GIS usually computes along the spatial dimension. Yet many processes and systems investigated with GIS have a significant temporal dimension: urban planning and land use, hydrology, fires, ecological change. Agent simulations with environmental change depend on developing Temporarl GIS. There have been efforts at addressing Temporal GIS, but only in preliminary research. Many issues are untouched in representation, usability, and particularly analysis.
From this search for Temporal GIS, these papers [1 2 3] and these books [4 5].
The source for the estimated year of a city's entry into the 1 million club is
Four Thousand Years of Urban Growth: An Historical Census by Tertius Chandler. Most of his research was in methods of estimating population size in absence of a precise historical record [ described in
this about.com article ]. The rapid urbanization in the industrial and post-industrial periods (displayed here) were accompanied by the endeavor for accurate (though still estimated) population census.
This map referencs Chandler's lists of the populations of the largest global metropolitan regions for the years 1750, 1800, 1825, 1850, 1875, 1900, 1914, 1925, 1950, 1975. The 1 million threshhold was estimated by linear interpolation between sample years.