– Chlorine-based aerosols, especially
and other halon
gases, are the principal agents of ozone depletion. From the 1930s until the 1980s, CFCs were
used all over the world and widely dispersed through the atmosphere.
Almost every year since its discovery in 1985, the Antarctic ozone hole has grown. In
2000 the region of ozone depletion covered 28.3 million km³ (larger than North America). Since
then, the maximum size of the hole seems to be shrinking.
Now, however, this phenomenon seems to be spreading to other parts of the world.
About 10% of all stratospheric ozone worldwide has been destroyed in recent years, and levels
over the Arctic have averaged 40% below normal. Ozone depletion has been observed over the
North Pole as well, although it is not as concentrated as in the south.
– The discovery of stratospheric ozone losses brought about a remarkably
quick international response. In 1987 an international meeting in Montreal, Canada produced the
Montreal Protocol, the first of several major international agreements on phasing out most of the
CFCs by 2000. As evidence accumulated, showing that losses were larger and more widespread
than previously thought, the deadline for the elimination of all CFCs (halon, carbon tetrachloride,
and methyl chloroform) was moved up to 1996, and a $500 million fund was established to assist
poorer countries in switching to non-CFC technologies. Fortunately, alternatives to CFCs for most
uses already exist. The first substitutes are hydrochlorofluorocarbons (HCFCs), which release
much less chlorine per molecule. Eventually, scientists hope to develop halogen-free molecules
that work just as well and are no more expensive than CFCs.
The Montreal Protocol is often cited as the most effective international environmental
agreement ever established. There is some evidence that the CFC ban is already having an
effect. CFC production in most industrialized countries has fallen sharply since 1989, and CFCs
are now being removed from the atmosphere more rapidly than they are being added. In 40 years
or so, stratospheric ozone levels are expected to be back to normal. Unfortunately, there may be
a downside to stopping ozone destruction. Ozone is a potent greenhouse gas. Some models
suggest that lower ozone levels have been offsetting the effects of increased CO2. When ozone
is restored, global warming may be accelerated. As so often is the case, when we disturb one
environmental factor, we affect others as well.
The Montreal Protocol bound signatory nations not to purchase CFCs or products made