Ozone Hole Science Revisited


Scientists are commemorating the discovery 20 years ago that man-made chlorofluorocarbons (CFCs) used chiefly in refrigerators and air-conditioners were responsible for creating the "ozone hole" over the Antarctic. The scientists concluded that CFCs would drift into the stratosphere where they would produce chlorine compounds that react with ice particles and sunlight to efficiently destroy ozone molecules that shield the surface from ultraviolet light streaming from the sun. In 1987, the world adopted the Montreal Protocol to eventually eliminate the production of CFCs. Activists often cite the Montreal Protocol as a model for a future treaty addressing man-made global warming by banning the emission of greenhouse gases. A Nobel Prize in chemistry was awarded in 1995 to the three scientists who identified the ozone/CFC connection.

This neat story of the scientific identification of a man-made cause for stratospheric ozone depletion followed by a successful international response to the threat is now being challenged by some very recent research. News@nature.com (sub required) is reporting a new analysis by Markus Rex, an atmosphere scientist at the Alfred Wegener Institute of Polar and Marine Research in Potsdam, Germany, which finds that the data for the break-down rate of a crucial molecule, dichlorine peroxide (Cl2O2) is almost an order of magnitude lower than the currently accepted rate.

What this could mean according to the Nature news article is that:

"This must have far-reaching consequences," Rex says. "If the measurements are correct we can basically no longer say we understand how ozone holes come into being." What effect the results have on projections of the speed or extent of ozone depletion remains unclear.

The rapid photolysis of Cl2O2 is a key reaction in the chemical model of ozone destruction developed 20 years ago2 (see graphic). If the rate is substantially lower than previously thought, then it would not be possible to create enough aggressive chlorine radicals to explain the observed ozone losses at high latitudes, says Rex. The extent of the discrepancy became apparent only when he incorporated the new photolysis rate into a chemical model of ozone depletion. The result was a shock: at least 60% of ozone destruction at the poles seems to be due to an unknown mechanism, Rex told a meeting of stratosphere researchers in Bremen, Germany, last week.

Other groups have yet to confirm the new photolysis rate, but the conundrum
is already causing much debate and uncertainty in the ozone research community. "Our understanding of chloride chemistry has really been blown apart," says John Crowley, an ozone researcher at the Max Planck Institute of Chemistry in Mainz, Germany.

"Until recently everything looked like it fitted nicely," agrees Neil Harris, an atmosphere scientist who heads the European Ozone Research Coordinating Unit at the University of Cambridge, UK. "Now suddenly it's like a plank has been pulled out of a bridge." …

Nothing currently suggests that the role of CFCs must be called into question, Rex stresses. "Overwhelming evidence still suggests that anthropogenic emissions of CFCs and halons are the reason for the ozone loss. But we would be on much firmer ground if we could write down the correct chemical reactions."

Of course, it may be that Rex's research has gone wrong somehow or that another chemical mechanism involving CFCs will turn out to be chiefly responsible for ozone depletion. Nevertheless, it is good to keep in mind that all scientific results are provisional and may change in the light of new evidence.

By the way, for anyone who cares about my own take on the ozone hole/CFC issue, in chapter 8 of my 1993 book, Eco-Scam: The False Prophets of Ecological Apocalypse, I concluded:

Despite a great deal of continuing scientific uncertainty, it appears that CFCs do contribute to the creation of the Antarctic ozone hole and perhaps to a tiny amount of global ozone depletion. If CFCs were allowed to build up in the atmosphere during the next century, ozone depletion might eventually entail significant costs. More ultraviolet light reaching the surface would require adaptation—switching to new crop varieties, for example—and it might boost the incidence of nonfatal skin cancer. In light of these costs, it makes sense to phase out the use of CFCs.