The climate factor of rainfall drives arid ecosystems, interaction (9). This is effected through a hierarchy of influences. At one extreme is the macroclimate which, by virtue of scale, is larger than arid ecosystem and thus outside its influence. At the other extreme is the microclimate or microenvironment, which is determined by the vegetation (Noy-Meir,1980).

   The strength and sign of the feedback (10) remains uncertain. The nature of the landscape surface undoubtedly influences the characteristics of the atmosphere passing over it. It has been proposed that this link can, under conditions of desertification, operate as a positive feedback (see section 4.1.3). The relative strength and scale of this influence compared with those of the mesoscale or global circulation mechanisms is not clear. For example the two periods of continent-wide drought associated with the modern epochs of desertification in the Sahel of Africa are now known to have been associated with the El Niño oceanic circulation anomaly of the Pacific Ocean.