Human activities have had tremendous impact on modern forests. Since agriculture began about 11,000 years ago, large forest areas have been cleared for farms and cities. Beginning in the 1800's, great expanses of forest have also been eliminated because of logging and industrial pollution. The destruction and degrading of forests is called deforestation.

Severe deforestation now occurs around the world, even in the most remote rain forests and boreal forests. Until the late 1940's, rain forests covered about 8.7 million square miles (22.5 million square kilometers) of the earth's land. Today, they cover less than half that area. Millions of acres or hectares of rain forests are destroyed each year. Since 1800, huge areas of temperate forests have also been cleared. Many parts of eastern North America, for example, have less than 2 percent of even degraded forests remaining.