The Tate goes to town

Global Cities starts with a momentous statistic, emblazoned across the bridge in Tate Modern's Turbine Hall. This year, it says, for the first time in history, more than half the world's population will live in cities. The exhibition is therefore about a subject - half the world - that is vast even for the vastness of the Turbine Hall. The designers, Pentagram, had to build an extra level to create a double-decker exhibition.

How to deal with such immenseness? First, by taking 10 cities from five continents to represent the whole. Second, by portraying them with aerial views and imposing statistics: Cairo is eight times as densely populated as London; Istanbul has grown by 27 per cent in a decade; 40 per cent of Los Angelenos were born outside the US.

Next, the cities are humanised, with films and photos of people at play in parks and beaches, with the buzz of pedestrians and traffic.

In a series of installations, artists give their takes on the cities shown. They instinctively favour pathos and intimacy in contrast to the hard structures of towers and roads, and the equally hard statistics.

Yang Zhengzhong films a sweet-looking girl blowing puffs of air towards another film of Shanghai Street, in such a way that it looks as if her breath is pushing the street backwards. Ozman Bozkurt shows the parks that people inhabit on the fringes of highways in Istanbul. Francis Alys wanders through posh parts of London making music by casually tapping a drumstick against the iron railings.

Between these extremes of the personal and the numerical, the overwhelming impression is of the sheer intractability of cities. They are among the most ancient and enduring of human inventions, but are beyond the control of any one human agency. You are boggled by size, shocked by poverty and charmed by the ingenuity with which people adapt to hostile urban environments.

The Tate's version of Global Cities, which was first shown at last year's Venice Architecture Biennale, also commissioned architects to design responses to the issues on show, all of whom had the sense not to pretend that they could come up with a single overarching solution. One, Fritz Haeg, is content to persuade his fellow Los Angelenos to convert their sterile, water-guzzling front lawns into vegetable patches.

Another, Nigel Coates, responds with half-serious playfulness. His Mixtacity is a model of a delirious version of the Thames Gateway, where people fly in mescaline-fuelled helicopters between giant statues and warped towers. The real Thames Gateway is unlikely to be as exciting.

Global Cities is grand in conception and theme, but a possibly unintended message is that relative modesty often works best. The movement of hundreds of millions will take its own course, but if you try to persuade Californians to grow their own vegetables, you might get somewhere.

Until 27 August (www.tate.org.uk).

Global Cities
Tate Modern
Bankside, Holland Street, SE1 9TG