IT has been proved by the works of many scientists that the changing dynamics of increased urbanisation has resulted in the change in energy balance, especially in the metropolitan cities like Mumbai. This has led to the creation of urban heat islands.

The metropolitan cities in India in general and Mumbai in particular have experienced rapid urbanisation since 1970.The manifestation of this process is seen in the rapid rise of population and growth of urban infrastructure.  Many undesirable changes have taken place within the city as well as in the surrounding areas and this have brought a corresponding change in the level of temperature in Mumbai.

Several factors give rise to the UHI effect, like increase in build up areas, use of constructional materials that effectively retain heat, emission of heat from household activities, industries and vehicle, and simultaneous shirking of open space, reduction in natural vegetation, reclamation of the low-lying areas in the coast, etc. All these factors have pushed Mumbai for the generation and storage of heat.

The study carried by Maral and Mukhopadhyay (2015) reveals that the city and the suburbs of Mumbai remain warmer than the peripheral station of Mumbai Metropolitan Region (MMR), especially during winter and summer. A statistically significant difference has been observed in the average maximum and minimum temperature of stations in greater Mumbai Island the stations located in the periphery of MMR. The findings of the study show that areas like Santacruz and Colaba are warmer than that of the peripheral stations during day time and night time of winter by 0.9 degree Celsius and during the daytime and nighttime of summer by 0.6 degree to 1.1degree C.

It is observed from our study  that the decadal mean of the maximum temperature for all seasons at Colaba in successive decades has shown a rise from 0.10 C to 0.60 C, except in winter when the  maximum temperature showed a slight dip of 0.10 C from 1991-2000 to 2001 2010. The average monsoon and post-monsoon maximum temperature of Colaba for the decades 1991 -2000 and 2001 -2010 remain unchanged.

It is observed that though decadal mean of winter, summer and post monsoon minimum temperature at Colaba do not show consistent rise in successive decades, the mean minimum temperature of 2001 -2010 decade for all seasons are higher than the minimum of three previous decades, i.e.  1971 -1980, 1981-90 and 1991 -2000.

Decadal mean of summer and monsoon minimum temperature at Santacruz in successive decades from 1971 to 2010 show arise from 0.20 to 0.60 C. However, consistent rise in mean of winter and post monsoon minimum temperature has not been observed for the successive decades.

Mean of minimum temperature for the decade of 2001- 2010 for Santacruz in winter, summer, and monsoon are higher than three previous decades.

It can be summarised that mean of winter and summer maximum and minimum temperature for the decades 2001-2010 for both Colaba and Santacruz are higher than the mean of the previous decade.

There are many contentious issues which are intricately interrelated and are responsible for the rise of temperature in urban areas, especially in Mumbai. One important issue is the change of the land use pattern.  

Land use pattern of Greater Mumbai has been altered considerably over the last five decades to accommodate the changing functions of the city. The comparison of land use function in different decades has been attempted after the construction of land use maps for selected three decades beginning from 1971, 2001 and 2012. This has been done by applying geographical information system (GIS) to facilitate the comparable areas.

The observations reveal that Greater Mumbai has witnessed rapid growth of built up area during the period 1971 and 2012.The built up area has increased during this period from about 37 per cent in 1971 to around 48 per cent in 2001 and further to as high as 56 per cent in 2012. One of the major components of the built up area is the increase in residential areas, which show a considerable increase from 25% in 1971 to 33 per cent in 2012.

The changing land use functions indicate certain changes in the inner city.  The original commercial area in old Mumbai was located in the eastern waterfront near Masjid rail station, known as Dana Bunder, has been shifted from Mumbai to New Mumbai. Thus, there is a decline of commercial land use function in the old city.  Similarly, old textile producing centre of Mumbai, originally located in the central part of old island city, has been closed down after the historic textile workers strike in 1982. The textile land remained locked for many years. Subsequently, it was allowed to accommodate corporate business activities. Thus, there is a decrease of industrial area and rise of area under transport, trading and commerce.

However, most striking feature here is the decline of the natural area, which consists of forest, lake, marshy land and open scrub land. It is derived from our comparative study of land use maps in temporal frame in Mumbai is the decline of natural areas. Natural areas have experienced substantial decrease from almost 50 per cent in 1971 to 33 per cent in 2001 and further to as low as 27 per cent in 2012. The conversion of wetland to built-up area occurred largely up to 15 per cent of the total land during 1971 and 1991.  During 1991 -2001 the number went down to 12 per cent for the area covered by forest and plantation, which was eventually converted to build-up area for residential and commercial purposes

Open space comprising of playground, barren open land covered by scanty forest, and recreational ground have also witnessed substantial decrease. Decline of natural area plantations like chikoowadi, banana, mangoes garden, etc. have been encompassed by build-up area. Decline of this open space has serious implications on micro climate of the city in terms of increase of temperature and increased intensity of urban flood.

 RISE OF FLOOR SPACE INDEX

The city has grown and expanded not only horizontally but also vertically. The vertical rise has been possible due to rise of Floor Space Index (FSI). As per the planner this vertical rise was required to accommodate the rising population and diversified function of the city. It is to be noted that since 1991 with the introduction of the reform of Indian economy, many of the economic sectors like industrial, financial sectors opened up and allowed foreign finance to flow in these sectors. There has been an increase in demand for new land. 

In the old city, the open space was limited and urban renewal process was extremely slow because rigid old tenancy system as well as the restrictive urban land ceiling policy. Thus, the suburban land came under pressure of corporate business activities. To accommodate the increasing demand for land, Government of Maharashtra changed the then FSI rule; the city has been allowed to grow vertically.

 Over different plan periods, Great Mumbai had different FSI levels. The Developmental Plan (D P),1967 prescribed differential FSI in the island city and in the suburbs ranging from as low as.0.5 to high 4.5-- varying across geographical location and uses: residential , commercial,  industrial, etc. Subsequently, the DP, 1991prescribed low and uniform ESIacross the island city and suburb: 1.33 for the island city and 1.0 for the suburbs.

However, Development Plan for 2014 to 34 adopts a variable FSI regime: 58.12 per cent of the city area has been proposed under the FSI of 3.5.  Then, 31.87 per cent of the area of the city, which is well accessed by public transport system, mainly areas in proximity to rail and the upcoming metro stations, has been allocated FSI 5.0.

Besides, high FSI of 6.5 and 8 have been provided for the areas in the immediate vicinity of a number of major railway stations. For example, for Andheri railway station area, which cover 4.55 per cent area of the city, a high FSI has also been allocated to the smaller areas where commercial demand is high. However, 5 per cent of the area of the city is allocated only 2 FSI.

Adoption of variable FSI in different plan periods has eventually resulted in high demand for the suburban land. As a result,  rapid urban renewal took place around the cluster of high rise buildings (30-40 story), already came up in Andheri ,  extended up to Jogeshwari, Goregaon east, Malad link road, one of the largest  concentrations being  Kandivali east. The high rise building activities have been spreading all across the city as well as the suburbs.

Buildings are constructed to accommodate large commercial complexes, IT sector offices, hotels, malls, etc. Multinational organisations have constructed high rise buildings, creating a ‘canyon effect,’ which does not allow the heat to be released by various surfaces to escape into the atmosphere. The condition gives rise to heat plume keeping the city warmer than the surrounding area. In other words, urban heat islands develop when urban cooling rate is also low.