NASA and National Oceanic and Atmospheric Administration (NOAA) scientists
report this year's ozone hole in the polar region of the Southern Hemisphere
has broken records for area and depth.
The ozone layer acts to protect life on Earth by blocking harmful
ultraviolet rays from the sun. The "ozone hole" is a severe depletion of the
ozone layer high above Antarctica. It is primarily caused by human-produced
compounds that release chlorine and bromine gases in the stratosphere.
"From September 21 to 30,
the average area of the ozone hole was the largest ever observed, at 10.6
million square miles," said Paul Newman, atmospheric scientist at NASA's
Goddard Space Flight Center, Greenbelt, Md. If the stratospheric weather
conditions had been normal, the ozone hole would be expected to reach a
size of about 8.9 to 9.3 million square miles, about the surface area of
North America.
From September
21-30, 2006 the average area of the ozone hole was the largest ever
observed, at 10.6 million square miles. This image, from Sept. 24, the
Antarctic ozone hole was equal to the record single-day largest area of
11.4 million square miles, reached on Sept. 9, 2000. Satellite instruments
monitor the ozone layer, and we use their data to create the images that
depict the amount of ozone. The blue and purple colors are where there is
the least ozone, and the greens, yellows, and reds are where there is more
ozone.
The Ozone Monitoring
Instrument on NASA's Aura satellite measures the total amount of ozone
from the ground to the upper atmosphere over the entire Antarctic
continent. This instrument observed a low value of 85 Dobson Units (DU) on
Oct. 8, in a region over the East Antarctic ice sheet. Dobson Units are a
measure of ozone amounts above a fixed point in the atmosphere. The Ozone
Monitoring Instrument was developed by the Netherlands' Agency for
Aerospace Programs, Delft, The Netherlands, and the Finnish Meteorological
Institute, Helsinki, Finland.
Scientists from NOAA's Earth System Research Laboratory in Boulder, Colo.,
use balloon-borne instruments to measure ozone directly over the South
Pole. By Oct. 9, the total column ozone had plunged to 93 DU from
approximately 300 DU in mid-July. More importantly, nearly all of the
ozone in the layer between eight and 13 miles above the Earth's surface
had been destroyed. In this critical layer, the instrument measured a
record low of only 1.2 DU., having rapidly plunged from an average
non-hole reading of 125 DU in July and August.
"These numbers mean the ozone is virtually gone in this layer of the
atmosphere," said David Hofmann, director of the Global Monitoring
Division at the NOAA Earth System Research Laboratory. "The depleted layer
has an unusual vertical extent this year, so it appears that the 2006
ozone hole will go down as a record-setter."
The ozone hole of 2006 is the most severe ozone hole (least amount of
ozone) observed to date. NASA's Aura satellite observed a low value of 85
Dobson Units (DU) on Oct. 8 in a region over the East Antarctic ice sheet.
Dobson Units are a measure of ozone amounts above a fixed point in the
atmosphere. This severe ozone hole resulted from the very high ozone
depleting substance levels and the record cold conditions in the Antarctic
stratosphere.
Observations by Aura's Microwave Limb Sounder show extremely high levels
of ozone destroying chlorine chemicals in the lower stratosphere
(approximately 12.4 miles high). These high chlorine values covered the
entire Antarctic region in mid to late September. The high chlorine levels
were accompanied by extremely low values of ozone.
The temperature of the Antarctic stratosphere causes the severity of the
ozone hole to vary from year to year. Colder than average temperatures
result in larger and deeper ozone holes, while warmer temperatures lead to
smaller ones. The NOAA National Centers for Environmental Prediction
(NCEP) provided analyses of satellite and balloon stratospheric
temperature observations. The temperature readings from NOAA satellites
and balloons during late-September 2006 showed the lower stratosphere at
the rim of Antarctica was approximately nine degrees Fahrenheit colder
than average, increasing the size of this year's ozone hole by 1.2 to 1.5
million square miles.
The Antarctic stratosphere warms by the return of sunlight at the end of
the polar winter and by large-scale weather systems (planetary-scale
waves) that form in the troposphere and move upward into the stratosphere.
During the 2006 Antarctic winter and spring, these planetary-scale wave
systems were relatively weak, causing the stratosphere to be colder than
average.
As a result of the Montreal Protocol and its amendments, the
concentrations of ozone-depleting substances in the lower atmosphere
(troposphere) peaked around 1995 and are decreasing in both the
troposphere and stratosphere. It is estimated these gases reached peak
levels in the Antarctica stratosphere in 2001. However, these
ozone-depleting substances typically have very long lifetimes in the
atmosphere (more than 40 years).
As a result of this slow decline, the ozone hole is estimated to annually
very slowly decrease in area by about 0.1 to 0.2 percent for the next five
to 10 years. This slow decrease is masked by large year-to-year variations
caused by Antarctic stratosphere weather fluctuations.
The recently completed 2006 World Meteorological Organization/United
Nations Environment Programme Scientific Assessment of Ozone Depletion
concluded the ozone hole recovery would be masked by annual variability
for the near future and the ozone hole would fully recover in
approximately 2065.
"We now have the largest ozone hole on record for this time of year," said
Craig Long of NCEP. As the sun rises higher in the sky during October and
November, this unusually large and persistent area may allow much more
ultraviolet light than usual to reach Earth's surface in the southern
latitudes.
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