Humphrey Ngala Ndi, Kevin Nfor Ntani & Rodrique Feumba
University of Yaounde I, Cameroon
Takem Mbi Bienvenu-Magloire
National Institute of Cartography, Cameroon
Ever since Luke Howard published his study on the urban heat island in London in 1818, the phenomenon has been reported on worldwide. The urban heat island (UHI) refers to a situation where city temperatures are higher than those of the neighbouring rural areas. This results from the predominance of construction materials that tend to absorb more solar energy in the city than in under-developed rural areas (Gartland 2008). The causes of the heat island phenomenon have been covered adequately in the existing literature (Vargo 2013, Gartland 2008, Adinna, Enete, & Okolie, 2009, and Golden 2004), among many others. Few of such studies have covered the African city, whose structure and layout differs significantly from the western city. As a result, the phenomenon has not received such attention as has benefitted the western city in terms of illustration through case studies. In view of this, discussions on UHI in the context of cities in developing countries are relevant for producing such case studies that should sensitise city administrators on a problem whose health impact could become far reaching on city populations. With a more recent history of urbanisation than other continents, it is obvious that UHI is now being perceived in many African cities. This study examines the surface temperature implications of rapid urbanisation and wetland decline in the city of Yaounde, Cameroon.
STUDY CONTEXT AND METHODS
The geographical limits of Yaounde are nearly contiguous with the Mfoundi administrative division that hosts it. The city lies between latitudes 3º 47´ and 3º 56´ north of the Equator and longitudes 11º 10´ and 11º 45´ east of the Greenwich Meridian. At this latitude, solar radiation is constant throughout because the sun is overhead most of the year resulting in very minimal diurnal temperature ranges, usually less than 3oC (mainland) and 1o C (coast).
Yaounde experiences the classical guinea equatorial climate with four seasons as follows: long dry season (November to March); long rainy season (March to June); short dry season (July to August); and the short rainy season (September to November). It receives an average annual rainfall of about 1700mm and experiences an average annual temperature of 25oC. Unlike other equatorial settlements, the climate of Yaounde is moderated by relief and continentality. It does not only lie on a plateau area, but is also set amidst many hills and mountains altering the patterns of wind circulation, rainfall and temperature. Thus compared with settlements like Douala and Limbe, located at a similar latitude, Yaounde has a milder climate.
The urbanised area of Yaounde has evolved dramatically from about 86.79km2 in 1989, to over 183.61km2 in 2015 (over 140% increase) representing 62.75% of the total surface area of the Mfoundi division (Landsat images). Lying within the Mfoundi river basin, the city is dissected by numerous wet valleys with either flowing rivers and streams, or stagnant waters that form numerous wetlands. (Fig.1). Even though wetlands fall under the ambit of fragile and protected zones, they have undergone profound ecological disturbances from human activities like housing construction, farming and waste disposal. This has resulted in a rapid deterioration and decline in this ecosystem in the Mfoundi river basin. The rapid urbanisation of the basin has transformed its surfaces from those characterised by high specific heat capacities to those with low specific heat capacities.
Data for the study was derived from a range of sources. The Landsat images of Yaounde (February 1989 and February 2015) were analysed in ENVI 4.5 to obtain a false colour composition for land use and land cover. For the purpose of the study, the classes sought for were bare surfaces, built-up areas, water surfaces and vegetation since the study objective was to explain the development of the UHI in Yaounde from the perspective of rapid urbanisation characterised by the development of bare surfaces and built-up area (low specific heat capacity), and retreating wetlands -water and vegetation, (high heat capacity). Land use and land cover changes were estimated from the images and their evolutions ascertained over the selected time interval spanning 26 years. These changes were corroborated by temperature data for the city covering a similar period. The evolution of temperatures in a neighbouring suburban area was compared with the in-city case to ascertain the UHI in Yaounde.
Cities experience climatic conditions that differ from those of surrounding rural areas. They generate more aerosol laden smoke, alter wind speed, and in developing countries where many city streets are dusty, the physical and chemical composition of the atmosphere is altered to favour the development of condensation nuclei at a faster rate than in natural settings. These reduce the albedo over cities resulting in heat retention in the troposphere.
The more a place becomes urbanised the more its surfaces change from those with high albedos to those with low albedos: from green to grey
Outlined below are some of the factors that explain the green to grey changes that have resulted in the development of the heat island in the city of Yaounde:
- Evolution in the built-up area
Tremendous changes have occurred in the land use pattern of Yaounde within the past 26 years. The built-up area has increased by over 140% between 1989 and 2015. From an urbanised surface area of 86.79km2 in 1989, the urban surface of the city has increased to 183.61km2. This area includes habitats, asphalt concrete, and paved surfaces among others (Fig. 2).
Fig. 2: The evolution in the surface area of Yaounde between 1989 and 2015
Source: Landsat ETM (http://glcf.umd.edu/data/landsat)
At the city centre, the high rise flats are mostly roofed with concrete slaps coated with tar, a thick black liquid derived from hydrocarbons. A majority of residential houses have white metal roofs with high albedos and low specific heat capacity, but many new houses are now roofed with coloured metallic material and tiles which have low albedos and higher specific heat capacities. In addition, higher rise buildings have become common even in the outlying quarters, helping to trap heat from being dissipated by free wind circulation.
- Population growth and related
In thirty years, the population of Yaounde grew by over 540%, moving from 318,700 inhabitants in 1976, to over 2,040,768 inhabitants in 2009, while built up area grew by 140% from 1989 to 2015. Although the population has been doubling every ten years since 1976, the absolute increases in numbers have been significantly greater in recent decades. For example, whereas it doubled between 1976 and 1987, only 330,000 were added to the number of urban dwellers in 1987, whereas over 751,000 people were added to the city’s population between 1987 and 1997. It is estimated that Yaounde is currently growing at the rate of 7% per annum (Poverty Reduction Strategy Paper 2003).
Population growth is accompanied by an increase in automobile ownership and use, a significant contributor to air pollution. Heat in the city is obtained from automobile fumes, power stations, central heating systems and people themselves (Waugh 1995). There is an estimated 6000 registered taxis in Yaounde, most of which are run on diesel engines (Daniels 2010), and diesel cars constitute the greatest share of passenger car market. Today, threats to clean air in both developing and developed countries come from vehicle emissions. Because of increased car usage in Yaounde, Talla & Meukam, (2014) have observed that the air quality of the city and other towns in Cameroon will degrade rapidly in the next few years because over 74% of the vehicles imported from the second-hand market in Europe, North America and Japan are generally more than 15 years old.
In the absence of cheap and reliable mass transportation in Cameroonian cities in general, and Yaounde in particular, the number of private cars and urban taxis has increased. Aggravating the prevalence of pollutants emanating from cars is the dust generated from the over 900km of unasphalted streets in the city (Evouna 2008). Added to motor cars are an uncountable number of motorcycle taxis, emitting aerosol-laden smoke into the atmosphere increasing the greenhouse effect that helps to retain heat in the earth’s atmosphere (Fig. 3). The presence of aerosols and other particles in the atmosphere constitute condensation nuclei that provides a greater possibility for the absorption of solar energy.
- The soil and water bodies of Yaounde
As a result of the dense network of rivers and streams in Yaounde, the valleys are mainly covered with hydromorphic soils which contain a mixture of sands and decomposing organic matter (Endamana et al. 2003). The water bodies are generally grey to dark in colour resulting from the high presence of organic matter ensuing from decomposing water plants, sewage and green algae (Fig. 4)
These water bodies are able to absorb significant amounts of heat energy because of their dark-grey colour, which heat is gradually released in weak terrestrial radiation at night.
- Vegetation and wetlands
Vegetation is an important component in the regulation of ambient temperature. Trees provide shade from sunlight that helps to reduce the solar energy absorbed by the earth thereby lowering ground surface temperature. They equally absorb CO2, a greenhouse gas. In the process of evapotranspiration, plants release water to the surrounding air moderating ambient heat. Urban growth has naturally occurred at the expense of vegetation cover and wetlands. Between 1989 and 2015, nearly half of the vegetal cover of Yaounde has disappeared whereas the surface occupied by water has also declined by more than 5%. This is evident from figures 6 & 7 which show the relationship between vegetation, water and built-up area in Yaounde.
Figures 5 & 6 indicate that only 108km2 of vegetal cover of Mfoundi is left of the over 204km2 existing in 1989. As indicated earlier, the implications for energy absorption and radiation cannot be over emphasised.
The decrease in surface water also means that the city has lost some the temperature moderating influence of the higher specific heat capacity of water. However, the presence of organic matter, mud and sand in most rivers and lakes in Yaounde can also modify the water-energy balance with implications on surface temperatures.
Fig. 6: The evolution of vegetation, water and built-up area in Yaounde. Left: February 1989: Urbanised area of 86.79 sq.km. Right: February 2015: Urbanised area of 184.61 sq.km.
Source: Landsat ETM (http://glcf.umd.edu/data/landsat)
Average annual temperatures in Yaounde over a thirty-eight year period (1971-2008) have shown a steadily increasing trend recording an increase in average annual temperature of about 1oC (Fig. 7). This is corroborated by the land use changes experienced in the city in favour of low albedo, increased terrestrial heating and radiation as discussed earlier.
In spite of the paucity of data, average annual temperature data for the predominantly rural locality of Nsimalen, located about 15km away from Yaounde recorded over five discontinuous years showed average annual temperatures significantly lower than those of Yaounde. This scenario is also illustrated in figure 7. The heat island of Yaounde is therefore evident.
Conclusions and way forward
In spite of its location at the equatorial belt where solar insolation is highest on the globe, the population of Yaounde is now becoming aware that the night time heat experienced in the city especially in the dry season is an anomalous situation. As a general rule the dry season is chilly in the mornings and night because with clear skies, terrestrial radiation that heats the earth cannot be trapped in the troposphere at night as would in the rainy season with cloudy skies. Clear skies are rarer in many cities today because of air pollution and the presence of aerosols in the city’s atmosphere that trap terrestrial radiation and re-radiate it back to the earth’s surface.
Although equatorial cities including Yaounde appear greener than most western cities, the expected soothing effects of vegetation via evapotranspiration on temperature are diminished by a nearly constant insolation resulting from the existence of approximately same duration of day and night, the over head sun, and the thinness of the ozone layer at the equator.
As evident as the phenomenon of urban heat may be in Yaounde and other cities in Cameroon, little conscious effort by the relevant authorities is being made to mitigate it (City Council and the Ministry of Urban Development and Housing). Although two master plans for the city have been adopted, their desired effects on town planning are hard in coming. For example, in the 1982 master plan, the following were envisaged: the creation of five large parks covering 1230ha; three natural reserves covering 18800ha and green parks covering 900ha. None of these were achieved (PDU 2008). In addition, about 11500 informal new housing units are produced each year in Yaounde with only 2500 of them being modern and 8500 being traditional. This follows the “built out” model of urban development in which new urban residents are directed towards the previously underdeveloped peripheral areas of the city. Recent research however shows that land cover changes occurring in urban peripheries have the capabilities of affecting temperatures within the urban cores themselves (Stone et al., cited by Vargo 2013).
Sprawl is an important feature of the urban landscape of Yaounde and the municipal authorities are completely dispossessed at its rapid pace. There is the urgent need for innovative policies that will promote the participation of city populations in the governance of city space at different scales. In the case of Yaounde, the City Council ought to decentralise the management of urban space to the seven districts councils in the city for timely and efficient environmental and habitat governance. With the pace of urban expansion, the city council alone has proven its limitations in the deliverance of a basic regulatory document like the building permit which may record delays of up to a year. This scenario, in part, explains the predominance of informal housing structures and the non respect of building norms especially at urban fronts in the city of Yaounde.
Humphrey Ngala Ndi is Associate Professor of Geography at the University of Yaounde I, Cameroon
Kevin Nfor Ntani is a Masters student in Cartography, Remote Sensing and Geographical Information Systems at the University of Yaounde I, Cameroon
Rodrique Feumba is an Assistant lecturer of Geography at the University of Yaounde I, Cameroon.
Header Image: Yaounde, Cameroon. Credit: Humphrey Ngala Ndi