Processes that reduce the productivity of arid and semi-arid lands, collectively known as desertification, affect more than 3 billion hectares or more than 80% of such lands. The degradation process results in redistribution of water and nutrients, loss of find soil fractions and replacement of palatable plants with undesirable plants. These changes frequently uncouple rainfall and productivity on a temporal scale. This uncoupling is probably due to modification of the nitrogen cycle as a result of the desertification processes. Changes in vegetation can produce changes in fluxes of radiatively active gasses and a variety of organic volatiles to the atmosphere. Redistribution of soil water may produce "hot" spots for denitrification and for ammonia volatilization that differ in extent both spatially and temporally from undesertified ecosystems. Dust from desertified areas can modify the chemistry of rainfall in areas distant from the dust source.