From Climate Central's Michael D. Lemonick:

Scientists know plenty about how Earth’s climate works, and greenhouse-gas pollution is messing with sea level, weather patterns, and more. There’s also plenty they don’t know yet — how global warming might affect tornadoes, for example, or how quickly the massive ice caps on Greenland and Antarctica could slide into the oceans.

But there are also plenty of unknown unknowns, as ex-defense secretary Donald Rumsfeld famously put it — climate disasters nobody has even thought of yet. Last week, one of these unexpected disasters was suddenly revealed: a paper in Science argued that powerful thunderstorms threaten to rip a hole in the atmospheric ozone layer that protects the planet’s surface from dangerous ultraviolet (UV) radiation from the sun. If thunderstorms start to become more frequent as the Earth warms, the consequences could include increased skin cancers, crop damage and other nasty consequences. “If you were to ask me where this fits into the spectrum of things I worry about, right now it’s at the top of the list,” lead author James Anderson of Harvard told the Harvard Gazette in an interview.

It all sounds pretty apocalyptic, and it could turn out to be. But one crucial fact was greatly underplayed in the wide news coverage triggered by the new study: the chain of reasoning laid out by Anderson and his co-authors has a couple of unproven links. “I view this as a hypothesis,” said Andrew Dessler, a climate scientist at Texas A&M University. “It’s a bold idea,” he said, “and it might be right. But there’s really no evidence for it yet.”

Or at least there’s no smoking gun. Anderson and his colleagues start with a new and unexpected observation, based on high-altitude flights and satellite measurements: powerful updrafts from powerful summer thunderstorms can send moisture much higher into the stratosphere than anyone had thought. “I didn’t believe it at first,” Kerry Emanuel, an atmospheric scientist at MIT told the New York Times. “But we’ve come to see that the evidence is pretty strong . . . ”

Once water vapor gets into the normally dry stratosphere, it can in theory interact with manmade chlorine compounds known as chlorofluorocarbons, or CFC’s, to destroy ozone molecules. It’s the same sort of reaction that has been destroying ozone over Antarctica for more than 25 years. The discovery of the so-called “ozone hole” over the south polar region in 1986 was so disturbing, in fact, that 197 nations agreed to ban CFC’s, widely used in air conditioners and refrigerators, just a year later, in an agreement known as the Montreal Protocol.

CFC’s take a long time to degrade, though, so they’ll still be a threat for several more decades. And while the Antarctic ozone hole doesn’t affect densely populated areas, ozone destruction in temperate latitudes, where thunderstorms are common, could bathe hundreds of millions of people in harsh, cancer-causing UV radiation.

The chemical reactions that could rip a hole in mid-latitude ozone come right out of the textbook and lab experiments. So far, however, that’s the only place they’ve been seen. “What our study shows is that observed water vapor concentrations are high enough and temperatures are low enough over the U.S. in summertime to initiate the chemistry that is known to lead to ozone losses,” said Harvard atmospheric scientist David Wilmouth, one of the paper’s co-authors, in an email.

But it doesn’t show that the reactions are actually happening: nobody has made those observations yet. “NASA doesn’t fly chemistry experiments on climate missions,” Dessler said. “I suspect this paper was partly a plea to change that.”

It’s even less clear that powerful thunderstorms are on the rise due to climate change. It’s plausible that they could be, since warm, moist air is clearly increasing, and since both are key factors in thunderstorm formation.

It also seemed plausible a few years ago, however, that climate change would lead to more hurricanes, and more recent research suggests otherwise. “I wouldn’t rule it out,“ Dessler said, “but the whole connection to climate change is just really speculative.”

Even if thunderstorms were to increase in frequency or intensity, nobody has any definitive idea on how quickly that might happen. At the same time, sunlight is gradually breaking CFC molecules into less harmful substances. This takes a long time: it could be several decades before these ozone-destroying chemicals are rendered mostly harmless. But if it takes thunderstorms that long or longer to get significantly more powerful, the danger could be gone.

Wilmouth said as much in his email. “One of the areas where our understanding is most limited is the link between climate change and the storms that convectively inject water into the stratosphere,” he wrote. “Accordingly, it is impossible to predict the potential severity of ozone losses in a future climate.”

Nevertheless, even the potential for drastic ozone loss is scary enough to warrant a lot more research — and since nobody knows how soon an ozone hole might open up over New York or Chicago or Miami, it makes sense to get on it. “We need to have the measurements to determine whether it’s there or not,” Dessler said.

The ozone-thunderstorm connection is no longer an unknown unknown, in other words. It’s now firmly in the category of known unknowns — things we now realize might be a threat. Figuring out just how great the threat might be is something scientists can’t afford to ignore.

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