A new Harvard study presents evidence that ozone over the United States, especially over the central part of the country, thins during the summer.
A quick refresher: ozone in the stratosphere is what protects us and much of life from what would be a deadly dose of ultraviolet radiation from the sun. (Conversely, periodic high ozone levels near the ground develop from human activity, and pose a threat to those with chronic heart and/or lung disease, including asthma. The latter is an entirely different problem, and is not the focus of this article.)
There is annual thinning of ozone over the polar regions, and some of the thinning is linked to manmade chemicals being put into the atmosphere. Some of the most damaging compounds are, for the most part, no longer being used due to an international agreement, the Montreal Protocol. Among these compounds were propellants formerly used in aerosol cans, some refrigerants, carbon tetrachloride, and trichloroethane. Catalytic chemical reactions in the stratosphere involving these compounds had been causing significant reductions in year-round ozone concentration. These were linked with some of the increases in many types of skin cancer. Since these chemicals were banned, there have been some limited improvements in global ozone concentrations, as had been predicted. However, the annual springtime increase in the polar ozone holes continues, as expected. While manmade chemicals are making significant contributions to the polar ozone holes, at least the banning of some of them has helped to a small but still significant extent. The greatest improvement tied to the Montreal Protocol has been where most people live — away from the polar regions.
However, skin cancer increases continue due to lifestyle changes, including many people migrating to lower latitudes where UV levels are greater. UV exposure in Phoenix is considerably greater than in Halifax, as you might guess.
The Harvard study, in the Proceedings of the National Academy of Sciences, indicates ozone depletion appears to occur from large complexes of powerful thunderstorms which are most common in the central United States. This mechanism was not known to exist. Since this process is due to naturally occurring convection, one could paraphrase Bart Simpson, “we didn’t do it!” Not much, yet, anyway.
The water vapor injected from these towering storms which spike above the the troposphere into the stratosphere, in combination with observed temperature variations create the same catalytic reactions over the central U.S. as occurs over the polar regions. More UV will reach the surface in these regions, exposing people, livestock and plant life to more radiation. Humankind’s relative disconnect from this phenomenon is fading, however. Climate models project the ongoing warming puts more water vapor into the atmosphere through evaporation over much of the globe. That will fuel these thunderstorm complexes. They will become more common with continuing warming in the plains and Midwest. There is some evidence this has already been occurring.
Peer-reviewed epidemiological studies show for every 1 percent decrease in ozone, there is a 3 percent increase in skin cancer. On the whole, the amount of ozone in the stratosphere provides the marginal protection for humans, livestock and plant life from UV radiation, and is a fragile component of the atmosphere. The Centers for Disease Control report 3.5 million new cases of skin cancer in our country alone annually. Anything which contributes to more ozone thinning, as seen above, will cause further increases in skin cancer, including often deadly melanomas. The contributions linked with these giant convective complexes will be more sporadic and seasonal, and will require new methods of tracking. Early measurements suggest these contributions are significant.
Because the Montreal Protocol did achieve some of the resulted improvements in ozone thinning related to those specific banned compounds, there may have been a mistaken impression the overall thinning of the ozone layer had been “fixed.” This entire newly discovered mechanism for mid-latitude thinning demonstrates the problem is not only not “fixed” but actually has facets we didn’t know about at the time of the Montreal Protocol.
Story topics: By Don Paul