This month, when the United Nations and the World Meteorological Organization release their first major report on global climate change in six years, two things are likely to happen. Some people will dismiss it. And Susan Solomon will grow hoarse explaining why they shouldn't.

A no-nonsense 51-year-old atmospheric chemist, she's a co-leader of the massive new study, along with Qin Dahe, a climatologist from the China Meteorological Administration in Beijing. Solomon will become the public face of the U.N. report, in charge of presenting the best scientific thinking on the subject of global warming and the evidence that it is caused by the burning of fossil fuels. "The science is strong," she says, "and we'll be presenting a consensus view."

To reach that consensus, Solomon logged more than 400,000 air miles over the past four years and held dozens of meetings with the report's more than 500 authors. "This much I can say: the climate is changing and quite noticeably," she says shortly before the report is released. In her paper- and book-crammed office at the National Oceanic and Atmospheric Administration in Boulder, Colorado, she suggests that policy makers (and the rest of us) have reached a critical moment in our dealings with, or failings to deal with, climate change: "The effects will vary from region to region, and the challenge that society will face is to get people to think beyond their own backyards and to make judgments about the risks they're willing to take."

Maybe as the climate continues to warm, the ice caps won't melt; maybe a rising sea level will be offset by some other unforeseen event. She's reminded of the scene in Dirty Harry in which the cop played by Clint Eastwood confronts a criminal: "You've got to ask yourself one question: Do I feel lucky? Well, do ya, punk?" Solomon says, "That's what we as a society have to decide. Will we choose to go down the same path, or will we make some changes in our behaviors? You could say that the gun of climate change is pointed at us. So, how lucky do we feel?"

Scientists who study climate change or other environmental problems often feel like Cassandra, the mythical prophetess of the Trojan War who was cursed to foretell the future but not to be believed. But Solomon doesn't see herself that way at all. She's more optimistic than many of her climatology peers, and with good reason: she was a prime mover behind one of the most important global environmental turnarounds in history, repairing the hole in the ozone layer above Antarctica.

Solomon made her first trip to the McMurdo Station in Antarctica in August 1986. She and her 15-man team went there to study a puzzling new phenomenon that British researchers had reported the year before: earth's protective ozone layer had begun to thin above the Antarctic. The planet was losing its best natural defense against the sun's ultraviolet radiation, which causes cancer and cataracts in people and can damage crops.

The plane landed in the middle of the day, in dark that was "like twilight at noon," Solomon recalls. Solomon was just 30. She'd completed her doctorate at the University of California at Berkeley five years earlier, and she was an out-and-out geek, a theoretician who used a computer to model the chemistry of the atmosphere. Although she had no field experience, she'd been chosen leader of the first National Ozone Expedition to the Antarctic in part because she was one of the few scientists to take the ozone hole seriously. And she had devised a theory to explain it.

The suspected culprit was chlorofluorocarbons (CFCs), because earlier studies had shown that these compounds, most of which are gases at room temperature, could damage earth's ozone shield. Invented in the 1920s, CFCs (made by linking atoms of chlorine, fluorine and carbon) had been put to use as aerosol propellants in hair spray and the like; CFCs were excellent coolants, too, and soon were in every refrigerator and air conditioner. But when released into the lower atmosphere—as they were with every squirt of an aerosol spray can—they drift with the winds up into the stratosphere. There, ultraviolet rays kick chlorine atoms out of CFCs, and each freed chlorine atom goes on to break apart as many as 100,000 ozone molecules.

The scientists who demonstrated the ozone-eating ability of CFCs—they would later earn a Nobel Prize in Chemistry for the work—believed that the degradation high in the atmosphere would be slow, taking perhaps hundreds of years. Yet the ozone shield was disintegrating quickly. "And the thinning wasn't supposed to show up over Antarctica first," Solomon recalls, "since that's not where these chemicals were being used."

She came up with an explanation after noticing something that others had overlooked: the ozone depletion was occurring in the lower stretches of the ozone layer, about eight miles above the earth, rather than in the stratosphere's upper reaches. A self-confessed "weather-weenie" who loves watching summer afternoon thunderheads over the Rockies, Solomon knew that unusual clouds often form above the Antarctic each austral winter and spring. They are so full of ice crystals they shimmer like abalone shells. The crystals contain nitric and sulfuric acids, and Solomon speculated that when chlorine compounds came into contact with these cold, acidic clouds, the chlorine was transformed into rabid ozone-eating chemicals.

"It was radical," says Mack McFarland, a chemical physicist at DuPont Corporation who previously worked with Solomon at NOAA. "She was suggesting a completely new area of chemistry for the atmosphere—that there could be reactions not just between molecules but on molecules in the ozone layer and at incredibly low temperatures. Other people had a hard time accepting that."

Yet it was viewed as an idea worth testing, and Solomon was put in charge of the 1986 expedition, which was organized by NOAA and NASA. En route to Antarctica, a reporter in New Zealand asked her what it was like to be the only woman among so many men. "Gosh," she quipped. "They are all men, aren't they?"

In their three months at McMurdo, Solomon and her team worked out of a small wooden cabin outfitted with computers and a visible light spectrometer, an instrument the size of a portable sewing machine. The scientists analyzed light for wavelengths that indicate the presence of ozone, chlorine dioxide and other ozone-depleting chemicals. They took turns standing on the cabin roof and directing moonlight through a tube and into the spectrometer. It was so cold, dipping to minus 40 degrees, that one night Solomon's right eyelid froze shut. Not a problem. "I'm a theoretician," says Solomon. "I'd never collected any data for any experiment before, and I just loved it."

Solomon and her colleagues not only confirmed that the ozone hole was real, but they also showed that it was being caused by chemical reactions between CFCs and the polar stratospheric clouds, just as Solomon had predicted. "Scientists were surprised but relieved," McFarland recalls. "We knew the cause and could do something about it."

Only a year after Solomon's research was published, governments started to act. Representatives from 24 nations met in Montreal in 1987 and signed a treaty to limit the use of CFCs. (The United States and other countries had already banned CFCs as propellants in aerosol cans, based on the early studies of their potential for harming the atmosphere, but the chemicals were still being used in refrigerators, air conditioners and cleaning solvents.) Today, more than 180 countries, including the United States, have signed the "Montreal Protocol on Substances that Deplete the Ozone Layer" as well as amendments calling for a complete ban on CFCs. In 2000, President Cinton awarded Solomon the National Medal of Science for her ozone hole research; the citation praised her "for exemplary service to worldwide public policy decisions and to the American public." She was one of the youngest members to be elected to the National Academy of Sciences, the country's most elite scientific organization.

The earth's ozone layer has stabilized in the past ten years, and although CFCs from old refrigerators are still floating up into the atmosphere, almost no new ones are being produced. Climate scientists predict that the ozone hole over Antarctica will disappear by the end of this century and the ozone layer over the rest of the planet will thicken back up. "With luck, I'll live long enough to see the layer close to being fully restored," Solomon says.

Part of that environmental success can be credited to the company that manufactured most of the world's CFCs, DuPont. It announced in 1988 that it would voluntarily stop CFCs production, and company chemists soon devised replacement chemicals for air conditioners and refrigerators.

Solomon knows the current climate crisis won't be as easy to solve as the ozone problem. (The ozone hole does not contribute noticeably to global warming.) For one thing, the health dangers of global warming, such as a possible increased range for malaria and other tropical diseases caused by warm-weather microbes, may not be immediate or dramatic. And there's no simple alternative to the burning of fossil fuels. But she's hopeful that the science presented in this month's report will inform public attitudes about global warming. "I don't think people realize how much solid data scientists have or what the data mean," she says, referring to the dramatic increase in global temperatures in the past few decades. "I'm an optimist," she adds with a smile. "I really do believe that we and our governments will do the right thing."

Virginia Morell is the author of Ancestral Passions and Blue Nile, and a co-author of Wildlife Wars.