No Growth For Ozone Hole
Good news: The Antarctic ozone hole didn't get any bigger this year.
Indeed, the ozone layer may even be in better shape.
The ozone hole is a thinning of the Earth's protective ozone layer. It tends to occur over the Antarctic in winter as manmade chemicals become trapped in the upper atmosphere.
The thinning could become a hazard over populated areas because ozone blocks some of the sun's cancer-causing radiation. Some spray propellants that contribute to the thinning have been banned.
Scientists at NASA and the National Oceanic and Atmospheric Administration reported Tuesday that satellite measurements show the hole peaked at about 10 million square miles (26 million square kilometers) this year, similar in size to the last three years.
"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 NOAA scientist Samuel Oltmans.
Over the longer term, 30 to 50 years, the severity of the ozone hole in Antarctica is expected to decrease as chlorine levels in the atmosphere decline.
Using instrumented balloons to make ozone profile measurements at the South Pole, researchers from NOAA reported that the ozone reached a minimum of 100 Dobson units on Sept. 28. That compares to a minimum of 98 Dobson units in 2000, said Bryan Johnson, a scientist with NOAA's Climate Monitoring and Diagnostics Laboratory in Boulder, Colorado.
In other words, at its worst, the ozone layer is thicker than it was when it was at its worst last year. The record low of 88 Dobson units was set in 1993.
The Dobson unit is the measure of ozone in the air overhead. In the tropics, ozone levels are typically between 250 and 300 Dobson units year-round. In temperate regions, seasonal variations can produce large swings in ozone levels, reaching as high as 475 Dobson units in some areas, and as low as 300.
The ozone hole is the region with total ozone below 220 Dobson units.
Recovery of the ozone layer back to levels observed before 1980 is expected to take at least 50 years.
By RANDOLPH E. SCHMID
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