More than 35 percent of the planet's surface is used, at least indirectly, for harvesting food and other materials. Grazing, agriculture, and timber harvesting disturb topsoil, increasing soil erosion. More than 75 million tons of soil are blown or washed into the oceans each year. Natural ecosystems shrink in the face of society's need to use land. Some species, unable to survive in such reduced areas, become extinct.
Life requires water.
On land, the amount and frequency of rainfall determine the success of crops, as well as the survival of natural ecosystems. It takes about 10 days, on average, for a drop of water that becomes airborne vapor in one place to return to Earth's surface as rain or snow in another. Precipitation varies by both season and geographic area. As one result, highly specialized ecosystems have developed, from deserts to rain forests. In the event of global warming, regional rainfall patterns may shift.