The study, published this week in the Proceedings of the National Academy of Sciences, builds on a groundbreaking 2012 paper that was among the first of its kind to tackle ozone loss over the United States. According to lead author James Anderson, an atmospheric chemist at Harvard University, most previous studies have focused on the poles, where the icy temperatures are well suited for the chemical reactions that lead to ozone depletion. In fact, the famous “ozone hole” — a massive deterioration of ozone in the atmosphere spurred by industrial pollution and discovered in the 1980s — largely occurred over the Antarctic.
The Antarctic ozone hole finally appears to be recovering — mostly thanks to an international agreement called the Montreal Protocol, which prompted major global efforts to curb the emissions of chlorofluorocarbons, the chemicals that cause ozone to break down in the atmosphere. But ozone-destroying chemicals still linger in the atmosphere, and depletion still occurs on smaller scales. Recent studies have found a new “hole” in the Arctic being carved by leftover atmospheric pollutants.
And now, scientists say this process is not necessarily limited to the poles. The new study suggests that the right conditions — including chilly upper-air temperatures previously thought to exist only in the Arctic and Antarctic — occur in the United States as well.
In the past, this idea has drawn its share of scientific skepticism. But the latest version is apparently being taken seriously by none other than Mario Molina, one of the researchers who won a Nobel Prize in 1995 for his co-discovery of the threat to the ozone layer.
“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,” Molina said in a statement.
For ozone depletion to occur, a set of specific conditions is necessary. Certain chemicals must be present in the atmosphere — namely chlorine, one of the major components of chlorofluorocarbons, which can chemically react with ozone and break it up into other molecules. These chemical reactions also require the presence of water vapor, and they occur more quickly at lower temperatures.
Although these conditions are known to occur at the poles, scientists were skeptical until recently that enough water vapor was capable of rising into the stratosphere — the layer of the atmosphere where ozone is found — in the area over the United States, or that temperatures there could dip low enough. But the basis for the new study was finally laid several years ago, according to Anderson, when researchers discovered that some storm systems in the Great Plains were indeed capable of forcing water vapor all the way up into the stratosphere. Anderson and several colleagues published evidence of this phenomenon in their 2012 paper.
“That’s what triggered our first concern,” Anderson told The Washington Post. “And then the question became, well, how frequent are these storms? Do we have 100 a year? Do we have 50?”
In fact, the new study suggests that the region sees a whopping 4,000 of these storms on average each summer. An analysis of radar data from the central United States suggested that nearly 40,000 storms extended more than a mile into the stratosphere between 2004 and 2013. The researchers also examined summer temperature measurements from the stratosphere in this region and found that they were much colder than scientists had previously believed.
“In fact, those temperatures border on the same temperatures we see in the Arctic stratosphere,” Anderson said. This combination of conditions — low temperatures and water vapor being frequently injected into the stratosphere — “make the situation far more serious than we originally thought,” he added. They suggest that the region is prime territory for the destruction of ozone, meaning there’s possible concern for public health. The depletion of ozone allows for greater penetration of ultraviolet radiation through the atmosphere, putting humans at an elevated risk of developing cancer.
Following the publication of the original 2012 paper, some experts expressed the idea that chlorine levels in the atmosphere weren’t high enough for large amounts of ozone depletion to occur over the United States, according to Andrew Dessler, an atmospheric scientist at Texas A&M University who was previously a PhD student under Anderson. But he noted that the new study incorporates observed atmospheric chlorine measurements in a model suggesting these depletion reactions can indeed take place under the conditions observed over the Great Plains.
That said, chlorine pollution in the atmosphere is also steadily decreasing, thanks to the Montreal Protocol. And some scientists say it’s likely that by the end of the century, chlorofluorocarbon levels will have returned to their pre-ozone-hole conditions. This means the threat of ozone depletion will probably continue to diminish in the coming decades.
This raises the question of how diminishing pollution levels and progressing climate change interact with each other in the future. In the new paper, Anderson and his colleagues suggest that climate change may significantly alter weather patterns over the central United States, potentially causing more of these storm systems to occur, and also that by cooling the stratosphere it could favor chemical reactions that deplete ozone.
“There’s a rapidly growing scientific literature connecting the increase in carbon dioxide and methane with the increasing number and severity of storm systems over the central United States,” Anderson said.
But Dessler cautioned that the reductions in atmospheric chlorine may lessen the impact of these potential future storms. “In 50 years, we might be having more water being injected, but the amount of chlorine is lower,” he said. “I don’t think it’s obvious that climate change combined with decreasing chlorine is going to make this process more important as time goes on.”
Still, pointing out the possibility that climate change could have an effect is part of what launched Anderson’s 2012 research into the spotlight in the first place. Previously, scientists had been adamant about keeping a clear separation between the pollution-related destruction of ozone and the long-term warming of the climate, which are physically caused by different mechanisms — the first by chlorofluorocarbons and the latter by carbon dioxide and other greenhouse gases.
According to Anderson, the next research step is to start tackling the uncertainties and verifying the chemical reactions that the authors say may be destroying ozone over the United States. And in the meantime, any immediate concerns about public health should be addressed with regular — perhaps weekly or even daily — UV forecasts for vulnerable regions, he suggested.
“It’s really surveillance and watching the system continuously and carefully with very high spatial resolution that provides what people will need,” he said.