Global Warming Forecast to Delay Ozone Layer Recovery

Darryn Waugh, a professor in the Department of Earth and Planetary Sciences at Johns Hopkins University, and his colleagues reported Thursday that climate change could provoke variations in the circulation of air in the lower stratosphere in tropical and southern mid-latitudes, including Australia and South America.

The circulation changes would cause ozone levels in these areas never to return to levels that were present before decline began, even after ozone-depleting substances have been wiped out from the atmosphere.

In tropical and southern mid-latitudes, Waugh says, "Global warming causes changes in the speed that the air is transported into and through the lower stratosphere. You're moving the air through it quicker, so less ozone gets formed."

Researchers at NASA Goddard Space Flight Center in Greenbelt, Maryland collaborated with Waugh in the study. The team forecast effects on ozone recovery by means of simulations using a computer model known as the Goddard Earth Observing System Chemistry - Climate Model.

On September 12, 2008, the Antarctic ozone hole reached its maximum size for the year. Though larger than it was in 2007, the 2008 ozone hole was still smaller than the record set in 2006. (Image courtesy NASA)

Waugh says this research will help scientists attribute ozone variations to the right agent.

"Ozone is going to change in response to both ozone-depleting substances and greenhouse gases," he says, "If you don't consider climate change when studying the ozone recovery data, you may get pretty confused."

The research is published in the current issue of "Geophysical Research Letters," a publication of the American Geophysical Union.

Dan Lubin, an atmospheric scientist who has studied the relationship between ozone depletion and variations in the ultraviolet radiation that reaches the Earth, says Waugh's findings could cause health problems for people living in the tropics and southern mid-latitudes if ozone levels never return to pre-1960 levels in those regions.

"The risk of skin cancer for fair-skinned populations living in countries like Australia and New Zealand, and probably in Chile and Argentina too, will be greater in the 21st century than it was during the 20th century," says Lubin, who is at Scripps Institution of Oceanography in La Jolla, California and did not participate in the research.

Ozone is a gas which is naturally present in the atmosphere and absorbs harmful ultraviolet radiation from the Sun. In the stratosphere, ozone blocks ultraviolet light that can cause skin cancers, cataracts, and other damage to animals and plants if it reached the surface.

This protective ozone layer has been in decline in the stratosphere since the 1970s due to an increase in atmospheric concentrations of human-made substances such as chlorofluorocarbon and bromofluorocarbon compounds such as refrigerants, solvents, and foam blowing agents.

Since the late 1980s, most countries have adhered to the Montreal Protocol, an international treaty to phase out production of such ozone-depleting substances.

The ozone layer has not grown thinner since 1998 over most of the world, and it appears to be recovering because of reduced emissions of ozone-depleting substances. Antarctic ozone is projected to return to pre-1980 levels by 2060 to 2075.

Not all regions face worse prospects for ozone recovery as a result of climate change, the Johns Hopkins scientists found.

In polar regions and northern mid-latitudes, restoration of ozone in the lower stratosphere will suffer little impact from increasing greenhouse gases, their projections indicate.

In the upper stratosphere, climate change causes a drop in temperatures that slows down some of the chemical reactions that destroy ozone. So, the Johns Hopkins team concludes, recovery might be reached in those parts of the atmosphere earlier than forecast, even decades before the removal of ozone-depleting substances.

While scientists have long suspected that climate change might be altering the dynamics of stratospheric ozone recovery, Waugh's team is the first to estimate the effects of increasing greenhouse gases on the recovery of ozone by region.

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