An area larger than the size of India and China combined has been desertified and eroded since 1950. On the other hand a recent science exhibition in Paris on global warming predicted that Paris will be the new Venice in only 50 years time, all of its boulevards and roads flooded as the oceans rise while Venice itself will be completely submerged under water.
The USA has about 170 Million/ha in cropland, 62 Million/ha potential cropland and 138 Million/ha unsuitable. Per capita this amounts to 0.68 ha, compared to 0.23 ha per person worldwide (year 2000).
World land degradation is about 7-10 Million/ha/yr on a total arable area of 1500 Million/ha. In the history of world farming as much as 2000 Million/ha may have been rendered unproductive. Erosion alone has destroyed some 430 Million/ha. Worldwide natural erosion is estimated at 10 Giga tons/yr but human-induced erosion is more than 2.5 times higher, 26 Giga tons/yr.
Soil loss in China averages at 40t/ha/yr. New Zealand lost 30 mm in 100 years of farming. It takes more than 10,000 years to regenerate 25 cm of lost top soil.
Soil formation is roughly an inch (25 mm) in 1000 years or 600 tons/ha. 1 ton/ha per year is common for natural ecosystems but under cultivation, soil formation increases somewhat. 'Sustainable' agriculture without ploughing limits annual soil losses to around 10tons/ha for flat land, to 40tons/ha for land with 10% slope, which are still remarkably higher than natural replenishment. Thus even "sustainable" agriculture degrades soil within about 60 years.
In every continent the main causes of soil degradation differ: Europe suffers most from deforestation although agriculture is a close contender. In Africa, its cause is overwhelmingly overgrazing, whereas in North America it comes with agriculture. The figures for Oceania are dominated by Australia and New Zealand where overgrazing is by far the largest contributor. World-wide these figures average out. Degradation due to fuelwood cutting, is large in the poor continents, while absent in the rich continents, where fossil fuel has taken its place.
Here is a description of each cause:
Deforestation: forest soils contain much organic matter, indeed often more than can be converted by the soil organisms. When a forest is cleared, the trees are burnt, which leads to an immediate loss in organic matter, but above the soil. Some of the organic matter in the soil is burnt too. But in the years following, soil organisms become starved of a carbon source and burn the remaining organic soil content. It all leads to massive emissions of carbondioxide. In the wet tropics, forest soils do not contain much fertility. The tropical rains make farming a nightmare.
Fuelwood: cutting forest for fuelwood is another form of deforestation. Fuelwood is usually converted to charcoal, which burns cleanly. In the process, all hydrogen and oxygen are removed, so that carbon remains. Humans need enough fuelwood for cooking, to be problematic. In arid regions, even the last tree and shrub is used, leaving the landscape barren.
Overgrazing: when insufficient amounts of grass litter are left for the soil, the soil organisms die and the soil loses fertility. Sparse cover lets raindrops erode the surface. It is a common practice that leads to desertification.
Agriculture: most agricultural practices are harmful to the soil.
Industrialisation: industries can pollute soils, mining operations do.
Factors affecting erosion can be summarised as follows:
Heavy rains on weak soil: rain drops loosen soil particles and water transports them down hill.
Vegetation depleted by drought: rain drops are free to hit the soil, causing erosion during rainfall. Winds blow away the fine particles during droughts.
Steep slopes: gravity 'pulls harder': water flows faster; soil creeps, slips or slumps downhill.
Sudden Climate Change
Rainfall: erosion increases unexpectedly rapidly as rainstorms become more severe.
Drought: water dries up and the soil becomes a playball of winds. Soil biota die. A sudden rain causes enormous damage.
Changing winds: areas previously sheltered, become exposed.
Change of land (deforestation): the land loses its cover, then its soil biota, porosity and moisture.
Intensive farming: the plough, excessive fertiliser and irrigation damage the land, often permanently.
Housing development: soil is bared; massive earthworks to landscape the subdivision; soil is on the loose.
Road construction: roads are cut; massive earthworks, leaving scars behind. Not enough attention paid to rainwater flow and maintenance of road sides.