Cities have existed for millennia, but their triumph is remarkably recent. As recently as 1950, only 30% of the world’s population were urban dwellers. This week, a United Nations report suggested that more than 80% of people are now urbanites, with most of those living in cities. London became the first city to reach a million inhabitants in the early 19th century. Now, almost 500 have done so.

Jakarta, with 42 million residents, has just overtaken Tokyo as the most populous of the lot; nine of the 10 largest megacities are in Asia. The UN report revealed the scale of the recent population shift to towns and cities thanks to a new, standardised measure in place of the widely varying national criteria previously used. The urbanisation rate in its 2018 report was just 55%.

Jakarta’s explosive growth – its population has grown almost 30-fold since 1950 – demonstrates both the costs of rapid urbanisation and the difficulties of addressing them. It is choked by traffic and pollution, regularly floods and is sinking fast due to the overextraction of groundwater. The government is now building a new administrative capital more than 1,000km away, in Borneo. But such projects have an uninspiring record. The new city of Nusantra is behind schedule and short on funding and would-be inhabitants.

Most thriving cities are more spontaneous affairs. The typical image of urbanisation is of young people lured by the promise of prosperity. But the late urbanist Mike Davis also pointed to waves of migration driven by growing rural desperation thanks to agricultural deregulation and punishing fiscal policies enforced by the International Monetary Fund and World Bank. In other cases, he wrote in his book Planet of Slums, “rural people no longer migrate to the city; it migrates to them” as urban sprawl encroaches on their land.

The benefits of cities are obvious. They are hubs of productivity, creativity and diversity that boost social and economic development. New York City’s gross product – $1.8tn last year – easily exceeds the GDP of Turkey or Saudi Arabia. That helps to explain why some cities are growing stronger as political – even diplomatic – entities.

Yet as in 19th century London, opportunities and services exist alongside monstrous inequality, overcrowded housing and substandard infrastructure. Diseases spread more rapidly in crowded conditions with highly mobile populations. The rational decisions of individuals – to find educational opportunities for their children, escape rural desperation or seek excitement – play out on a grand scale, demanding intervention.

Most of the land that has been urbanised in the last 55 years was used for agriculture, endangering food security and despoiling the environment. Improved urban-rural links are needed to prevent the entrenchment of rural deprivation as older people are left behind, with few services. Global heating is creating new dangers, with urban dwellers disproportionately exposed to heatwaves and rising seas – and the poorest hit hardest. António Guterres, the UN secretary general, argued last year that sustainable and equitable urbanisation and fighting climate change are two sides of the same coin. While cities generate a huge share of emissions, they can also foster more efficient use of resources.

Cities have always represented both opportunity and threat. They grow as if of their own accord, but their new inhabitants cannot flourish that way. Humans are a new urban species, still learning to adapt to the environment we created.

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