Rainforests receive more than 200 days of rain per year, or as much as 240 inches of water. They are characterized by thousands and even millions of plants and trees growing thickly over the land, which soak up rainwater from the soil and return it to the air through transpiration. At least half of that water then falls back down onto the forest as rain again. Most rainforests are located in the central region of the earth, near the equator, where temperatures range from 70 to 90 degrees Fahrenheit or warmer. At least fifty percent of the northern half of South America is covered in rainforest, as is most of Central America, the middle portion of Africa and southern Asia, and even small parts of northern Australia. Together, rainforests cover approximately seven percent of the earth's land surface.

Some rainforests are believed to have existed for 100 million years. At least half of...