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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