November 3, 2008 NOAA

The ozone hole over Antarctica,
which fluctuates in response to temperature and sunlight, grew to the size of
North America in a one-day maximum in September that was the fifth largest on
record, since NOAA satellite records began in 1979.

High resolution (Credit: NOAA)
NOAA scientists, who have monitored the ozone layer since 1962, have determined
that this year’s ozone hole has passed its seasonal peak for 2008. Data is
available at

The primary cause of the ozone hole is human-produced compounds called
chlorofluorocarbons, or CFCs, which release ozone-destroying chlorine and
bromine into the atmosphere. Earth’s protective ozone layer acts like a giant
umbrella, blocking the sun’s ultraviolet-B rays. Though banned for the past 21
years to reduce their harmful build up, CFCs still take many decades to
dissipate from the atmosphere.
According to NOAA scientists, colder than average temperatures in the
stratosphere may have helped play a part in allowing the ozone hole to develop
more fully this year.
“Weather is the most important factor in the fluctuation of the size of the
ozone hole from year-to-year,” said Bryan Johnson, a scientist at NOAA’s Earth
System Research Laboratory in Boulder, which monitors ozone, ozone-depleting
chemicals, and greenhouse gases around the globe. “How cold the stratosphere is
and what the winds do determine how powerfully the chemicals can perform their
dirty work.”

High resolution (Credit: NOAA)
NASA satellites measured the maximum area of this year’s ozone hole at 10.5
million square miles and four miles deep, on Sept. 12. Balloon-borne sensors
released from NOAA’s South Pole site showed the total column of atmospheric
ozone dropped to its lowest count of 107 Dobson units on Sept. 28. Dobson units
are a measure of total ozone in a vertical column of air.
In 2006, record-breaking ozone loss occurred as ozone thickness plunged to 93
Dobson units on Oct. 9 and sprawled over 11.4 million square miles at its peak.
Scientists blamed colder-than-usual temperatures in the stratosphere for its
unusually large size. Last year’s ozone hole was average in size and depth.
Starting in May, as Antarctica moves into a period of 24-hour-a-day darkness,
rotating winds the size of the continent create a vortex of cold, stable air
centered near the South Pole that isolates CFCs over the continent. When spring
sunshine returns in August, the sun’s ultraviolet light sets off a series of
chemical reactions inside the vortex that consume the ozone. The colder and more
isolated the air inside the vortex, the more destructive the chemistry. By late
December the southern summer is in full swing, the vortex has crumbled, and the
ozone has returned-until the process begins anew the following winter.
The 1987 Montreal Protocol and other regulations banning CFCs reversed the
buildup of chlorine and bromine, first noticed in the 1980s.
“These chemicals-and signs of their reduction-take several years to rise from
the lower atmosphere into the stratosphere and then migrate to the poles,” said
NOAA’s Craig Long, a research meteorologist at NOAA’s National Centers for
Environmental Prediction. “The chemicals also typically last 40 to 100 years in
the atmosphere. For these reasons, stratospheric CFC levels have dropped only a
few percent below their peak in the early 2000s.”
“The decline of these harmful substances to their pre-ozone hole levels in the
Antarctic stratosphere will take decades,” said NOAA atmospheric chemist Stephen
Montzka of the Earth System Research Laboratory. “We don’t expect a full
recovery of Antarctic ozone until the second half of the century.”
NOAA understands and predicts changes in the Earth’s environment, from the
depths of the ocean to the surface of the sun, and conserves and manages our
coastal and marine resources.