Desertification measured in million hectare.

"Every year about 12 million hectares worldwide are lost to land degradation, and the rate is increasing."
The International Fund for Agricultural Development (IFAD)

The ability to establish estimates for desertification (marginal and desert) are still in early stages of development, even with the use of satellite imagery. Part of the problem is how the term is defined.

Everyone seems to agree though, that the quality of soil world-wide, is degrading. And so we use this number as a best guess and placeholder for now.
CO2 Level Forest Cover Desertification GDP Sea Life Population
2009 1.72 2009 3,952 2009 12.1 2008 60,587,016 2009 75% 2010 1.10
2008 1.66 2008 3,989 2008 12.0 2007 54,347,038 2008 69% 1990 1.11
2007 2.12 2007 4,077     2006 48,244,879 2007   1980 1.13
Moves Toward Sustainable As Numbers Go DOWN Moves Toward Collapse As Numbers Go UP

A continual increase of any one criterion may cause a catastrophic collapse of civilization.
Meter Desertification