It happened in the Brazilian port of Belem while he was making a series of films on the destruction of the rainforest. Tracking the camera along a stack of mahogany being swung into a freighter, he noticed the word "London" stamped on it.
"I suddenly realised the impact that cities had on the world," he said yesterday. "I started to take an interest in the connection between urban consumption and human impact on the biosphere."
Now Professor of Environmental Planning at Middlesex University, he has just completed a study on London's effect on the planet. He has found that although it covers less than 400,000 acres, it needs nearly 50 million acres - 125 times its area - to provide it with food, timber and other resources and to absorb its pollution. "This means that, although it contains only 12 per cent of Britain's population, London requires an area equivalent of all the country's productive land to service it - though, of course, this extends to the wheat prairies of Kansas, the tea gardens of Assam, the copper mines of Zambia and other far-flung places."
The city, he calculates, burns the equivalent of two supertanker loads of oil every week and takes in 1.2 million tonnes each of timber and metal, over 2 million tons each of food, paper and plastics, and 1 billion tons of water every year. In return, it churns out more than 15 million tons of waste and 7.5 million tons of sewage sludge annually - and emits 60 million tonnes of carbon dioxide as its contribution to global warming.
Similar calculations could be done elsewhere. Vienna takes in so much material that it adds 36,000 tons to its weight every day. Aligarh City in India imports 1,000 tonnes of soil daily for use in construction, affecting natural drainage and thus increasing flooding in the surrounding region. Mexico City has sunk by more than 20 feet over the last century because it has drawn so much water from the aquifer beneath it. And an expanding "ring of destruction" surrounds many west African cities as trees are cut down to provide people with fuel. In all, says Giradet, "cities occupy 2 per cent of the world's land surface but use some 75 per cent of the world's resources and release similar percentages of wastes".
Cities have always depleted the environment around them, and have often paid the price. When archaeologists excavated Ur in Mesopotamia - one of the world's first cities - they found it had been buried by a layer of mud in around 2,500BC, the result, it is thought, of flooding caused by felling forests in the surrounding hills. The fall of Rome may have partially resulted from exhaustion of cropland. "Hell," wrote Shelley, "is a city much like London - a populous and smoky city."
But there has never been anything like the present explosive urban growth. As we report on page one today, the number of people living in towns and cities will outstrip those in the countryside for the first time in human history, by the turn of the millennium. Within another 30 years there will be twice as many urban as country people.
Next weekend in Marmaris, Turkey, the United Nations will bring together more than 130 politicians, officials, academics and other experts, to plan the "intellectual framework" for a giant conference to be held in Istanbul in June - Habitat II (Habitat I was 20 years ago).
"The future of humanity will be shaped largely by urban conditions," writes Professor Klaus Topfer, the German minister of urban development and a participant at Marmaris, in a forthcoming issue of Our Planet, the magazine of the UN Environment Programme. "The quality of life for generations to come - and the chance to solve conflict within nations and between them - will depend on whether governments find ways of coping with accelerating urban growth."
TWENTY years ago, in the run-up to the last Habitat conference, the prophetic Barbara Ward, economist and environmentalist, reached for a simile to describe the coming cataclysm.
"One has the obscure feeling," she wrote, "that only the distant billennia of geological time can provide any adequate concept of the scale of the upheaval. The Indian subcontinent detaching itself from Antarctica and sweeping across the Indian Ocean to its violent collision with Asia's landmass across the Himalayas, the sea pouring in to change the Caribbean or the South China Sea into a chain of islands, the grinding of continental plates against each other, heaving up the Andes and leaving volcanic chains where Asia and Europe collide - these surely are the images that are appropriate to the scale of the 20th-century urban deluge."
London became the first city since ancient Rome to reach a million people in around 1800, during Shelley's lifetime: it then took about 130 years to grow to 8 million. But it has taken Mexico City only 50 years to grow from 1 million to 15 million, while Bombay has gone from 5 to nearly 15 million in less than quarter of a century, and will exceed 27 million in another 30 years.
In 1800, only 50 million people lived in towns and cities worldwide. By 1975 there were 1.5 billion, by the millennium this will have doubled to 3 billion, which means that then there will be more people living in urban areas than made up the entire population of the Earth in 1960. And by 2025 there will be 5 billion. Nine-tenths of this growth is taking place in developing countries. About half is due to population increase, the rest comes from the greatest mass migration in history.
Pushed by rural poverty and official neglect of the countryside, pulled by the hope of a better life in the cities, tens of millions of country people uproot themselves every year to join the swelling urban slums. The migrants find no houses waiting for them, no water supplies, no sewerage, no schools - and no welcome, for they are usually resented by wealthier citizens and ignored, at best, by the authorities. They have to settle on land no one else wants, that is too wet, too dry, too steep or too polluted for normal habitation. They throw up makeshift hovels, made of whatever they can find - sticks, fronds, cardboard, tar-paper, petrol tins, perhaps (if they are lucky) corrugated iron.
In Brazil they call these slums favelas, in India bustees, in Tunisia gourbevilles, in Chile cullampas ("mushrooms", after the way they spring up overnight) and in Argentina villas miserias. In Bombay 3 million people are squeezed into such shantytowns, in Bogata and Kinshasa 60 per cent of the citizens live in them; in Addis Ababa, 79 per cent. They already contain one-third of all the urban dwellers in developing countries. But they are considered "illegal", and can be demolished without notice.
Infant mortality rates in city slums in Bangladesh are 50 times higher even than in the deprived countryside the migrants left behind; in Manila they are three times higher - and tuberculosis is nine times more common - in the slums than in the rest of the city. Worldwide, the UN estimates, at least 250 million urban dwellers cannot get safe drinking water and many of those who do have to rely on standpipes that run for only a few hours a day. At least 400 million are without even latrines for sanitation. By 2000, most children born in Third World towns will be to such desperately poor families. Already more than 100 million homeless children struggle to survive on the streets.
Rich-world cities have ceased to grow rapidly, but their relative prosperity has brought its own swiftly increasing problems, quite apart from their enormous impact on the world's environment and resources. Pollution from car exhausts has raised death rates in cities all over the industrialised world. Meanwhile congestion has limited traffic to average speeds slower than in the days of the horse in cities as diverse as London and Milan, Utsunomiya in Japan and Trondheim in Norway.
Increasingly, cities are suffering simultaneously the problems of poverty and affluence: pollution and destitution. In Mexico City one quarter of all babies are born with enough lead in their blood permanently to damage their brains and physical development, while particulates help kill 12,500 people each year. Worldwide, the World Health Organisation estimates, more than one in every three urban dwellers - 1.1 billion people - have to breathe unhealthy air.
Many inner cities of the industrialised world are sunk in dire poverty. Men have a better chance of living to 65 in Bangladesh, the world's 12th poorest country, than in Harlem, New York, part of one of the wealthiest.
It is hard to imagine the cities long being able to withstand existing pressures, let alone accommodating the explosive growth of the next decades. But, as the organisers of Habitat II stress, the similarity of the problems all over the world at least provides common cause in tackling them.
George Wilhelm, the conference's deputy secretary-general, likes to quote Shakespeare: "What are cities, but its people?" A former Brazilian minister of the environment in Sao Paulo, he remains an optimist. The concentration of people in cities is also "a concentration of opportunities", he says.
Nowhere is this more true than in the apparently hopeless slums. Visiting Rio, I was surprised to find favela after favela, which their own people had turned into solidly built communities, with running water and sewerage, entirely by themselves, with little or no official help. One was once so dire that its people had called it the Scorpions' Nest: after they had rebuilt it they named it Happiness Park. Given security of tenure to their land, people will work hard to improve their slums.
The authoritative World Resources Report - which will be published next week by the World Resources Institute, the UN Development Programme, the World Bank and the UN Environment Programme - gives examples of transformations wrought by people of the cities themselves, often with help from citizens' organisations or enlightened authorities.
In Orangi, Karachi's largest slum, residents organised themselves into groups of 20 to 40 families, installed sewage disposal, provided simple health care and family planning, and brought down infant mortality four- fold in just nine years. There is a similar success story in Cali, Columbia's second largest city. In Chattanooga, Tennessee, the city authorities and local people cleaned up some of the worst air pollution in the United States. Some 1,200 local authorities in 33 countries have established grassroots campaigns to implement the recommendations of the 1992 Rio Earth Summit.
Curitiba, a Brazilian city of 2.3 million people, recycles 70 per cent of its waste, has increased the amount of green space per inhabitant ten-fold, and has so improved public transport that it burns 30 per cent less petrol per head than other Brazilian cities. Jaime Lerner, three times mayor of the city, says the key is giving residents joint responsibility for solving its problems. Evidence from around the world bears him out. When "solutions" have been imposed by experts, they have rarely worked. When local people - particularly women - have been involved in design and implementation,they have succeeded much more frequently, and at much less cost.
International organisations are increasingly promoting this. The UNDP has a project for searching out and helping grassroots urban groups. And Habitat II will include citizens' groups (the first to do so officially), rather than just governments at the heart of its deliberations.
"We are trying to change the very way the United Nations operates," says Dr Wilhelm. "After all, it is not called the United Governments."