I was hoping to report some good news on the ozone hole, which appears each year over Antarctica early in October, the austral spring. After all, it’s been more than 25 years since the world’s nations signed the Montreal Protocol, limiting the production and emissions of ozone-depleting substances such as chlorofluorocarbons (CFCs) that were widely used as refrigerants and propellants. By now, one might hope that the human impact on the stratosphere, first predicted in 1974 and documented in the early 1980s, might show some signs of recovery.
Alas, the size of this year’s ozone hole over Antarctica was among the largest ever.[1] And the remaining ozone layer was exceptionally thin. Frankly, I was surprised: the ozone hole had shown signs of stabilization in recent years, and a turn-around seems overdue.
All this stems from the innate properties of chlorofluorocarbon (CFC) molecules, which were designed and manufactured to be resistant to chemical destruction. Thus, CFCs remain in the atmosphere for a long time and mix into the stratosphere, where they release active chlorine. Active chlorine catalyzes a reaction that destroys ozone at high altitudes.
During the past few decades, the decline of ozone is a mirror image of the rise in active chlorine in the stratosphere, which was at low levels before the ozone loss began. Besides CFCs, and chlorine from exceptional, catastrophic volcanic eruptions, there are no other significant sources of chlorine in the stratosphere. These observations represent environmental chemistry at its best, and the scientists involved, Sherwood Rowland, Mario Molina and Paul Crutzen, received the Nobel Prize for their work in 1995.
Without the ozone layer, greater amounts of ultraviolet light pass through the atmosphere to the Earth’s surface. Ultraviolet light is widely recognized as a cause of skin cancer, cataracts, and other harmful effects to humans and wildlife. Indeed, some researchers suggest that ultraviolet light made life on land impossible until the ozone layer had formed about 400 million years ago.
Unfortunately, nearly all the chlorofluorocarbons in the stratosphere survive the ozone-destroying reaction and persist to react again. Loss of ozone has also been reported over the North Pole, although the stratospheric temperatures in the North are not low enough to allow the ozone-destruction reactions to proceed as rapidly. Atmospheric chemists predict that it might take up to 100 years for CFCs to disappear from the atmosphere, allowing full recovery of the ozone hole over Antarctica.
A few countries are known for continuing, clandestine emissions of CFCs, and a few other types of molecules, notably methyl bromide (CH4Br), also participate in ozone destruction. Most, but not all, of these substances are also regulated by the Montreal protocol, but a few countries, including the USA, still ask for exemptions to allow the continued use of a small amount of methyl bromide in agriculture. Nitrous oxide (N2O) is also a major cause of ozone destruction in the stratosphere. Humans cause nitrous oxide emissions to the atmosphere from the widespread use of nitrogen fertilizer.[2]
Reductions in N2O emissions would be difficult to regulate under the Montreal Protocol and must accompany better management practices in agriculture. Although work remains to be done on a variety of ozone-depleting substances, the elimination of CFCs was a major success story for the environment. Someday, I hope to see evidence of a recovery of ozone in the stratosphere.
While some human impacts on the environment are short-lived—for instance, a forest begins to regrow on a clearcut landscape within a year—other impacts, like the extinction of species, last forever. Unfortunately, recovery of the ozone hole appears to be taking the better part of forever.
References
Cadoux, A., B. Scaillet, S. Bekki, C. Oppenheimer, and T.H. Druitt. 2015. Stratospheric ozone destruction by the Bronze Age Minoan eruption (Santorini Volcano, Greece). Scientific Reports 5: doi: 10.1038/srep12243
Chang, C-B., R. Feng, Z. Gao, and W. Gao. 2010. Skin cancer incidence is highly associated with ultraviolet-B radiation history. International Journal of Hygiene and Environmental Health 213: 359-368.
Manney, G.L., et al. 2011. Unprecedented Arctic ozone loss in 2011. Nature 478: 469-475.
Ravishankara, A.R., J.S. Daniel and R.W. Portmann. 2009. Nitrous oxide (N2O): The dominant ozone-depleting substance emitted in the 21st century. Science 326: 123-125.
Rowland, F.S. 1989. Chlorofluorocarbons and the depletion of stratospheric ozone. American Scientist 77: 36-45.
Solomon, S. 1990. Progress towards a quantitative understanding of Antarctic ozone depletion. Nature 347: 347-354.
[1] http://ozonewatch.gsfc.nasa.gov/
[2] [2]See this blog of August 13, 2015 at http://blogs.nicholas.duke.edu/citizenscientist/nitrous-oxide-no-laughing-matter/