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An international agreement to limit ozone-depleting chemicals appears to be helping the ozone layer to recover, but it is unlikely to be restored to pre-1980 levels, a new study suggests.

Ozone in the Earth's stratosphere filters out ultraviolet light from the sun, and helps to protect DNA from damage.

Ozone depletion is linked to a higher risk of skin cancer and cataracts in humans, and harmful effects on plants.

Worries about ozone layer not yet over: Report

But the latest study, published today 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.

Giant ozone hole may be forming over Tibet, experts warn

Chinese scientists have warned a 2.5-million-square-kilometer (one-million-square-mile) ozone hole may be forming over the Tibetan plateau.

While it does not yet qualify as a regular ozone hole, like the ones over the two poles, the area has seen a dramatic drop in ozone density in recent years, the Xinhua news agency said on Thursday, citing China's Scientific Report journal.

The decrease in ozone over the plateau was caused by atmospheric air movements rather than the global greenhouse effect, Xinhua quoted the journal as saying.

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