Cities on the twenty-ninth day

Guest post by
Blogs, 3 October 2016

Population predictions for the world's largest cities in the 21st century (part 1): rapid urbanisation, if managed sustainably, could ease the pressure of exponential growth set for urban areas.

Lillies spread over a pond; cities must be governed and managed well to ensure population growth is sustainable (Photo: hqworld)

One of the better early books to raise awareness of our planet's vulnerability to rapid population growth and environmental impact was Lester Brown's The Twenty Ninth Day (1979). The book applied a metaphor often used in French schools to teach children about the power of exponential growth. 

Question: if a lake will be covered by water lilies in 30 days, and the area covered – starting with one leaf – doubles every day, on what day is the lake half-covered (and would presumably slow-to-act humans finally respond)? Answer: the 29th day.

When exponential growth meets finite resource

The Twenty Ninth Day was published a few years after The Limits to Growth (1972), which remains one of the most important counsels on the earth's limits to support exponential growth in population and wealth, and the excessive strain this can put on natural systems and resources.

Much of the world's growth and impact on the planet over the last 100 years has been exponential, and in areas like biodiversity loss and climate change we have already passed 'safe' limits, where ecosystems no longer function properly.

Leaping towards full capacity

In 1500, just a few years after Columbus sailed the ocean blue, the global population was around 460 million, less than 10 per cent of which was urban. Average life expectancy was around 33 years. Wealth per person was US$ 130 (in constant 2015 US$).

In 1800, 300 years later, population had doubled to 990 million (still less than 10 per cent urban). Average life expectancy had edged up to 40 years and per capita wealth was $200.

In just 100 years, from 1800 to 1900, the population almost doubled to 1.65 billion (less than 20 per cent urban) and personal wealth had more than tripled to $680.

That staggering pace of population growth and wealth increased over the next 100 years. In 2000 the global population was 6.1 billion (edging close to the 50 per cent urban mark reached in 2008). Per person wealth was $6500 and, amazingly, average human life expectancy doubled to 78 years.

In 2100, five of the world's 10 largest cities are expected to be in South Asia and Africa, and all of these cities will likely have more than 50 million residents

By the end of this century there will likely be about 11 billion humans living on the planet (80 per cent in urban areas) and wealth might be a staggering $40,000 per person (based on annual GDP with 83 year life expectancy).

In the latest Environment & Urbanization paper we now have an early glimpse of likely growth patterns for the rest of this century; when the global lily pond might be fully covered. The view should give everyone pause. 

The paper suggests the last doubling of human population will mostly be driven by South Asia and Africa. And this growth will be concentrated in cities; in 2100, five of the world's 10 largest cities are expected to be in South Asia and Africa, and all of these cities will likely have more than 50 million residents.

A chart showing the world's growing wealth and health

How cities grow and are managed, the focus of Habitat III next month, the United Nations' first urbanisation conference in 20 years, is critical for everyone: resource needs and planetary impacts are on track to more than double yet again.

Students graduating today will see the world's cities double in population during their careers. This rate of urbanisation is unprecedented. And could generate enormous wealth, providing money to help reduce poverty and limit environmental damage.

Most of the world's wealthy live in cities. Well managed cities, that nurture and focus well-intentioned residents, are the best hope to slow local and global environmental damage.

Cities and sustainable growth in the lily pond

When cities are governed and managed well, urbanisation can be a powerful antidote to environmental degradation – increasing population density can accelerate the provision of basic services such as effective waste management, or drive more efficient consumption trends. 

Working directly with cities to build urban spaces that are large, vibrant and equitable may get us more quickly to a scenario where population growth is sustainable – where overwhelming the lily pond is prevented and the strain from our ever more urbanised planet is easier to manage.

Daniel Hoornweg ( is associate professor in the Faculty of Energy Systems and Nuclear Science at the University of Ontario Institute of Technology (UOIT). This is the first of two blogs on this topic; the second focused on the shifting power of cities.

This blog draws on Population predictions for the world's largest cities in the 21st century, by Daniel Hoornweg and Kevin Pope, to be published in the April 2017 issue of Environment & Urbanization. The paper is available under open access until 10 October 2016.