Article: Earth's ozone depletion is finally slowing

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http://www.newscientist.com/news/news.jsp?id=ns99994010

 

 

Earth's ozone depletion is finally slowing

 

 

17:05 30 July 03

 

NewScientist.com news service

 

 

Almost 30 years after it was first reported that pollutants were

destroying the Earth's protective ozone layer, there is clear evidence

that the global CFC ban has had an impact.

 

For the first time, it has been shown that the rate of ozone depletion

in the upper stratosphere - 35 to 45 kilometres up - is slowing down.

" This is the beginning of a recovery of the ozone layer, " says Michael

Newchurch, at the University of Alabama in Huntsville, who led the new

research.

 

Between 1997 and 2000, the average rate of ozone depletion slowed by the

equivalent of seven per cent per decade. But the authors say that it

will take at least 40 years before the depletion stops and recovery

begins. " Ozone is still depleting but just not as fast. We are still

decades away from total ozone recovery, " Newchurch says.

 

The researchers used data from three NASA satellites and three

international ground stations. The satellites measured the sunlight that

filtered through the atmosphere at sunset and sunrise. Because ozone and

other constituents absorb light at known wavelengths, the researchers

were able to calculate the amount of ozone and other gases.

 

 

Refrigerators and aerosols

 

 

" This is very important research because it is the first time that a

slowing has been shown in an region of the atmosphere where chlorine has

been demonstrated to affect the ozone trend, " says Neil Harris, from the

European Ozone Research Coordinating Unit in Cambridge, UK.

 

" It's evidence that agrees with a 2002 computational model that

predicted rate of ozone depletion would slow, he says. Unlike Newchurch,

Harris thinks ozone depletion is now reaching a plateau and will soon

start recovering: " Ozone should be at pre-1980 levels by 2020. "

 

The role of CFCs in ozone depletion was proved in 1974 - the scientists

involved later won the Nobel Prize for chemistry. The chemicals were

commonly used in refrigerants and aerosol propellants until the 1987

Montreal Protocol cut and then banned their use. The ozone layer

protects life on Earth from the Sun's harmful ultraviolet rays.

 

The time lag between the CFC ban and the start of ozone recovery

revealed by the new data is due to the slowness of the cycle involved.

CFCs last between 45 to 100 years in the atmosphere.

 

 

Tipping the balance

 

 

But while the destructive effects of CFCs appear to have been conquered,

the global warming almost certainly induced by rising levels of carbon

dioxide could tip the balance the other way, says Harris.

 

" By about 2030 to 2040 we predict a 10 per cent increase in ozone

levels, because of these climatic effects, " he told New Scientist. " The

greenhouse gas will cause a rise in low-atmospheric temperature, but a

chilling of the upper stratosphere. The lower temperature there will

slow down the chemical reaction that depletes ozone, so ozone levels

will build up. "

 

He adds: " Scientists have been so concerned about ozone depletion that

they haven't really thought about what effect such a high ozone level

could have - life on Earth needs a certain amount of UV. "

 

Journal reference: Journal of Geophysical Research

(DOI:10.1029/2003JD003471)

 

 

 

Gaia Vince