To get a more accurate picture of desertification, one must first have a concise definition and understand the causes. Desertification is a process of land degradation that reduces its productivity, and its impact may be confined to relatively small scales (Dregne, et al. 1992). It is in this latter light that one can begin to see the human-biosphere link in desertification in the Sahel belt and the causes. At issue is direct human reinforcement through acceleration or deceleration of biophysical phenomena such as soil erosion, land degradation, and agricultural practices. Additionally, the variables used to judge desertification need to be addressed individually as they indicate desertification at different rates and are effects of the human-biosphere link in different proportions depending on the micro-ecosystem. As detailed by The Center for International Earth Science Information Network (CIESIN) at Columbia University, a significant source in understanding present desertification; “In the context of land degradation and desertification, researchers speak of "fast" and "slow" variables. In terms of biophysical variables, crop yield, for example, would be a fast (or quickly changing) variable whereas soil fertility, which affects yield, is a slow (slowly changing) variable.” (SEDAC/CIEN 2009) The most commonly cited forms of unsustainable land use are over-cultivation, overgrazing, deforestation, and poor irrigation practices (UNCCD 2008). Let us examine each of these significant causes individually.