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.

Data based on Population in 2008, Land in 1999, 2.43 births per death in 2004. There are many world-population counters on the internet, and all vary about half a billion people, but that's pretty good as there is no world census.