Rainforests are characterized by high rainfall, with definitions setting minimum normal annual rainfall between 1750‚Äì2000 mm (68-78 inches). From 40 to 75% of all species on Earth are indigenous to the rainforests. Rainforests are responsible for 28% of the worlds oxygen turn over, processing it through photosynthesis from CO2 and storing it as carbon through biosequestration. Tropical rainforests have been called the "jewels of the Earth", and the "world's largest pharmacy", because of the large number of natural medicines discovered there. There are two types of rainforest, tropical rainforest and temperate rainforest. Both types have been subjected to heavy logging and agricultural clearance throughout the 20th century and the area covered by rainforests around the world is shrinking.