Arctic

Even a small depletion of stratospheric ozone in the Arctic region would give cause for considerable concern due to the higher populations in the higher latitudes of the Northern Hemisphere.

Arctic ozone depletion has not been as marked as over the Antarctic for two reasons. Firstly, stratospheric temperatures seldom fall below -80C due to frequent exchange of air with the mid latitudes. Without these low temperatures, the formation of polar stratospheric clouds is precluded, and ozone destruction rates are much lower. Secondly, the polar vortex of wind in the Arctic region is much weaker than the corresponding one in the Southern Hemisphere, and usually dissipates in late winter before sunlight returns to initiate a rapid ozone destruction. The differences between the two polar regions result in part from the larger land mass in the Northern Hemisphere, which causes more irregular airflow 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 and spring concurrent with observation of elevated levels of reactive chlorine, made available through man-made emissions of CFCs and other ozone depleting chemicals (ODCs). Ozone losses have increased greatly during the 1990s in the Arctic.

The springtime ozone hole in the Arctic is expected to grow larger during the coming decades as a result of global warming, before recovering after 2020. 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 -80C when polar stratospheric clouds can form, this stratospheric cooling may result in more ozone depletion in the Arctic. Furthermore, 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, enhancing ozone depletion.