For millions of years the basic atmospheric composition is believed not to have changed, although some rare compounds such as carbon dioxide are known to have varied. The ozone molecules concentrated mainly between altitudes of 15 to 35 km have determined the temperature structure of the stratosphere and by absorbing the harmful ultraviolet radiation have safeguarded on this planet. But over the past half-century, humans have placed the ozone layer in jeopardy. Unwittingly we have released into the atmosphere chemicals that are destroying part of the life -protecting ozone layer and thus upsetting the delicate balance established by nature.
Complex chemical reactions involving man-made chlorofluorocarbons facilitate destruction of the stratospheric ozone, most dramatically demonstrated in Antarctica, during spring. The ozone hole at the time of its maximum spread, in September early October covers an area twice greater than Europe.
More disturbing, however, is the discovery of ozone decline over middle latitudes, where a large percentage of the world's population resides. In the northern latitude major ozone decline was observed during February and March in 7 of the last 11 years.
Since 1957, WMO has played a major role in arranging for systematic measurements of the ozone, and communicating the potential problem for ozone depletion. The first international statement warning of the danger for ozone depletion was issued by the WMO Executive Committee in 1975 and since 1976 WMO Global Ozone Research and Monitoring Project was launched. The data collected and analysed by WMO were essential for coordinating preparations of authoritative ozone assessments, which surfed as base for international actions to safeguard the ozone layer. For example the Vienna Convention for the Protection of the Ozone Layer in 1985 or its Montreal Protocol in 1987 were adopted with active participation by WMO.
Currently WMO's Global Atmosphere Watch (GAW) makes measurements of ozone levels at 160 stations around the world. Ozone data are collected at the WMO Ozone and Ultraviolet Data Centre, operated by Canada.