Recent assessments which suggest ozone erosion has now permanently stabilised fail to take into account the potential for volcanic eruptions, solar storms and other natural phenomena to distort the picture, it says.

Ozone, a molecule of oxygen, exists in a thin layer in the stratosphere, where it helps to filter out ultra-violet light from the Sun. Without it, plant and human DNA can be damaged, causing destruction of crops and initiating skin cancer.

In the early 1970s, scientists first theorised that chlorine compounds, used as refrigerants and aerosol propellants, could eat away at this vital shield.

In the following decade, observations proved them right, with the discovery of a large and growing "ozone hole" at the planet's poles.

The scare led to the Montreal Protocol in 1989, one of the world's few successful environmental treaties, which outlaws ozone-depleting substances.

Research published in 2000 and 2002 recorded that, following the ban, there had been no acceleration in ozone loss above any region for the previous decade, and this could point to a recovery in the early 21st century.

But the latest study, published on Thursday in the British science journal Nature, warns that this conclusion could be hasty as it is based on short-term data.

It notes that the data comes from a period of calm in major volcanic eruptions. For instance, Mount Pinatubo in the Philippines in 1991 and El Chichon in Mexico in 1982 disgorged ozone-destroying sulphates into the upper atmosphere.

And, it adds, big bursts of solar activity, which generally run in 11-year cycles, can also cause local depletions of the ozone layer. The last solar peak was in 1999-2003.

Neither of these big events has been properly factored in, which suggests that the image of stabilisation, and the prospect of early restoration, could be wrong.

The paper, authored by Elizabeth Weatherhead of the University of Colorado at Boulder and Signe Bech Andersen of the Danish Meteorological Institute, says that only towards the end of this decade will we get a fuller picture about the benefits of Montreal.

In any cause, Earth's atmospheric chemistry has been altered by greenhouse gases and other factors -- and as a result, ozone concentrations may stabilise at lower (or possibly even higher) levels than in 1980, the benchmark year, they warn.

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