More than 80 per cent of people worldwide now live in towns and cities, according to a major United Nations report, and this figure is set to rise further, underscoring the need to ensure urban areas benefit both our health and the planet.

The most recent edition of the World Urbanisation Prospects report, published in 2018, found that of people live in urban areas, but that estimate relied on countries’ widely varying definitions of what constitutes an urban or rural settlement. For instance, Denmark defines urban areas as those inhabited by as few as 200 people, but this figure is 50,000 in Japan, muddying our view of urbanisation globally.

To gain a clearer picture, at the UN in New York and her colleagues defined urban areas as either cities inhabited by at least 50,000 people, with at least 1500 individuals per square kilometre, or towns with at least 5000 inhabitants and a density of at least 300 people per km². They classed the remaining areas as rural. “We used the same definition for all countries for the first time,” says Hertog.

Next, the researchers analysed satellite and national survey data from 237 countries and regions to estimate the degree of urbanisation worldwide in 2025. This revealed that 45 per cent of the world’s population now live in cities, mostly in those with fewer than 250,000 people, while 36 per cent live in towns – meaning 81 per cent of people are urban-dwellers. The remaining 19 per cent live in rural regions.

Using a statistical model that accounted for factors like population ageing and migration trends, the team also estimated that by 2050, 83 per cent of people worldwide will live in urban, rather than rural, regions. The actual number – rather than proportion – of people living in both towns and cities is set to rise until 2050, while the number of rural inhabitants is predicted to peak in the 2040s – mostly driven by the Democratic Republic of the Congo – before declining until 2050, says Hertog.

The fresh estimates will help the UN assess progress towards its , which aims to “make cities and human settlements inclusive, safe, resilient and sustainable” by 2030, says Hertog. The results will also help shape policies to reduce global warming by feeding into reports produced by the Intergovernmental Panel on Climate Change, she says.

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The drivers of urban population growth vary between regions. In eastern and southern Asia, it’s mainly driven by people migrating from rural to urban areas within countries, says Hertog. “People move in search of education and employment, but also social life,” she says. In Europe and North America, international migration plays a large role, while in sub-Saharan Africa, it’s mainly down to birth rates exceeding deaths, she says.

Increasing urbanisation can either benefit or harm the environment. For instance, if a city expands its borders due to population growth, but public transport links are not planned appropriately, this can lead to urban sprawl, where people rely heavily on cars – increasing carbon emissions, says Hertog. On the other hand, careful planning can provide transportation that is more energy-efficient than what is available in rural regions, she says.

Urbanisation also has health effects. For instance, people are generally more exposed to air pollution and extreme heat in cities, both of which have been linked to and , says at King’s College London. What’s more, a lack of green space in some urban areas is linked to and , he says.

But urbanisation can also bring health benefits. “Healthcare is more responsive; it’s more comprehensive in cities compared to rural areas,” says Mechelli. “There are also many social advantages – you’re more likely to connect with someone who shares the same values as you than in rural areas, where you might need to drive 2 hours to find someone who shares the same interests,” he says.

It’s not that we should reverse urbanisation, or people shouldn’t live in cities, says Mechelli. “The report tells us it’s now more urgent than ever to think about how to make our cities more liveable, and this means greener – with all the benefits that brings.”

Amsterdam has set itself and halving its use of virgin materials by 2030. Behind those lofty aims is a host of projects accelerating the transition to a more environmentally sound city – something, perhaps, more places can learn from.

To fulfil its material wants and needs, humanity currently uses more than 100 billion tonnes of materials – metals, plastics, stone and more. Only about 30 per cent of that makes up anything of any permanence; the rest is burned or thrown away to rot in landfill. It’s an awful state of affairs, and many advocate switching as fast as we can to a circular economy, in which materials are reused in perpetuity.

The trouble is, this transition requires rethinking and rewiring the systems we use to make and maintain just about everything. Overwhelming? Just a bit. But the problem is increasingly being broken down into city-sized chunks. The world’s urban centres, after all, consume about 75 per cent of all materials and are responsible for around half of carbon emissions.

To find out more, 91av paid a visit to Amsterdam and, in partnership with researchers from the , explored the work going on there – and what it can teach us about circularity.

We have corrected this video to properly identify spokesperson Sacha Stolp, director of innovation for futureproof assets for the City of Amsterdam.

More than two dozen of the biggest cities in the US are sinking, which could affect thousands of buildings and millions of people.

The problem has been reported before, particularly in coastal areas. But by using satellite technology, which sends radar signals towards Earth’s surface and measures the time it takes for them to bounce back, scientists have found that it affects 25 out of 28 of the country’s biggest cities.

“By comparing multiple images taken over time from the same area, we can detect tiny vertical movements of the ground, down to a few millimetres per year,” says team member at Virginia Tech. “It’s like taking a high-resolution time-lapse of Earth’s surface and watching how it rises or sinks over time.”

Fort Worth, Houston and Dallas exhibited the highest subsidence rates of all the large cities, exceeding 4 millimetres per year, on average. For New York, Chicago, Columbus, Seattle and Denver, the average subsidence was greater than 2 millimetres per year.

“Houston – the fastest sinking city out of the 28 most populated US cities – has 42 per cent of its land area subsiding faster than 5 mm per year, and 12 per cent subsiding faster than 10 mm per year,” according to the researchers.

They say that most of the subsidence is caused by groundwater extraction, however, in some cities, such as New York, Philadelphia and Washington DC, the sinking is primarily caused by “glacial isostatic adjustment”.

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“During the last ice age, these areas were covered by massive ice sheets. The sheer weight of the ice pushed down on Earth’s crust, like sitting on a memory foam mattress,” says Shirzaei. When the ice melted thousands of years ago, the pressure lifted, and the ground began to slowly rebound, he says.

“But this rebound isn’t uniform,” says Shirzaei. “In some areas, like the US East Coast and Midwest, the land is still sinking rather than rising, because they’re near the forebulge, a zone that had been pushed up by the weight of the ice nearby and is now collapsing.”

In Seattle, Portland and San Francisco, plate tectonics is probably to blame for the subsidence.

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“We need to start treating subsidence like the slow-moving disaster it is,” says Shirzaei. The scientists also found that some cities are sinking at different rates in different spots, or sinking in some places and rising in others. “This uneven movement creates angular distortion and stress, potentially leading to cracks in walls and foundations, misaligned windows and doors, or worse, structural failure,” says Shirzaei.

at Kyoto University, Japan, has used similar satellite data to show that . “A key challenge that remains for the geophysics community is how to attribute the observed trends to specific causes, whether they be anthropogenic or natural geological processes,” he says.

Urban trees lit by streetlights tend to have tougher leaves that are eaten less by insects than trees that see dark nights. The researchers who discovered the pattern say it could have a detrimental effect on biodiversity in cities by preventing the flow of energy up the food chain.

and his colleagues at the Chinese Academy of Sciences decided to investigate the effect of artificial light on trees after Zhang noticed that Japanese pagoda (Styphnolobium japonicum) and green ash (Fraxinus pennsylvanicaa) trees on the streets of Beijing seemed to suffer relatively little insect damage compared with other trees in the city.

The researchers collected around 5500 leaves from 180 trees at 30 sites in Beijing, some near the characteristic orange glow of sodium streetlights and others where nights are dark, and measured their size, toughness, water content and nutrient levels. They also recorded any evidence of insect damage.

Leaves taken from under streetlights were tougher and had less insect damage. On Japanese pagoda trees, the amount of leaf damage was 2.1 per cent in areas with streetlights and 5.3 per cent in dark areas, while the amount of leaf damage on green ash trees was 2 per cent near streetlights and 4.1 per cent in dark areas.

The researchers weren’t available to answer questions, but they say in their paper that a decrease in leaves eaten by insects will mean less energy flowing up the food chain to insects and birds, leading to a cascading effect that further reduces biodiversity.

The team acknowledges that more investigation is needed because the mechanism leading to lower leaf damage isn’t yet understood. It could be that more light makes insects more visible to predators, reducing their numbers and therefore their effect on trees, for example.

at the University of Oxford says the study is interesting but doesn’t show a causal link. He suggests future studies should take plants from areas with and without streetlights, place them in a controlled environment, and then observe insect habits to see if they show a preference for trees grown in dark conditions.

Lewis also points out that measuring herbivory is complex: more damage may mean a leaf has lower nutritional value and insects therefore have to eat more of it. Holes caused by insect damage can also grow as the leaf grows in size, he says.

“My hunch is that this could be quite a subtle effect,” he says. “In the middle of Beijing, the subtle effects of light pollution on insect herbivory, compared to the effects of how urbanised [the area is], how much pollution there is, how much semi-natural habitat there is – it’s likely to be trivial. It’s important, but it’s probably not the principal threat to insect diversity and ecosystem function.”