- Stratospheric ozone has decreased since l 979 by about 5.4 percent at northern mid-latitudes in winter and spring;
about 2.8 percent at northern mid-latitudes in summer and fall; and abeut 5.0 percent at southern mid-latitudes on a
year-round basis. There is no discernible trend in the tropics and sub-tropics.
- Every springtime in Antarctica (fall in the northern hemisphere) more than 60 percent of stratospheric ozone is de-
stroyed for a period of several months, creating what is commonly known as the hole in the ozone layer.
- Stratospheric ozone levels in the Arctic region have been unusually low in six of the last nine years in late winter and
spring, with typically a l 5 percent depletion.
Underlying causes of change
- Human activities have caused a six-fold increase in the abundance of stratospheric chlorine, and a smaller increase in
stratospheric bromine. These increases have been caused through the production and emission of chlorofluorocarbons
(used as aerosel propellants, solvents, air-conditioning fluids, and refrigerants), halons (used as fire retardants), and
methyl chloroform and carbon tetrachloride (used as solvents).
- The annual springtime "Antarctic ozone hole" is caused by anthropogenic chlorine- and bromine-containing chemicals.
- The weight of scientific evidence suggests that the observed mid-latitude loss of ozone is, in large part, due to anthropo-
genic chlorine- and bromine-containing chemicals.
Projected impact of human activities on the ozone layer
- Stratospheric ozone depletion is projected to peak within the next several years because of the effectiveness of the
Montreal Protocol and subsequent amendments and adjustments. The expected depletion is about 6 percent at north-
ern mid-latitudes in winter and spring, about 3 percent at northern mid-latitudes in summer and fall, and about 5
percent at southern mid-latitudes on a year-round basis. These changes would be accompanied by increases in ground-
level ultraviolet radiation of 7 percent, 4 percent, and 6 percent, respectively.
- Assuming full compliance with the Montreal Protocol and subsequent amendments and adjustments, the ozone layer in
Antarctica and at mid-latitudes should fully recover by the middle of the next century. Thus important progress has been
made in this area.
Social and economic consequences of projected changes
- Stratospheric ozone depletion leads to an increase in ground-level ultraviolet radiation, which can cause adverse conse-
quences for human health (melanoma and non-melanoma skin cancer, eye cataracts and possible suppression of the
human immune system), ecological systems (loss of productivity in terrestrial and aquatic ecosystems), air quality (in-
crease in oxidant levels), and accelerated degradation of materials.
Technologies, policies, and measures to mitigate the projected changes
- Long-lived chlorofluorocarbons are being replaced by shorter lived halocarbons, which are environmentally more be-
nign or by non-halogen-containing chemicals. The substitutes include, hydrochlorofluorocarbons (HCFCs),
hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs).
- However, even these shorter-lived chemicals are only transitional substitutes because the HCFCs still lead to ozone
destruction, and all of them contribute to global warming.
Status of international agreements
- The Vienna Convention for the Protection of the Ozone Layer was signed in 1985, the Montreal Protocol on Substances
that Deplete the Ozone Layer was signed in 1987 and was amended and adjusted in London in 1991, Copenhagen in
1993, and Vienna in 1996.
- The production and consumption of all chlorofluorocarbons, carbon tetrachloride, methyl chloroform, and halons has
been banned in developed countries as of January l 996, and is to be banned by the year 2010 in developing countries.
- Control measures for other halocarbons, such as methyl bromide and the HCFCs, have also been negotiated.
"Protecting Our Planet, Securing Our Future" UNEP / U.S. NASA / World Bank, 1997