Ayisha Yahya, BBC News 16 Jul 09;
It has been assumed that global warming would cause an expansion of the world's deserts, but now some scientists are predicting a contrary scenario in which water and life slowly reclaim these arid places.
They think vast, dry regions like the Sahara might soon begin shrinking.
The evidence is limited and definitive conclusions are impossible to reach but recent satellite pictures of North Africa seem to show areas of the Sahara in retreat.
It could be that an increase in rainfall has caused this effect.
Farouk el-Baz, director of the Centre for Remote Sensing at Boston University, believes the Sahara is experiencing a shift from dryer to wetter conditions.
"It's not greening yet. But the desert expands and shrinks in relation to the amount of energy that is received by the Earth from the Sun, and this over many thousands of years," Mr el-Baz told the BBC World Service.
"The heating of the Earth would result in more evaporation of the oceans, in turn resulting in more rainfall."
But it might be hard to reconcile the view from satellites with the view from the ground.
While experts debate how global warming will affect the poorest continent, people are reacting in their own ways.
Droughts over the preceding decades have had the effect of driving nomadic people and rural farmers into the towns and cities. Such movement of people suggests weather patterns are becoming dryer and harsher.
The Intergovernmental Panel on Climate Change warned recently that rising global temperatures could cut West African agricultural production by up to 50% by the year 2020.
But satellite images from the last 15 years do seem to show a recovery of vegetation in the Southern Sahara, although the Sahel Belt, the semi-arid tropical savannah to the south of the desert, remains fragile.
The fragility of the Sahel may have been exacerbated by the cutting of trees, poor land management and subsequent erosion of soil.
Namibia
The broader picture is reinforced by studies carried out in the Namib Desert in Namibia.
This is a region with an average rainfall of just 12 millimetres per year - what scientists call "hyper-arid". Scientists have been measuring rainfall here for the last 60 years.
Last year the local research centre, called Gobabeb, measured 80mm of rain.
In the last decade they have seen the local river, a dry bed for most of the year, experience record-high floods. All this has coincided with record-high temperatures.
"Whether this is due to global change or is a trend anyway, it's hard to distil actually out of the [data] but certainly we've had record highs of temperature," said Joh Henschel, director of Gobabeb.
"Three years ago we had the hottest day on record, 47 degrees Celsius."
The mean annual evaporation is several hundred times higher than the actual rainfall. This is an intense environment.
Fluctuation
His colleague Mary Seely agrees.
"Deserts and arid areas always have extremely varied rainfall," she said.
"You would have to look at a record of several hundred years to maybe say that things are getting greener or dryer. For the last few years there has been higher than average rainfall.
"That said, there is even greater variability in the rainfall and the weather patterns than there has been in the past."
Though positioned on the Atlantic coast, the rain that falls on the Namib desert actually comes from the Indian Ocean, having travelled across Africa.
It is therefore hard to explain an increase in rainfall without accepting that higher temperatures globally are causing shifts in established patterns.
The thing these scientists are most keen to work out is what is man-made change and what is natural fluctuation.
Since 1998 the centre has observed a steady but unmistakable trend of rising levels of C02.
They are sure this increase has not been caused locally, since Gobabeb is in a pristine, isolated part of the world with no local sources of pollution.
This is a change that comes about on a global level.
Manufacturing green
Meanwhile, elsewhere on the continent, things are moving at a faster pace.
Global warming may be greening the desert in small, barely measurable ways but, in parts of Egypt, the greening is being advanced in an artificial way, and on an industrial scale.
Egypt has an expanding population and water is becoming an ever more a precious resource.
Waiting to find out if the deserts are greening is not a realistic option.
Remote sensing, radar imagining from space, began in 1981 and showed scientists what was going on under the Saharan sand.
The aquifer, a collection of reservoirs trapped underground between layers of permeable rock, was studied and mapped for the first time.
Tapping into this supply has meant deserts areas can, with skill and judgement, be transformed into farmable land.
Thank to the work of people like Mr el-Baz, the greening of the desert is happening in Egypt in a controlled way.
Out of the newly irrigated desert we now see the commercial growing of oranges, limes and mangoes.
Further, the Egyptian government is actually sponsoring people to settle in the desert to farm, using the water supply they can now tap into and pump out from under the sand.
The programme is part of an ambitious and controversial plan to reclaim 3.4 million acres of desert.
The trend in other parts of the continent may be a migration of people into the cities and away from arid and semi-arid places, but in Egypt, where the desert is undeniably getting greener, the reverse is true.
