"The ozone layer must be replenished so as to provide long-term protection against harmful UV radiation."
The ozone layer of the atmosphere protects life on earth by filtering out some of the harmful ultraviolet (UV) radiation from the sun. Thinning of this layer therefore poses a threat. In humans, it increases the risk of conditions such as skin cancer, suppression of the immune system and eye cataracts.
Thinning is a result of the release into the atmosphere of substances which destroy ozone. These include chlorinated compounds that are to be found for example in fridges, air conditioning systems and foam plastics.
Since 1987 there has been an international agreement called the Montreal Protocol, which requires signatory states to ban and restrict the use of ozone-depleting substances. The measures taken have been very successful. Thanks to the Montreal Protocol, emissions of these substances are now falling. So too, with certain exceptions, are levels of ozone-depleting gases in the atmosphere. Most current evidence suggests that the ozone layer is no longer thinning, and there are even signs that it is set to increase in thickness again.
What are the challenges?
Many ozone-depleting substances remain in the atmosphere for a long time. Although emissions of most of them have been reduced or halted altogether, it will therefore be several decades before their thinning effect on the ozone layer is eliminated.
International efforts under the Montreal Protocol must continue, to further reduce production and consumption of ozone depleters. It is also important to ensure that new substances of this kind do not end up on the market.
The thickness of the ozone layer is difficult to determine, partly owing to its natural variability. It is also affected by both climate and by levels of certain greenhouse gases in the atmosphere. Rising emissions of nitrous oxide, for example, could delay ozone recovery.