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Midwest ozone shield may be thinning, thanks to rising thunderstorm activity

Surges of wet, icy air are reaching the stratosphere after all, creating polar-like conditions for ozone depletion.

Shortly after Sunday’s hellacious thunderstorms moved east, I came across a provocative piece of new research showing that these severe events create conditions that may be thinning the ozone layer across the central United States, including Minnesota.

Like you, perhaps, my first reaction — as a fellow who spends his summer sunlit hours outside, on a sailboat if not on a bike, tending the garden if not making paragraphs on the back deck — was to wonder how much this raises the risk of skin cancer.

So let me say right away that these findings do not justify those glib reports urging Midwesterners to start hoarding sunblock and rental DVDs, or the flip conclusion that climate change has found a way to kill us more quickly.

But this is an intriguing  piece of research on an important and underexamined question, enabled by new techniques and resources, conducted by an estimable Harvard-led team of scientists who are careful to say what they don’t know.

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For example, they don’t yet know for sure whether serious ozone depletion is occurring over the Midwest — only that the conditions for it are unexpectedly and even uniquely ripe, and increasingly so because of climate change, which is making severe storms more frequent.

Just demonstrating that much strikes me as an impressive achievement, and among others impressed is Mario Molina, the atmospheric chemist who was awarded the Nobel Prize for the discovery in the mid-1980s that the ozone layer was being eaten away by chlorine and bromine compounds from broken-down fluorocarbon propellants.

Molina was not involved in the new study, but offered his assessment in Harvard’s announcement of the findings:

These developments were not predicted previously and they represent an important change in the assessment of the risk of increasing UV radiation over the central U.S. in summer.

The stats on skin cancer

The study, published in the prestigious Proceedings of the National Academy of Sciences, does not attempt to draw any direct link to cancer incidence. But in the announcement the authors offer this sobering context:

Stratospheric ozone is one of the most delicate aspects of habitability on the planet. There is only marginally enough ozone in the stratosphere to provide protection from UV radiation for humans, animals and crops.

Medical research specific to the United States has determined that a 1 percent decrease in the amount of ozone in the stratosphere corresponds to a 3 percent increase in the incidence of human skin cancer. There are now 3.5 million new cases of skin cancer each year reported in the U.S. alone.

Thus, for each 1 percent reduction in ozone, there would be an additional 100,000 new cases of skin cancer annually in the United States.

The paper itself concludes that the convergence of strong storms, certain air circulation patterns and other factors in a region stretching from the Rocky Mountains to the Mississippi River “define why the central United States in summer represents a unique case, in the global context, for the risk of regional ozone loss.”

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Also, one that hasn’t been investigated much until now, because so much attention has focused on the Arctic and Antarctic situations.

Before turning to the specifics of the new findings, let’s review some ozone layer basics.

How ozone forms and fades

Atmospheric ozone is created when sunlight acts on oxygen, which occurs most often in a two-atom molecule called dioxygen or O2. Solar energy splits that molecule, and some of the freed atoms stick to an unsplit molecule, forming ozone or O3.

There isn’t very much ozone in the atmosphere compared to other gases, but what there is concentrates in a thin layer of the lower stratosphere that starts at about 12 miles above the surface and ends at about 20. That layer is sufficient to shield us from much ultraviolet radiation, including most at the cancer- and cataract-encouraging frequency designated UV-B. (Also, at a higher altitude and in combination with dioxygen, virtually all at the UV-C frequency, which is even worse).

This shield is thickest at the tropics, where solar energy and its effect on oxygen are strongest, and thins toward the poles. It also thickens and thins with the seasons and, especially, with wind patterns in the stratosphere. And though ground-level ozone (aka smog) is unstable, in the stratosphere it’s notably long-lived — until attacked by chlorines and bromines.

To do their work effectively, those chemicals need moist air and low temperatures, which is why the shield thins dramatically over the Arctic and almost to nothingness over Antarctica at certain times of year, creating the famous “hole.”

So that’s where the Harvard team, led by Jim Anderson, focused its Midwestern research: on the atmospheric effect when large volumes of rapidly cooling moisture ascend from the surface in summer thunderstorm conditions (forming, for example, the hail that then falls to earth).

It was long assumed that injections of chilly water vapor never reached as high as the stratosphere. But using aircraft observations and radar data gathered at the height of summer (July and August) from 2002 to 2014, the Anderson team found that, on average, 4,000 storms per year were penetrating the stratosphere across the central U.S., typically by a mile or more, and lowering temperatures to Arctic levels for a time.

Depletion measurements next?

Anderson’s co-author, Steven C. Wofsy, said of the findings:

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Every year, sharp losses of stratospheric ozone are recorded in polar regions, traceable to chlorine and bromine added to the atmosphere by industrial chlorofluorocarbons and halons. The new paper shows that the same kind of chemistry could occur over the central United States, triggered by storm systems that introduce water, or the next volcanic eruption, or by increasing levels of atmospheric carbon dioxide. We don’t yet know just how close we are to reaching that threshold.

Anderson added that looking for actual losses scattered across the central states is a bigger challenge than measuring depletion focused at the poles.

Rather than large continental-scale ozone loss that occurs over the polar regions in winter characterized, for example, by the term Antarctic ozone hole, circumstances over the central U.S. in summer are very different.  In particular, because of the very frequent storm-induced injection events detailed by studies at Texas A&M and the University of Oklahoma using advanced radar methods, this structure of highly localized but numerous regions of potential ozone loss requires carefully specified observational strategies and systematic surveillance in order to provide the basis for accurate weekly forecasts of ozone loss.

That’s a somewhat lengthy way of saying this: Current forecasts of ozone make no allowance at all for ozone loss, despite the known consequences, and while measuring depletion might be complicated it’s really rather important to protecting public health and also agriculture, so let’s maybe get started.

In the meantime, it never really hurts to use more sunblock, does it?

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The full paper, “Stratospheric ozone over the United States in summer linked to observations of convection and temperature via chlorine and bromine catalysis,” can be found here but access is not free.