Atmospheric levels of a key ozone-depleting chemical are on the increase but the rise appears to be a symptom of climate change rather than additional sources of the destructive substance, according to international researchers including three from the University of Wollongong.
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Investigations were prompted when scientists identified levels of hydrogen chloride had began rising in 2007 – but only in the northern hemisphere – when they should have been falling because of curbs agreed under the Montreal Protocol to protect the ozone layer.
Hydrogen chloride releases chlorine in the stratosphere, depleting ozone and allowing more ultraviolet radiation to reach the Earth, increasing skin cancer and damaging crops and other species.
Findings based on that satellite observations and model simulations and published in Nature on Thursday rule out any "rogue" source of emissions from undisclosed sources because the abundance of the chemical is falling at other layers of the atmosphere and in the southern hemisphere.
"The overall burden of chlorine is still decreasing," said David Griffith, director of the University of Wollongong's Centre for Atmospheric Chemistry, and a co-author of the report. "It's a good news story about ozone."
It's not so positive news on the climate change front, however, since the increased abundance of chlorine in the northern hemisphere's stratosphere is attributed to a slowdown in atmospheric circulation leading to slower mixing at some levels.
Climate change, through increased greenhouse gas emissions, "is changing the way radiation is absorbed in the atmosphere and distributed, which would drive things such as this circulation," Professor Griffith said.
Although it was beyond the scope of the paper to examine how long the circulation slowdown will last, or other possible consequences, Professor Griffith said the study showed the recovery of the ozone layer wouldbe a slow process, taking decades.
"Our results show that atmospheric variability and perhaps climate change can significantly modify the path towards full recovery," he said. "It will be a bumpy ride rather than a smooth evolution."
Professor Griffith said the work also underscored the general success in tackling ozone depletion and a range of chemicals that were phased out in a matter of years in contract to dealing with global warming.
For ozone, it was a "problem created by man, problem recognised, solution proposed, solution implemented," he said. "For climate change, the culprits have been recognised but no-one's prepared to stop producing [carbon dioxide]."
SMH
