Of six billion humans, nearly a fifth are threatened directly or indirectly by desertification, experts warn ahead of the UN's annual World Day to Combat Desertification.

China, India, Pakistan, Central Asia, the Middle East, as well as a major part of Africa and swathes of Argentina, Brazil and Chile are in the front line of this unacknowledged crisis.

Desertification does not necessarily mean the advance of the desert, which is typically conveyed by dramatic images of wind-blown sand overwhelming farms on the fringes of the Sahara and Gobi.

A subtler, yet bigger, problem is degradation which hits so-called drylands -- areas with low rainfall and high evaporation that account for more than 40 percent of Earth's cultivated surface.

These vulnerable lands are progressively at risk of overgrazing, deforestation and other forms of exploitation, to which climate change is now a powerful addition.

In April, the UN's top scientific authority on global warming warned that higher global temperatures could have brutal effects on rainfall patterns, runoff from snowmelt and river flows in scores of countries that already battle water stress.

Between 80 and 200 million more people could be at risk of hunger by 2080, the Intergovernmental Panel on Climate Change (IPCC) estimated.

Some 70 percent of Earth's 5.2 billion hectares (13 billion acres) of agricultural drylands "are already degraded and threatened by desertification," says the UN Convention to Combat Desertification (UNCCD), which hosts Sunday's commemorative day.

By 2025, Africa could lose as much as two-thirds of its arable land compared with 1990, and there could be declines of one-third in Asia and one-fifth in South America, it says.

Migration -- from the Sahelian regions to the West African coast, from sub-Saharan Africa to Europe, from Mexico to the United States -- will be an inevitable consequence as poor people are driven off their land.

As many as 135 million are at risk of being displaced, says the UNCCD.

Desertification is "a forgotten emergency," says SOS-Sahel, a non-governmental organisation (NGO) which lobbies for Saharan countries.

But it is not exclusive to poor countries.

Severe droughts and water depletion in the United States have left nearly a third of US land affected by desertification, a figure that is also applicable for Spain, whose south has been afflicted by successive dry summers, as well as reckless development and water use.

Marc Bied-Charreton, chairman of the French Scientific Committee on Desertification, says that the direct costs of desertification are around 60 billion dollars per year, a tally that does not take into account economic migration and other phenomena.

Despite its obvious threat factor, desertification is failing to gain traction as an issue, he complains.

"The Global Environment Facility [GEF] is starting to make a small effort. It has committed 200 million dollars over four years," notes Bied-Charreton.

"By comparison, at meeting last December with economists and the World Bank, we concluded that 10 billion dollars a year is needed over the next decade to restore all the fertility in degraded land."

Bied-Charreton says that, with money and effort, desertification can be reversed in all but five percent of cases.

"It entails replanting over three to four years, at an annual cost of around 300-400 euros per hectare (160-212 dollars per acre), to limit erosion and encourage water penetration in the soil," he says.

"This is often hard, manual work which has to be carried out over large areas, and you have to persevere, for up to a decade, to progressively win back your land. But Nature is incredibly strong and these ecosystems are fundamentally resilient."

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