Eruption's threat to the ozone layer

The atmosphere's protective ozone layer could be imperilled by a large volcanic eruption anytime in the next couple of decades, a Nobel prizewinning scientist has warned.

Global warming could pose an additional risk and increase the rate at which ozone was being depleted, said Professor Sherwood Rowland, from the University of California.

He correctly predicted, more than a decade before the hole in the Antarctic ozone layer was discovered, that chlorofluorocarbons (CFCs) could damage the ozone layer.

Professor Rowland, who shared the 1995 Nobel Prize in Chemistry, had some good news, however. Thanks to the 1987 Montreal Protocol banning the production of CFCs, the international community is reducing the rate at which ozone-destroying chemicals are entering the atmosphere.

But over the next 50 years, he believes, it will be a race between the decreasing concentrations and the increasing efficiency of ozone depletion provoked by global warming.

Ozone is formed about 24 kilometres up in the stratosphere and shields the earth's surface from the sun's harmful ultraviolet radiation.

Professor Rowland and Mario Molina (with whom he shared the Nobel Prize) showed in 1974 that this protective layer could be eroded when CFCs, from air-conditioning units, refrigerators, and aerosol cans, reached the stratosphere.

Volcanoes can increase ozone depletion by throwing huge quantities of sulphur dioxide gas into the stratosphere where it forms sulphuric acid aerosols. These aerosol droplets provide reactive surfaces to increase the efficiency with which chlorine from CFCs reacts with and removes the ozone.

Professor Rowland said: "A hundred years ago, aerosols would have had no effect on ozone, because there was no chlorine there to take it out. The worry about low ozone in the next couple of decades is the possibility of volcanic eruptions putting surfaces in there for chlorine to take out ozone more efficiently."