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The following points are among the most interesting observations of the articles:
There may be a practical limit to the size of one-downtown cities (excerpt follows)
How big can cities get?
Get ready for a century of urban sprawl
The rise of the megacity - usually defined as having a population of more than 10 million - is one of the most extraordinary phenomena of the modern world. New York City was the first to reach megacity status, sometime around 1940. Today there are at least 20, including three in India and two in China.
There does seem to be a practical limit to the size of a city with a single downtown. In recent years, a succession of megacities have surged through 10, 12 and 15 million inhabitants before seizing up with traffic congestion and choking with pollution. People and business have fled, taking with them the wealth that made the cities what they were. Against all expectations, these megacities have abruptly stopped growing.
Mexico City is a typical case in point. Its population soared to 16 million people in the mid-1980s and was predicted to double by 2000. Instead, it is stuck at 18 million. Likewise, São Paulo in Brazil hit a wall at 18 million, and Kolkata in India, once predicted to reach 40 million people, is stable at 13 million.
Something new is happening. Single megacities are being replaced by urban archipelagos, only some of which have a dominant megacity at their heart. Helped along by the boom in cheap communications, extensive transport networks and cultural changes in work and living, they have become "the largest, most complex man-made structures ever created", says Herbert Girardet, professor of environmental planning at Middlesex University, London.
For example, London's population count may have reached a plateau, but it is spawning a sprawling urban region covering most of the south-east of England. The original city is now flanked by urban centres west towards Reading and Oxford, north towards Cambridge, and, as the government plans it, east along the Thames estuary. Likewise, the people of Mexico City fled to Toluca and Cuernavaca; Kolkata dispersed to a rash of new urban centres across West Bengal; São Paulo is embracing a new "golden urban triangle" stretching to Rio de Janeiro and Belo Horizonte; Hong Kong is merging with Chinese urban centres round the Pearl river delta; and 70 million people are linked by bullet train from Tokyo to Osaka. This last is the largest and densest megalopolis of them all.
A 200-kilometre journey west along the banks of the Yangtze takes you from Shanghai to Nanjing, through a largely built-up hinterland. Suzhou, one of the cities on that route, is a metropolis of 5 million people constructed in 15 years on the model of Singapore, which it has now overtaken in size. Heading south-west from Shanghai by 2010 it will be possible to travel the 170 kilometres to Hangzhou in just 27 minutes aboard a new maglev train, uniting a crescent-shaped urban corridor containing some 75 million people. Right now, the Yangtze delta is the fastest growing urban area in the world.
2007 will mark the first time in human history that the majority of people on this planet will be urban-dwellers.
Super-dense cities can be just as bad as sprawling cities regarding their environmental effects.
It appears that cities such as Copenhagen, Singapore and London are more or less optimal. As a graphic example:
Regarding the graph above, here is an excerpt from the article:
So is there an inevitable conflict between eco-efficiency and pleasant living?
A study by Jeff Kenworthy and Peter Neuman suggests not. They found a strong inverse relationship between urban density and the amount of energy used by cars driving within the city limits (see Graph above). However, they also showed that super-dense is not a good thing either. Energy use in transport is far higher in a sprawling city such as Houston than in more compact, low-rise cities such as London or Copenhagen, but up the density any more and you run into another problem. Dense cities heat the air around them. Stone, concrete and asphalt absorb more solar energy, and reflect less, than natural surfaces such as grass, water and trees, so they pump up the temperature at night. Vehicles, air conditioning and electrical appliances also give off heat, while tall buildings cut down winds that can disperse the heat. So cities are usually about 1 °C warmer than the surrounding countryside during the day, and can be up to 6 °C warmer at night.
The denser the city, the worse the effect. In hot climates, where many of the world's super-dense megacities are found, air conditioning is used to keep the indoor temperature bearable. On a hot day in many of these cities, air conditioning can consume more energy than any other single activity.
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