Antarctic Ozone Hole Smaller But Not Recovering

Antarctic Ozone Hole Smaller But Not Recovering



October 2007

 The Antarctic ozone hole 
this year is relatively small, but its size is not a sign of ozone 
recovery, the World Meteorological Organization, WMO, said Thursday. 
Instead, the smaller size of the ozone hole is related to the mild 
temperatures in the Antarctic stratosphere during the 2007 southern 
hemispheric winter. 
Since 1998, only the ozone holes of 2002 and 2004 have been smaller than 
this year's hole, said the UN's climate and weather agency.

The ozone hole over Antarctica has shrunk 30 percent as compared to last 
year's record size. Measurements made by the European Space Agency's 
Envisat satellite show that this year's ozone loss peaked at 27.7 million 
metric tons, compared to the 2006 record ozone loss of 40 million tonnes. 
The 2007 ozone hole is relatively small both in terms of area - about 25 
million square kilometers - and the amount of ozone destruction, which 
occurs when ozone reacts with chlorine, bromine, nitrogen, hydrogen, and 
oxygen gases. 
Most ozone resides between 10 and 40 kilometers above the Earth's surface. 
This atmospheric region is called the stratosphere and it contains about 
90 percent of all the ozone in the atmosphere. 
"The stratosphere still contains more than enough chlorine and bromine to 
cause complete ozone destruction in the 14 to 21 kilometer altitude 
range," the WMO said. 
Scientists emphasize that the smaller ozone hole this year is not a sign 
of recovery. 
"Although the hole is somewhat smaller than usual, we cannot conclude from 
this that the ozone layer is recovering already,” said Ronald van der A, a 
senior project scientist at Royal Dutch Meteorological Institute. 
"This year's ozone hole was less centered on the South Pole than in other 
years," he said, "which allowed it to mix with warmer air, reducing the 
growth of the hole because ozone is depleted at temperatures less than -78 
degrees Celsius." 
During the southern hemisphere winter, the atmospheric mass above the 
Antarctic continent is kept cut off from exchanges with mid-latitude air 
by prevailing winds known as the polar vortex. This leads to very low 
temperatures, and in the cold and continuous darkness of this season, 
polar stratospheric clouds are formed that contain chlorine.

As the polar spring arrives, the combination of returning sunlight and the 
presence of polar stratospheric clouds leads to splitting of chlorine 
compounds into highly ozone-reactive radicals that break ozone down into 
individual oxygen molecules. 
A single molecule of chlorine has the potential to break down thousands of 
molecules of ozone, scientists say. 
First recognized in 1985, the Antarctic ozone hole is caused by the 
presence of ozone destroying gases in the atmosphere such as chlorine and 
bromine. These originate from manufactured products like the refrigerants 
chlorofluorocarbons, CFCs. They have still not vanished from the air but 
are on the decline as they are banned under the Montreal Protocol, signed 
in 1987. 
The Antarctic ozone hole typically persists until November or December, 
when the winds surrounding the South Pole, known as the polar vortex, 
weaken, and ozone-poor air inside the vortex is mixed with ozone-rich air 
outside it. 
The amount of ozone-depleting gases reached a maximum in the Antarctic 
stratosphere around the year 2000. This amount is now declining slowly at 
a rate of about one percent per year. 
The stratosphere is expected to contain enough chlorine and bromine to 
cause severe ozone holes for another 10 to 20 years, the WMO says. 
During this period, the severity of the ozone hole will be determined by 
the meteorological conditions of the stratosphere during the Antarctic 
winter. These conditions are related to global warming, says the WMO. 
Increasing amounts of greenhouse gases in the atmosphere will lead to 
lower temperatures in the stratosphere. This increases the risk of severe 
ozone holes in upcoming decades. 
Ozone in the stratosphere absorbs some of the Sun's biologically harmful 
ultraviolet radiation, a beneficial role. 
By contrast, excess ozone at Earth's surface that is formed from 
pollutants is considered bad ozone because it can be harmful to humans, 
plants, and animals. 
For more basic information about the ozone hole, visit: 
http://www.theozonehole.com/ozonedestruction.htm 

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