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Population ageing and shrinking are demographic phenomena with far-reaching implications for sustainability in the current context of extensive and rapid urbanization. This Perspective rationalizes their interface by (a) identifying the challenges and opportunities that ageing and shrinking urban populations will have for implementing the sustainable development goals (SDGs), and (b) discussing some emerging interventions to capitalise on the opportunities and reduce the challenges to achieving sustainability. We argue that a diverse set of context-specific technological, socioeconomic, institutional and governance interventions would be needed to leverage effectively the opportunities and minimize the risks posed by ageing and shrinking urban populations for long-term sustainability.

The establishment of interdisciplinary Master’s and PhD programs in sustainability science is opening up an exciting arena filled with opportunities for early-career scholars to address pressing sustainability challenges. However, embarking upon an interdisciplinary endeavor as an early-career scholar poses a unique set of challenges: to develop an individual scientific identity and a strong and specific methodological skill-set, while at the same time gaining the ability to understand and communicate between different epistemologies. Here, we explore the challenges and opportunities that emerge from a new kind of interdisciplinary journey, which we describe as ‘undisciplinary.’ Undisciplinary describes (1) the space or condition of early-career researchers with early interdisciplinary backgrounds, (2) the process of the journey, and (3) the orientation which aids scholars to address the complex nature of today’s sustainability challenges. The undisciplinary journey is an iterative and reflexive process of balancing methodological groundedness and epistemological agility to engage in rigorous sustainability science. The paper draws upon insights from a collective journey of broad discussion, reflection, and learning, including a survey on educational backgrounds of different generations of sustainability scholars, participatory forum theater, and a panel discussion at the Resilience 2014 conference (Montpellier, France). Based on the results from this diversity of methods, we suggest that there is now a new and distinct generation of sustainability scholars that start their careers with interdisciplinary training, as opposed to only engaging in interdisciplinary research once strong disciplinary foundations have been built. We further identify methodological groundedness and epistemological agility as guiding competencies to become capable sustainability scientists and discuss the implications of an undisciplinary journey in the current institutional context of universities and research centers. In this paper, we propose a simple framework to help early-career sustainability scholars and well-established scientists successfully navigate what can sometimes be an uncomfortable space in education and research, with the ultimate aim of producing and engaging in rigorous and impactful sustainability science. Electronic supplementary material The online version of this article (doi:10.1007/s11625-017-0445-1) contains supplementary material, which is available to authorized users.

The city of Cape Town, South Africa’s most southwestern city, sits on a peninsula in the heart of the geographically restricted Cape Floristic Region, which is home to exceptional biodiversity. Within the city boundary are some 3,350 plant species, 190 of which are endemic to the city itself. Like all South African cities, Cape Town continues to grapple with development discrepancies that persist from unjust apartheid governance in the past and present-day challenges of urban sprawl. The population of the city is 3.7 million. Extreme poverty, with nearly 17 % unemployment, and extensive informal settlements characterize much of the City and stand in stark contrast to wealthy suburbs with freestanding homes. For the populace of Cape Town, the natural environment presents both considerable ecosystem service advantage with, for example, a flourishing tourism industry and provisioning opportunities for the urban poor, but also a significant hazard with, for example, exposure to flooding in winter from a high water table. The value of ecosystem services is an emerging concept in the environmental management arena, and environmental and conservation issues are still seen as separate to other areas of city development, and tend to receive a lower prioritization. South Africa has good environmental legislation, but this is sometimes weakly enforced due to conflicting demands, fiscal constraints, and/or lack of implementation mechanisms. Climate change predictions for the region suggest likely biodiversity impacts, but how these will play out remain unknown. An emerging interest in the role of ecosystem services in broader City management and novel conservation approaches involving civic interests all show considerable promise for the conservation of urban biodiversity in the city of Cape Town.

Several studies in recent years have forecasted global urban expansion and examined its potential impacts on biodiversity and ecosystem services. The amount of urban land near protected areas (PAs) is expected to increase, on average, by more than three times between 2000 and 2030 (from 450,000 km2 circa 2000) around the world. During the same time period, the urban land in biodiversity hotspots, areas with high concentrations of endemic species, will increase by about four times on average. China will likely become the nation with the most urban land within 50 km of its PAs by 2030. The largest proportional change, however, will likely be in Mid-Latitudinal Africa; its urban land near PAs will increase 20 ± 5 times by 2030. The largest urban expansion in biodiversity hotspots, an increase of over 100,000 km2, is forecasted to occur in South America. The forecasts of the amount and location of urban land expansion are subject to many uncertainties in their underlying drivers including urban population and economic growth. Nevertheless, the direct impacts of urban expansion on biodiversity and ecosystem services will likely be significant. The forecasts point to the need to reconcile urban development and biodiversity conservation strategies. Urbanization will also have impacts on food and food security. While the direct loss of cropland to urban expansion is of concern to the extent that high-yielding croplands are lost, the indirect impacts of urbanization due to dietary changes to more meat-based food products can also be substantial. Presently, regional and global studies that forecast impacts of future urban expansion on biodiversity and ecosystem services are in their infancy and more analyses are needed especially focusing on interactive effects of factors that drive urbanization. We conclude by highlighting the knowledge gaps on implications of future urbanization and suggest research directions that would help fill these gaps.

This chapter reviews recent literature on global projections of future urbanization, covering the population, economic and physical extent perspectives. We report on several recent findings based on studies and reports on global patterns of urbanization. Specifically, we review new literature that makes projections about the spatial pattern, rate, and magnitude of urbanization change in the next 30–50 years. While projections should be viewed and utilized with caution, the chapter synthesis reports on several major findings that will have significant socioeconomic and environmental impacts including the following: By 2030, world urban population is expected to increase from the current 3.4 billion to almost 5 billion; Urban areas dominate the global economy – urban economies currently generate more than 90 % of global Gross Value Added; From 2000 to 2030, the percent increase in global urban land cover will be over 200 % whereas the global urban population will only grow by a little over 70 %. Our synthesis of recent projections suggest that between 50%–60% of the total urban land in existence in 2030 will be built in the first three decades of the 21st century.