Cities are all about density – groups of strangers agreeing to spend their lives in close proximity, whether for protection, mutual opportunity or simply the need to be together. In a world whose human population is now more than 50% urban, this condition is shared by most of humanity.The simplest definition of density is the amount of people divided by the land they occupy. On this basis, the 7 billion living humans divided by the land surface of the Earth means there is a population density of roughly 50 people for every sq km. Evenly spread over all the world’s mountains, deserts and other terrains, we’d be standing about 150 metres away from our neighbour.While there is no internationally agreed definition of the boundary of a city, measuring the density of “urban agglomerations” – including adjacent suburbia as well as the administrative city proper – gives a decent comparative picture.The UN’s Habitat data, collected from national census offices, gives the number one spot to Dhaka, with a density of 44,500 people per sq km. It’s mostly Asian cities in the top of the list: Mumbai is second, while Manila is fourth, with Singapore high up as well. Medellin is South America’s most dense city at third overall, with Casablanca and Lagos in Africa close behind.But different measures give different results. The UN’s Demographic Yearbook has data for the “city proper”. This smaller measurement of area makes Manila the world’s most densely populated city, with the centres of Paris, Athens and Barcelona topping the European list. New York is the densest city in North America, while sprawling Sydney is Australia’s.But what causes high density? While the roots of urban growth are complex and interlinked, there are factors that simply force people to live in high-density environments. Prime among these are natural boundaries. Malé, the capital of the Maldives, is an island city with an area of just 5.8 sq km and a population of more than 130,000 people. It doesn’t have super tall buildings or other spectacular architecture, but having to build on all possible scraps of land means it has a density of 23,000 people per sq km.This island effect can lead to real extremes. Before it was abandoned and became a hallowed site of modern ruins, the offshore coal mine of Hashima Island near Nagasaki had more than 5,000 people living on an area barely 400 metres long, giving it a density above 83,500 people/sq km.Another cause of density are defensive boundaries. From Iron Age hill forts to medieval castles, military protection encouraged populations to huddle together within walls. In Europe, by the end of the 17th century, the ultra-sophisticated, space-hungry bastions and ravelins that surrounded many cities were in danger of strangling life within. The effect of this military encirclement still applies today in the besieged Gaza Strip – which is completely blocked in by Israel and Egypt and has the fifth highest population density of any territory on the planet.And then there’s the exploitation of verticality. Ancient Rome is believed to have reached a population of over 1 million inhabitants (although this figure has been disputed), many of whom were packed into Insulae, apartment blocks rising up to six storeys above shops – an urban form that would not look out of place in contemporary Europe.In the early 18th century, the author Daniel Defoe was amazed by the high-rise flats of Edinburgh – sometimes as tall as 12 storeys – teetering over the Royal Mile. “I believe, this may be said with truth,” he wrote, “that in no city in the world so many people live in so little room as at Edinburgh.”But it was the invention of the lift that really allowed density to rise. By unlocking the third dimension, the skyscraper became possible. This led to the canyons of Manhattan – which even today with its huge amount of commercial space has a density of 26,000 people/sq km – and eventually to Hong Kong with its forests of 70-storey housing blocks crowded at the foot of the mountains.It’s not just about physical restrictions. People must find places to live, and when there is a strong economic force drawing people with few resources into a city with little regulation, then slums grow. From the Rookeries of old London, to 20th-century population explosions in favelas and barrios, economic pressure can force the poor into ultra-dense living conditions. This creates packed neighbourhoods often without political recognition and thus no recourse to protection from crime, drugs, disease, natural disaster, deprivation and lack of education.Kibera, a slum in Nairobi, is infamous for its horrendous poverty and its metal shacks built millimetres away from railway lines, while Dharavi in Mumbai has a reported density of over 200,000 people/sq km. Rocinha, the largest favela in Rio de Janeiro, has a density of 48,000 people/sq km and over the years has accumulated much of the infrastructure of the city down the hill, gaining new problems of rising living costs and the threat of gentrification.Probably the densest urban environment humans have ever created was Kowloon Walled City, a block on the Hong Kong mainland which was demolished in the 1990s. This phenomenally dense cluster of 12-storey apartments around a tiny courtyard had alleyways like tunnels and Jumbo Jets swooping directly overhead into the old Kai Tak airport. When the famously lawless walled city was subject to a census in 1987 it had more than 30,000 people living there – a density of well over 1,000,000 people/sq km.Part of the solutionBut does this mean that density is inherently terrible? At times, it has been closely associated with urban, and moral, decay. The slums of the 19th century that had grown in the industrialised nations inspired reformers to imagine lower-density settlements. In the UK, Ebeneezer Howard designed Garden Cities, where density would be tightly controlled, both for the health giving benefits of access to open space, but also for order, because slums were where revolutions came from.Modernist architecture and urbanism made the control of density one of its main concepts, with Le Corbusier and his followers advocating huge, dense buildings surrounded by natural landscape, giving an overall more civilised urban form. And in the postwar rebuilding programme, this notion of “towers in the park” with strictly controlled dwellings per hectare limits, and the construction of the New Towns, meant there was a determined political effort to lower urban densities.Then something strange happened. Not only did modernist urbanism not seem to alleviate urban problems, but the aversion to high density began to be overturned. In the early 1960s Jane Jacobs tried to counter the ideas of Howard and Le Corbusier through her deep observation of ordering systems in high-density neighbourhoods, ideas that would later be taken up by the New Urbanists, who reacted against both modernist planning and the sprawl of American suburbia.In 1990s Britain, Richard Rogers and the Urban Task Force advocated high density residential development along the lines of the city of Barcelona (density of 15,000 people/sq km), with its consistent superblocks as a civilised counterpart to suburbia. This argument for density is echoed by geographers such as Richard Florida, who point out that the entire point of the city is the dense proximity of people, leading to what he calls “collision density”, and all the innovations of modern life.Higher density city environments can also be more efficient, with greater public transport use and shorter journeys. Clustering dwellings together also means they share in each other’s energy loads – so density can have a significant effect on reducing carbon emissions. “Anyone who believes that global warming is a real danger should see dense urban living as part of the solution,” as Harvard’s Ed Glaeser puts it.Source: The Guardian