Ever since the infamous ozone hole over Antarctica was discovered in 1985, ozone problems have most often been associated with the Southern Hemisphere. The "hole" shows up nicely on maps, and is easy to picture hovering over the South Pole.

The atmospheric conditions over the Arctic are a lot more complicated -- and hard to visualize -- but that does not mean that they may not become as serious. While an easy-to-see hole has not formed in the Arctic, the ozone there has thinned dramatically during the last decade, and scientists predict that the problem will get worse before it gets better.

What's more, ozone-destroying chemicals like chlorine are not the only culprits destroying the ozone layer; greenhouse gases are also involved. Scientists now know that the build-up of carbon dioxide and other greenhouse gases not only warms Earth's lower atmosphere, it cools the stratosphere, setting off a whole series of reactions that are all bad news for ozone.

In the spring of 1997, ozone values over the Canadian Arctic were the lowest ever recorded for March, as much as 45 percent below normal. David Wardle, a senior scientist with the Meteorological Services Branch of Environment Canada, says that we should expect levels to drop even lower at times.

"There is little doubt that we will see more frequent severe depletions in the arctic spring, and that we will see more severe cases of it," he says, adding that the ozone levels will continue to change from year to year.

Wardle is co-author of a 1998 report that predicted the increased ozone thinning in the Arctic. He says that since the report was released, evidence has mounted that the Arctic stratosphere will become colder and less friendly to ozone.

The problem is that the greenhouse gases radiate energy away from the stratosphere, driving temperatures there down. At temperatures of -80 degrees Celsius or lower, clouds composed of ice, nitric acid and sulphuric acid start to form.

These polar stratospheric clouds help trigger a series of reactions that destroy ozone far more effectively than the reactions that take place in warmer air. With the return of sunlight in the spring, ozone-depleting substances that are normally stable break down and release large volumes of chlorine and bromine, which deplete ozone.

So far ozone levels have not dropped to the very low levels observed over Antarctica. This is partly because the Arctic has more ozone to begin with, and also because the stratosphere has historically been warmer over the Arctic than over the Antarctic, so the polar stratospheric clouds did not regularly form over the North Pole.

Over both poles, winter winds form a vortex that isolates the polar stratosphere and causes it to become extremely cold. The Arctic vortex has been less stable in the past, allowing incursions of warmer air from the south, but the build-up of greenhouse gases is changing the circulation patterns.

The ozone layer protects all life on Earth from the harmful effects of excessive ultraviolet radiation. A thinner ozone layer can increase rates of skin cancer among humans, and it can also harm plants and animals, particularly those adapted to life in the Arctic.

"If life forms are adapted to certain conditions and thrive in them, it is only common sense that if you change the stress on them by increasing the UV radiation, there will be a reaction," says Wardle.

The ozone loss in the Arctic could also serious health problems for more people than the ozone hole over Antarctica. That's because ozone-depleted air from the Arctic drifts south each spring over North America, Europe and Russia -- areas with lots of people.

With sunlight just now starting to return to the arctic, scientists should soon know the status of the Arctic ozone layer for this year. In the long-term, the future of the layer will depend not only on getting rid of ozone-depleting chemicals, but also on controlling greenhouse gases.

For more David Wardle can be contacted at (416) 739-4632 or david.wardle@ec.gc.ca. Information on the Arctic ozone can also be found at www.exp-studies.tor.ec.gc.ca.