As a result of the dynamics displayed in the l’previous section, the number, the volume, the form and the density of the cities, and also the efficiency of their environmental management are key-issues for achievement of the global sustainability.
The rural-to-urban migration, the primary cause of the urban growth, is due to two main reasons: the rural expulsion factors (poverty, lack of land and work opportunities) and the city attractiveness factors (better job offers and social services). The relationship between the urbanisation and the environment has been generally analysed from two different perspectives. In some theories, the urbanisation is the centre of the study, since it is a global process that not only means a concentration of population, but also a deep transformation of the rural world, and their subsequent environmental implications. On the contrary, in some other studies, the urbanisation is seen as a secondary topic, as there are no valid universal relationships between the urbanisation, and the population and the environment.
It is difficult to evaluate the environmental consequences of the urbanisation, since there are many factors to be considered, such as the state political measures (which cannot often meet the growth), the adaption and the implementation of effective urban planning, the evolution of the economic development, etc. In few words, the interferences between the volume and the distribution of the cities, and the environmental impacts are numerous, complex and poorly-known.
Nevertheless, there is a unanimous opinion that the urban growth has a huge impact on the environment. Water and soil contaminations, the most noticeable ones, are due to the non-existence, in some cases, or to inadequacy, in some others, of the sewer systems, of the lack of control on the industrial waste and the lack of regulation on modern chemical contaminants.
A new investigation line, which has not been developed in the consulted literature, would be the study on whether the differential impact of the urban population is the result of the simply individual effects accumulation, derived from the great urban concentration of inhabitants; or whether, contrarily, there is a differentiating individual behaviour among the urban inhabitants when using resources, consuming or moving, etc.
Other significant impacts related to the urban growth are the reduction of the carrying capacity —that is, a unit of surface cannot maintain such a big population volume as in the rural areas, as a result of a higher environmental pressure of the urban population—, the expansion of the poverty bags —and the previously mentioned subsequent environmental implications—, the industrial pollution, etc.
However, some scholars state that cities should be proportionally resource-thirstier than little towns, since they are more suitable (because of their greater density) for the public transport use and, consequently, relatively less contaminant in terms of air pollution.
Lastly, urbanisation also affects the food industry, since it occupies cultivable land as the cities grow and reduces the number of family exploitations as farmers increasingly immigrate to the cities.