The statistics paint a grim picture. According to the World Resources Institute, more than 80 percent of the Earth’s natural forests already have been destroyed. Up to 90 percent of West Africa’s coastal rain forests have disappeared since 1900. Brazil and Indonesia, which contain the world’s two largest surviving regions of rain forest, are being stripped at an alarming rate by logging, fires, and land-clearing for agriculture and cattle-grazing.
Among the obvious consequences of deforestation is the loss of living space. Seventy percent of the Earth’s land animals and plants reside in forests. But the harm doesn’t stop there. Rain forests help generate rainfall in drought-prone countries elsewhere. Studies have shown that destruction of rain forests in such West African countries as Nigeria, Ghana, and Côte d’Ivoire may have caused two decades of droughts in the interior of Africa, with attendant hardship and famine.

Deforestation may have catastrophic global effects as well. Trees are natural consumers of carbon dioxide—one of the greenhouse gases whose buildup in the atmosphere contributes to global warming. Destruction of trees not only removes these “carbon sinks,” but tree burning and decomposition pump into the atmosphere even more carbon dioxide, along with methane, another major greenhouse gas.

http://www.nationalgeographic.com/eye/deforestation/effect.html