For decades we have been warned that the Sahara is on the march, eating away valuable soil and turning ever larger tracts of land into deserts. Billions have been spent to fight desertification. But is the desert really moving southwards or have the experts got it wrong? Tom Nevin digs up some startling facts.

The question of deserts on the march engulfing arable land at a terrifying rate is once again on the international agenda. A United Nations conference on desertification in Dakar, Senegal heard late last year that more than 250m people face hunger and starvation as a result of expanding deserts around the globe. Africa, the conference was told, is one of the worst affected regions with virtually three-quarters of its landmass severely or moderately influenced by drought due to changing climatic conditions and harmful agricultural practices.

For years, some scientists have warned of the desertification of arable lands on the fringes of the world's great deserts. Deforestation, over-cultivation and drought mean an annual expansion of deserts. The Sahara, they warn, is moving southwards, engulfing once arable land and insidiously extending its borders. Or is it? Not all scientists agree.

Almost since the beginning of the century, both French and British colonial scientists have spoken about the advancing sands of the Sahara. They blamed decreasing rains and the destructive ways of the farmers and nomadic cattle herders. In 1949, the French geographer Andre Aubreville, coined the phrase 'desertification' to describe how, as he put it, "deserts are being born today under our very eyes" In 1972 American scientists said "the desert is advancing at a rate of about 50 kilometres a year over a 8,350km front". They did not present any evidence but these grim figures and the spectre of a monster desert on the move caught the world's imagination.

The United Nations Environment Programme (UNEP), has frequently claimed that, "each year 21m hectares of once productive soil are reduced by creeping deserts to the level where nothing will grow on it". That's a lot of land, about as big as Britain or, in African terms, the size of Cameroon. But how do we know this? Where do the figures come from? To find out, we must travel back in time and go flying.

Meet Hugh Lamprey. Hugh is a bush pilot turned environmentalist. For 17 years he flew the skies over Sudan in his single-engine Cessna 210. On this flight at 3,000 metres over the Sudanese province of Kordofan we can see how the land changes from plains of grassland in the south to arid desert the north.

Hugh Lamprey says, "I've been flying this region for nearly 20 years and I've watched the Sahara moving south, four or five kilometres a year. You don't have to be rocket scientists to work that out. In 20 years the desert has invaded southwards by 100km". If the same is true for the entire 6,000km southern Sahara frontier, 600,000 square kilometres of once fertile land has died. And that's roughly the size of Kenya. The sun is setting on a dying landscape as we fly home. But things are seldom as they seem.

The idea of an advancing desert caught on, especially after drought and famine devastated the Sahel in the early 1970s and something was finally done. The United Nations Conference on Desertification, held in Nairobi in 1977 and 'Plan Action' was launched to halt the Sahara's march. Part of the plan was to plant millions of trees on sand dunes, but after $6bn was spent over 16 years, there was little to show for the effort or expense.

At the 1995 Earth Summit environmental conference in Brazil, the secretary-general of UNEP, Mostafa Tolba, described the result of the campaign as 'pathetic'. On the strength of what he had to say, the summit adopted the desertification convention, the only measure accepted that was of direct benefit to the third world in general and Africa in particular.

Mr Tolba blamed the failure so far to stop the desert's encroachment on a lack of funds. To do the job properly, he said, UNEP would have to spend a staggering $450bn over the next 20 years.

Mr Tolba believed that while desert invasion may be accelerated by drought, it is rarely caused by it. In his view, human activity was largely to blame, especially bad farming and ranching methods. Take cattlemen for example. Thousands of herds, made of up to 100 head each, graze the desert fringes. When they come across vegetation that's where they stay until the area is bare. Then they move on and lay waste to the next green patch. And so it goes on, day after day. The damage is devastating.

It was UNEP's conviction that because the problem was caused by human activity the solution could also be provided by human action. All that had to be done was to teach the people who live off the land to take care of it. However, not all scientists were convinced it was that simple.

Now meet Andrew Warren, a geographer with the University of London and an expert on deserts. He says there are three basic reasons why deserts expand. They are short-term drought, longer term climatic changes and the destruction of soils and vegetation through human activity. "You need a different response for each one," he says. "The real problem is how to tell the difference between them"

And that is a problem that neither UNEP nor the scientific community has been able to solve in the 20 years since the launch of Plan Action. So the question was whether there was any point in throwing money at a problem that was so badly understood. What was needed, said the desert scientists, was better science.

But this is the age of science. We're only a handful of days away from the 21st century. Let's use it. Let's take a look at the big picture. Perhaps if we can see the entire problem at a glance, we'll also be able to spot the answer.

We have to go awfully high up to see the Sahara all at once. How do we do that? We go by satellite! Here in space nearly 160km straight up, the continent of Africa stands out in stark contrast against the sea. The few scraps of cloud do little to conceal the unmistakable line that is made where the desert stops and the vegetation begins. We take a couple of snapshots so we can compare them with photographs taken from the same position every year since 1982.

Enter Elf Helen. Elf and other researchers at Luanda University in Sweden studied satellite pictures of the Sahara taken over a 10-year period. But instead of finding the answers, they were confronted by another mystery. They concentrated on the area identified by bush pilot Lamprey in Sudan where the concept of desertification was born. Through most of the 1980s the Swedish team photo-surveyed the area by satellite and in 1994 had to conclude that the Sahara is not migrating southwards.

Instead, what the Swedes discovered was that the desert did advance during times of drought, sometimes quite a long way, but when the rains came the Sahara retreated. The space shots showed that the desert boundary can swing back and forth by dozens of kilometres a year, depending on the rainfall in the area. The Scandinavian scientists concluded that the Sahara is no further south than it was a millenium ago.

Now what? The evidence was there for all to see. The Sahara had been tried and found not guilty of territorial expansion The textbooks, especially those about the Sahel's nomadic herdsmen, had to be rewritten.

It now seemed certain, noted New Scientist magazine at the time, that the Sahara's cattlemen know far more about the staying power of the deserts vegetation that botanists trained in the west. Their herding methods, based on constant migration, actually work better than settled farming. Their cycle is perfect: they move their cattle in, remove the grazing and fertilize the area, at the same time spreading grass seed. When the rains come, and eventually they always do, the seeds take root and grow. So the amount of vegetation visible at any one time is a poor indicator of the state of the land.

In the face of new evidence, the UNEP dropped its 'shifting sands' theory and said, "whether the deserts of the world are advancing or receding is not the issue..." The UN agency insisted that much more good than harm could be done by upgrading land management and farming methods in the arid areas of the world. There can be no denying that and it will take a lot of money to do it. But it now looks as if the big drawcard, the creeping deserts, is no longer the star of the show, and it will be much more difficult to raise they money needed to help the nomads of Sahel.

The riddle of Sahara's shifting sands has finally been solved, and the answer is that they are shifting back, just as much as they are shifting forward.

There can be no denying that the life of the Sahel nomad is a wretched one and efforts must be made to better it. But the way to do it is not by flushing out phantoms that don't exist.

Ways forward to help the people of Sahel must be found within those parameters. It simply is not possible to change the ways of one of natures greatest creations, no matter what amount of money you throw at it.