Lingering atmospheric pollutants and a blast of frigid air have carved an unusually deep hole in Earth’s protective ozone layer over the Arctic, and it threatens to get deeper. Atmospheric scientists are analyzing data from weather balloons and satellites for clues to how the ozone will fare when sunlight—a third factor in ozone loss—returns to the Arctic in the spring. But they are already worrying about how extra ultraviolet light might affect humans and ecosystems below and wondering whether climate change will make such Arctic holes more common or severe.
Record cold temperatures in the Arctic stratospheric ozone layer, 15 to 35 kilometers up, are the proximate cause for this year’s losses, because they help to unleash ozone-destroying chemicals. “This winter has been stunning,” says Markus Rex, an atmospheric chemist at the Alfred Wegener Institute in Potsdam, Germany. By next week, about 25% of the Arctic’s ozone will be destroyed, he says.
This time of year, the stratosphere tends to warm up with the breakdown of the polar vortex, a cyclone that traps cold air. But if a strong vortex persists another month as light returns to the Arctic after the dark winter, ozone losses will get much bigger, Rex says. Conditions are ripe for losses to surpass a record Arctic ozone hole observed in the spring of 2011, he adds.
At Earth’s surface, ozone is a caustic chemical and a health hazard. But in the stratosphere, it shields the planet from ultraviolet light. Scientists noticed in the 1980s that chlorine-containing chemicals commonly used in refrigerants were reacting to form compounds that ate away stratospheric ozone, especially over the poles. The 1989 Montreal Protocol led to the phaseout of those chemicals, but their long atmospheric lifetime means that seasonal ozone losses will persist well into this century. Every year, a major ozone hole opens up over Antarctica, where winters are colder and polar vortices are stronger and more stable than over the Arctic.
But this year, the Arctic could be the poster child. Cold temperatures have allowed nitric acid, mostly from natural sources, to condense and form the peculiar, iridescent clouds that have been spotted all over northern latitudes this winter. “They’re beautiful, but once I see them, I’m concerned—they’re dangerous,” Rex says. That’s because the clouds catalyze the reactions that mobilize chlorine into active chemicals that can react in the presence of sunlight to destroy ozone.
An instrument on the NASA AURA satellite has detected record lows of the inert forms of chlorine and rising amounts of the active ones, notes Gloria Manney, an atmospheric scientist at NorthWest Research Associates in Socorro, New Mexico. “Conditions are primed,” she says. “The last ingredient we need is sunlight.” Weather models are predicting some warming of the stratosphere this week, she adds, but probably not enough to halt the ozone destroying brew.
The Arctic vortex tends to behave erratically, with blobs of cold air often dipping into more heavily populated northern latitudes. The influx of ozone-poor air could cause problems for people there, who are unused to wearing sunscreen in March, Rex says. “If we get such a deep minimum, then people need to be informed,” he says. The extra radiation could even adversely affect phytoplankton, which typically bloom in the Arctic Ocean each spring, Rex suggests.
Ross Salawitch, an atmospheric chemist at the University of Maryland, College Park, says the health hazards shouldn’t be sensationalized. “The worst-case scenario would be folks in high northern latitudes being in a type of ultraviolet environment that people are exposed to all the time in San Diego.”
For Salawitch, the bigger question is what role climate change might be playing. The notoriously mercurial polar weather is the main factor determining how much ozone is destroyed each spring, he says. But climate change is also expected to cool the stratosphere over the long run. The same greenhouse gases that trap heat in the lower atmosphere allow the stratosphere to more effectively radiate energy into space.
On its own, the stratospheric cooling could make bad ozone years in the Arctic more common. It should also make polar vortices stronger, and more stable. But there is evidence that storminess at lower latitudes—another thing that is expected to increase in a warming world—will make stable polar vortices less common.
Which effects will win out? Salawitch offers a parallel to hurricanes. Climate change is expected to make tropical hurricanes less frequent but more intense. Persistent Arctic vortices, too, could become scarcer but stronger. “When you have cold winters, they tend to be whoppers.” And that could mean that Arctic holes like this year’s could get deeper in the future.