|This ozone map indicates the current amounts of ozone in the atmosphere over the Arctic. The area with the least ozone is located is over Canada's Arctic islands, Alaska, and Arctic Russia. The different coloured zones on this map represent Dobson Units (DU), the standard measurement used to quantify ozone levels. DUs represent the physical thickness of the ozone layer if it were compressed in the earth's atmosphere at standard temperature and pressure. One DU is equivalent to 0.01 mm thickness.|
The Ozone Hole
Most of the Earth's ozone is contained in the stratosphere, a layer of the atmosphere 10 to 40 kilometres above the surface of the earth. An "ozone hole" first appeared over Antarctica in the early 1980s. Since 1982, the hole it is actually a thinning of the ozone layer has formed there almost every spring. By spring 2000, this hole extended over 17 million square miles, an area larger than North America! Similar but less dramatic thinning happens in the Arctic each year. Scientists now agree that the main cause of ozone depletion is atmospheric pollutants.
The stratosphere above the Arctic loses about 12% of its ozone each year. The ozone hole is more serious in the Antarctic, where about 60% of the stratospheric ozone is lost each spring. There are two explanations for the difference. To start with, the Arctic has more ozone than the Antarctic. The Antarctic vortex is far stronger, so atmospheric conditions inside it remain undisturbed in winter and its temperatures fall much lower. In the Arctic, greater atmospheric circulation and mixing during winter makes its vortex less stable. There, events called sudden stratospheric warmings allow warmer air to enter the vortex. These can warm the stratosphere enough to prevent the cold polar clouds, where ozone is destroyed, from forming or reduce their lifespan. However, due to the presence of people, plants, and animals, increased UV radiation from ozone loss is still a concern over the Arctic.
Although the polar ozone "holes" have attracted the most attention, stratospheric ozone has also thinned over other areas of the globe. Worldwide, stratospheric ozone has fallen by 17% over the past 17 years, mostly in winter, but also in summer. In early 1993, scientists recorded low concentrations over much of Canada. These appeared to be linked to the injection of aerosols into the atmosphere from the eruption of Mt. Pinatubo in 1991. By 1994, ozone amounts over Canada had recovered to pre-Pinatubo levels, but were still lower than normal. Another reason for the decrease in mid-latitude ozone could be the mixing of Arctic air. Large depletions in the Arctic could cause this ozone-deficient air to dilute the ozone content of air farther south.