The movement of water masses of different densities from the top to the bottom of the ocean, caused by variations in ocean salinity and temperature. 

Substances that break down stratospheric ozone, principally chlorofluorocarbons, freons and halons used as refrigerants, industrial solvents and propellants in aerosol spray cans.  

These substances are stable and long-lived in the lower atmosphere, but drift up to the stratosphere where they break down through the action of ultraviolet radiation. This releases highly reactive atoms (chlorine and bromine) that react with ozone molecules and break them apart.  

See also ozone layer. 

The reduction of the amount of ozone in the lower stratosphere above Antarctica that has occurred each spring since around 1980. 

A layer of ozone in the stratosphere that limits the amount of harmful ultraviolet light passing through to lower layers of the atmosphere.