high intensity residential area and industrial area together with commercial area. The area was divided into several urban grid cell where any of three regions were dominant, therefore the grid cell was partitioned into vegetation and impervious part. The authors used RayMan model to invesitgate human thermal comfort using Universal Thermal Climate Index (UTCI) with four meterological parameters i.e., air temperature, humidity, wind speed, radiation and mean radiant temperature based on three class i.e., no thermal stress, moderate heat stress and no thermal stress. This study was performed for a very short period to investigate the effectiveness of cool and green roofs for reducing LST and therefore improving human thermal comfort and Wang, Li and Sodoudi. 2022 came up with a very positive result both strategies were found feasible solutiong for mitigiating the heat stress due to climate change and urbanization, also cool roofs performed better than green roofs with regular irrigation.

Impact of cool roofs and green roofs on LST over different landuse classes (Top-left), Spatial distribution of UTCI both daytime and nighttime (Top-right), Number of hours for each thermal sensation class for reference, cool and green roof scenarios (Bottom-left), Impact of green and cool roofs on urban surface energy balance components: (a) net all-wave radiation (Rn), (b) sensible heat flux (SH), (c) latent heat flux (LH), and (d) ground heat flux (Bottom-right); Source: [4]

Conclusion

Urban heat island is one of the most pressing challenges of our day, alongside the rise in global temperature and its impact on human life. The abundance of resources in urban heat islands makes it evident and worthwhile to review this topic. The study found that comparatively higher temperatures were recorded in urban areas than in other LULC classes, and with the rapid urbanization process, LST was increasing with the growing trend of urban areas. In addition, this study reviewed the interaction between urban heat island and expansion of urban areas through spatio-temporal aspects, where it was found that LST was higher in urban core areas in comparison to non-urban areas due to high vegetation loss. Finally, the study concluded by reviewing some mitigation strategies for reducing land surface temperatures to improve the quality of the city environment. As it is difficult to restrict the growth trend of urbanization, a cool and green roof could be a feasible solution for future city planning, ensuring a sustainable urban living environment considering human thermal comfort.

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