The realisation that more people than ever before are on the move across the world came later to us in Britain than it did to the Brazilian photojournalist Sebastiao Salgado. Since the late 1970s, he has been producing a series of epic photo-essays that, taken together, amount to a one-man campaign to raise awareness in the first world of the disasters and privations of the third. His latest exhibition of pictures - 350 of them lining the unlovely caverns of the Barbican Gallery - deals with the subject of human migration: the movement of peoples who, whether as a result of war, environmental change, economic deprivation or the redrawing of borders, have crossed countries and continents looking for a better life. It is the result of six years' work in nearly 40 countries and, accompanied by a large book, Migrations: humanity in transition (Aperture), is just starting the third year of its global tour. Block-busting statistics like these are synonymous with Salgado, who over the past decade has become that slightly uncomfortable thing, a celebrity photojournalist: uncomfortable in the sense that few reporters want to achieve fame on the back of other people's misery; no bad thing, however, if you need access to media that put "name recognition" high on their lists of editorial values.
He identified the migration project in 1993, but his earlier work had already pointed in that direction. In the late 1970s and early 1980s, he worked in Latin American countries where rural communities were trying to hang on to their traditional ways of life. In 1984 and 1985, he worked with Medecins Sans Frontieres in the Sahel region of West Africa, where thousands were dying as a result of drought and famine and the remaining population was struggling to survive. Between 1986 and 1992, he travelled through 26 countries in search of manual labour - a celebration as much as an epitaph - as it declined in the face of the technological revolution. This included the pictures that are still his best known, of the gold miners of the Serra Pelada in Brazil clambering up and down the steep cliffs of the open-cast mine like an army of mud-covered ants.
One of the qualities that marks out Salgado is the way his first career as an economist has shaped his photojournalism. It was while working for the International Coffee Organisation in Rwanda in 1971 that he first used a camera for his work, and began to feel that he could say what he wanted to in photographs rather than in an economic report. Two years later, he gave up his job and joined a photo agency in Paris. But in the scope, the research and the organisation of his mammoth reportages, the economist is still in evidence. When very many photographers are struggling to sell a single story, Salgado has established a system by which the majority of the individual stories that make up one project are guaranteed publication before he sets off. He makes out a schedule of countries and story outlines, then pre-sells them for release at regular intervals to the few remaining news magazines and supplements still able to make such a long-term investment. Added to this he takes commissions from charities and NGOs, which then use his pictures in their own campaigns. He sets up publishing deals, alerts exhibition venues and at the same time as editing magazine stories, he is earmarking images that will eventually reappear as part of a book, an exhibition or a film.
In many of the countries he travelled through for the manual labour project, he realised that communities were breaking up. The former Yugoslavia was already being divided by a nationalist civil war, and later the wars in Chechnya and Kosovo would swell the thousands of refugees and economic migrants - north Africans, Turkish and Iraqi Kurds, Somalis, Afghans, Russians - making their way across Europe to the prosperous and stable cities in the west. In the United States, the Mexican border was still the scene of daily attempts by thousands of migrants from Central America and Mexico to get into southern California. In South America, the big cities, notably Buenos Aires and Salgado's own city, Sao Paulo, were exploding as people came in from the countryside looking for any kind of work. In Africa, civil wars, famine and drought still worked in concert to drive entire populations in search of the most basic means of survival. And then there were people going nowhere: Vietnamese boat people still held in detention centres in Hong Kong; generations of Palestinians stuck in camps in northern Lebanon.
All these places and more make up the "Exodus" exhibition, which opened in Paris three years ago. Then, as now, the overwhelming impression was of the sheer volume of photographs, the number of countries, the constant sense of people moving, almost always on foot or crammed into some makeshift vehicle; huge numbers of men, women and children, trailing along highways, gathered in camps. Going from image to image, shifting from scenes that have a cinematic grandeur - thousands of Rwandans lost inside a dust cloud in the early morning, walking towards a camp in Tanzania; women carrying their goods to market backed by the mountains of Ecuador; lowering clouds over the skyline of Sao Paulo - to close-up portraits that pick off details of individual lives, it began to feel as if the pictures were moving and I was standing still. It was like watching a train going by, each picture a window through which - in the best traditions of photojournalism - I was being given a glimpse into someone else's world. But when the train has gone by, what emotions are left behind?
Even within the main body of the exhibition, there is a bias towards pictures of children, and at its end visitors are faced with a coda: wall after wall of portraits of sober, watchful children who have posed obediently for the camera. None of them is identified by name, only by the location of the transit camp, refugee centre, prison, orphanage or other collection point they have ended up in. Faced with this delegation, despite an inevitable sense of being manipulated, the plea is, as it is intended to be, inescapable.
Salgado has said that he doesn't want his pictures to make people feel guilty. He wants us to know what is happening to our fellow members of the human race, and to do what we can to help. The people in the book, he says, "allowed themselves to be photographed, I believe, because they wanted their plight to be made known. When I could, I explained to them that this was my purpose. Many just stood before my camera and addressed it as they might a microphone."
And what did the people make of him? In a few of the pictures, you can catch a look of suspicion or disdain. But for the most part people do, as he says, stand obediently before his camera, some with that terrible passivity that has come to stand for victimhood, just as small snapshots held up to the camera have become the universal language of the disappeared.
Salgado is not the only photographer to have covered these stories, nor are the situations in these countries unknown to most of us. Some photographers have concentrated far more intensively on a particular situation: Gilles Peress's unparalleled record of the Rwandan massacres, collected in his book The Silence, is a prime example. But for Salgado, each individual essay represents a piece of evidence harnessed to support a central truth: in this case, populations are shifting, borders are breaking down, more people live in cities than ever before, the social and political structures that once contained them are no longer adequate. The world is shrinking, not just because of new technology but because more of us are crammed together. We need to empathise and to adapt. We need to share our wealth. We have to learn to live alongside one another. If the message is dogmatic, it is meant to be. On Salgado's website, there's a button: How to Help. Go there and see.
"Exodus: photographs by Sebastiao Salgado" is at the Barbican Gallery, London EC2 (020 7638 5403) until 1 June, or visit Salgado's website: www.sebastiaosalgado.com.br
Liz Jobey is deputy editor of Granta magazine