PUNTA ARENAS, Chile -- The worst of the ozone hole has pulled back once more to Antarctica this southern spring, leaving behind a shadow of uncertainty for the people living at the bottom of the Americas.
How many will develop skin cancer in years to come? How many more decades must their children live with dangerous ultraviolet rays? Will the global treaty to save the ozone survive until then?
The people of wind-blown Punta Arenas, like the local evergreens forever bent eastward from westerly gusts, are adjusting to the intense radiation that pours each year through the gap in the ozone layer. At least that's what some say.
"People are better informed. They're buying more sun-block and putting it on their children," said pharmacist Gerardo Leal. "They've gotten used to it," taxi driver Rene Bahamonde said.
But on a "red alert" day when UV rays could have damaged eyes, Bahamonde's dark glasses sat unused. And local health chief Dr. Lidia Amarales said many of the 150,000 Punta Arenans take few precautions against a damaging sun as they go about their business on the quiet streets that slope downward to the broad, chill waters of the Strait of Magellan.
The reason: It's cool here.
"When it's 30 degrees somewhere [86 Fahrenheit], people don't go out into the sun. Here, with 13 degrees [55 Fahrenheit], they go outside," Amarales explained.
This is a gray, drizzly corner of South America, but clouds are no protection against UV. It rarely exceeds 70 Fahrenheit, and "without the heat, they don't 'feel' the radiation," Amarales said. "We need to change habits."
The stratosphere's layer of ozone, a form of oxygen, for countless millennia filtered out almost all the sun's cancer-causing ultraviolet-B rays. But in the 1970s scientists warned that manmade chemicals such as chlorofluorocarbons (CFCs), used in aerosol sprays and refrigerants, were destroying ozone through chain reactions high in the skies. By the 1980s, satellite images showed the world, an "ozone hole" had formed over Antarctica.
Air currents and intense cold in the polar region, combined with chlorine from CFCs, created a vast expanse of ozone-thin atmosphere that briefly reached the tip of South America each southern spring. Measured in Dobson units, ozone here was found in October 1992 to have thinned to 147 units, less than half the normal 333. Ultraviolet radiation, in its most damaging wavelengths, multiplied many times.
The world's nations took action in 1987, signing the Montreal Protocol, phasing out some CFCs and some other ozone-damaging compounds. As a result, chlorine has declined in the lower atmosphere since the mid-1990s, while the rate of growth of bromine, another targeted chemical, has slowed.
It will take decades to purge the atmosphere. Experts watch year by year for positive signs, and in fact this September's maximum ozone hole, at 11 million square miles, was markedly smaller for the second straight year from the huge 17-million-square-mile hole of 2002.
But Dutch climatologist Henk Eskes, a leading ozone analyst, cautioned that climatic changes make it hard to draw conclusions.
"There's a lot of variability, from one year to the next, because of wind patterns and dynamical situations," he said.
Punta Arenas' own specialist, Claudio Casiccia, is equally noncommittal. "If this trend continues for four, five, six years, then I think that's a sign the ozone is recovering," said the Chilean, who monitors the skies via sophisticated instruments on the roof of his Ozone Laboratory at the University of Magallanes.
Computer models suggest ozone should recover globally by 2040-2050, Casiccia said. "But global ozone is one situation" -- it was depleted by 6 percent over the United States, for example -- "and Antarctica is another situation."
Casiccia and others worry that unforeseen new compounds might damage the ozone shield. And they are troubled by the granting of exemptions from the Montreal Protocol.
On Nov. 26, at negotiations in Prague, a dozen nations, including the United States, Canada, and some European countries, won continued exemptions for use of methyl bromide, an ozone-depleting agricultural pesticide that was to have been phased out this year.
Chemist Mario Molina, who shared a 1995 Nobel Prize for identifying the ozone threat, protests that substitutes are available. "Any exemptions for further emissions will delay the recovery of the ozone layer," he said by telephone from Cambridge, Mass.
Environmentalists at the Prague meeting worried that continued exceptions for rich countries could undermine the Montreal treaty itself, by discouraging poorer countries from meeting their own later deadlines -- to end CFC use by 2010, for example.
In Punta Arenas, ozone is a here-and-now concern for Dr. Jaime F. Abarca, the city's lone dermatologist, who with Casiccia's help conducted the only detailed studies of local sunburns, skin cancer and UV-B levels.
Abarca reported in a British dermatology journal that incidence of skin cancer leaped from 65 cases recorded between 1987 to 1993, to 108 in the following seven-year period. He says additional research, with international aid, is urgently needed.
The annual shrinking of the hole -- the area where ozone drops below 220 Dobson units -- doesn't mean the danger ends for Punta Arenas. Even in December, below-normal ozone levels combine with a higher-angled, more direct sun to produce "red alert" days, announced via 131 prominent signs posted at schools, office lobbies, and other key points, modeled on a traffic light showing five alert levels ranging from green to violet.