Rising greenhouse gases threaten Arctic ozone layer

Extremely low winter temperatures in the atmosphere over the arctic are becoming more frequent and more extreme because of climate patterns associated with global warming, a study published Wednesday by Nature Communications found.

In addition, the extremely low temperatures are causing reactions among chemicals humans pumped into the air decades ago, leading to further reductions in the ozone layer of the Earth's atmosphere.

The findings call into question the commonly held assumption that the 2010 global ban on the production of ozone-depleting chemicals called chlorofluorocarbons, or CFCs, and halons would bring an end to ozone layer loss, they said.

"We're in a kind of race between the slow and steady decline in CFCs, which take 50 to 100 years to go away, and climate change, which is causing polar vortex temperature extremes to become colder at a rapid pace," study co-author Ross Salawitch said in a press release.

"The increasingly cold temperatures create conditions that promote ozone depletion by CFCs," said Salawitch, a professor of atmospheric and oceanic science at the University of Maryland in College Park.

This means that, despite the fact these compounds are slowly going away, "Arctic ozone depletion is on the rise as the climate changes," he said.

The ozone layer protects the Earth from damaging ultraviolet, or UV, radiation but is slowly being depleted due to greenhouse gas emissions.

Despite drastic reduction in the industrial production of CFCs and halons following the Montreal Protocol in 1987 and the global ban that followed in 2010, these long-lasting compounds are still abundant in the atmosphere, the researchers said.

Atmospheric chlorine and bromine produced by humans is not expected to fall below 50% of their highest levels until the end of this century, the World Meteorological Organization estimates.

Their presence in the stratosphere comes from the breakdown of CFCs, halons and other ozone-depleting substances, Salawitch and his colleagues said.

Normally, chlorine within the Arctic polar vortex is non-reactive, but clouds provide the right conditions for it to change form and react with bromine and sunlight to destroy ozone.

The polar vortex is a low-pressure system that forms in the stratosphere at an altitude of about 7.5 to 31 miles over the Arctic every autumn and remains through the winter and spring.

For this study, the researchers projected ozone loss out to 2100 based on the long-term temperature trend in the polar vortex and the expected decline in chlorine and bromine in the atmosphere.

Their predictions were based on the output from 53 top climate models used by the Intergovernmental Panel on Climate Change.

Combining these projections with analyses of meteorological data from the past 56 years, the researchers confirmed that the Arctic is already experiencing a significant trend toward lower stratospheric temperatures and associated increases in ozone losses.

In addition, these trends are occurring at rate consistent with the fastest climate models, the researchers said.

The lowest Arctic polar vortex temperatures and the highest ozone losses on record occurred in 2020, beating the previous records set nine years ago in 2011.

This trend toward more frequent and more extreme low temperatures in the polar vortex concerns the researchers because these conditions promote the formation of clouds, which in turn deplete ozone in the polar stratosphere.

The effects of increasing greenhouse gas emissions, and the associated changes to global climate, are causing the extreme cold winters in the stratospheric layer of the polar vortex are not fully understood.

However, it is known that global warming occurs in part because greenhouse gases trap heat closer to Earth's surface, allowing cooling of the upper layers in the stratosphere, where the ozone layer is located.

As warming at the Earth's surface causes changes to prevailing wind patterns, these changes also produce lower temperatures in the polar vortex, they said.

Recent years have also seen a rapid increase in methane, a more powerful greenhouse gas than carbon dioxide, in the lower atmosphere.

As this gas travels to the stratosphere, it increases humidity, which leads to conditions that promote ozone-destroying chemical reactions in the Arctic, the researchers said.

Because ozone filters much of the sun's potentially harmful UV rays, its depletion over the Arctic could result in more reaching the surface of Earth over Europe, North America and Asia when the polar vortex moves south.

Substantial reductions in greenhouse gas emissions over the coming decades, however, could lead to a steady decline in conditions that favor large ozone loss in the Arctic stratosphere, according to the researchers.

"All but one of the climate models we looked at show that exceptionally cold winters in the polar vortex will get colder over time and the more greenhouse gas emissions there are, the steeper the trend, which means greater ozone depletion," Salawitch said.

"We've now seen ... record ozone loss in 2011 and now in 2020, so this paper is really a wake-up call that something is happening in the atmosphere that's really important for ozone, and it looks like greenhouse gases are driving it," he said.

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