While it does not yet qualify as a regular ozone hole, like those over the two poles, the area has seen a large 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 by 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," the journal said.

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 ground monitoring and from the Total Ozone Mapping Spectrometer, a satellite-borne instrument used to measure global ozone levels.

Chemicals 

Without the ozone layer, crops
are in danger of being destroyed

Ozone, which is formed by an electrical discharge in oxygen, exists in a thin layer in the stratosphere where it helps to filter out ultra-violet light from the sun.

It is broken down by chemicals such as those used as refrigerants and aerosol propellants in the last 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 on 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 that 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, the Nature report said.

*Image of ozone hole courtesy of Nasa