It turns out that the ozone hole may not be completely healed.
The ozone layer, which protects Earth from the
Sun’s ultraviolet radiation, has been severely damaged by chlorofluorocarbons
and other harmful compounds, with a large hole forming each year over
Antarctica. Although the Montreal Protocol banned such compounds in 1989, it
will take many years for the ozone layer to fully recover, according to Anne
Douglass, Natalya Kramarova, and Susan Strahan of NASA’s Goddard Space Flight
Center. They presented their study at
the American Geophysical Union’s annual meeting in San Francisco.
The ozone hole forms every year over
Antarctica; however, its extent varies from one year to the next due to
fluctuations in temperature in the lower stratosphere over Antarctica and winds
that transport ozone from the tropical latitudes around the equator to the
poles. The largest hole ever, wider than the entire Antarctic continent,
occurred in 2006. The hole in 2011 was nearly as extensive. In contrast, the
2012 hole was far smaller, even though it formed at the same rate as those of
previous years; Douglass and team determined that this hole disappeared more
quickly than normal, thanks to a greater quantity of ozone brought to the
southern polar region by winds.
Unfortunately, the small 2012 hole does not
signal a smooth decline in the size of the annual ozone gap. Levels of
atmospheric chlorine have been falling since the Montreal Protocol was enacted,
but they will have to decline to pre-1990 levels before the ozone layer can
begin to stabilize between 2015 and 2033; until then, the ozone hole will
continue to vary dramatically year to year, as it did from 2011 to 2012, for
example. As chlorine continues to decline, the annual holes will become
progressively smaller until complete recovery is achieved sometime between 2058
and 2090.
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