Urbanization

Abstract

How has the concentration of the world's population in urban settlements changed in the past 300 years? Where and when has urban growth occurred, and why? What has happened to the distribution of cities by size? The first purpose of this chapter is to answer these questions by laying out the evidence on urban growth since 1700. Has this urbanization resulted in environmental change? Is further urbanization likely to do so in the years ahead? Neither the social nor the physical sciences have answered these questions, yet the broad outlines of an answer certainly can be sketched. Until the middle of the twentieth century, urbanization levels were too low and the number of large cities was too small for there to be anything other than local climatic and hydrologic impacts. To the extent that urbanization produced environmental modification, it was in urban-centered gradients of agricultural land use and mineral and forest exploitation as urban demands diffused into the surrounding countryside. As late as 1900, there were barely 43 cities in the world exceeding 500,000 population, of which only 16 exceeded 1,000,000. But since 1950, the number of large cities has increased very rapidly-close to 400 now exceed 1,000,000. Sprawling metropolitan areas have formed even larger agglomerations, and some very large urban regions with populations in the tens of millions have emerged. The question that arises in these cases is whether or not changes in the biosphere are unfolding at a regional scale that, in turn, might have global impacts. There is little to suggest that historic urban developments were active agents in climatic change. There is significant evidence that the modern metropolis has climatic and hydrologic consequences that increase with city size and urban densities. There is at least the suggestion that these consequences may be compounded at a regional scale in the largest agglomerations. But if our analysis is correct, regional-scale impacts may be more likely in the years ahead in Third World nations, where very large urban agglomerations are emerging, rather than in the most economically advanced countries, where a transformation is unfolding that is resulting in dispersed and relatively low-density urban networks. The very regions in which environmental alterations are most likely are those regions in which increasing shares of the world's population are concentrating. © 2008 Springer Science+Business Media, LLC.