Daniel Hoornweg

Population predictions for the world’s largest cities in the 21st century

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.

An urban approach to planetary boundaries.

The achievement of global sustainable development goals subject to planetary boundaries will mostly be determined by cities as they drive cultures, economies, material use, and waste generation. Locally relevant, applied and quantitative methodologies are critical to capture the complexity of urban infrastructure systems, global inter-connections, and to monitor local and global progress toward sustainability. An urban monitoring (and communications) tool is presented here illustrating that a city-based approach to sustainable development is possible. Following efforts to define and quantify safe planetary boundaries in areas such as climate change, biosphere integrity, and freshwater use, this paper modifies the methodology to propose boundaries from a city’s perspective. Socio-economic boundaries, or targets, largely derived from the Sustainable Development Goals are added to bio-physical boundaries. Issues such as data availability, city priorities, and ease of implementation are considered. The framework is trialed for Toronto, Shanghai, Sao Paulo, Mumbai, and Dakar, as well as aggregated for the world’s larger cities. The methodology provides an important tool for cities to play a more fulsome and active role in global sustainable development.

Solid Waste and Climate Change

Urbanization and the growth of cities is driven largely by the ability of cities to use materials more efficiently, to bring people together, and to provide better access to health care, education, and employment. Accompanying that urbanization and growth, however, is an increasing stream of waste. As more people move from the countryside to urban areas, their per capita waste levels are rising, commensurate with the higher-consumption lifestyles associated with cities.

Low-Carbon Waste Management

Waste management is a significant source of urban GHG emissions, with inventories suggesting that it contributes 5 % of the total, on average. Landfills are the dominant source of urban waste GHG emissions, due to their production of methane from the degradation of organic waste. A number of management strategies (e.g., LFG collection, oxidizing covering materials) can be implemented to reduce emissions from landfill operations. Organic waste can also be diverted to other treatment options (composting, anaerobic digesters) that reduce both direct process emissions and indirect emissions through the use of coproducts such as energy and soil amendments. Thermal management practices provide co-benefits such as improved material recovery and energy services, while studies of health implications have generally been inconclusive or have demonstrated no convincing evidence to directly link these treatment approaches with health outcomes. “Three R” approaches to waste management have additional benefits outside the recovery of valuable materials (e.g., aluminum, steel), in that they also can provide a significant indirect emission savings. Further to waste management infrastructure, systemic approaches, such as extended producer responsibility and product service systems, should be employed to shift waste mitigation incentives from cities to manufacturers towards higher diversion rates.