A NASA satellite has shown that the area of ozone depletion over the Antarctic -- the well-known ozone "hole" -- is a bit less in 1999 than it was last year.
"This Antarctic year's ozone depletion area, or ozone 'hole,' is very large, but slightly smaller than that of 1998," said Dr. Richard McPeters, principal investigator for the instrument that made the measurements.
This year's study found that an ozone "low" had formed between New Zealand and Antarctica on Sept. 17. This sort of ozone low, commonly referred to as a "mini-hole," is a result of the redistribution of ozone by a large weather system. The "mini-hole" moved eastward along the rim of the Antarctic "ozone hole" for a number of days after Sept. 17.
Preliminary data from the satellite show that this year's Antarctic ozone depletion covered 9.8 million square miles on Sept. 15. The record area of Antarctic ozone depletion of 10.5 million square miles was set on Sept. 19, 1998.
The ozone levels are expected to decrease over the next two weeks. The lowest amount of total-column ozone recorded to date this year was 92 Dobson Units on Oct. 1. In contrast, ozone levels of 90 Dobson Units were observed at one point last year. Dobson units measure how thick the ozone layer would be if all the overhead ozone molecules in a column of atmosphere could be brought down to the Earth's surface.
Globally, the ozone layer averages approximately 300 Dobson Units, which would correspond to a layer about 1/8th of an inch (3 millimeters) thick at the Earth's surface, about the thickness of two stacked pennies. In contrast, during the annual Antarctic ozone "hole," the amount of ozone in the ozone "hole" is about 100 Dobson Units, about 1/25th of an inch, or approximately the thickness of a single dime.
The slightly decreased size of the ozone "hole" from last year is not an indication of the recovery of Antarctic ozone levels. The current year-to-year variations of size and depth of the ozone "hole" depend primarily on the variations in meteorological conditions.
The Antarctic ozone losses are caused by chlorine and bromine compounds released by chlorofluorocarbons (CFCs) and halons. Due to international treaties regulating the production of these gases, the amount of chlorine in the stratosphere is close to maximum value and, in some regions, is beginning to decline. In the next century, chlorine-induced ozone losses will be reduced as chlorine amounts throughout the stratosphere decline, and ozone levels will begin to recover. The actual rate of recovery will likely be affected by the increasing abundance of greenhouse gases in the atmosphere. Detecting the recovery of the ozone hole will require a number of years of measurements.
Ozone molecules, made up of three atoms of oxygen, comprise a thin layer of the atmosphere that absorbs harmful ultraviolet radiation from the Sun. Most atmospheric ozone is found between 6 and 18 miles above the Earth's surface.
Ozone shields life on Earth from the harmful effects of the Sun's ultraviolet radiation. Scientists and others have a keen interest in ozone depletion. Increased amounts of ultraviolet radiation that reach the Earth's surface due to ozone loss might increase the incidence of skin cancer and cataracts in humans, depress the human immune system, harm some crops and interfere with marine life.
These measurements were obtained between mid-August and early October using the Total Ozone Mapping Spectrometer (TOMS) instrument aboard NASA's Earth Probe (TOMS-EP) satellite. NASA instruments have been measuring Antarctic ozone levels since the early 1970s. Since the discovery of the ozone "hole" in 1985, TOMS has been a key instrument for monitoring ozone levels over the Earth.
TOMS ozone data and pictures are available on the Internet:
TOMS-EP and other ozone-measurement programs are important parts of a global environmental effort of NASA's Earth Science enterprise, a long-term research program designed to study Earth's land, oceans, atmosphere, ice and life as a total integrated system.
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