Urban expansion in Dhaka and its heat island effect
Rapid urbanisation has been heating up Dhaka for decades, it is going to get worse in the future
Dhaka City is one of the most densely populated cities in the world, comprising a large human settlement. The population of this megacity is increasing rapidly.
Rapid Urban Growth (UG) has caused an increase in the number of impervious surfaces, in energy consumption, domestic heating, exhaust emissions, and a decrease in the number of green land and water surfaces that have changed 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 the Urban Heat Island (UHI) effect.
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Due to the proliferation of impervious surfaces in the city area, higher surface temperatures have been 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 identify the potential heatwave zones and ensure a sustainable city environment by taking preventive measures.
Many researchers throughout the world have performed various simulation studies to identify the relationship between UG and UHI change which helped the cities to develop future sustainable development strategies.
Machine Learning Algorithms such as Cellular Automata (CA) based Artificial Neural Network (ANN) have demonstrated successful prediction results by integrating Remote Sensing (RS) and Geographic Information System (GIS) techniques.
Without UHI's concept, the analysis of UG itself cannot correctly describe the thermal environmental change. Therefore, in a study titled 'Spatio-Temporal Trends of Urban Growth and Its Heat Island Effects Using Machine Learning Algorithms for Dhaka City,' we analysed both UG and UTFVI and used these two components to visualise present and future thermal trends of the city.
In the study, which was a part of my MSC in Civil Engineering at MIST, we have considered Dhaka city to be located between the latitude 23.58°N and 23.90°N, and longitudes 90.33°E and 90.50°E containing approximately 304.78 square km area.
As a part of the study, data from multi-spectral Landsat 4-5 TM and Landsat 8 OLI satellites 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. A Support Vector Machine (SVM) supervised the classification technique used in the Land use/Land cover (LULC) classification for UG analysis with the help of the ENVI 5.3 software.
The 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 were divided into six categories (none, weak, medium, strong, stronger, and strongest) to describe the urban health and heat distribution in the study area. Finally, ANNs were used for predicting seasonal surface temperature and UTFVI using the MOLUSCE plugin of QGIS.
A BBC Bangla article on June 1, 2021, citing a few national and international researchers said that in the last 20 years, the temperature of Dhaka has increased by 3˚C while the world is trying to keep the average global temperature rise within 1.5˚C since pre industrial levels. Moreover, compared to rural areas, the temperature of Dhaka increased by 2.74˚C.
If this trend continues in the next 20 years, the average temperature will increase by 5˚C and Dhaka will become 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 as these two types of calamities have been the biggest challenges for the country so far. Now some new challenges have been added, including rising temperatures in urban areas.
Temperature is rising due to overcrowding in the city. The human body has its own temperature called metabolic heating. In fact, Dhaka recently experienced its hottest day in 26 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 urbanisation. According to a United Nations (UN) report, 54 percent of the world's population lived in urban areas in 2014 which is expected to increase at 66% by 2050. Unplanned and rapid urbanisation creates pressure in cities and brings changes in ecosystems, biodiversity, landscape, and environment.
The foremost long-term consequence of increasing urbanisation is the change in land surfaces from pervious to impervious layers that is caused by UG. UG is the major root cause for converting wetland, vegetated and agricultural surfaces into an urbanised area.
Projections show that urbanisation combined with the overall UG rate of the world's population could add another 2.5 billion urban people by 2050, where 90 percent of that increase will be concentrated in Asia and Africa. According to the World Bank, for the year 2019, 37 percent of Bangladesh's population lived in urban areas and is expected to increase to 50 percent by 2030. Half of the urban population in 2019 lived in the four major divisional centre cities, i.e., Dhaka, Chattogram, Khulna, and Rajshahi.
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 environments consist of more impervious layers, higher LST is usually recorded in city areas, which dramatically contributes to the formation of UHIs and reduces the environmental sustainability of the cities.
The UHI effect is considered to be the main driver behind urban micro-climate warming. UTFVI has significant negative impacts on the local wind patterns, humidity, air quality, and contributes to indirect economic loss, discomfort and higher mortality rates.
Our study analysed several factors which actively or passively contributed to the environmental degradation of Dhaka city. Dhaka city has expanded at an alarming rate in the past few decades. From 2000 to 2010, the urban area was increased from 79.74 km2 to 99.23 km2 with a net increase of 6.39 percent by replacing 3.17 percent bare land, 1.71 percent vegetation cover and 1.52 percent water bodies.
Similarly, from 2010 to 2020, the urban area of Dhaka increased from 99.23 km2 to 133.52 km2 with a net increase of 11.25 percent by replacing 8.29 percent bare land, 1.82 percent vegetation cover and 1.14 percent water bodies.
Our ML algorithm predicted that from 2020 to 2040, the urban area of Dhaka city will increase from 133.52 km2 to 187.57 km2 with a net increase of 17.73 percent by replacing 9.46 percent bare land, 4.72 percent vegetation cover and 3.55 percent water bodies.
The average surface temperature of the city is increasing day by day, providing evidence for local warming which also contributes to global warming.
In summer, the strongest UTFVI value increased from 10.4 percent to 26.1 percent in a 20-year interval period (2000 to 2020). Our model predicted the strongest UTFVI value to increase from 26.1 percent to 42 percent in a 20-year interval period (2020 to 2040).
On the other hand, in winter, the highest temperature range was <32˚C and the area was around 36.61 percent in winter 2020. The strongest UTFVI increased from 4.28 percent to 13.5 percent in a 20-year interval period (2000 to 2020). Our model predicted that the strongest UTFVI value will increase from 13.5 percent to 28.22 percent in a 20-year interval period (2020 to 2040).
In broad strokes, the UHI effect mitigation can be done in two ways. One is by using albedo materials to cover the urban surface and the other is by increasing evapotranspiration.
According to some researchers, the roofs in the cities represent about 21 percent to 26 percent of the city area. Therefore, if the roofs are made green by vegetating, it will act as a major role in mitigating the UHI effect.
Specifically, green roofs absorb heat and filter the air, keeping the temperature low. Increasing the amount of vegetation is one of the most effective strategies to mitigate the effects of the urban microclimate.
Moreover, proper plantation and shading from trees can significantly decrease the energy required for cooling, decrease the rate of heat convection inside buildings and decrease the radiation exchange of the wall with the sky. Plants utilise heat energy to continue their evapotranspiration process, making the environment cool.
Additionally, proper urban planning through the construction of buildings built with a wind path is created to let cool air flow from the river/water bodies into the city may also reduce the UHI effect. Natural ventilation is the most effective passive cooling technique that can provide cooling during the day and the night.
Finally, high albedo roofing materials, i.e, light coloured roofs do not get warmed significantly by reflecting solar radiation can be used on our buildings and pavements also contribute to the reduction of the UHI effect.
Major Md Altamas Karim, psc (retd) served in Bangladesh Army from 1995 to 2016 in the corps of engineers.
