2001 OZONE HOLE ABOUT THE SAME SIZE
AS PAST THREE YEARS
Satellite data show the area of this
year's Antarctic ozone hole peaked at about 26 million square kilometers
-- roughly the size of North America -- making the hole similar in size to
those of the past three years, according to scientists from NASA and the
National Oceanic and Atmospheric Administration (NOAA). Researchers have
observed a leveling-off of the hole size and predict a slow recovery.
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Image 1 - Level of ozone on
September 17, 2001 - maximum levels for this year
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Over the past several years the annual
ozone hole over Antarctica has remained about the same in both its size
and in the thickness of the ozone layer. "This is consistent with
human-produced chlorine compounds that destroy ozone reaching their peak
concentrations in the atmosphere, leveling off, and now beginning a very
slow decline," said Samuel Oltmans of NOAA's Climate Monitoring and
Diagnostics Laboratory, Boulder, Colo.
In the near future -- barring unusual
events such as explosive volcanic eruptions -- the severity of the ozone
hole will likely remain similar to what has been seen in recent years,
with year-to-year differences associated with meteorological variability.
Over the longer term (30-50 years) the severity of the ozone hole in
Antarctica is expected to decrease as chlorine levels in the atmosphere
decline.
The total area of the ozone hole is one
measure of its severity. The ozone hole area is defined as the size of the
region with total ozone below 220 Dobson units. A Dobson unit is a unit of
measurement that describes the thickness of the ozone layer in a column
directly above the location being measured, a quantity called the
"total column ozone amount."
Prior to the springtime period in
Antarctica, when ozone depletion occurs, the normal ozone reading is
around 275 Dobson units. "Last year the ozone hole was of record
size, but it formed very early and then collapsed quickly," said NASA
scientist R.D. McPeters of the Goddard Space Flight Center, Greenbelt, Md.
"This year the hole was about 10 percent smaller."
Data from NOAA's polar-orbiting
operational environmental satellites and estimates of the area made by
NASA scientists using measurements from the Total Ozone Mapping
Spectrometer aboard NASA's Earth Probe satellite give similar sizes.
Each spring when the Sun rises over
Antarctica, chemical reactions involving chlorine and bromine from
man-made CFCs (chlorofluorocarbons) and bromine-containing compounds occur
in the stratosphere and destroy ozone, causing the "ozone hole."
Measurements of this year's ozone hole made at the South Pole and above
the Antarctic show that atmospheric ozone depletion reached levels typical
of the past 10 years.
Using instrumented balloons to make
ozone-profile measurements at the South Pole, researchers from NOAA
reported that the September decline in ozone was similar to recent years
with almost all of the ozone in the 15-20 kilometer (9-12 mile) altitude
region destroyed.
"Total column ozone over the South
Pole reached a minimum reading of 100 Dobson units on Sept. 28, 2001,
compared to a minimum of 98 Dobson units in 2000," said Bryan
Johnson, a scientist with the Climate Monitoring and Diagnostics
Laboratory. The record low of 88 Dobson units was observed in 1993.
"The severity of the ozone
depletion within the hole reached about the same levels as the past few
years and the highly depleted region filled about three-fourths of the
Antarctic polar vortex," said Jim Miller, a scientist with NOAA's
Climate Prediction Center in Camp Springs, Md. "This year the vortex
has been more stable and somewhat colder than average." Year-to-year
fluctuations in the geographical size of the polar vortex and the size of
the region with low temperatures will alter the size of the ozone hole
over the next decade during the period that levels of ozone-destroying
chemicals in the atmosphere begin a slow decline.
Thinning of the ozone layer is a concern
because the ozone layer protects the Earth from harmful effects of the
Sun's ultraviolet radiation, which contributes to skin cancer and
cataracts in humans. Total recovery of the ozone layer to levels observed
before 1980 will take at least 50 years, and expected changes in climate,
including a cooler stratosphere, could delay this process. NASA is
committed to obtaining critically important observations to examine and
document the recovery of this life-protecting atmospheric gas.
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