The unusually large hole that opened up in the ozone layer over the Arctic last year was likely caused by a chain of events sparked by record-high sea surface temperatures in the North Pacific, according to a new study reported in Advances in Atmospheric Sciences.
When you hear about holes in the ozone layer, it’s most likely referring to the patch above Antarctica in the southern hemisphere during austral spring around September, October, and November each year. The Arctic in the northern hemisphere is typically too warm for polar stratospheric clouds to form, which is the key driver of ozone depletion processes in the springtime.
Back in Spring 2020, however, an unprecedented hole formed over the Arctic. Scientists from the Chinese Academy of Sciences have now used satellite data and simulations to show that the hole was the product of record-high North Pacific sea surface temperatures that occurred between February and March. These warmer sea surface temperatures helped to weaken an important planetary wave (which help transfer heat from the tropics to the poles, and cold air from the poles to the tropics to maintain atmospheric balance), which affected the Aleutian low, a large atmospheric low-pressure center that frequently dwells over the Aleutian Islands near the Gulf of Alaska each winter.
The result of this knock-on effect was an extremely cold and persistent stratospheric polar vortex between February and April 2020, allowing the formation of polar stratospheric clouds that break down the ozone layer.
The ozone layer is a region of the stratosphere between 15 and 30 kilometers (9.3 to 18.6 miles) above Earth's surface that has a high concentration of the gas ozone. The layer absorbs much of the Sun's harmful ultraviolet rays, acting as an invisible shield for our planet. This layer is degraded by chlorofluorocarbons (CFCs) — human-made chemicals once widely used in aerosol sprays, solvents, and as refrigerants — after they are wafted up into the stratosphere.
While CFCs have been phased out under the Montreal Protocol in the late 1980s, they continue to lurk in Earth’s atmosphere for some time. This is especially problematic when there’s the formation of polar stratospheric clouds, high altitude clouds that can help to increase the chemical reactions involving the CFCs that lead to ozone depletion.
The Montreal Protocol is rightly regarded as a remarkable success — to date, it's the only United Nations environmental agreement to be ratified by every country in the world, and the ozone layer, as a whole, is in considerably better shape than it was three decades ago. However, as this study shows, the problem of CFCs and ozone depletion continues to haunt our planet.
“The formation of the record Arctic ozone loss in spring 2020 indicates that present-day ozone-depleting substances are still sufficient to cause severe springtime ozone depletion in the Arctic stratosphere.” lead author Professor Yongyun Hu from the Laboratory for Climate and Ocean-Atmosphere Studies at Peking University explained in a statement.
“These results suggest that severe ozone loss is likely to occur in the near future as long as North Pacific warm SST [sea surface temperature] anomalies or other dynamical processes are sufficiently strong.”