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.
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 online.
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."
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."
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