Researchers announced last month that the total ozone levels in the Arctic dropped up to 30% during the annual springtime breakup of the polar vortex, a new record loss in the Arctic. The ozone destruction was caused by human-emitted chlorofluorocarbons (CFCs) present in the stratosphere. Strangely, observations
show that levels of CFCs and other ozone depleting gases are at a maximum now and are beginning to decline, thanks to the adoption of the Montreal Protocol in 1987. So why are we seeing record ozone losses?
b>Figure 1. Ozone loss in the Arctic the past 15 years. Source: Nature magazine
The answer is that ozone destruction is critically dependent upon the number of Polar Stratospheric Clouds (PSCs) in the stratosphere, since the chemical reactions that destroy ozone go much faster on cloud particles. Very cold temperatures cause more PSCs, and the temperatures in the stratosphere during the winter of 2005 were the coldest on record (50 years). Indeed, temperatures in the Arctic stratosphere have been declining steadily the past 20 years or more. The main reasons:
1) Lower ozone in the stratosphere due to destruction by CFCs means less UV light is absorbed. Since this absorption heats the air, less heating occurs. This lowers the temperature of the stratosphere, creating more PSCs, which creates ozone destruction, further lowering the temperature in a positive feedback loop.
2) The greenhouse effect acts to cool the stratosphere. Yes, the surface and lower atmosphere are warmed by the greenhouse effect, but this means that the atmosphere must cool somewhere else to compensate. Much of this compensating cooling happens in the stratosphere. This can best be understood by considering that the Earth is in "radiative equilibrium"--the amount of solar energy coming in is balanced by the amount of energy going out. Surface warming must be balanced by upper-atmosphere cooling, since the amount of solar radiation the Earth receives does not change.
Should we be concerned about this loss of ozone in the Arctic? Markus Rex, one of the authors of the Nature
study, remarks: "So UV-levels in spring have been higher than in "normal" winters. But they have not been higher than during summer or even in fall. So it is important not to overstate the problem. On the other hand the depletion of the ozone layer in spring certainly can have a significant effect on ecosystems, although so far the ozone layer has not been thinner in spring than later during the year. Plants and animals have adapted to the normal seasonal cycle of total ozone and UV and are not used to high UV-exposure during the part of their life cycle that takes place in spring (e.g. germination, growths of buds, alga blooms in the Arctic ocean, ... ). To my knowledge (which is quite limited in this area) the effect of increased UV-levels during spring on the various ecosystems is subject to ongoing research."
In the Antarctic, where Springtime ozone losses are much more severe (over 70% some years), only modest impacts on ecosystems have yet been found (for example, 6-12% declines in oceanic phytoplankton), so the current observations of record ozone loss in the Arctic should not cause undue alarm. Still, these results from the uncontroled global-scale experiment we humans are performing on our planet should make us all a little nervous. And the ozone destruction isn't going away for at least another 50 years--it will take that long for the atmosphere to flush out the accumulated burden of CFCs we've added.