Public debate over the possible effects of an eroding ozone layer often has been characterized by inflammatory claims and doomsday rhetoric.
Vice President Gore's popular book, "Earth in the Balance: Ecology and the Human Spirit," says ozone depletion has become so bad that hunters in Patagonia are finding rabbits blinded by increased ultraviolet. Anglers, Gore reports, are catching blind fish. Other accounts add that Patagonian sheep are going blind.
The reports have gained credibility because Patagonia is at the tip of South America, not far from the Antarctic ozone hole. Yet, efforts to link the rabbit and fish claims to ozone depletion have failed. And the sheep, it turned out later, just had eye infections.
While ozone scientists have not made such claims, some have adopted similarly dramatic language. In a booklet called, "Report to the Nation: Our Ozone Shield," researchers at the National Oceanic and Atmospheric Administration say, "life as we know it is possible in part because of the protection afforded by the ozone layer," but that it is now "in jeopardy." They called the discovery that industrial chemicals are to blame "a dark message for the world."
Even environmentalists have found some of the debate hard to take. "Unfortunately, we have very little hard data on the effects of ozone depletion," said Michael Oppenheimer of the Environmental Defense Fund. "To begin with, no one knows how much increase in ultraviolet there's been. All we can really say is that there has been a likelihood of some increase in some areas. When there isn't much data, there's room for lots of wild speculation."
One of the most widely repeated claims is that during the past decade skin cancer rates -- known to be linked to UV exposure -- have doubled. While true, the fact is that skin cancer rates have been climbing steadily through most of this century.
According to data from the American Cancer Society, for example, the skin cancer rate in the United States now is nearly eight times higher than it was 30 years ago. Because there is a long lag between exposure and disease, the 732,000 Americans who are expected to get skin cancer this year will be suffering for their exposure to ultraviolet long before CFCs became a threat.
Decline Found in Planktonic Organisms
The rising trend, cancer researchers say, is the result not of ozone depletion or increased UV (if anything, UV levels have declined in populated areas because of increasing low-altitude air pollution), but of lifestyle changes such as wearing skimpier clothing in summer, spending more time outdoors and spending more of that time in southerly latitudes.
Still, it is true in principle that if ultraviolet levels increase, they should raise the number of skin cancer cases -- at least among light-skinned people -- and assuming the trend toward less tanning and more use of sunscreens does not continue. One rule of thumb scientists use is that for every decline of 1 percentage point in ozone, there is a 1.3 percentage point increase in the amount of DNA-damaging UV.
The only credible claim of a biological effect that has already occurred comes from those who study microscopic creatures in the ocean surrounding Antarctica. Ray Smith of the University of California at Santa Barbara has found evidence of a 6 to 12 percent decline in the number of these planktonic organisms while the ozone hole is open. Some other scientists criticize Smith's methods and say the results probably overestimate the damage.
"One problem is that we only began studying this after the ozone hole was in existence," said Deneb Karentz of the University of San Francisco, who also studies Antarctic plankton. "If there were organisms that were extremely sensitive to UV, they could be gone now."
Despite the hole, the planktonic organisms are not helpless. Karentz has discovered that virtually all of them can protect themselves against increased ultraviolet with a natural sunscreen, a colorless molecule that absorbs UV-B. Algae make the sunscreen in response to an increase in UV. Planktonic animals extract it from the algae they eat and, instead of digesting the molecules, distribute them in their own bodies.
So effective is the molecule, Karentz said, that an Australian firm is working to synthesize the substance and sell it as a natural sunscreen for humans.
Testing for Effects of Higher UV Levels
Botanists have found that land plants also make their own natural sunscreens, molecules called flavenoids. These molecules are analogous to the melanin that human skin makes to tan itself for protection from ultraviolet, except that plants can do it much faster. Some plants respond within hours to changes in UV level, increasing their flavenoid content each morning as the sun moves higher, lowering it if the day turns cloudy to take in more of the visible wavelengths needed for photosynthesis, and dropping it to low levels through the night.
Flavenoid content also varies with latitude: The farther south a species' growing range, the higher the flavenoid content. But each species has a limit. If it is transplanted still farther south, the plant may not be able to make enough flavenoid to protect itself and its growth will be retarded.
While scientists find no solid evidence that land plants are suffering from increased ultraviolet bombardment, experiments are underway to see what might happen in the future if UV levels go up significantly.
Alan Teramura of the University of Maryland, for example, has been growing various food crops and tree species under sunlamps that boost the UV levels by as much as 50 percent. This is several times as much extra ultraviolet as is expected in 2000, when ozone depletion is projected to reach its worst.
At these high doses, about two-thirds of the 50 varieties tested displayed reduced yields. The most sensitive soybean varieties, for example, lost as much as 25 percent of their yield. Other varieties were unchanged and a few increased their yields slightly. Because soybeans are Maryland's leading field crop, researchers are working to develop varieties with enhanced UV-B resistance.
The effect on perennial species such as loblolly pines was more insidious. Teramura has found that with each passing year, the trees' growth was stunted by a larger amount. Studies are continuing to see whether the effect of massive UV doses will eventually kill the trees.
"The effects in trees that we see now may be small and subtle but they accumulate over time," Teramura said.
Still, "I'm optimistic," said the Environmental Defense Fund's Oppenheimer. "I believe the Montreal Protocol is already solving the problem before it gets out of control."
The amount of ozone depletion varies depending on the distance from the pole or equator. Depletion is greatest over polar regions and less severe over temperate zones. There is no measured depletion over the tropics. Losses shown here are approximations published in a National Oceanic and Atmospheric Administration booklet on the ozone problem.