In spite of recent discoveries related to the mechanism of the Antarctic ozone hole (AOH), we do not as yet have a sufficient scientific base to answer important policy questions: is the AOH a completely new phenomenon, or is it a recurrent one? Is it produced by human activities? And what can and should be done about it? I suggest here a hypothesis concerning the cause of the AOH, which may provide at least partial answers.
The AOH is more than a scientific curiosity. Its dramatic discovery in 1985 raised fears about the fate of global ozone and provided the impetus for an international effort to limit and roll back the worldwide production of chlorofluorocarbons (CFCs), synthetic chemicals widely used in refrigeration and industrial processes.
There is little doubt now, based on the recent measurements, that chlorine chemistry is the immediate cause of the seasonal (October) ozone decrease at around 18 km in the southern polar regions—rather than purely meteorological effects based on dynamics, or direct solar influences related to the solar cycle. See, for example, Levy [1988]. It is also probable that the major source of the chlorine is man-made CFCs—although no precise estimate exists so far of the chlorine contributed by volcanoes and by oceanic salt particles carried into the stratosphere by storms. Natural sources of chlorine would dilute the effects of CFC increases. Contributions by volcanoes have been summarized by Symonds et al. [1988].
Yet how does one explain the sudden onset and rapid growth of the AOH phenomenon? Starting from essentially zero in the mid 1970s, the thinning reached, within just a few years, about 50% of the vertical ozone column—and essentially, saturation in the lower stratosphere. This rapid change presents us with an important clue. Clearly, the CFC content of the atmosphere has not risen quite so rapidly, nor should one expect any kind of trigger effect related to the chlorine concentration. This point becomes particularly relevant if, as has been frequently suggested, the chlorine from CFCs superimposes upon natural sources of stratospheric chlorine.
The research results further suggest that, in addition to the chlorine, ozone destruction requires the presence of ice particles, "polar stratospheric clouds" that can form in the coldest part of the Earth's atmosphere, the lower Antarctic stratosphere [Levi, 1988]. But it is highly unlikely that the water vapor content could have increased so suddenly within a few years' time. Increased emissions of methane should lead to increased injection of water vapor into the stratosphere [Singer, 1971]. Recent measurements have confirmed and extended this hypothesis [Blake and Rowland, 1988].
This line of reasoning leads me to propose that the trigger for the AOH has been a gradual cooling of the stratosphere, which took the temperature below the freezing point; this cooling could have taken place as part of a general climate fluctuation of the Earth. And indeed, there has been an unusual surface temperature increase since about 1975; under some theoretical models of climate change, such a surface warming should be accompanied by a cooling of the upper atmosphere [Ramanathan, 1988].
If this hypothesis is borne out by appropriate measurements, then the AOH should disappear, or at least become less pronounced, if the stratosphere warms again, perhaps in conjunction with a cooling of the Earth's surface. Conversely, a further cooling of the stratosphere could induce an Arctic ozone hole and a larger Antarctic hole. The policy implication here is that the AOH would not be much affected by further slow increases of atmospheric CFC, nor could it be removed if the CFC concentration were to decrease. In other words, the AOH phenomenon should be reasonably insensitive to stratospheric chlorine concentration, somewhat more sensitive to stratospheric water vapor concentration, and extremely dependent on the exact value of the temperature minimum in the lower Antarctic stratosphere.
The signing of the Montreal Protocol in September 1987, calling for a freeze and roll-back of CFC production, was spurred by the belief that the AOH may just be the precursor of a general global decline in stratospheric ozone. The hypothesis presented here would make this possibility less likely and could therefore affect the future of global control of CFCs.
The science leading up to the discovery and initial explanations for the AOH has been nothing short of magnificent. It is essential now to develop the measurements and theories to answer the fundamental questions posed at the beginning.
S. Fred Singer
Chief Scientist
U.S. Department of Transportation
References
Blake, D. R., and F. S. Rowland, Worldwide increase in tropospheric methane, Science, 239, 1129, 1988.
Levi, B. G., Ozone depletion at the poles: The hole story emerges, Phys. Today, 7, 17, 1988.
Ramanathan, V., The greenhouse theory of climate change: A test by an inadvertent global experiment, Science, 240, 293, 1988.
Singer, S. F., Stratospheric water vapour in-crease due to human activities, Nature, 223, 543, 1971.
Symonds, R. B., W. 1. Rose, and M. H. Reed, Contributions of Cl- and F- bearing gases in the atmosphere by volcanoes, Nature, 334, 415, 1988.