On April 5, the World Meteorological Organization announced that the stratospheric ozone in the Arctic had suffered an unprecedented drop over the winter and early spring, causing a large area of very thin ozone much like the annual ozone hole that forms in the Antarctic. In an interview with News Bureau physical sciences editor Liz Ahlberg, U. of I. atmospheric sciences professor Don Wuebbles, an expert in stratospheric ozone, explains the processes behind ozone depletion and the implications of the record low levels.
What protection does the ozone layer offer? What are the risks of ozone depletion?
The stratospheric ozone layer is extremely important to life on Earth because it protects us from very harmful levels of solar ultraviolet radiation. Ozone is a molecule made of three oxygen atoms that absorbs ultraviolet radiation, preventing the most energetic wavelengths from reaching Earth's surface. Ninety percent of atmospheric ozone is in the stratosphere. A decrease in the amounts of stratospheric ozone is of concern because more of the harmful levels of ultraviolet radiation can then reach the Earth. We talk about there being good ozone and bad ozone - good ozone is the ozone in the stratosphere that protects us from this UV radiation, whereas bad ozone is that in the smog we breathe in urban areas that has direct health effects.
How much did the Arctic ozone deplete? What causes such sharp seasonal fluctuations?
There was a 40 percent decline in total ozone over a large region of the Arctic this winter and early spring. This was a very unusual event because most years it is not cold enough in the lower stratosphere over the Arctic to have such an event. This year the Arctic stratosphere was extremely cold leading to a very strong polar vortex and the formation of polar stratospheric clouds in the lower stratosphere. Those clouds act as reaction sites to convert less reactive forms of chlorine and bromine in the lower stratosphere to much more reactive forms that can then react catalytically to destroy stratospheric ozone.
Then spring starts setting in with warmer temperatures and wave activity that break up the vortex, essentially spreading the depleted levels of ozone throughout the planet. Ozone is constantly produced and destroyed so the Arctic will fill in with higher levels of ozone from other latitudes.
These are essentially the same type of processes that produce the Antarctic ozone hole we have been seeing every year for more than 25 years, but we don't call the Arctic decrease in ozone a "hole" because it is so unusual.
We've been hearing about the "ozone hole" over the Antarctic for some time. What is the significance of now having record low levels in the Arctic as well?
It is likely the decrease in Arctic ozone will continue to be an extremely unusual event, in which case there will be very little long-term impact. The Antarctic ozone decrease occurs annually and will do so until the long-lived CFCS (chlorofluorocarbons) and other gases that are the sources of the reactive chlorine and bromine in the stratosphere are finally removed from the atmosphere.
How do the atmospheric conditions and UV exposure in the Arctic differ from the Antarctic?
Because of the presence of the land masses in the Northern Hemisphere, and winds blowing over those land masses, waves are produced that are then transported into the stratosphere where they keep the vortex weaker and temperatures too warm to form polar stratospheric clouds, at least in a normal year. In contrast, Antarctica is surrounded by ocean water and the polar winter vortex is much stronger around Antarctica.
Most of the time period of the high depletion over the Arctic this year were also times of strong lower level clouds, so very little of the increased levels of UV radiation reached Earth's surface. The ozone hole over Antarctica occurs every year so there is much more exposure to higher levels of UV for life in the Antarctic region.
The UN adopted the Montreal Protocol on Substances that Deplete the Ozone Level back in 1989. Wasn't that supposed to prevent further ozone loss? How long will recovery take?
The Montreal Protocol international agreement has resulted in the banning of the major compounds causing the high levels of chlorine and bromine in the stratosphere, but many of these compounds have very long atmospheric lifetimes so it will take a number of decades to clear the atmosphere sufficiently to allow ozone to recover. The Antarctic ozone hole may be with us for another 40-60 years.