Arctic Ozone

Topic Summary

Extensive ozone losses have occurred over the Arctic in the 1990s, and there is some concern that serious depletion episodes could become even more frequent over the next 10-20 years. Concentrations of ozone-depleting chemicals will be at or near peak levels during that time, and changes to the Arctic stratosphere arising from global warming could create more favourable conditions for depletion processes.

Large increases in ultraviolet radiation at the earth's surface as a result of deep ozone depletion could be highly damaging to sensitive Arctic life forms. Ozone losses over the Arctic could also reduce ozone amounts over the middle latitudes as a result of the mixing of air masses.

Deep ozone losses over both the Arctic and Antarctic are the result of special conditions that occur over polar regions in the winter and early spring. As winter arrives in each hemisphere, a vortex of winds develops around the pole and isolates the polar stratosphere. Without milder air flowing in from the lower latitudes and in the absence of sunlight, air within the vortex becomes very cold. At temperatures of -80°C or less, clouds made up of ice, nitric acid, and sulphuric acid begin to form in the stratosphere. These are called polar stratospheric clouds (PSCs), and they give rise to a series of chemical reactions that destroy ozone far more effectively than the reactions that take place in warmer air. The destruction of ozone begins with the return of sunlight in the spring and continues rapidly until the vortex dissipates and warmer temperatures prevent the formation of PSCs.

Over the Antarctic, these processes commonly lead to the formation of a massive ozone hole. Over the Arctic, however, ozone amounts have not yet fallen to the very low levels observed in Antarctica. This is partly because the Arctic has more ozone to start with, but it is also a result of the more variable atmospheric circulation of the Northern Hemisphere, which makes the Arctic vortex less stable. As a result, incursions of air from the south often keep the Arctic stratosphere too warm for PSC formation.

Arctic ozone depletion could be further enhanced over the next few decades, however, as a result of climatic changes caused by increasing accumulations of greenhouse gases such as CO2 in the atmosphere. Although the buildup of these gases causes warming at the earth's surface, it also contributes to cooling in the stratosphere. Since temperatures in the Arctic stratosphere often come within a few degrees of the threshold for PSC formation, further cooling of the stratosphere could cause PSCs to form more frequently and increase the severity of ozone losses. Preliminary studies with atmospheric models suggest that this effect could delay a recovery of the Arctic ozone layer by a decade or more.

A number of natural phenomena also affect Arctic ozone levels over time periods ranging from days to years. These include weather systems, the quasi-biennial oscillation (a periodic reversal of the direction of stratospheric winds over the equator), El Niņos, slight variations in solar radiation associated with the sunspot cycle, and volcanic eruptions.

Continued monitoring and research are essential if we are to reduce present uncertainties in our understanding of depletion processes and improve our capability to predict how the ozone layer is likely to respond to changing atmospheric conditions and stresses in the future. Canada's involvement in ozone research and monitoring reflects our special concern as a northern polar nation for the fate of the Arctic ozone layer.

The future of the Arctic ozone layer will depend primarily on our success in ridding the atmosphere of ozone-depleting chemicals, but our ability to control greenhouse gases will also be important. The linkages between these issues mean that we cannot treat either of them in isolation. Instead, they indicate the importance of developing a comprehensive strategy for moderating the human impact on the atmosphere.

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