By Jonathan Fildes
Technology reporter, BBC News, Oxford
Desert sands, and the dunes that they form, are constantly on the move
A plan to build a 6,000km-long wall across the Sahara Desert to stop the spread of the desert has been outlined.
The barrier - formed by solidifying sand dunes - would stretch from Mauritania in the west of Africa to Djibouti in the east.
The plan was put forward by architect Magnus Larsson at the TED Global conference in Oxford.
A 2007 UN study described desertification as "the greatest environmental challenge of our times".
"The threat is desertification. My response is a sandstone wall made from solidified sand," said Mr Larsson, who describes himself as a dune architect.
The sand would be stabilised by flooding it with bacteria that can set it like concrete in a matter of hours.
North African nations have promoted the idea of planting trees to form a Great Green Belt to prevent the spread of the sand.
A similar proposal - known as the Green Wall of China - has also been proposed to stop the spread of the Gobi Desert.
In 2007, the UN issued a report that said that one third of the Earth's population - about two billion people - are potential victims of desertification.
It is concerned that the slow creep of the sands will displace people and put new strains on natural resources and societies.
Problem areas include the former Soviet republics in central Asia, China and sub-Saharan Africa.
"It affects about 140 countries," Mr Larsson told BBC News.
Mr Larsson showed pictures of a village called Gidan-Kara in Nigeria which had had to be moved because of the creep of the dunes. He said it was one of many examples.
The architect's proposed wall across the desert would be a complement to, rather than a replacement, of the Great Green Belt proposal.
"It would provide physical support for the trees," he said.
Crucially, he said, it would leave a barrier even if the trees were removed.
"People are so poor in these countries and these regions that they chop them down for firewood."
The wall would effectively be made by "freezing" the shifting sand dunes, turning them into sandstone.
"The idea is to stop the desert using the desert itself," he said.
The sand grains would be bound together using a bacterium called Bacillus pasteurii commonly found in wetlands.
Moving dunes displace both people and crops
"It is a microorganism which chemically produces calcite - a kind of natural cement."
Mr Larsson got the idea for using the bacteria from a team at the University of California Davis, which had been investigating its use for solidifying the ground in earthquake prone areas.
Mr Larsson envisages injecting the dunes with the bacteria on a massive scale or using a barrage of giant bacteria-filled balloons.
"We allow the dune to wash over this structure then we would pop the balloon," he told BBC News.
The scheme would also have advantages for nearby populations, he said. For example, it could be excavated he said to provide shade, shelter or as a structure to collect water.
However, Mr Larsson admitted that the scheme faced numerous practical problems.
"There are many details left to explore in this story: political, practical, ethical, financial. My design is fraught with many challenges," he said.
"However, it's a beginning, it's a vision; if nothing else I would like this scheme to initiate a discussion," he added.
TED Global is a conference dedicated to "ideas worth spreading". It runs from the 21 to 24 July in Oxford, UK.