December, 1998

The arrival of the Antarctic spring last September produced the biggest ozone hole yet observed. At some 26 million square km, the ozone "hole," really a thinning in the concentration of ozone gas in the stratosphere, reached about three times the size of Australia, according to researchers at the US National Oceanic and Atmospheric Administration (NOAA). The ozone hole also lasted longer in 1998. Developing during each Antarctic spring since the late 1970s, this ozone thinning has generally disappeared by late November. In 1998 the ozone hole lasted into December. The region of unusually depleted ozone has grown in size each year since it was discovered in the early 1980s, but some reports point to hopeful evidence that the loss of stratospheric ozone may soon start to slow.

The antarctic ozone hole is a reduction in concentration of ozone gas (O3) in the stratosphere (the layer of atmosphere between about 20 and 40 km above the earth's surface). Ozone is an important component of the atmosphere because it selectively absorbs the ultraviolet (UV) wavelengths of sunlight, reducing the amount of UV that reaches the earth's surface. Too much ultraviolet light can damage living tissues, so decreasing stratospheric ozone threatens both humans and animals with skin diseases, including cancer. Although ozone may be thinning worldwide, the ozone hole is especially dramatic near the South Pole for several reasons. First, strong winds circling Antarctica isolate the atmosphere near the pole the winter. Second, high-altitude clouds of ice crystals accumulate in the extreme cold of the polar winter.  As the sun returns in the southern spring, solar energy initiates chemical reactions on the surfaces of these ice crystals, with ozone-depleting chemicals such as chlorofluorocarbons (CFCs) and halons breaking down ozone (O₃) into molecular oxygen (O₂). Toward the middle of the southern summer (November and December) stratospheric ice clouds melt, reducing the rate of reactions breaking down the ozone. At the same time, the strong winds circling the pole begin to weaken as the southern atmosphere warms, and ozone-rich air from warmer latitudes begins to mix with Antarctic air. For both these reasons, the ozone "Zhole" diminishes in the middle of the summer. At the same time, however, the ozone-poor polar air drifts north during the southern summer. The net effect is a gradual reduction of ozone at all latitudes.

There is some ozone thinning at the North Pole, too. However the Arctic lacks the strong, circling winds of the Antarctic, so northern ozone loss is much less dramatic than the southern ozone hole.

Most of the worrisome ozone-depleting gases are human-made, including the gases used in refrigerators, air conditioners, and inflating styrofoam. Production of the most serious culprits has been phased out following an international agreement known as the Montreal Protocol. However the gases produced and used earlier continue to collect in the upper atmosphere. And some ozone depleting chemicals continue to be in use. One important ozone destroyer now targeted for phasing out is methyl bromide, a fruit fumigant (see methyl bromide link below).

On the positive side, atmospheric scientists have begun to find reduced levels of some ozone-depleting chemicals in the atmosphere. These reductions are a heartening sign that the Montreal Protocol may be starting to be effective in slowing the loss of ozone. Atmospheric scientists hope that concentrations of these chemicals will drop to pre-1970 levels by the middle of the next century. That is a long time for the ozone hole to continue threatening the health of humans and wildlife, but it also evidence that progress may be possible through international accords such as the Montreal Protocol.

For further information, see these related web sites:

Satellite images of the ozone hole (TOMS site) (click on the large TOMS title for further info)

Movie! Click here to see watch the formation of the 1998 ozone hole (905.0K)
This movie comes from NASA's TOMs site, above, which provides a full explanation of what you see. The black circle is the region where the sun has not yet returned.

Frequently asked questions about the ozone hole.
(Scan down through preliminary disclaimer information to get to an excellent summary of the ozone problem.)

Other satellite images from NASA

To read more, see

Environmental Science, A Global Concern, Cunningham and Saigo, 5th ed.
Stratospheric ozone depletion: pages 395-396

Environmental Science, Enger and Smith, 6th ed.
Ozone depletion: pages 364-365