Changes Brought On by Climate Change Threaten Layer That Protects Earth From Harmful Radiation
Stratospheric clouds above the Arctic, like those seen here over Kiruna, Sweden, provide ideal conditions for chemical reactions that transform chlorine to a form that depletes the Earth’s protective ozone layer. Unless greenhouse gas emissions are reduced, climate patterns that favor the formation of such clouds will continue to accelerate ozone loss, new research shows.
Extremely low winter temperatures over the Arctic are becoming more frequent and more extreme because of climate patterns associated with global warming, according to a new study from an international team of scientists including a University of Maryland researcher. The result is increased chemical reactions among substances humans pumped into the air decades ago, leading to greater ozone losses.
The new findings call into question the commonly held assumption that ozone loss would begin grinding to a halt following the 2010 global ban on the production of ozone-depleting chemicals called chlorofluorocarbons (CFCs) and halons.
The study—jointly conducted by UMD, the Alfred Wegener Institute’s Helmholtz Centre for Polar and Marine Research, and the Finnish Meteorological Institute—appeared yesterday in the journal Nature Communications.
“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,” said Ross Salawitch, a professor in the Department of Atmospheric and Oceanic Science, the Department of Chemistry and Biochemistry and the Earth System Science Interdisciplinary Center. “The increasingly cold temperatures create conditions that promote ozone depletion by CFCs. So, even though these compounds are slowly going away, Arctic ozone depletion is on the rise as the climate changes.”
New data from the study showed the lowest Arctic polar vortex temperatures and the highest ozone losses on record in 2020, beating the previous records set in 2011. During the winter of 2019-20, the coldest temperatures in the Arctic polar vortex were about 10 C (18 F) colder than the average over the past few decades.
The polar vortex is a relatively self-contained, low-pressure system that forms in the stratosphere at an altitude of about 12 to 50 kilometers (7.5 to 31 miles) over the Arctic every autumn and stays for varying durations throughout the winter to spring.
A trend toward more frequent and more extreme low temperatures in the polar vortex concerns the researchers, because those conditions promote the formation of clouds. That in turn promotes ozone loss in the polar stratosphere because clouds provide the right conditions for the atmospheric chlorine to change form and react with bromine and sunlight to destroy ozone.
Despite drastic reduction of the industrial production of CFCs and halons since the Montreal Protocol in 1987 and the global ban that followed in 2010, 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 researchers projected ozone loss out to the year 2100 based on the long-term temperature trend in the polar vortex and the expected decline in chlorine and bromine compounds. They based their predictions on the output from 53 top climate models used by the Intergovernmental Panel on Climate Change.
“All but one of the climate models we looked at show that exceptionally cold winters in the polar vortex will get colder over time,” Salawitch said. “And the more greenhouse gas emissions there are, the steeper the trend, which means greater ozone depletion.”
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. What’s more, their observations reveal that these trends are occurring at a rate consistent with the fastest climate change models.
“We have been saying that a train is coming for a number of years now,” said Salawitch, whose earlier publications showed extreme Arctic winters getting colder. “We’ve now seen the train whizzing by with 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.”
Because ozone filters much of the sun’s potentially harmful UV radiation, a depleted ozone layer over the Arctic can result in more UV radiation reaching the surface of the Earth over Europe, North America and Asia when the polar vortex dips south.
But there is hope for avoiding future ozone depletion, according to the researchers. Their study shows that substantial reductions in greenhouse gas emissions over the coming decades could lead to a steady decline in conditions that favor large ozone loss in the Arctic stratosphere.
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