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Dhaka's temperature is rising because of Urban Heat Island Effect (UHI) effect. This phenomenon was first recognized by Luke Howard in his famous book The Climate of London, which was first published in 1818 4. An Urban Heat Island is a metropolitan area or a portion of a metropolitan area where surface temperature is significantly higher compared to its surrounding (less urbanized) area 5,6. It is usually quantified as the difference of air temperature between these two areas 7.
Reasons of Dhaka’s Rising Temperature [Factors of Urban Heat Island Effect]
Temperature (Land Surface Temperature) of Dhaka increased at a rate of 1.26°C per decade between 1991 and 2014.1
There is a significant positive impact of population density on the temperature (land surface temperature), at the neighborhood scale. This rise of temperature is particularly higher slum settlements and unplanned mixed-use areas in Dhaka.1
In Dhaka, the Urban Heat Island effect varies seasonally. Considering the whole city, the Urban Heat Island effect is comparatively higher during the winter in Dhaka. 12
Urban Heat Island effect is more prevalent during night times
In Dhaka, the Urban Heat Island effect varies seasonally. It is particularly prominent surrounding the built-up areas during hottest season 11. However, considering the whole city, the Urban Heat Island effect is comparatively higher during the winter. 12
The reasons behind this seasonal variation are manifold. During Summer, night-time cooling effect is reduced (mainly due to the cloudy sky) due to the lowest air temperature difference between urban and surrounding (rural) areas 13. High surface winds within the city in the Summer also attribute low Urban Heat Island effect. Lack of cloud cover, however, particularly in winter season enhances the effect of Sky View Factors (SVF) and Urban Heat Island 10,13. Night-time prevalence of Dhaka’s Urban Heat Island effect is mainly due to the construction materials (mainly concreate), which store heat during daytime and gradually emits at night. 12,14
Road geometry, particularly the Sky View Factors (SVF) of roads govern solar radiation and therefore the air temperature differentials in areas on and near to street 10. There is positive correlation between Sky View Factors (SVF) and solar radiation in case of Dhaka, irrespective of seasonal variation. In Dhaka, SVF of roads is factored by the height of the buildings and width of the roads. Wider roads or roads surrounded with low-height buildings thus produce higher air temperature comparatively 15. Additionally, combinations of two factors: street orientation and vegetation coverage in Dhaka’s road is crucial in determining the Urban Heat Island effect 16.
Satellite-date based analyses observed consistent high land surface temperature (LST) is in urban-built up areas, particularly in impervious surfaces, sand-filled barren lands, and industrial zones 11,14,18. High heat flux in landfill areas is distinctive of Dhaka’s heat map, because of using sand as the primary land-filing material 18. Micro-analyses found that maximum air temperature in Dhaka reaches between 1300 – 1400 hours, and as the day progresses, the impact of solar radiation on air temperature increases, as well as the UHT effect 10,19.
Although Urban Heat Island is more prominent in winter season, discussion about it usually takes place more during the summer seasons, most obviously because of the increasing uncomfortable weather condition, impact on human health, and increasing stress on urban heat management systems.
There is no doubt that the problems caused by Urban Heat Island will increase in future as the Urban Heat Island effect intensifies with increasing population and economic activity 9. However, the relationship between population and Urban Heat Island effect is not explicit. Many of the articles calling for direct relationship between population density doesn’t hold ground in recent analyses with latest available data. Urban population size is positively correlated with night surface temperature but very weakly associated with the day surface temperature of cities 6. Actual population density, through metabolic heating of human contribute insignificantly to urban heat flux 7. Population density can be, however, work as a proxy variable of the other anthropogenic factors of Urban Heat Island. Thus, it is often observed that the increased heat flux in location with greater population density 20.
What can we do to stop temperature rise in Dhaka?
Like climate change, control measures of Urban Heat Island can be classified in two groups: adaptation and mitigation. Among adaptive strategies, reducing anthropogenic heat release and appropriate design changes of urban structures (use of high albedo materials, designing cooler etc) are suggested 9. Planting trees (roof garden, urban park etc.), on the other hand, is regarded as the most prevalent and effective mitigation strategies 7,9. Positive role of urban vegetation in lowering the day-temperatures is also evident globally 7.
In Dhaka urban parks, playground, green spaces can play a positive role in cooling down adjacent areas in Dhaka 13,21. This cooling effect is significant during day-times 13. Efforts or plans to reduce the Sky View Factors (planting green covers, using intelligent architectural designing) could be one of the best solutions for Dhaka in future to reduce UHI effects.
To prepare for the changing scenario and guide sustainable urban growth, in 2008, the government of Bangladesh developed the Dhaka Metropolitan Building Construction Regulations 2008 22. With the aim of improving the environment of Dhaka, this new guide incorporates the rules of floor area ratio (FAR), ground coverage and site setbacks.
The Floor Area Ratio (FAR) is particularly aimed to create comfortable urban climatic condition for Dhaka in future. However, there is doubt about its usefulness. FAR guideline have the potential to improve wind flow, but shows little promise in controlling air or surface temperature 15. Instead of a general guideline for the whole Dhaka, it is suggested to develop area specific guideline or at least have provision to tweak the guideline based on locational demand. 13, however, suggested that vegetation coverage within the “mandatory setback of each plot” can significantly reduce UHI effect in Dhaka. Government may develop and encourage to follow a detail guideline for building design particularly focusing on building geometry to reduce UHI effect. This should be included in the Dhaka Metropolitan Building Construction Regulations 13.
References
1. Hossain, Md. A., Sultana, S. & Siddiqui, M. R. Effects of the Nature of Urban Development on Land Surface Temperature (LST) at the Neighbourhood Scale in Dhaka City, Bangladesh. Environment and Urbanization ASIA 13, 284–303 (2022).
2. World Bank. Urban population (% of total population) | Data. https://data.worldbank.org/indicator/SP.URB.TOTL.in.zs (2018).
3. Ritchie, H. & Roser, M. Urbanization. Our World in Data (2018).
4. Mills, G. Luke Howard and The Climate of London. Weather 63, 153–157 (2008).
5. Ahmed, B., Kamruzzaman, Md., Zhu, X., Rahman, Md. & Choi, K. Simulating Land Cover Changes and Their Impacts on Land Surface Temperature in Dhaka, Bangladesh. Remote Sensing 5, 5969–5998 (2013).
6. Zhao, L., Lee, X., Smith, R. B. & Oleson, K. Strong contributions of local background climate to urban heat islands. Nature 511, 216–219 (2014).
7. Peng, S. et al. Surface Urban Heat Island Across 419 Global Big Cities. Environ. Sci. Technol. 46, 696–703 (2012).
8. Souch, C. & Grimmond, S. Applied climatology: urban climate. Progress in Physical Geography: Earth and Environment 30, 270–279 (2006).
9. Rizwan, A. M., Dennis, L. Y. C. & Liu, C. A review on the generation, determination and mitigation of Urban Heat Island. Journal of Environmental Sciences 20, 120–128 (2008).
10. Kakon, A. N. & Nobuo, M. The sky view factor effect on the microclimate of a city environment: A case study of Dhaka city. in Proceedings of the 7th International Conference on Urban Climate, Yokohama, Japan vol. 29 (2009).
11. Dewan, A. M. & Corner, R. J. Impact of Land Use and Land Cover Changes on Urban Land Surface Temperature. in Dhaka Megacity: Geospatial Perspectives on Urbanisation, Environment and Health (eds. Dewan, A. & Corner, R.) 219–238 (Springer Netherlands, 2014). doi:10.1007/978-94-007-6735-5_12.
12. Burkart, K. & Endlicher, W. Human bioclimate and thermal stress in the megacity of Dhaka, Bangladesh: application and evaluation of thermophysiological indices. in Health in megacities and urban areas 153–170 (Springer, 2011).
13. Islam, N. & Wong, N.-H. Urban greenery as a mitigation strategy for urban heat island effect in high density commercial districts of Dhaka, Bangladesh. in Sustainability Matters: Asia’s Energy Concerns, Green Policies and Environmental Advocacy 167–232 (World Scientific, 2014).
14. Trotter, L., Dewan, A. & Robinson, T. Effects of rapid urbanisation on the urban thermal environment between 1990 and 2011 in Dhaka Megacity, Bangladesh. AIMS Environmental Science 4, 145–167 (2017).
15. Kakon, A. N., Mishima, N. & Kojima, S. Simulation of the urban thermal comfort in a high density tropical city: Analysis of the proposed urban construction rules for Dhaka, Bangladesh. Build. Simul. 2, 291 (2009).
16. Kushol, S. A. S., Ahmed, K. S., Hossain, M. M. & Rahman, I. Effect of street morphology on microclimate in residential areas following FAR rule in Dhaka City. in PLEA (2013).
17. Raja, D. R. Spatial analysis of land surface temperature in Dhaka metropolitan area. J Bangladesh Institut Plann ISSN 2075, 9363 (2012).
18. Mia, M. B., Bhattacharya, R. & Woobaidullah, A. S. M. Correlation and Monitoring of Land Surface Temperature, Urban Heat Island with Land Use-Land Cover of Dhaka City Using Satellite Imageries. International Journal of Research in Geography (IJRG) 3, 10–20 (2017).
19. Rob, M. A., Kabir, M. H. & Rabby, Y. W. An Assessment of Microclimatic Variations A Study in Dhaka City, Bangladesh. (2015).
20. Brazel, A., Selover, N., Vose, R. & Heisler, G. The tale of two climates Baltimore and Phoenix urban LTER sites. Climate Research 15, 123–135 (2000).
21. Sudah, S. S. Impact of Neighborhood Parks On Surrounding Build Area In Dhakacity. in Proceedings of the 5th International Conference on Engineering Research, Innovation and Education 347–353 (2019).
Cite this article: Siddiqui M R (2023) Why is Dhaka's Temperature Rising? Rezwan Siddiqui. https://www.rezsid.com/research/why-is-dhakas-temperature-rising
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