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Ozone layer may take decades to mend
Though ozone-depleting chemicals such as chlorofluorocarbons are beginning to fade from the atmosphere, there's no telling when the ozone layer will begin to recover, NOAA scientists say.
"Depletion has occurred mainly over the last two decades," said Elizabeth Weatherhead, a University of Colorado scientist at NOAA's Air Resources Laboratory.
Most scientists believe we're living with the lowest ozone-layer levels ever recorded, she added. But there's hope on the horizon.
"We should expect to be able to detect recovery in most regions of the world within the next 15 to 45 years," said Weatherhead. "That's based on full compliance with the Montreal Protocol and its amendments and no other complicating factors such as major volcanic eruptions or enhanced stratospheric cooling."
Ozone is the thin band around the Earth that protects living organisms from harmful ultraviolet radiation. The Montreal Protocol is an international agreement aimed at phasing out ozone-depleting chemicals.
Most developed countries have adopted the treaty, but many developing countries have not complied for economic reasons. Most scientists agree that the ozone layer will fully recover only if all countries adopt the protocol and stop producing ozone-depleting substances.
Ozone depleting chemicals such as CFCs, halons and other substances commonly found in coolants, foaming agents, fire extinguishers and solvents linger in the atmosphere for different periods of time. It can take days or even years for some chemicals to reach the stratosphere. Though many CFCs have been banned, their impact is only starting to affect the ozone layer.
Weatherhead and colleagues analyzed predictions from a NASA chemical model and forecasts from models used in the World Meteorological Organization's 1998 ozone assessment to estimate how much time is needed to detect ozone recovery in mid-latitudes, the most populated areas of the world.
The results of the study, published in the Sept. 16 issue of the Journal of Geophysical Research, suggest that it will be several decades before an increase in overhead ozone, also known as total column ozone, can be detected.
Though there are other ways to detect signs of ozone recovery, total column ozone measurements provide a complete picture of how much ozone is present over a region.
"The reason why total column ozone is so important is because it's the single significant factor that determines how much UV radiation gets through the atmosphere, meaning it's the most important factor for humans and the environment," said Weatherhead.
"We can't really expect much of an improvement in the UV levels reaching the biosphere until we can detect an increase in total column ozone," said Weatherhead.
The first place that recovery of total column ozone will likely appear is in the Southern Hemisphere near New Zealand, southern Africa and southern South America, she said.
Because it has more landmass than the Northern Hemisphere, the Southern Hemisphere is experiencing more ozone depletion and thus has more to recover from.
"The most encouraging factor is that we do measure ozone depleting substances in the atmosphere, and it shows they're declining," said Weatherhead. "If things keep going in that direction, hopefully the ozone layer will get better. It depends on if countries maintain what they say they will maintain in terms of emissions."
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