Along with the mild winter across much of the eastern United States and the return of something more like real winter in Alaska, here’s something else we can blame on the polar vortex: a rare “hole” in the ozone layer over the Arctic in February and March 2020.
The stratospheric ozone layer is a natural barrier to high-energy ultraviolet (UV) light that can cause skin cancer and other DNA damage in people, other animals, and plants. In the late 1970s, scientists discovered that CFCs—short for chlorofluorocarbons—being used in cooling systems and aerosol sprays were damaging the ozone layer. The discovery led to an international treaty to phase out and eventually eliminate their use.
While it isn’t on par with the ozone hole that develops each year over the South Pole, this year’s severe event is a record for the Northern Hemisphere. Since ozone observations began in the 1970s, events like this have only happened two other times, during the winters of 1996-97 and 2010-11. In each case, the extreme ozone loss was the result of unusual weather that kept the Arctic colder and more isolated than normal in late winter.
What’s cold got to do with it?
In the lower atmosphere, CFCs are inert, but exposure to ultraviolet light in the stratosphere breaks them down into more reactive gases. The process is accelerated within polar stratospheric clouds. Commonly known as noctilucent (“night-shining”) clouds, they only form at temperatures below -78°C. It rarely gets that cold in the Arctic, even in the winter. That’s why when we talk about “the ozone hole,” we generally mean the one that happens every spring over the South Pole, where that kind of extreme cold is widespread.
But the winter of 2019-20 was highly unusual, explained Craig Long of NOAA’s Climate Prediction Center via email. “The cold temperatures in the Northern Hemisphere polar region were present all winter long without ‘weather’ disrupting the circulation pattern,” he wrote. “There have been previous years where part of the winter has been cold, 2010-2011 for example, but not for the entire winter. This winter is also interesting in that the stratosphere and troposphere [lower atmosphere] were coupled throughout most of the winter. By this I mean that the polar region (60°-90°N) had cold anomalies throughout the troposphere and stratosphere.”
Abundant amounts of polar stratospheric clouds throughout the dark winter months created a much bigger reservoir of reactive CFC byproducts than usual. As the Sun returned through late February and early March, ozone destruction occurred rapidly.