A plan to build a 6,000km-long wall across the Sahara Desert to stop the spread of the desert has been outlined.

一项计划在撒哈拉沙漠建立一条6000公里长的墙来组织沙漠蔓延的方案已经被提出。

dunes² that they form, are constantly on the move" width="466" border="0" src="/upimg/090725/4_031934_1.jpg" />
Desert sands, and the dunes that they form, are constantly on the move

The barrier - formed by solidifying³ sand dunes（沙丘） - 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 TED⁴ Global conference in Oxford⁵.

A 2007 UN study described desertification（荒漠化，沙漠化） as "the greatest environmental challenge of our times".

"The threat is desertification. My response is a sandstone wall made from solidified⁶ sand," said Mr Larsson, who describes himself as a dune¹ 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.

Ballooning idea

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 Soviet⁷ 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 complement⁸ to, rather than a replacement⁹, 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（砍，切） 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（砂粒） would be bound together using a bacterium¹⁰ called Bacillus pasteurii（巴斯德氏芽孢杆菌） commonly found in wetlands.

"It is a microorganism which chemically produces calcite（方解石） - 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 prone¹¹ areas.

Mr Larsson envisages¹²（正视，想象） injecting the dunes with the bacteria on a massive scale or using a barrage¹³（弹幕，掩护炮火） 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 excavated¹⁴（挖开，凿通） 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, ethical¹⁵, financial. My design is fraught¹⁶（充满的） with many challenges," he said.

"However, it's a beginning, it's a vision; if nothing else I would like this scheme to initiate¹⁷（开始，创始） a discussion," he added.

TED Global is a conference dedicated¹⁸ to "ideas worth spreading". It runs from the 21 to 24 July in Oxford, UK.