Scientists from NASA and the Commerce Department's National Oceanic and Atmospheric Administration (NOAA) have confirmed the ozone hole over the Antarctic this September is not only much smaller than it was in 2000 and 2001, but has split into two separate "holes."

Comparison of the first split ozone hole on record (right, 2002) and the Antarctic ozone hole at the same time one year earlier (left, 2001). The hole is dark blue and magenta. In 2001, the ozone layer thinning over Antarctica reached 26.5 million square kilometers, larger than the size of the entire North American continent. Due to higher Antarctic winter temperatures, the 2002 'hole' seems to be about 40% smaller. Credit: NASA

The researchers stressed the smaller hole is due to this year's peculiar stratospheric weather patterns and that a single year's unusual pattern does not make a long-term trend. Moreover, they said, the data are not conclusive that the ozone layer is recovering.

Paul Newman, a lead ozone researcher at NASA's Goddard Space Flight Center, Greenbelt, Md., said this year, warmer-than- normal temperatures around the edge of the polar vortex that forms annually in the stratosphere over Antarctica are responsible for the smaller ozone loss.

Estimates for the last two weeks of the size of the Antarctic Ozone Hole (the region with total column ozone below 220 Dobson Units), from the NASA Earth Probe Total Ozone Mapping Spectrometer (EPTOMS) and the NOAA-16 Solar Backscatter Ultraviolet instrument (SBUV/2), are around 15 million square kilometers (6 million square miles). These values are well below the more-than 24 million sq. km. (9 million sq. mi.) seen the last six years for the same time of year.

The stratosphere is a portion of the atmosphere about 6-to-30 miles above the Earth's surface where the ozone layer is found. The ozone layer prevents the sun's harmful ultraviolet radiation from reaching the Earth's surface. Ultraviolet radiation is a primary cause of skin cancer. Without protective upper-level ozone, there would be no life on Earth.

"The Southern Hemisphere's stratosphere was unusually disturbed this year," said Craig Long, meteorologist at NOAA's Climate Prediction Center (CPC). The unusual weather patterns were so strong, the ozone hole split into two pieces during late September. NOAA's CPC has been monitoring and studying the ozone since the early 1970s. "This is the first time we've seen the polar vortex split in September," said Long.

At South Pole Station, balloon-borne ozone-measuring instruments launched by NOAA's Climate Monitoring and Diagnostics Laboratory (CMDL) reveal the vertical structure of the developing ozone hole. Bryan Johnson, a scientist with CMDL, said the main ozone depletion region, from 7-to-14 miles above the Earth, has large ozone losses, similar to the last few years. At more than 15 miles above the Earth, surface measurements show higher-than-normal ozone concentrations and higher temperatures.

The combination of these layers indicate total ozone levels in a column of atmosphere will be higher than observed during the last few years, Johnson said. However, some layers may still show complete ozone destruction by early October, when ozone depletion is greatest.

In 2001, the Antarctic ozone hole was larger than the combined area of the United States, Canada and Mexico. The last time the ozone hole was this small was in 1988, and that was also due to warm atmospheric temperatures.

"While chlorine and bromine chemicals cause the ozone hole, temperature is also a key factor in ozone loss," Newman said. The Montreal Protocol and its amendments banned chlorine- containing chlorofluorocarbons (CFCs) and bromine-containing halons in 1995, because of their destructive effect on the ozone layer. However, CFCs and halons are extremely long- lived and still linger at high concentrations in the atmosphere.

The coldest temperatures over the South Pole typically occur in August and September. Thin clouds form in these cold conditions, and chemical reactions on the cloud particles help chlorine and bromine gases to rapidly destroy ozone. By early October, temperatures usually begin to warm, and thereafter the ozone layer starts to recover.

NOAA and NASA continuously observe Antarctic ozone with a combination of ground, balloon, and satellite-based instruments.