We project populations to 2100 for the world’s larger cities. Three socioeconomic scenarios with various levels of sustainability and global cooperation are evaluated, and individual “best fit” projections made for each city using global urbanization forecasts. In 2010, 757 million people resided in the 101 largest cities – 11 per cent of the world’s population. By the end of the century, world population is projected to range from 6.9 billion to 13.1 billion, with 15 per cent to 23 per cent of people residing in the 101 largest cities (1.6 billion to 2.3 billion). The disparate effects of socioeconomic pathways on regional distribution of the world’s 101 largest cities in the 21st century are examined by changes in population rank for 2010, 2025, 2050, 2075 and 2100. Socioeconomic pathways are assessed based on their influence on the world’s largest cities. Two aspects of the projections raise concerns about reliability: the unlikely degree of growth of cities suggested for Africa and the growth of cities in coastal settings (and likely global immigration). Trends and the effect of sustainable development on regional distribution of large cities throughout the 21st century are discussed.

I. Introduction Large cities (metropolitan urban areas) are hubs of economic development and innovation, with larger cities underpinning regional economies and local and global sustainability initiatives. Larger cities tend to be more challenged by the “boundary issue”, except for Singapore. City boundaries are as ephemeral and arbitrary as other political boundaries; however, a systems approach to cities requires assessment of the overall urban agglomeration. Larger urban areas are often comprised of many smaller cities, e.g. metropolitan Sydney, Australia is administered by 38 local governments; Toronto, Canada by 29; Lagos, Nigeria by 16; and Jakarta, Indonesia by 10. Urban agglomerations, or metropolitan areas, are typically defined through common employment- and commuter-sheds (extent of area in which people typically travel for regular employment). Consistent with: (i) Brinkhoff’s database;(1) (ii) citymayors.com; and (iii) the World Bank data on the 100 largest urban areas,(2) Kennedy et al.(3) define boundaries of the world’s 27 megacities (urban areas of more than 10 million people). In this paper the approach is extended to include all urban areas expected to have more than 5 million people before 2100 (online supplementary information (SI), Table S4). “Future Five” cities are those expected to have populations greater than 5 million by 2050 (122 cities) and 2100 (181 cities). Box 1 Abbreviations used in this paper View larger version In 2010, the baseline year for this paper, 757 million people resided in the 101 largest cities, with a population of 36 million for the largest city (Tokyo), and 3.5 million for the 101st largest city (Addis Ababa); these cities were home to 11 per cent of the world’s population. By the end of the century, the world population is projected to grow by 6.9 billion to 13.1 billion; the percentage of people residing in the 101 largest cities is estimated to be 15 per cent to 23 per cent. In all scenarios, projected total populations in the world’s largest cities are growing. When cities are governed and managed well, urbanization can be a powerful antidote to environmental degradation, as population density, and the effective provision of basic services, enables substantial gains in efficiency of resource consumption and waste management. Urbanization is also associated with higher rates of education and health care provision.(4) Urbanization, particularly through large cities, is a key driver of economic development.(5) Larger cities have a disproportionate impact on regional and global geopolitical power dynamics.(6) Bettencourt(7) and Bettencourt and West(8) further promote the agglomeration of urban areas with minimal borders (e.g. barriers to travel, overlapping utilities), as they show that cities scale super-linearly (~1.15) for economy, while they scale sub-linearly (~0.85) for infrastructure costs. If externalities associated with increased density, e.g. congestion, are addressed, doubling a city’s effective size more than doubles the economy and wealth generation, with less than a doubling of infrastructure costs and material flows.(9) Working with cities, instead of national governments, may offer urban practitioners a more straightforward approach to sustainable development as requirements and achievements may be easier to define and monitor, and a systems approach may emerge.(10) The Sustainable Development Goals and comprehensive global inventories such as greenhouse gas emissions can be down-scaled to larger cities.(11) Considering their sheer economic heft and material flows (and subsequent pollution), sustainable development globally is not possible without sustainability being firmly anchored in most of the world’s larger cities. This report considers how socioeconomic scenarios of development may affect the world’s 101 largest cities, with the National Center for Atmospheric Research’s (NCAR’s) three basic “shared socioeconomic pathways” (SSP1, SSP2 and SSP3 – see below and SI for details).(12) The approach is expanded using the United Nations World Urbanization Prospects (WUP)(13) for predictions of urban growth in the larger urban areas (each city’s “best fit” growth, or decline, presented in SI). Synergies between the growth and development of the 101 largest cities and their effect on global sustainability are highlighted. The Shared Socioeconomic Pathways (SSPs) are: SSP1 (Sustainability): The world makes relatively good progress towards sustainability, with sustained efforts to achieve development goals, while reducing resource intensity and fossil fuel dependency (rapid development of low-income countries; reduced inequality; rapid technology development; open, globalized economy). Population of 7 billion, 90 per cent urbanized.

SSP2 (Middle of the Road): This pathway assumes extension of current trends in urbanization, along with similar middle of the road assumptions about population growth, technological change and economic growth. Estimated population of 9.5 billion, 80 per cent urbanization.

SSP3 (Fragmentation): Urbanization follows the slow pathway. In high-income countries, low population growth (especially ageing), slow economic growth and technological changes reduce the incentives for urban expansion. In the developing regions, the population grows rapidly, particularly in rural areas, causing significant land use change and environmental degradation. Population of 13.5 billion, pockets of extreme poverty and moderate wealth, many countries with rapidly growing populations.

III. Results: Key Findings The reliability of data remains a concern. A consistent and explicit definition of each large urban area is needed for more accurate and reliable data collection. Estimates in this paper should be seen as educated guesses; however, they are sufficiently robust to influence critical infrastructure development in key cities. What the data reveals is that regardless of socioeconomic scenarios, sub-Saharan Africa would have the most dramatic rise of large urban areas, growing from 7 in 2010 to approximately 20 in 2050 and over 30 in 2100. In all scenarios, by 2100 the SSA region will have the most cities in the largest 101 (ranging from 30 to 40). Current low urbanization rates are not consistent with scenario projections: the urbanization of Africa will have a significant impact on future sustainability. There will be far-reaching benefits to ensuring that sub-Saharan Africa and other developing regions optimally progress, with enhanced local economic opportunity and adequate infrastructure, education and social policy, to curb population growth and resource depletion. Along with African cities, coastal cities are key sites of urban growth. Almost half of the larger cities are coastal, and some 25 are located in seismically active regions. Urban resilience is especially critical to long-term growth scenarios for these cities. Regarding the effects on high-income areas, globally, an unsustainable socioeconomic pathway (SSP3 – fragmentation) has a dramatic effect on the rank of the largest cities in high-income regions. “Fragmentation”, i.e. unsustainable development, significantly reduces the number of high-income cities in the largest 101 (21 in SSP1 vs. 5 in SSP3 in 2100). In the 21st century, most high-income countries and their number of cities will decline in rank, relative to cities from low- and middle-income countries, especially if a more sustainable development trajectory is not followed.(29) A more sustainable socioeconomic pathway would correspond to a higher fraction of the global population living in the 101 largest cities, compared to a less sustainable pathway. Larger, faster-urbanizing cities promote sustainable development. Different socioeconomic scenarios apply very different growth pressures to large cities (Table 5). Lagos’ projected population in 2100, for example, varies from 61 million to 100 million (SSP1 vs SSP3). Depending on the path of development, the world population could range from 7.5 to 8.3 billion in 2025, 8.2 to 9.9 billion in 2050, 7.9 to 11.4 billion in 2075, and 6.9 to 13.1 billion in 2100, with more sustainable progress favouring lower population predictions. Table 5 Population projections of the 10 largest cities in 2025. 2050. 2075 and 2100 View larger version In all scenarios, the urbanization growth rate between 2010 and 2025 has significant implications for the urbanization rates in 2075 and 2100, with larger city growth in the first part of this century leading to more sustainable conditions after 2075. Cities may emerge as polycentric nodes of governance.(30) To date, the world is not on SSP1 – a sustainable scenario. Strategic development of the current and future larger cities that promotes resource efficiency, as well as cooperation within and across global urban areas, is essential for sustainable development.

IV. Discussion The size and shape of 21st century cities will determine much of the overall attainment of sustainable development. Various growth and sustainability scenarios (SSP1, SSP2 and SSP3) highlight the influence that urbanization and the world’s larger urban areas have on total global population, resource consumption, and quality of life. Greater sustainability (SSP1) is characterized by a faster rate of urbanization, especially of larger cities (Figure 2A). This scenario suggests a total global population around 8 billion, peaking in 2050. Compare this to the fragmentation scenario (SSP3 – Figure 2C) where the total population does not peak before 2100, and likely exceeds 14 billion. Projecting city growth, typified by Lagos growing from 10.6 million in 2010 to 88.3 million in 2100, calls for a healthy measure of scepticism; many variables could change. Projections used here are linked to the 2050 WUP city estimates that, especially in Africa, are suspect, and here these projections are extended for yet another 50 years, suggesting caution in their use. Africa’s projected city growth is striking. For example, among Tanzania, Niger, Kenya, Malawi, Uganda, Zambia, Somalia, Chad, Mali, Senegal, Mozambique, Burkina Faso, Madagascar, Rwanda, Guinea, Liberia, Ghana, Togo and Cameroon (as well as Cambodia), none of these countries had a top city in 2010, yet all are projected to have at least one in 2100. Each country has a projected rate of urban population growth (SI, Section V), and despite this estimate being relatively conservative, significant city growth is still projected. Niamey, Niger for example is projected to grow from under 1 million inhabitants today to 56 million in 2100. Growth rates like these appear overstated, especially in one of the world’s poorest countries with an insufficient economic base to support this rate of urbanization. However, in that same timeframe, under the UN medium fertility variant, Niger’s population is projected to grow from 15 million to 210 million. How these newly enlarged cities will be governed is also critical. Managing a city in excess of 50 million inhabitants is a daunting task, especially as many of these cities are woefully under-serviced today. Sixty-five years is a relatively short timeframe with regard to urban infrastructure. Much of it, e.g. London’s Underground and Rome’s streets, is long-lived and can take decades to develop. A key driver of dampened urban growth may be climate change and rising sea levels.(31) Forty-seven of the projected 101 largest cities in 2100 are coastal, including the three largest cities: Lagos, Dar es Salaam and Mumbai. However, in the absence of more accurate population projections, estimates are important. Infrastructure and resource development plans are developed with time horizons extending to the end of this century. Long-lived, large-scale urban infrastructure is being planned and built with future population estimates; these estimates should be clearly available and consistently used. Pending regular updates and refinements on urban area borders, the projections in this paper are considered a starting point. China is expected to experience a relatively large drop in the number of largest cities as overall population growth slows from 2030 onward. The size of cities will remain relatively consistent in the second half of the century; however, as many cities in regions like South Asia and Africa are growing considerably, the relative ranking of Chinese cities will decline. China, with its large economy, may consolidate the urban population in a few targeted cities. Population projections of Chinese cities may also be conservative as migrant workers might not be included in population estimates. Concerns with suggested findings are most apparent in sub-Saharan Africa, where the extreme growth rates for cities are not supported by current urbanization rates.(32) Potts highlights how in many African countries urbanization rates are well below what is commonly believed. Urbanization rates in the cities of countries like Côte d’Ivoire, Central African Republic, Kenya, Malawi, Nigeria, Tanzania, Zambia and Zimbabwe are often lower than overall national population growth rates and historically below 3 per cent per year for most. Africa’s low urbanization rates are mostly attributable to the lack of commensurate economic development.(33) A major change to this phenomenon is not yet evident; also, some 70 per cent of Africa’s urbanization is taking place in secondary cities.(34) Despite these concerns, with low rates of African urbanization in primary cities, the average aggregate population of the world’s 101 largest cities is still projected to increase from 757 million in 2010 to 2.3 billion in 2100: a three-fold increase in average city size. The “average” large urban area would increase from 7.5 million in 2010 to about 23 million in 2100. Managing these large megacities, more than 50 of which are in excess of 15 million (while the top 10 are all in excess of 50 million), will place inordinate demands on urban managers and citizens. Planning and skill development should begin now. Cities are complex systems impacting many disciplines. Economists, planners, political scientists, and businesses (existing and potential) are all interested in projected growth and relative rankings of cities. As the pace of city-building grows, engineers and planners have an acute need for urban “rules of thumb”. All of the 101 largest urban areas are served by local engineering faculties. The engineering and planning professions would be well served by developing a peer-reviewed self-collected and regularly updated (ideally annually) urban database that, as a minimum, includes population projections, resource flows, and quality of life indicators. Where practicable this data collection should incorporate local and national census data, and international standards consistent with agencies such as the Global Cities Indicator Facility (ISO 37120). Historically, the world’s largest cities drove the world’s largest economies. The emergence of cities like N’Djamena, Mogadishu, Blantyre and Lilongwe, which today are in impoverished economies, gives pause. So does the relative decline of the number of large Chinese cities. The city population projections provided in this paper are driven by national populations from shared socioeconomic pathways and estimated urbanization rates. These estimates will be further impacted by regional economies, resource availability, conflict, and possible geophysical and climate disasters.

V. Conclusions Projections are made here for the size of the world’s largest cities this century. The effect of socioeconomic pathways on regional distribution and the rank of the world’s largest cities is investigated to illustrate how sustainable development through our largest cities can impact overall global sustainability trends. Promoting global cooperation, accessibility to education, and adequate resources for effective development and optimal infrastructure deployment will significantly limit population growth, natural resource depletion, and local and global emissions. Efforts to improve city governance and urban management are urgently needed, especially in regions where current levels of service provision are already poor, yet cities are expected to grow markedly. Achieving global sustainability requires a collective effort with common but differentiated responsibilities to ensure that large, rapidly growing urban areas adhere to the requirements of SSP1 (sustainability). This includes, for example, a sharp reduction in consumption for high-income cities; re-shaping of current development trajectories in middle-income and rapidly industrializing cities (e.g. China); and future-oriented urban design and land-use planning for African cities. In the next decade, efforts toward development of “sustainable cities” will lay the framework for future growth that will substantially affect global sustainability throughout the 21st century. The projections presented here warrant caution as two broad trends do not appear to be fully reflected in city growth scenarios. First, urbanization rates suggested for Africa are well above current levels. Second, of the 101 largest cities 47 are coastal. Growth rates in these cities may be curtailed through climate change impacts of flooding from sea level rise and expected increased storm intensity. Larger cities have historically driven much of the world’s economic growth. Many of the cities projected to grow most rapidly here do not yet show commensurate economic strength. Also, SSP1 is based on higher rates of urbanization, especially in the early 21st century. These rates are not evident today, greatly reducing the likelihood that SSP1 can be achieved.