A February 3 NASA announcement that aircraft borne instruments had detected a high level of active chlorine over the Arctic led the White House and the governments of other industrialized nations to accelerate the phaseout of chlorofluorocarbons (CFCs), a family of manmade substances widely used as refrigerants.

The fear being advanced is that global stratospheric ozone is depleting rapidly because of a buildup of CFCs. While CFCs are very stable in the lower atmosphere, theoretical calculations suggest that they may rise to the stratosphere where, due to stimulation by ultraviolet light, they would break down to release a chlorine atom that catalyzes the conversion of ozone to oxygen molecules. This theory is plausible, but scientists have yet to establish how significant the CFC source is in relation to other, natural chlorine sources, such as volcanic gases and cloud-borne oceanic salt-spray. Observations that show stratospheric chlorine increasing as fast as CFCs would be strong evidence that CFCs are the major and perhaps only source. But we have yet to see that.

Aside from the origin of stratospheric chlorine, the case for a global depletion of stratospheric ozone has not been established. While thinning of the ozone layer over the Antarctic is a genuine, local phenomenon that scientists have been tracking since the mid-1980s, global "depletion" may be merely an artifact of the analysis. The reason is that ozone has large, natural variations that must be eliminated before scientists can establish a small, remaining trend.

Ozone records from single stations, dating back to the 1920s, show (naturally occurring) long-term trends whose causes are not known or understood. Observations from a global ground network of ozone stations cover only the last 35 years and show huge fluctuations on all time scales: daily, seasonal, and year-to-year. Some of these fluctuations are 100 times larger than the ozone depletion trend claimed by the U.S. Environmental Protection Agency (EPA).

Scientists have also documented an 11-year variation in stratospheric ozone that closely tracks the sunspot cycle. From a solar maximum to a solar minimum, ozone drops as much as 5 percent again, faster than the depletion trend claimed by the EPA.

Eliminating these natural variations, and particularly the solar-cycle variation, is tricky. The EPA "trend," for example, is highly dependent upon where the analysis starts and where it stops. Critics have compared it to claiming a "trend" in climate by starting temperature observations in January and finishing in July.

Ozone data taken with satellites are of much better quality than the ground-based data, but extend over only one solar cycle so far. Since each solar cycle is unique, it would be premature to draw firm conclusions from the observed decrease between 1979 and 1985, and the increase between 1986 and 1991. At least one more cycle of satellite observations is needed before an ozone trend can be established, and even then scientists could not be sure that the cause is CFCs.

It is clear that the science underlying the decision to accelerate the phaseout of CFCs is insubstantial and uncertain; it did not justify drastic policy actions.