posted 04-16-2002 10:58 PM            

Posted on Tue, Apr. 16, 2002

Ozone layer study predicts an unexpected depletion
GASES MAY UNDERMINE PARTIAL RECOVERY
By Glennda Chui
Mercury News

Some of the same chemical changes in the atmosphere that are responsible for the greenhouse effect could also worsen the depletion of the Earth's protective ozone layer over North America, Europe and parts of Asia, scientists in Australia report.

Their work focuses not on the famous ``ozone hole'' that forms over Antarctica, but on the health of the ozone layer over the mid-latitudes of the Northern Hemisphere, where most of the world's population lives.

Here, ozone depletion has been much less dramatic -- a loss of about 4 percent to 5 percent since 1980. With a global agreement in place that bans the production of chlorine compounds that destroy ozone, scientists thought the ozone layer would return to 1980 levels by the middle of this century.

But they did not consider the effects of two gases -- methane and nitrous oxide -- that contribute to the greenhouse effect by trapping heat against the Earth's surface, said scientists from the CSIRO Telecommunications and Industrial Physics laboratory in Australia. CSIRO stands for Commonwealth Scientific and Industrial Research Organization, and is funded by the Australian government.

When these gases are taken into account, the team reported, ozone levels could partly recover in the next 50 years, but then take a sharp downturn through the end of the century.

``There's a lot of talk about recovery, but very little talk about partial recovery and then getting worse again. That's why we thought it was important,'' said Ian C. Plumb, an atmospheric chemist who led the study.

He was quick to add that this finding, published in the Feb. 28 issue of Geophysical Research Letters, is not an ironclad prediction.

Missing variables

Like most such forecasts, it's based on a computer model that takes into account only some of the many variables that shape global climate. This one considers temperatures and interactions among chemicals in the atmosphere, but it leaves out such crucial factors as wind patterns.

``There's no single model that does everything,'' said Martyn Chipperfield of the University of Leeds in England, who is helping to assess the state of the ozone layer for the World Meteorological Organization. ``Our basic physical understanding of how the atmosphere works is lacking.''

Further, it's essentially impossible to see that far into the future.

People don't fully understand why levels of methane and other gases are fluctuating today, said Singh Hanwant, an atmospheric scientist at NASA's Ames Research Center in Mountain View. ``I don't think we have the capacity to make 100-year predictions,'' he said, adding that he felt the whole exercise was ``rather speculative.''

An important step

But other researchers said the study was an important step that could shake scientists out of their complacency when it comes to the ozone layer.

``The focus has been on polar regions, and how it might get worse before it gets better,'' Chipperfield said. ``What this paper points out is that after 2050, it might get worse.''

Ozone consists of three atoms of oxygen joined together. At ground level, it's an irritating pollutant, a prime component of smog; but high in the atmosphere, it forms a protective layer that screens out ultraviolet light, which can harm wildlife and cause skin cancer in people.

When researchers found that ozone levels over Antarctica were dropping sharply each spring, they scrambled to find the cause. The prime culprits, it turned out, were chlorine compounds used as refrigerants and as propellants in aerosol cans, among other things. Their production is being phased out under the 1987 Montreal Protocol.

Other culprits

But methane and nitrous oxide, whose levels have been increasing in the atmosphere, would not be so easy to control.

Nitrous oxide is given off by fertilized crops and by feedlots where livestock are fattened, said Pamela Matson, a professor of environmental studies at Stanford University. ``They're very closely linked to growing food -- a very essential human activity.''

Methane leaks from natural gas lines, landfills and waste treatment plants and is given off by cattle and flooded farm fields. ``All of these things, the source is likely to increase over time as the human population increases but also our consumption increases,'' Matson said.

While researchers have developed ways to reduce those emissions -- by applying fertilizer more carefully or giving special feed to cattle, for instance -- they have not been widely adopted, she said.

Of various scenarios tested by the Australian group, the one most damaging to ozone is one in which levels of nitrous oxide increase, but methane levels go down. Researchers said this shows just how complex the chemistry -- and the solutions -- can be.

Susan Solomon, a senior scientist in the National Oceanic and Atmospheric Administration's aeronomy lab in Boulder, Colo., said, ``It's very interesting. It's going to highlight the need to think about nitrous oxide and methane -- where they're coming from, and where they're going in the future.''