It borrows its name from the massive stone structure built by the Qin Dynasty. But the purpose of the Green Great Wall is not to hold back the barbarians — it's to stop the ever-encroaching deserts.

"About a quarter of all of China's land mass is desert, and those deserts, up until very recently, were expanding; they were growing at the rate of about 1,000 square miles per year," journalist and author Vince Beiser says.

It's a desertification rate that has laid waste to vast swathes of valuable farmland and regularly choked the suburbs of Beijing in clouds of dust.

When completed, the Green Great Wall will stretch more than 4,800 kilometres across the north of China, forming a living barrier along the edge of the giant Gobi Desert.

The plan involves the planting of 88 billion trees. ( Getty/AFP: STR )

The 50-year project involves the planting of more than 88 billion trees and the results so far, says Beiser, have been "amazing".

"You can drive through areas where they have planted just millions and millions and millions of trees," he says.

"I stood on top of a hillside in one place in inner Mongolia and as far as you could see it was desert land that had obviously been forested."

It's sold as a great patriotic effort to tame nature. And its success, to date, has relied on the involvement of tens of thousands of farmers and landholders following a regimented master plan.

"There's one area in the Kubuqi … they built this brand-new road through the desert, and all the way along the road for miles and miles and miles it's lined with trees," Beiser says.

"And then just behind those rows, is just sand, just sand as far as you can see."

A truly global problem

China is not alone in facing the threat of encroaching deserts.

Late last century, 197 states came together to ratify the United Nations' Convention to Combat Desertification.

Despite that, more than 24 billion tonnes of fertile soil is still lost annually to desertification, while 40 per cent of the Earth's land surface is now considered degraded.

China's massive planting program aims to prevent dust storms like this one in Gansu. ( Getty: Li Chunqing/Visual China Group )

The cost, says UN Secretary-General Antonio Guterres, isn't just environmental — it's social, cultural and economic.

"Dry land degradation reduces national domestic product in developing countries by up to 8 per cent annually," he recently warned.

In fact, the UN estimates desertification affects the lives of about 3 billion people.

"We have a short window of 10 to 15 years to reduce the pressure and if possible reverse the trends," senior UN official Ibrahim Thiaw says.

"Technologically we have the means, we have the knowledge right now. Maybe we need more of a political will and more public engagement."

Tens of thousands of volunteers have taken part in desertification control work in China. ( Getty: TPG )

Mr Thiaw has praise for the type of forestation work undertaken in places like China and India, but he worries that a focus on desertification primarily as an environmental challenge misses the bigger problem.

"Just like climate change, human activities are one of the main causes of land degradation," he says.

"We push international resources, international systems to produce food; we have our industry that is also impacting; we have cities that are encroaching into productive land."

Planting working bees are common in China as the country attempts to stop desert areas from increasing in size. ( Getty: TPG )

Beiser says that's particularly true of China, where the urbanisation of the country's interior is officially encouraged by the Government as a means of reducing overcrowding in coastal cities.

"There are lots more people moving into areas right next to the desert. With people comes livestock. The land just really can't support that many people," he says.

"In the Gobi Desert, for instance, the number of people has quadrupled just in the last few decades.

"You've got so many people cutting down trees for firewood, so many more farms and factories sucking up groundwater, so many more animals eating up the grass — it just dries up the land."

Africa opts for a different approach

In the Sahel region of Africa — essentially the southern fringe of the Sahara — 26 countries have come together to build their own version of the Green Great Wall, rather confusingly dubbed the Great Green Wall.

It too was originally envisaged as a winding strip of trees — twice as long as China's and several kilometres deep.

But that approach has now been abandoned.

The Great Green Wall project in Africa is taking a different approach. ( Getty/AFP: Seyllou Diallo )

"Actually, what's needed is not so much a line of trees as widespread adoption of sustainable land management," says Jonathan Davies, the global drylands coordinator with the International Union for Conservation of Nature.

As a result, the new approach focuses on creating a patchwork of differing vegetation, matching plant species to the differing environments in need of revegetation.

That, says Dr Davies, is its strength.

"In many cases [what's needed] is reviving farming practices that were there in the past and for different reasons, due usually to cultural policies, those practices were eradicated," he says.

"A common one is agroforestry, integrating trees like acacia species into the farming system, which obviously stabilises soil and promotes soil health."

Acacia trees are being planted to improve soil health. ( Getty/AFP: Seyllou )

The idea is to build a more resilient landscape, one that's less susceptible to disease and other natural pressures.

Beiser says China's mass forestation efforts have suffered, by contrast, because of their mono-crop approach.

"A few years back a pest, a particular kind of beetle, hit one big chunk of the Green Great Wall and wiped out 1 billion trees almost overnight," he says.

"In many other places they are finding that a lot of the trees simply die. They are just the wrong type of trees for that kind of environment and they just don't last very long."

Are the walls making a difference?

There are differing challenges in measuring the overall effectiveness of both the Green Great Wall and the Great Green Wall.

In China, the evaluation system is handled by the very same state forestry department responsible for planting. Because of that, says Beiser, the figures they produce can't be trusted.

"So, the Chinese Government says it's been a walloping success, that it has reduced sandstorms by a huge amount, that they've reclaimed enormous amounts of land, they've pushed back the desert and reclaimed thousands and thousands of square miles of land," Beiser says.

"Some of that is true to a certain extent, but it's also true that billions of these trees have died."

Even scientists within China have criticised the project's mono-crop approach.

China has been focused on desertification control planting for many years now. ( Getty Images: TPG )

The difficulty with measuring the effectiveness of Africa's project is very different.

According to Dr Davies the problem there lies in the complexity of the scheme, and the sheer number of stakeholders involved.

But the early signs are promising.

"In Niger, I think the figure now is about 7 million hectares of farmland has been put back under different forms of agroforestry," Dr Davies says.

"That's huge-scale, and you can really see the impact that is having on the lives and the stability of the farming communities.

"You've got to look at outcomes, not just in terms of the health of the land, but also how that translates into impacts on people's lives, on people's incomes, on the overall welfare of people throughout the region."

The benefits of an inclusive approach

Mohamed Bakarr, a lead environmental specialist at the Global Environment Facility, says the African project demonstrates the benefit of weaving economic and social incentives into environmental projects.

"It's not just about restoring degraded areas now, it's restoring those degraded areas and then harnessing them to transform the livelihoods of the people, as well as addressing their food scarcity problems.

"You can't pursue one goal and ignore the other, they have to be interlinked. Solutions have to be integrated, but the outcomes will deliver for all the three goals because at the end of the day you want the system to be resilient."

The UN's Mr Thiaw agrees.

"By restoring land, we store carbon, and we also restore biodiversity at the same time, while responding to multiple benefits for local communities," he says.

"So, we have a wonderful opportunity right now to actually build something that will be a symbol for future generations."

Next year marks the end of the UN-designated Decade for Deserts. The year 2021, however, will see the commencement of the Decade on Ecosystem Restoration.