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Visualising change over time is a common cartographic task, with solutions ranging from mapping change variables to multiple map frames, interactive maps and animations. In this graphic, a global time series of city population data are mapped through a series of overlapping proportional circle layers representing different points in time. This technique allows spatial time series data to be perceived in a single image, communicating both magnitude and growth in city populations (Figure 1).While this is not a new cartographic technique (see for example Burdett et al., 2011), it is not often used and is relevant for visualising global dynamic datasets. The dataset used is the United Nations World Urbanization Prospects (UNWUP) 2014 Revision, which estimates individual city populations from 1950 to 2014 from compiling national census data and includes projections up to 2030 (UN Department of Economic and Social Affairs, 2015). There are inevitable limitations with this dataset concerning variable census data quality and inconsistent city boundaries (Satterthwaite, 2010). The basic unit of measurement is metropolitan agglomerations, but there are inconsistencies in urban definitions between countries. Where no metropolitan data are available, population estimates revert to administrative populations (UN Department of Economic and Social Affairs, 2014). Cities such as Seoul and Jakarta for example are significantly under-estimated in Figure 1 due to the lack of metropolitan data. Despite these limitations, UNWUP remains a very useful open dataset for exploring global city population trends. Figure 1 maps city populations in 1950, 1990, 2015 and 2030 – coloured from very dark blue (1950) to very light blue (2030) – highlighting the regions in the globe with the largest urban growth, and the general time period when it occurred. Cities in Europe and Northwestern USA are dark due to relatively static populations that in many cases are similar to 1950 totals. In contrast, Latin America, Japan, South Korea and Indonesia are dominated by growth from 1950 to 1990. The rapid growth of cities in India, China and South/South-East Asia is more recent, focussed mainly between 1990 and 2015. Further growth is projected in many Asian cities by 2030, though it is sub-Saharan Africa where the greatest proportional growth is predicted for the next 15 years, and subsequently many African cities appear in very light blue in the map.

World city populations 1950–2030: Proportional circle time series map