The United Nations-sponsored International Day for the Preservation of the Ozone Layer, Monday, Sep 16, gives scientists the opportunity to remind the global community that, while progress has been made in slowing ozone depletion, there is still work to be done.

International treaties to protect the Earth's atmospheric ozone layer are having a positive impact, conclude the WMO and the United Nations Environment Programme (UNEP) in their latest scientific assessment of the natural shield that protects all living species from the harmful effects of the sun's ultraviolet (UV) rays.

The presence of chlorine in the atmosphere is diminishing, while bromine, another ozone-depleting substance continues to rise, although at a slower rate than the WMO recorded in 1998.

The ozone hole over Antarctica, discovered in the early 1980s, should disappear by the middle of this century, says Mo Lagarde, spokesman for the Geneva-based WMO.

The international community approved the Vienna Convention on the Protection of the Ozone Layer in 1985, and two years later 180 countries signed the Montreal Protocol on Substances that Deplete the Ozone Layer and its subsequent amendments.

These two legal instruments are seen as key for abolishing the production and consumption of ozone-depleting materials. They establish separate phase-out timelines for the industrialised North and the developing South, taking into account that poor countries need more time to accumulate the financial and technological resources to replacement those substances.

The WMO and UNEP have proven in this new evaluation that "the Montreal Protocol works," said Lagarde.

The hole -- or more accurately, the thinning -- in the ozone layer, caused by industrial substances that the Protocol controls, should begin to close the next decade, according to the international agency.

The Protocol's effectiveness is also evident in the upper reaches of the atmosphere, the stratosphere, where the presence of ozone-depleting gases is currently at its peak, or very close to it, said Lagarde.

The recovery of the protective ozone layer by mid-century, according to the predicted timeframe, requires that all nations must comply with the stipulations of the Montreal Protocol and its reforms and that all other related variables remain the same.

For example, another major volcanic eruption, like the 1991 eruption of Mount Pinatubo in the Philippines, could launch particles into that atmosphere that are harmful to the ozone layer, thus requiring a recalculation of the recovery process.

The WMO report notes that ozone depletion and the greenhouse effect, the cause of climate change, are interconnected through shared chemical and physical processes.

For example, the reduction of chlorofluorocarbons (CFCs) in the atmosphere credited to the application of the Montreal Protocol also means a decline in the contribution of those gases to the greenhouse effect.

However, the hydrofluorocarbons (HFCs) used to replace CFCs -- utilised as refrigerants, aerosols and solvent cleaners -- will increase the greenhouse effect, the accumulation of pollutants in the atmosphere that causes global warming.

These substances are produced by industrial sources and their presence in the atmosphere is a consequence of human activity, not natural processes, as proven by analysis of air trapped in snow since the late 19th century, in other words, prior to the widespread industrial use of ozone-depleting chemicals.

But these observations do not allow scientists to assert that the ozone hole has achieved its maximum size, says the WMO. The annual cycle of the hole's appearance is now beginning earlier, in late July or early August, and lasting longer.

In the 1980s, the ozone hole would begin to disappear in early November, but in the 1990s it only did so in the first days of December.

In global terms, in the 1997-2001 period, the ozone layer saw a three-percent decline in what is known as the ozone column, the depth or thickness of the atmospheric layer, compared to the average for 1964-1980, when the ozone hole had not yet been discovered.

But the lowest average ozone column occurred in 1992-1993, when it was recorded at five percent less than in 1964-1980.

In the tropics, the area between 25 degrees latitude North and South of the Equator, there have been no notable trends in the ozone column.

The differences appear in comparing the Northern Hemisphere at the middle latitudes (35 to 60 degrees North) and the same coordinates in the Southern Hemisphere.

From 1997 to 2001, the ozone column in the north was three percent below its pre-1980s average, while in the south it was six percent less for the same periods.

In the Arctic, the loss of ozone is manifest in the winter- spring season and shows marked variations that are linked to the changes in the meteorological conditions in the stratosphere from one winter to the next.

In the 1999-2000 season, characterised by the WMO as "an unusually and persistently cold Arctic stratospheric winter", ozone depletion was significant. In contrast, during the warmer 1998-1999 Arctic winter, there was relatively little depletion.

Three of the last four Arctic winters have been warm, with limited reduction of ozone, while six of the nine previous winters were cold, with greater ozone loss.

Although the depletion of the ozone over the Arctic is quite variable and difficult to predict, WMO scientists believe it is unlikely that the region could see a hole like that over Antarctica to form in the Arctic in the future, said Lagarde.

"The ozone layer will remain particularly vulnerable during the next decade or so, even with full compliance" by all states with the Montreal Protocol and other treaties that protect it, states the WMO report.

According to a UNEP "backgrounder" on the ozone problem, if the Montreal Protocol did not exist, by the year 2050 ozone depletion would have risen to at least 50 percent in the Northern Hemisphere's middle latitudes and 70 percent in the southern middle latitudes, "about 10 times worse than current levels".

The consequences would have meant that people living in those areas would be exposed, respectively, to twice and quadruple the amount of UV radiation.

The catastrophic human health consequences, according to UNEP projections, would have included 19 million more cases of non- melanoma skin cancer and 1.5 million cases of melanoma cancer.