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. Similarly, the removal of forest cover may alter rainfall distribution because of reduced evaporation of water from plants. Changes in patterns of precipitation could have dramatic effects, positive or negative, on all life on Earth. Throughout most of the Earth's history, Earth's systems have functioned unmonitored, but not unrecorded.