Natural Influences on Stratospheric Ozone
Despite all of our harmful chemicals going into the atmosphere, some people still argue that stratospheric ozone depletion is just part of a natural variation. Natural variations which influence the amount of ozone in the upper atmosphere include solar activity, volcanic eruptions and changes in atmospheric circulation - the planetary winds.
The Sun's Influence on Ozone
Stratospheric ozone is primarily created by ultraviolet (UV) radiation coming from the Sun. The Sun's energy release does vary, especially over the 11-year sunspot cycle. During the active phase of the 11-year sunspot cycle, more ozone is produced with the increased UV coming to Earth. This phenomenon can boost the average ozone concentration over the poles by about 4%, but when this is averaged out over the whole earth, the world average ozone increase is about 2%.
Observations since the 1960s have shown that total global ozone levels have decreased by 1-2% from the maximum to the minimum of a typical cycle. However, since downward trends in ozone levels are much larger than 1-2%, particularly at the higher latitudes, the Sun's output cannot be wholly responsible.
Unusual solar activity can cause the ozone levels in the upper stratosphere to be substantially depleted, but since most of the ozone is in the middle stratosphere, the effect on the total ozone column is negligible.
Atmospheric Winds and Ozone
A natural cycle in which prevailing tropical winds in the lower stratosphere vary over a time span of about two years can also influence the amount of ozone in the stratosphere. A change from easterly flow to westerly flow can bring up to a 3% increase in ozone over certain locations, but it is usually cancelled out when the total ozone of the Earth is averaged.
Volcanic Eruptions and Ozone
Volcanic eruptions are one of the few natural things that can have a diminishing effect on the ozone layer. Large eruptions can potentially inject significant quantities of chlorine (via hydrochloric acid - HCl) directly in the stratosphere where the highest concentrations of ozone are found. However, the vast majority of volcanic eruptions are too weak to reach the stratosphere, around 10 km above the surface. Thus, any HCl emitted in the eruption remains in the troposphere where it is quickly dissolved and washed out by rain. [Note that CFCs do not dissolve in water and can therefore reach the stratosphere through atmospheric mixing.] In addition, there is no historical record that shows significant increases in chlorine in the stratosphere following even the most major eruptions.
It is also possible that ice particles containing sulphuric acid from large volcanic eruptions may contribute to ozone loss. When chlorine compounds resulting from the break-up of man-made CFCs in the stratosphere are present, the sulphate particles serve to convert them into more active forms that may cause more rapid ozone depletion. In 1991 Mt. Pinatubo in the Philippines erupted tonnes of dust and gas high into the atmosphere which caused global reductions in the ozone layer for 2 to 3 years. Thus, whilst large volcanic eruptions may increase the rate of stratospheric ozone depletion, it is more probable that the presence of chlorine from man-made CFC emissions is the chief cause of ozone loss in the first instance.