Ozone depletion — causes and consequences


By Tom Kelly

Ozone depletion in the stratosphere, a layer of the atmosphere that extends from 10 to 50 kilometres above the earth's surface, is caused by the presence of chlorine and bromine. These are released by industrially produced substances that find their way into the stratosphere. Such substances include chlorofluorocarbons (CFCs), halons, carbon tetrachloride, hydrochlorofluorocarbons (HCFCs), methyl chloroform and methyl bromide.

An estimated 20 million tonnes of CFCs alone had been released into the atmosphere by 1989, of which about 15 million tonnes had probably reached the ozone layer by that time. (It takes about five years for CFCs to reach the ozone layer from around sea level). The bulk of the chlorine in the atmosphere now will remain active for more than 50 years, and under present phase-out programs its concentration will continue to increase for years to come.

Levels of chlorine in the atmosphere prior to the introduction of CFCs in 1928 were around 0.6 parts per billion (ppb), which represents a natural equilibrium. In 1984, when chlorine levels reached 2.0 ppb, the ozone hole over Antarctica was detected. By 1990, chlorine levels had reached 3.4 ppb, and estimates at that time suggested that chlorine levels would peak at 4.7 ppb early next century.

According to an article by John Gribbin in the New Scientist of September 18, 1993, the key to the appearance of the ozone hole over the Antarctic in the spring is the low temperatures coming out of winter, combined with sufficient sunlight to trigger photochemical reactions. The importance of the low temperature is that it facilitates the condensation and breaking down of the usually stable molecules that incorporate chlorine, releasing chlorine gas, which actively destroys ozone molecules.

A vital factor in this process is the presence of clouds of particles that serve as the site upon which the chlorine-releasing reactions take place. This function is usually served by polar stratospheric clouds, but the composition of the particles is unimportant. This explains the impact of volcanic eruptions such as that of Mount Pinatubo on stratospheric ozone depletion. Such eruptions provide a profusion of aerosol particles upon which reactions releasing chlorine can occur.

This fact in no way lessens the significance of industrially produced chlorine, since this is still the source of the destructive chlorine. A recent study has shown that chlorine released during some volcanic eruptions as hydrogen chloride doesn't reach the ozone layer because it is "washed out in the rain that always accompanies such eruptions" (New Scientist June 5, 1993).

The significance of low temperatures to ozone destruction also explains why ozone depletion is less advanced in the Arctic. The Arctic stratosphere doesn't get as cold as air over Antarctica. This is partly due to different wind patterns, influenced by mountain ranges in the northern hemisphere.

This partial protection of the northern ozone layer is under threat, however, from the greenhouse effect. John Gribbin reported in the New Scientist of November 28, 1992, that researchers have predicted that the lower stratosphere will cool significantly in coming decades and lead to the appearance of an Arctic ozone hole. Carbon dioxide and other greenhouse gases trap heat closer to the earth's surface. This warms the lower atmosphere, but robs the stratosphere of warmth. This in turn will create the conditions to release chlorine in its ozone-destroying form.

The circumstances upon which the viability of the ozone layer depends are also threatened by another factor. The loss of ozone from the stratosphere means that it is not there to absorb ultraviolet light from the sun. This also leads to a cooling of the stratosphere, further facilitating the release of chlorine and the destruction of ozone.

The problem of ozone depletion thus threatens to get even more out of hand than it is already, and the risks it poses can't all be dealt with by a bottle of sun block and a wide-brimmed hat. Not only does ozone depletion threaten human life through increased incidence of skin cancer. Increased ultraviolet rays as a result of ozone destruction will also damage crops and threaten the basis of the marine food chain, by killing off the phytoplankton that live near the ocean's surface.

By the end of 1992, scientists were measuring global ozone levels at 4% below normal. A Greenpeace London press release last week indicated that according to a group of British scientists, who are members of the Stratospheric Ozone Review Group, 1993 was the worst year on record for ozone depletion.

The fact that ozone depletion is not uniform means that some regions suffer far more than a 4% reduction in protection by the ozone layer, while others, usually close to the equator, suffer less depletion.

A 50% loss of ozone above southern Argentina and southern Chile for a few days in October 1992 led to a doubling of ultraviolet radiation reaching ground level.

Last northern winter and spring brought the lowest ever ozone levels over North America and Europe. Ozone levels in this region declined by 6% between 1969 and 1989, but by 1993 the total loss since 1969 amounted to 14%. In February 1993 Finland and Sweden reported ozone levels 30-40% lower than normal.

Greenpeace points out, "One of the most disturbing aspects of past ozone studies has been the consistent underestimation of the rate and extent of ozone loss. Scientists did not predict the formation of the Antarctic ozone hole. They did not expect the extent of mid-latitude depletion now being measured. Nor did they expect the depletion recently measured in the Arctic."

The lessons to be drawn from this brief look at ozone depletion are that the problem is too serious and too complex to take chances with. The processes we're dealing with require urgent measures to prevent further damage: humanity doesn't have the capacity to repair the sort of global ecological disaster that is looming.

With our environmental security at stake, it is far better to err on the side of minimising our impact on the ozone layer, rather than constantly being surprised that the situation is much more serious than previously thought. If governments are unwilling to adopt this attitude, we will have to pressure them to do so, or replace them with governments that will.