The Ozone Hole: The Arctic
Unlike the Arctic, the Antarctic is isolated from the rest of the world during the winter and spring by a natural circulation of wind called the polar vortex. This prevents atmospheric mixing of stratospheric ozone, thus contributing to the depletion of ozone. However, even small depletions in the Arctic region would give cause for considerable concern due to the higher populations in the higher latitudes of the northern hemisphere.
Do Ozone Holes Form Over The Arctic?
Arctic ozone depletion has not been as marked as over the Antarctic for two reasons: a) the stratospheric temperatures are seldom below -80°C due to frequent exchange of air masses with the mid latitudes; b) the Arctic air vortex usually dissipates in late winter before sunlight returns to initiate the ozone destruction. The differences between the two regions result in part from the larger land mass in the northern hemisphere, which causes more activity in the atmosphere.
Nevertheless, analysis of satellite data reveals that the loss of ozone in the northern hemisphere is now proceeding faster than previously thought. In 1989, NASA's Airborne Arctic Stratospheric Expedition, the first comprehensive research expedition to explore the Arctic region, found that the Arctic stratosphere in winter has almost as much chlorine monoxide as is found in Antarctica, the same destructive chlorine that causes the Antarctic ozone hole. While no Arctic ozone losses comparable with those in the Antarctic have occurred, localised Arctic ozone losses have been observed in winter concurrent with observation of elevated levels of reactive chlorine, made available through man-made emissions of CFCs. Ozone losses have increased greatly in the 1990s in the Arctic and in late 1997 were the greatest ever observed, according to measurements by NASA satellites.
The rate of loss in mid-latitudes has reached 8% per decade in late winter and spring, and significant loss is now encroaching on the growing season. This compares with the yearly-averaged global mean decrease in the amount of ozone of about 3% in the last decade.
Climate Change and Ozone Depletion in the Arctic
An ozone hole in the Arctic is expected to grow larger over the coming decades as a result of man-made greenhouse gas emissions which may cause climate change, before recovering after 2020. Loss of ozone in the Arctic by 2020 could be about double what would occur without greenhouse gases. Though greenhouse gases cause atmospheric warming at the Earth's surface, they cool the stratosphere by trapping more heat below, in the troposphere. Since ozone chemistry is very sensitive to temperature, particularly at -80°C when Polar Stratospheric Clouds can form, this stratospheric cooling may result in more ozone depletion in the Arctic.
Temperatures are slightly warmer in the Arctic than the Antarctic during their respective winter and spring seasons, with the result that ozone loss in the northern hemisphere has been lower than that in the southern hemisphere. But the Arctic stratosphere has gradually cooled over the last decade, resulting in the increased ozone loss. Computer models predict that temperature and wind changes induced by greenhouse gas emissions may allow a stronger and longer-lasting atmospheric vortex to form above the Arctic, as in the Antarctic, causing an increase in ozone depletion.
Because of international controls on the emission of ozone-depleting chemicals, those gases are expected to peak about the year 2000. However, greenhouse gas emissions continue to increase, despite international efforts to control them, and Arctic ozone depletion may continue to worsen until the 2010s, with two-thirds of atmospheric ozone lost in the most severely affected areas.