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, 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.

"When low-ozone air currents in the lower layer enter the upper air layer, the overall ozone density is reduced."

Scientists have known about the thinning of the ozone density in the area at least since early this decade, but the report in the journal offers the most solid scientific evidence so far.

The article is based on comprehensive research and analysis of data from both ground monitoring and the Total Ozone Mapping Spectrometer, a satellite-borne instrument used to measure global ozone levels.

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. It was ravaged by chemicals such as those used as refrigerants and aerosol propellants in the 20th century.

Without the ozone layer, plant and human DNA can be damaged, causing destruction of crops and initiating skin cancer.

The Chinese findings came as a study published Thursday in the British journal Nature said optimism that the ozone layer may be restored within the next couple of decades was premature.

Recent assessments that suggest ozone erosion had now permanently stabilized fail to take into account the potential for volcanic eruptions, solar storms and other natural phenomena to distort the picture, the Nature report said.

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