The incredible shrinking ozone hole. (Earth News/Graph It!).

For two decades, an ominous hole in the ozone layer has ballooned over Antarctica, alarming scientists. The ozone layer, a thick band of bluish gas that wraps around the planet, shields Earth from life-threatening solar radiation. By last fall, the hole--actually a thinning of the ozone layer--had grown three times the size of the U.S.

But this September, the hole staged a sudden retreat: It shrank nearly by half, then split in two. The ozone hole normally fluctuates in size during the year, but "this is very abnormal," says NASA researcher Paul Newman. At 12.5 million square kilometers (4.8 million sq miles), the hole is now its smallest size since 1988.

About 90 percent of atmospheric ozone exists in the stratosphere, 16 to 48 km (10 to 30 mi) above Earth's surface. What destroys ozone? Mostly human-made chemicals called chlorofluorocarbons (CFCs). In recent decades, CFCs--used as coolants in refrigerators and air conditioners--have spewed into the atmosphere and broken down the ozone layer, especially over the South Pole, where frigid temperatures accelerate the destructive chemical reactions.

Why the sudden shrinkage? Temperature is a key factor, says Newman. Normally in September the stratosphere over Antarctica drops to -80[degrees]C; this year it was -15[degrees]C. The unusual warmth defused CFCs and allowed ozone molecules, which form spontaneously in the stratosphere, to regenerate and shrink the hole. Strong winds then tore the gap in two.

Although Newman deems this year's ozone-hole shrinkage a fluke, the hole could mend itself by 2050, he says--thanks to a 1987 ban on CFCs.

THINK ABOUT IT

Each year the ozone hole is measured daily between Sept. 7 and Oct. 13. The dots on this graph represent the hole's average size during this time period. If the hole's average size in 2002 was 12.5 million sq km, how would you expand this line graph to include that information?

OZONE HOLE SEPT. 24, 2002

NASA's Total Ozone Mapping Spectrometer (TOMS) sends data on atmospheric ozone to scientists who produce these images.

The colors correspond to the thickness of the ozone layer: Patches of dark blue and magenta show where the ozone is dangerously thin. This year's ozone hole shrank abnormally and split in two.