Desertification is a serious problem in most of the marginal regions of the world. The Sahara, for example, is expanding southwards through the Sahel due to human usage patterns. Plants respire water from the ground into the air, reducing the dryness of the air, which leads to more rainfall, which keeps the plants alive to respire more water, allowing marginal areas to support plant life in excess of what might be expected from other factors. When these conditions are upset by removal of plants through human usage or drought then these areas will quickly tend towards desert. Conversely, a few wet years or enforced establishment of plant cover can push them away from desert.