Copper turns green because of reactions with oxygen and carbon compounds in the air or from exposure to salt water. The basic oxidation of copper is like the rusting of iron, which produces a brown surface rather than a green one. The copper oxides only turn green with carbon compounds.
Green carbon compounds form naturally when copper is exposed to the elements for long periods of time. Both the green compounds and the basic brown copper oxides protect the copper underneath and can keep the copper from corroding for a long time. The Statue of Liberty is green because its copper has oxidized.