MEDELLÍN, Colombia: Land degradation will unleash a mass migration of at least 50 million people by 2050 — as many as 700 million unless humans stop depleting the life-giving resource, dozens of scientists warned Monday.

Already, land decay caused by unsustainable farming, mining, pollution, and city expansion is undermining the well-being of some 3.2 billion people — 40 percent of the global population, they said in the first comprehensive assessment of land health.

The condition of land is “critical,” said the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES).“We’ve converted large amounts of our forests, we’ve converted large amounts of our grasslands, we’ve lost 87 percent of our wetlands... we’ve really changed our land surface in the last several hundred years,” IPBES chairman Robert Watson said of the findings.

“The message is: land degradation, loss of productivity of those soils and those vegetations will force people to move. It will be no longer viable to live on those lands,” he told AFP. “Between now and 2050, we estimate the number could be 50 (million) to some 700 million people.”

The lowest number is a best-case-scenario projection, said Watson.It assumes “we’re actually starting to be much more sustainable, we’ve really tried hard to have sustainable agricultural practices, sustainable forestry, we’ve tried to minimize climate change.” The upper end of the range is based on a “business-as-usual” approach.

The main drivers of land degradation, said the report, were “high-consumption lifestyles” in rich countries, and rising demand for products in developing ones, fueled by income and population growth.

The problem of land decay does not only impact the people who live on it, the assessment underlined.It threatens food security for all Earth’s citizens, as well as access to clean water and breathable air regulated by the soil and the plants that grow on it.

Yet less than a quarter of land has managed to escape “substantial impacts” of human activity — primarily because it is found in inhospitable parts of the world.And even this small repository is projected to shrink to less than 10 percent in just 30 years’ time.

“People are pushing into those frontiers,” said Bob Scholes of the University of the Witwatersrand in South Africa, a co-author of the paper.“One of the consequences of global warming is that we are moving agriculture into areas” such as the icy, subarctic Boreal region, he told AFP.

“Tropical rainforests historically have had low human populations because it’s hard to get in there — we are now building roads into them, we are putting agriculture into them,” Scholes said.“In the extreme desert areas we are finding deep aquifers, and we’re pumping up, unsustainably, ancient water resources to irrigate.”

By 2014, more than 1.5 billion hectares of natural ecosystems had been converted to croplands, said the analysis. Crop and grazing lands now cover more than a third of the Earth’s land surface.This means not only a loss of soil, but also populations of wild plants and animals, and forests that suck up planet-warming carbon dioxide and produce oxygen.

The IPBES assessment took 100 volunteer experts from around the globe three years to compile, analyzing all the available scientific data.The end product covers the entirety of Earth’s land, as well as the lakes and rivers it supports. The analysis estimated that land degradation cost the equivalent of 10 percent of global economic output in 2010.