The Antarctic ozone hole in the Southern Hemisphere is known for its yearly variations. This year, it has a counterpart.
An ozone hole has developed over the Arctic, a rare occurrence that scientists say is the biggest such atmospheric opening ever recorded over the planet's northernmost regions.
Persistent cold temperatures in the polar region and unusually quiet ozone dynamics have caused record depletion of Earth's protective ozone layer over the Arctic, according to Paul Newman, chief scientist for Earth sciences at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
"It's an unusual event," Newman said. "There is some Arctic ozone depletion every year, but it's more extreme in 2020 than in most years."
Earth's atmospheric ozone layer functions like a protective skin, shielding the planet's surface from the sun's harmful ultraviolet radiation. But in the past century, human-made chemicals known as chlorofluorocarbons have been eating away at the ozone and causing it to thin — and gape — in places.
This year, frigid temperatures caused so-called polar stratospheric clouds to form at high altitudes in the Arctic. Scientists have long known that these types of clouds play a key role in the destruction of Earth's ozone because they provide a surface high in the atmosphere for chemical reactions to take place that release harmful forms of chlorine.
"It's the unusual temperatures this year that led to unusual levels of polar stratospheric clouds, which led to unusual ozone depletion," said Newman, who runs NASA Ozone Watch, an online portal that tracks the health of the planet's ozone layer.
Another factor that has influenced the formation of the Arctic ozone hole is an observed lack of ozone mixing within the stratosphere, he added. In a normal year, weather systems can churn big atmospheric waves that ripple up from the lower atmosphere through higher altitudes, mixing up ozone in the stratosphere. But this year has been oddly quiet, according to Newman.
"We've seen winters like this before, so the real question is: Why is it so dynamically quiescent in the Arctic?" he said.
Newman said scientists don't yet know what causes atmospheric mixing to slow, but he said it's likely a key factor in ozone depletion over the Arctic.
Previous ozone holes over the North Pole were detected in 2011 and 1997, but this year's depletion — an area measuring a bit larger than Virginia — is the largest in NASA's 41-year record, Newman said.
The Arctic ozone hole is expected to heal and will likely disappear in the next month or so.
"Our forecasts suggest that temperatures have now started to increase in the polar vortex," Vincent-Henri Peuch, director of the European Union's Copernicus Atmosphere Monitoring Service, which is studying the phenomenon, said in a statement. "This means that ozone depletion will slow down and eventually stop, as polar air will mix with ozone-rich air from lower latitudes."
The depletion event is not expected to pose significant health concerns, but Newman said any breach in the ozone layer is worrisome.
"Right now, the sun isn't very high in the Arctic, so it doesn't really make a difference, but if these low levels persist, you would see higher and higher UV levels," he said. "Low ozone leads to more UV radiation, so that's not a good situation."
In 1987, 197 countries signed an international treaty known as the Montreal Protocol to protect the ozone layer by phasing out ozone-depleting chemicals such as chlorofluorocarbons. Without this landmark multilateral agreement, Newman said, the situation in the Arctic would likely be much worse.
And although the Arctic is registering record low levels of ozone, the situation in the Northern Hemisphere pales in comparison to the Antarctic ozone hole, which is vastly larger and much more volatile.
The Antarctic ozone hole typically reaches its annual peak in September and October before rebounding in December and subsequently growing again the next spring. Last year, the Antarctic ozone hole was the smallest it has been since it was discovered in 1985, but at its greatest extent, in 2006, the opening measured an average of 10.3 million square miles — larger than the entire North American continent.
"If the situation right now in the Arctic was over Antarctica in September or October, we would be saying that the ozone hole is gone — that's how big of a difference we're talking about," Newman said. "This is unusual for the Arctic, but it's not at all comparable to the Antarctic ozone hole."