Here's some good news for a change: The badly damaged sky shield that shelters Earth from dangerous solar radiation is about to get stronger.
The buildup of man-made gases that has been chewing up the protective ozone layer in the stratosphere for the past 25 years has leveled off, atmospheric scientists say.
That means less danger of cancer, cataracts and other harm to plants and animals from the sun's searing ultraviolet rays.
This optimistic message comes from a draft report from the World Meteorological Organization to the nations that agreed in Montreal 11 years ago to stop pumping ozone-killing substances into the atmosphere.
The agreement, known as the Montreal Protocol, appears to be succeeding. The WMO's full report will be published early next year. A summary was made available recently.
According to the report, ozone-depleting chemicals - primarily chlorine and bromine - in the lower part of the atmosphere peaked in 1994 and are now disappearing. From 10 to 25 miles above the Earth, the destructive accumulation has almost stopped and is expected to peak in 1999. As a result, the ozone shield will start to thicken in the next decade, though it won't get back to 1980 levels for a half century or more.
"The Montreal Protocol is working," said Dan Albritton, an expert at the National Oceanographic and Atmospheric Administration in Boulder, Colo., and co-chair of an international group of 200 scientists who reassessed the ozone situation this summer for the WMO, the weather agency of the United Nations.
"We have peaked in the lower atmosphere and started downward," Albritton said. "A problem was discovered; decisions were taken; the atmosphere is responding."
Albritton said this is the first report to show "a downward trend in the total set of ozone-depleting compounds in the lowest part of the atmosphere."
Chlorine in the atmosphere is produced by chlorofluorocarbons, chemical compounds developed after World War II that once were widely used in refrigeration, air conditioning and aerosol sprays.
Under the Montreal Protocol, production of CFCs ended in 1996 in industrial countries, but developing nations, like China and India, have until 2006 to comply. Slower timetables apply to other ozone-killers, like halons and bromides.
According to the WMO report, without the cutoff of CFCs half of the ozone shield would have been lost by 2050 in the Northern Hemisphere, and 70 percent would be gone in the Southern Hemisphere - 10 times greater than today's loss.
As a result, UV radiation on the surface of the Earth would at least double in the Northern Hemisphere and quadruple in the south, compared with current increases of 5 percent in the north and 8 percent in the south.
"Furthermore, all these impacts would have continued to grow in the years beyond 2050," the report said.
Ozone, an unstable molecule composed of three oxygen atoms, can be useful or destructive depending on its location. Albritton labeled its various roles, with apologies to Clint Eastwood, as "the good, the bad and the ugly." CHLORINE HIGHLY DESTRUCTIVE "Good" ozone lives in the stratosphere, where its extra atom serves to block UV radiation. "Bad" ozone hangs out in the lower atmosphere, where it acts as a "greenhouse gas," trapping the Earth's heat and contributing to global warming. "Ugly" ozone is the low-lying stuff found in smog from burning fuel in cars and factories.
In the stratosphere, chlorine splits ozone molecules, rendering them useless against UV. "One chlorine atom can take out hundreds of thousands of ozone molecules," Albritton said.
Bromine, the other major ozone-depleting chemical, is still increasing, but is now considered less dangerous than before.
Unfortunately, the ozone news isn't all good:
* Global warming - both natural and man-made - raises temperatures on the Earth's surface but, paradoxically, lowers them in the stratosphere. As it gets colder, chlorine becomes even more destructive, Albritton said.
* The world's growing fleet of high-flying aircraft leaves trails of icy particles that contribute to the destruction. Volcanic eruptions, like Mount Pinatubo in 1992, also wreak havoc on the ozone layer.
* Some illegal production and smuggling of CFCs continues, despite the ban.
* Congressmen from California and Florida are trying to delay the phase-out of methyl bromide - the principal source of atmospheric bromine - because their farmers use it to fumigate local crops.
* The temporary ozone "holes" over the South and North Poles, which appear every spring, continue unabated. The extreme cold of the polar regions makes recovery there especially difficult.
Scientists and environmentalists cautioned that continued vigilance is necessary to save the ozone shield.
"This doesn't mean we've seen a recovery," said Beatrice Olivastri of Friends of the Earth, an environmental group. "What we've seen is a slowing of the damage."
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