Ozone depletion
Since the ozone layer absorbs ultraviolet radiation, which in turn causes skin cancer, ozone depletion has been cause for concern, particularly in the Southern Hemisphere.
Ozone in the Earth's atmosphere is generally created by ultraviolet light striking oxygen molecules, which consist of two oxygen atoms (O2), creating two single oxygen atoms, known as atomic oxygen. The atomic oxygen then combines with a molecule of O2 to create ozone, O3. The ozone molecule is also unstable and when hit by ultraviolet light it splits into a molecule of O2 and an atom of atomic oxygen, a continuing process called the ozone-oxygen cycle. Ozone can be destroyed by atomic chlorine, fluorine or bromine in the atmosphere. These elements are found in certain stable compounds, especially chlorofluorocarbons (CFCs), which may find their way to the stratosphere and there be liberated by the action of ultraviolet light. Most importantly, the chlorine atoms so generated destroy ozone molecules in a catalytic cycle. In this cycle a single chlorine atom would keep on destroying ozone forever, were it not for reactions that remove chlorine atoms from this cycle by forming reservoir species such as hydrochloric acid and chlorine nitrate. The reactivation of atomic chlorine from these reservoir species is normally slow, but is enhanced by the presence of polar stratospheric clouds which appear during arctic winters, leading to a strong seasonal cycle in ozone hole formation.
Substantial reductions of up to 50% in the ozone column observed in the austral (i.e. southern hemispheric) spring over Antarctica and first reported in 1985 (Farman et al 1985) are continuing (SORG 1990).
Coupled with this there has been a statistically significant downward trend in wintertime total ozone over the northern hemisphere of about 2-3% per decade for the past 30 years, although summertime ozone levels have remained approximately constant (Frederick 1990).
Most atmospheric scientists and environmental advocacy groups say that human-made substances, particularly Chlorofluorocarbons (CFCs), cause the ozone depletion, because a rise in CFC production has accompanied the ozone depletion and because a plausible chemical mechanism for CFC's role in ozone depletion has been proposed.
As a result, a worldwide ban on CFCs, the Montreal Protocol, was signed and entered into force in 1989.
Some atmospheric scientists (for instance Fred Singer, founder of SEPP and also a global warming skeptic) and industry-sponsored advocacy groups question or completely deny a link between CFCs and ozone depletion [1]. It is fairly common to see completely nonsensical arguments proposed to "prove" that CFCs cannot cause ozone depletion - for example, that they are heavier than air and so cannot reach the stratosphere. [1] (Think about it for a moment: if this were true, what would you be breathing?).
Links:
Observations
Explanations