Ozone Hole Grows

The seasonally variable hole in the ozone layer over Antarctica has set new records both for its average area and its thickness, scientists from NASA and the National Oceanic & Atmospheric Administration (NOAA) reported on Oct. 19.

In late September, the ozone hole's average sprawl was a record 10.6 million sq miles. Temperature variations in the stratosphere cause annual changes in the hole???s size, and colder than average temperatures result in larger and deeper ozone holes.

On the basis of satellite and stratospheric balloon measurements, researchers found lower stratosphere temperatures at the rim of Antarctica to be about 9 °F colder than average. If stratospheric weather conditions had been normal, they would have expected a hole of about 8.9 million to 9.3 million sq miles, or about the size of North America.

By early October, the amount of ozone in the hole directly over the South Pole was 93 Dobson units, down from about 300 DU in mid-July. One DU is equivalent to about 2.69 x 10¹⁶ ozone molecules in a column with a cross-section of 1 sq cm. The average thickness of the ozone layer over Earth's surface is 300 DU, and the so-called hole is actually an area where concentrations drop to about 100 DU.

Even more critical, the researchers said, ozone in a layer 8 to 13 miles above Earth was nearly nonexistent. There they measured a record low of 1.2 DU, compared with an average nonhole reading of 125 DU in July and August. They also measured extremely high levels of ozone-destroying chlorine chemicals in a layer at about 12 miles high over the entire Antarctic region in mid- to late-September.

"These numbers mean the ozone is virtually gone in this layer of the atmosphere," said David Hofmann, director of the Global Monitoring Division at NOAA's 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."

Since the enactment of the Montreal protocol, levels of banned ozone-depleting chemicals have declined since the mid-1990s. Scientists estimate the ozone hole will decrease in area by about 0.1 to 0.2% annually for the next five to 10 years, although seasonal fluctuations will continue to mask these changes. In an August report, the World Meteorological Organization and United Nations Environment Program concluded that the Antarctic ozone hole will recover by 2065, about 15 years later than previously predicted.