The world's productive land is a constantly changing resource. Climatic variations, natural disasters, and human intervention are ceaselessly at work changing the boundaries of productive land -- arable land, pasture land, and forest.

Arable land covers 3% of the world's surface. Despite the fact that this land is continually being lost to urbanization, the total area under cultivation is rising because of deforestation. Demand for agricultural land continues to increase in line with population growth, resulting in the clearing of marginal land, such as hillsides. The exploitation of marginal land is partly responsible for the erosion of the fertile soil layer, increased drought, the loss of essential soil nutrients, and salt contamination -- all reasons for abandoning the land.

Land used for pasture occupies twice the area of land now under the plow. Although livestock raising produces less protein per hectare than grain, especially in developing countries, it enables farmers to take advantage of marginal land that is less suitable for growing grain.

The loss of productive land can be attributed largely to the destruction of forests. The cultivation of land once forested, however, has not stopped the steady decrease in arable land or pasture land.

Finally, the land that produces our food, provides us with firewood and construction lumber, purifies the atmosphere, maintains precipitation levels, and slows down erosion is continually decreasing. It is estimated that one hectare of productive land is lost every 7.67 seconds.