A strong regularity in urban systems has long been identified : the hierarchical distribution of city sizes. Moreover, a closer observation of the evolution of this distribution shows that in the majority of city systems, there is a trend towards a more and more unequal distribution of city sizes. Why does the majority of urban systems show those strong regularities? What are the common growth processes involved? Several dynamic growth models have been proposed but no consensus has yet been reached because of the under-determination of models by those empirical laws. In this presentation we describe a new method of agent-based parsimonious modeling that we think can contribute to the identification of the common urban growth processes. This modeling method is based on intensive model exploration for quantitative evaluation of implemented mechanisms. The exploration tools were first developed for the evaluation of SimpopLocal, a model of the organization of urban systems when cities first emerged. The use of those exploration tools was then generalized into a modeling method that was applied for the first time with the construction of the MARIUS family of models which aims at reproducing the evolution of Soviet urbanisation between 1959 and 1989. Those two examples show how this new modeling method can help the construction of urban theories by helping the evaluation of assumptions made on urban processes.
Within the framework of the GeoDiverCity programme we are attempting at modelling the future evolution of cities. As cities are very complex systems, any exact prediction is impossible. However, the exploration of plausible futures is possible, with an increasing approximation according to the length of time duration.
The theory behind our modelling is that cities have to be considered not as isolated entities but as interdependent systems being embedded in complex situations summarised by three major aspects:
– the system of cities with which they have most of their interactions
– the territory to which they belong
– the historical period under consideration
Cities are depending on multiple interactions that occur with other cities in multiple networks for all kind of urban activities from local to global scales – that is why we always consider them as part of systems of cities; cities also are rooted in administrative and political territories that generate specific rules and constraints for their development, at local, regional, national and international levels; during the about ten thousands years period of their existence, the type of relations that cities have had with their environment has changed and despite its rather massive common features the urbanisation process has taken a wide range of variations in different parts of the world.
Analysing the evolution of systems of cities from large urban data bases, we suggest recognising that they share many common features but also exhibit a fundamental geo-diversity that is the expression of path dependence in their development. We can model the common dynamics of systems of cities from the interactions between cities, but for understanding and predicting their differentiated evolution we have to take into account their history. This does not mean building a narrative of successive events but a careful selection of a few specific historical regimes that contextualise the development of systems of cities all over the world (including for instance quality of natural environment, steps of the demographic transition, or relative situation in innovation networks), as well as a restricted set of events that may have more specifically occurred during the history when trying to predict the evolution of any individual city.