The ozone hole over Antarctica is starting to decrease in severity, the World Meteorological Organization (WMO) says.
In a report to the Antarctic Treaty Consultative Meeting under way here WMO said, however it may be five years before a ``statistically significant trend in the weakening of the ozone hole may emerge''.
Ozone, which blocks cancer causing solar rays, has been depleted by man-made gases and volcanic gases. Three British scientists discovered the formation of the hole over Antarctica in 1985.
WMO said the great destruction of ozone over any region of the world occurs of Antarctica during spring amounting to a ``large and rapid depletion'' of the ozone layer.
There stratospheric ozone layer represents a layer of pure ozone only about three millimeters (0.1 inch) thick. It is actually spread through the zone and is at its greatest concentration near the 17 kilometers (10.5 miles) above sea level.
In recent years the level of ozone in early October was reduced 70 percent of average total ozone in years before the discovery of the ozone hole.
WMO said extremely cold temperatures within the polar vortex lead to formation of polar stratospheric clouds (PSCs) over large areas in early winter.
The clouds are the key to the severe ozone depletion. The destruction of ozone comes about because of chemical reactions occurring on the surface of ice crystals in PSCs which release reactive chlorine that destroy ozone.
``The chemical reactions are induced by the return of sunlight after the winter polar darkness.''
WMO said the greatest depth of the ozone hole was recorded in 1993 and the largest area of the hole in 1994 when it was 24 million square kilometers (10 million square miles).
The latest hole, that in Spring 1996, was distinguished by its early development, forming around August last year. It was 22 million square kilometers (9 million square miles) in area.
``In 1996 the number of days where the ozone hole covered an area of greater than 10 million square kilometers (four million square miles) was about 80. This compares with 25 days in 1985.''
Balloon flights showed ``almost complete'' annihilation of ozone between 16 and 22 kilometers (10 and 14 miles).
The lowest October mean total column ozone above the British Antarctic station at Halley Bay occurred in 1993 and they have recorded slight increases in ozone levels in subsequent years.
``It is possible that the ozone hole is starting to decrease in severity on a long term basis,'' WMO said. ``However, it is likely to be five years or more before a statistically significant trend in the weakening of the ozone hole may emerge.''
The 1991 eruption of Mount Pinatubo in the Philippines had illustrated how volcanically induced changes can influence climate. The polar vortex around Antarctica was ``easily penetrated'' by aerosols thrown out by Pinatubo, leading to enhanced ozone depletion.
But WMO said human-made gases ``provide 'fingerprints' of climate change which are large enough to be observed with a high degree of confidence''.
It said the ozone hole was caused by human-made gases.
``Stratospheric chlorine levels are expected to reach a maximum before the end of this decade, because tropspheric chlorine levels have already reached their maximum in 1992 and the impact of this should be seen in the stratosphere, with a lag of about five years.''
WMO said if the Montreal Protocols on limiting ozone depleting gases are observed the stratospheric chlorine should reach its maximum between 2000 and 2005.