THE ENVIRONMENT
The impacts of environmental change on patterns of urbanisation and urban change are seen at a number
of geographic scales. At the planetary scale, global warming due to the greenhouse effect (caused in part
by the waste outputs of urban civilisation) may require the construction of coastal defences to protect cities
such Bangkok, Jakarta, Venice and London from as the danger of inundation. Equally, any significant
deflection of the North Atlantic Drift current (the Gulf Stream) which carries warm water from the
Caribbean to the coasts of Britain and northern Europe would affect local weather patterns and require a
costly response from cities in terms of urban infra-structure such as improved winter heating systems,
public transport and road maintenance. At the local scale, natural' phenomena such as earthquakes and
landslides can force the abandonment of settlements (for example, Third World squatter settlements con-
structed on marginal lands) or require major works of reconstruction, as in Mexico City after the 1985
earthquake.For reasons of clarity we have examined each of these important 'trigger factors' independently In practice, of course, they are interrelated and operate simultaneously along with other local-scale factors
to influence urban change. These trigger factors are integral elements in the processes of globalisation.