Friday, 17 September, 2021

Trends of Urban Growth and Its Heat Island Effects

Major Md Altamas Karim, psc

Dhaka is the most densely populated city in the world, containing a large human settlement. The population of this Mega city is increasing rapidly. The transition of the non-built-up area to the built-up area causes significant changes in the city's thermal heat environment. Rapid Urban Growth (UG) has caused an increase of impervious surfaces, energy consumption, domestic heating, exhaust emissions, and decrease of green land and water surface that changes the thermal conditions of the city in various ways.

Urban areas tend to have higher temperatures than their rural surroundings due to gradual surface modifications that include replacing the natural vegetation with impervious surfaces causing the phenomenon known as Urban Heat Island (UHI). Due to excessive presence of impervious surfaces in the city area, higher surface temperature is recorded that dramatically contributes to UHI formation and reduces environmental sustainability. The Urban Thermal Field Variance Index (UTFVI) is widely used to describe the UHI effect. Predicting the impacts of future UTFVI can be an effective approach to identifying the potential heat wave zones and ensuring a sustainable city environment by taking essential steps.

Many researchers throughout the world have performed various simulation studies to identify the relationship between UG and UHI change that helped the cities to develop future sustainable development strategies. Machine Learning Algorithms such as Cellular Automata (CA) based Artificial Neural Network (ANN) has demonstrated successful prediction results by integrating Remote Sensing (RS) and Geographic Information System (GIS) techniques.

Without UHI's concept, only analysis of UG cannot correctly describe the thermal environmental change. Therefore, the study will analyze both UG and UTFVI and predict these two components to visualize present and future thermal trends.

Three Multi-spectral Landsat 4-5 TM and Landsat 8 OLI satellite data were acquired for 2000, 2010, and 2020 from the United States Geological Survey (USGS) to explore the UG and UHI change in the study area. Support Vector Machine (SVM) supervised classification technique used in the Land use/Land cover (LULC) classification for UG analysis using ENVI 5.3 software. Surface temperature was estimated using geometric and radiometric corrected thermal bands from Landsat images to retrieve UHI change. Using the estimated surface temperature data, UTFVI was retrieved. The estimated UTFVI values divided into six categories (none, weak, medium, strong, stronger, and strongest) to describe the urban health and heat distribution in the study area. Finally, ANN was used for predicting seasonal surface temperature and UTFVI using MOLUSCE plugin of QGIS.

BBC Bangla published news on 01 June 2021 referring few national and international researchers informing that in the last 20 years the temperature of Dhaka increased by 3˚C where the world is trying to keep its temperature rising within 1.5˚C. Comparing to rural areas, temperature of Dhaka increased by 2.74˚C. If this trend continues in the next 20 years, temperature will increase by 5˚C and Dhaka will be unsuitable for habitation. This overheating will add to the environmental problems for cities. Government policies in Bangladesh are largely focused on natural disasters such as cyclones and floods because these two have been the biggest challenges so far. Now some new challenges have been added, including rising temperatures in urban areas.

Temperatures are rising due to overcrowding in the city. The human body has its own temperature called metabolic heating. Dhaka experienced the hottest day in twenty-six years, with the highest temperature recorded at 40.7˚C on 26 April 2021 as per BMD data.

Cities are the engines of development and this development pathway is accelerating urbanization. According to the United Nations report, 54% of the world’s population lived in urban areas in the year 2014, which is expected to increase at 66% by 2050. Unplanned and rapid urbanization creates pressure in cities and brings changes in ecosystems, biodiversity, landscape, and environment. The foremost long-term consequence of increasing urbanization is the change of land surfaces from pervious to impervious layers, which is led by UG. UG is the major root cause for converting wetland, vegetated and agricultural surfaces into the built-up area. Projections show that urbanization combined with the overall UG rate of the world’s population could add another 2.5 billion urban people by 2050, where 90% of that increase will be concentrated in Asia and Africa. According to the World Bank data for the year 2019, 37% of Bangladesh's population is living in urban areas, which is expected to increase 50% by 2030. Half of the urban population for year 2019 lives in the four major divisional headquarters, i.e., Dhaka, Chattogram, Khulna, and Rajshahi. Containing a large human settlement, Dhaka City is the most densely populated urban centre in the world.

Rapid UG has occurred in the last few decades, primarily due to the significant increase in urban development, which has adversely influenced the urban Land Surface Temperature (LST).

As urban environment consists of a more impervious layer, therefore, higher LST is recorded in city areas, which dramatically contributes to the formation of UHI and reduce environmental sustainability of the cities. UHI is considered as the major reason behind urban micro-climate warming. UTFVI significantly causes negative impacts on the local wind patterns, humidity, and air quality, and indirect economic loss, reduction of comfort and increase of mortality rate.

This study analyzes several factors, which actively or passively contribute environmental degradation of Dhaka city. Dhaka city expanded at an alarming rate in the past few decades. From 2000 to 2010, the urban area increased from 79.74 km2 to 99.23 km2 with a net increase of 6.39% by replacing 3.17% bare land, 1.71% vegetation cover and 1.52% water bodies. From 2010 to 2020, the urban area increased from 99.23 km2 to 133.52 km2 with a net increase of 11.25% by replacing 8.29% bare land, 1.82% vegetation cover and 1.14% water bodies. Predicted result from 2020 to 2040; the urban area will increase from 133.52 km2 to 187.57 km2 with a net increase of 17.73% by replacing 9.46% bare land, 4.72% vegetation cover and 3.55% water bodies.

Average surface temperature is increasing day-by-day, indicating the local warming which leads to the global warming. In summer, the highest temperature range was ≥ 32˚C and during summer 2020 the area was around 22.37% within this range. Strongest UTFVI was increased from 10.4% to 26.1% in 20-year interval period (2000 to 2020). Predicted Strongest UTFVI will be increased from 26.1% to 42% in 20-year interval period (2020 to 2040). Similarly, in winter, highest temperature range was 29 - <32˚C and the area was around 36.61% in winter 2020. Strongest UTFVI was increased from 4.28% to 13.5% in 20-year interval period (2000 to 2020). Predicted Strongest UTFVI will be increased from 13.5% to 28.22% in 20-year interval period (2020 to 2040).

According to some researchers, roofs in the cities represent about 21% to 26% of the city area. Therefore, if the roofs are made green by vegetating, it will act as a major factor in mitigating the UHI effect. Green roofs absorb heat and filter the air, keeping the temperature low. Plants utilize heat energy to continue their evapotranspiration process, making the environment cool. Proper Urban Planning by Buildings built in such a way that wind path is created for cool airflow from the river/water bodies into the city may also reduce UHI effect. Natural ventilation is the most effective passive cooling technique that can provide cooling during day and night.


The writer is a retired army officer.