ANALYSIS OF SURFACE URBAN HEAT ISLAND IN KEY CITIES IN SOUTHWEST NIGERIA

Abstract

Land use land cover change is recognized as the most significant regional human induced disturbance to the environment and it is also identified as a vital climate forcing. This study therefore assessed the Surface Urban Heat Island (SUHI) intensity in six major cities (Ibadan, Akure, Lagos metropolis, Osogbo, Abeokuta and Ado-Ekiti) within the southwestern region of Nigeria. This was done by assessing the changes in the impervious surface and vegetation cover over the cities; investigating the Land Surface Temperature (LST) over the cities and selected surrounding rural/sub-urban. After this, the intensity of surface heat island for each city within the region was then examined. This was achieved by using remotely sensed Landsat data covering a period of thirty-two years (1984 to 2016), obtained from Earth Explorer. The result for this study showed major changes in the impervious surface as the impervious surfaces were shown to increase with time over all the cities. The areas initially covered by vegetation around the major urban regions were modified to urban areas over the years. This was clearly seen in the vegetation cover changes over the cities for period under consideration as the extent of vegetal cover had decreased tremendously as urban areas increased. The increase in urban areas at the locations indicated by the impervious surface index ranged between 30 % to 57.2 % between 1984 and 2016. Investigating the LST over the cities and the selected rural/sub-urban areas, it was observed that the mean land surface temperature had increased over the years with the most notable increase observed to be between 1984 and 2016. The mean surface temperature values ranged between 25.15oC and 28.8oC in 1984 but increased to a range of 30.15oC and 32.4oC in 2016. This showed that the areas are undergoing considerable warming of the land surface which had considerable impact on the general environment such as the thermal comfort situation of the inhabitants. In examining the Surface Urban Heat Island intensity across the states, the cities were used as the 3 major urban areas. It was revealed that the SUHI is in existence in all the cities both in 1984 and 2016 with an increased intensity in the latter year. The temporal analysis showed that the SUHI intensified massively owing to increase in impervious surfaces within the cities over time. Urbanization is a major contributor factor to the SUHI phenomenon amongst other factors as it causes the temperature of urban areas to increase more than that of the surrounding rural/sub-urban areas mainly as a result of the material makeup of urban areas which possesses high thermal capacity. In conclusion, changes in the land surface features and the associated impacts on the thermal environment, need to be monitored continually as human activities over land continue. This will help foster adequate policies in view of sustainable development.