Ozone holes in the greenhouse

Why millions face radiation threat

Joe Farman is a scientist of the old school. String and sealing wax. Smokes a pipe and drinks real ale. He has the faraway look in his eyes that you often see in men who have spent any length of time in Antarctica. He is retired now from the British Antarctic Survey, where he spent virtually his entire working life in a worthy though less than exalted capacity. Or he did until 1985, when he wrote one of the decade's most quoted research papers. He is the man who discovered the ozone hole over Antarctica. And the way it happened—or, rather, almost didn't happen—is revealing.

A quarter of a century ago, Farman was in charge of the BAS's Dobson meter, which for many years had been pointing up into the sky measuring the depth of the ozone layer in the stratosphere from the BAS's base at Halley Bay, on an ice shelf off West Antarctica. For several years his bosses had been trying to halt the observations and bring the old instrument home. After all, they pointed out, nothing interesting had happened for years, and satellites orbiting Earth were by then measuring ozone levels routinely. Ground-based observations were deemed superfluous.

But Farman resisted, and in 1982, he noticed a series of unusual and abrupt fluctuations in the ozone readings, just after the sun reappeared following the long polar night. It happened again the following year.

"I asked the Americans if they had seen anything similar from their satellites," he told me later. "They said they hadn't. So I assumed that my old machine was on the blink." But he was intrigued enough not to leave it at that. He found another Dobson meter back in Cambridge, and took it south in 1984 to check the readings. It recorded the same thing—only more so. Farman's data were by now unambiguous. He was seeing a deep hole opening in the ozone layer over the base. It lasted for several weeks before closing again. "We were sure then that something dramatic was happening," Farman said. In places, more than 90 percent of the ozone was disappearing in what appeared to be runaway reactions taking place in just a few days.

The ozone layer protects Earth's surface from dangerous ultraviolet radiation from the sun. Without this filter, there would be epidemics of skin cancers, cataracts, and many other diseases, as well as damage to vital ecosystems. Life on Earth has evolved to live under its protection, and would find things much harder without it.

For more than a decade, scientists had been concerned about the ozone layer, fearing that man-made chemicals such CFCs in aerosols might cause it to thin. But nobody had thought of a hole forming. Least of all over Antarctica, which was as far from the source of any ozone-destroying chemicals as you could get. And certainly not in a runaway reaction over just a few days. Earth was simply not supposed to work that way.

Farman bit his pipe and got to work. No more checking with NASA. He had his data and was intent on an urgent publication in the scientific press. Perhaps he sensed it was his moment of fame. He was certainly scared by what he had found—scared enough to miss all the office parties in

Cambridge in 1984 to finish his paper titled "Large Losses of Total Ozone in Antarctica." He posted it to Nature on Christmas Eve.

The editors didn't quite share Farman's sense of urgency. It took them three months to accept his paper, and another two months to publish it. When the paper finally appeared, NASA scientists were confused. They still had no inkling of anything amiss over Antarctica. But they could hardly ignore the findings of two Dobson meters, however ancient. They re-examined the raw data from their satellite instruments and were shocked to find that their satellites had seen the ozone hole forming and growing over Antarctica all along, even before Farman had spotted it. But the computers on the ground that were analyzing the streams of data had been programmed to throw out any wildly abnormal readings. And the data showing the ozone hole had certainly fitted that category. The episode, as Farman was not slow to point out, was a salutary lesson for high-tech science. It was also a triumph for the string-and-sealing-wax school, and for the dogged collection of seemingly boring and useless data about the environment.

Paul Crutzen—who had unraveled much of the complex chemistry of the ozone layer—swiftly tied Farman's findings to specific chemical reactions involving CFCs that took place only in the uniquely cold air over Antarctica each spring. Below about -130°F, unique clouds form in the stratosphere above Antarctica. These are called polar stratospheric clouds. It turned out that the runaway reactions happened only on the surface of the frozen particles in these clouds. The reactions required both the cold to create the clouds and solar energy to fuel them. And there was a window of a few weeks when both were supplied—after the sun had risen, but before the air warmed enough to destroy the clouds. After that, the air warmed and the ozone recovered, though the repair job took some months.

Farman's discovery and Crutzen's analysis finally pushed the world into taking tough action against ozone-eating chemicals. The Montreal Protocol was signed in 1987. Slowly, very slowly, the amount of CFCs and other ozone-eaters in the stratosphere is declining. And the Antarctic ozone layer is equally slowly starting to heal, though it could be a century before it is fully repaired, even if every promise made by government negotiators is met. But it had been a close call.

And things could have been a lot worse. "Looking back, we were extremely lucky that industrialists chose chlorine compounds, rather than the very similar bromine compounds, to put in spray cans and refrigerators early in the last century," says Crutzen. Why so? Bromine compounds make refrigerants that are at least as effective as their chlorine equivalents. But atom for atom, bromine is about a hundred times better than chlorine at destroying ozone. Pure luck determined that Thomas Midgley, the American chemist who developed CFCs, did not opt for their bromine equivalent. "It is a nightmarish thought," says Crutzen, "but if he had chosen bromine, we would have had something far worse than an ozone hole over Antarctica. We would have been faced with a catastrophic ozone hole, everywhere and at all seasons during the 1970s, before we knew a thing about what was going on."

The world has been very lucky Or has been lucky so far. The same combination of low temperatures and accumulating gases that combined so devastatingly over Antarctica can also occur over the Arctic in some years. The conditions are not quite so favorable for ozone destruction, because the atmosphere is not quite so stable and the extremely cold temperatures occur less frequently. But there have been some near misses.

One occurred in January 2005. Anne Hormes, who runs the German research station at Ny-Alesund, in Svalbard, told me the story when I visited there a few months later. Temperatures in the lower stratosphere above Svalbard had for a few days fallen to -I44°F, fully 14 degrees below the threshold necessary for the formation of polar stratospheric clouds, and extremely low even by the standards of Antarctica. "We feared that a real, big ozone hole would form," she said. "And if the temperature had stayed that cold for a few more weeks, till the sun came up to drive the chemical reactions, we would certainly have seen one." It would have been the Arctic's first full-fledged ozone hole, and in all probability a major world environment story.

Her concern is shared. The ozone expert Drew Shindell, of the Goddard Institute for Space Studies, says: "Overall winter temperatures are going down in the Arctic stratosphere—2005 was very cold. But actual ozone loss is very time-critical. So far, we have been lucky." But he doubts that our luck will hold. How so? Why are the risks of an ozone hole still growing, even though the chemicals that cause it are now in decline in the stratosphere?

The problem is this. In the lower atmosphere, greenhouse gases trap heat. But in the stratosphere, they have the opposite effect, causing an increase in the amount of heat that escapes to space from that zone of the atmosphere. This is happening worldwide, but some of the most intense stratospheric cooling is over areas with the greatest warming at the surface. Like the Arctic, where the air increasingly resembles the air high above Antarctica.

There is another risk factor, too. The warmer troposphere, with stronger convection currents taking thunderstorm clouds right up to the boundary with the stratosphere, may be injecting more water vapor into the stratosphere. As far as we know, the stratosphere has always been very dry in the past. So extra water vapor is potentially a big change. And more water vapor will make more likely the formation of the polar stratospheric clouds within which ozone destruction takes place. "If it gets a lot wetter, that will make ozone depletion much worse," says Shindell. There is some evidence that that is happening, though data are scarce. "Water vapor levels in parts of the lower stratosphere have doubled in the past sixty years," he says.

No hole formed in the Arctic ozone layer in 2005, because the sun did not rise when the air was at its coldest. But the spring of 2005 nonetheless saw the largest Arctic ozone loss in forty years of record-keeping. More than a third of the ozone disappeared, and losses reached 70 percent in some places. Air masses with reduced ozone levels spread south across Scandinavia and Britain, and even as far south as Italy for a few days. One year soon, the sun will rise when temperatures are still cold enough for major runaway ozone destruction. And when it does, millions of people may be living beneath. This will be another unexpected consequence of global warming.

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