Green bananas ripen naturally over time by being left out until they turn yellow with brown spots. Placing green bananas in a brown paper bag helps to speed up the ripening process.
According to the New York Times, as the starches in green bananas converts into fruit sugar, ethylene gas is released. This gas can accelerate the rate at which other fruits and vegetables ripen if they are nearby. Warmer temperatures and higher humidity rates also help green bananas to ripen faster. The ripening process can be slowed down by refrigerating bananas or storing them in a cool, dry place.