The Antarctic's ozone hole forms in the stratosphere every winter. It's a region of low ozone concentrations that puts the local population at risk of heightened UV exposure when the sun returns. Fortunately, the local population is tiny, and the hole dissipates considerably before reaching populated areas of the Southern Hemisphere. Although the atmospheric circulation in the Northern Hemisphere tends to limit the formation of a similar hole in more populated areas, this past year's conditions proved to be an exception, and a large area of depleted ozone has formed over Scandinavia, resulting in a warning about UV exposure from the World Meteorological Organization.

The discovery of the Antarctic ozone hole is credited with generating the political will to push through a phase out of the chemicals that are thought to be responsible for its depletion. Despite the success of the Montreal Protocol, which called for the phase out of chloroflourocarbons and their relatives, CFC decay and the regeneration of the stratosphere's ozone is a multidecade process, and we've only recently seen the levels of ozone stabilize. Thus, holes at both hemispheres remain distinct possibilities.

A specific set of atmospheric conditions are required in order to eliminate ozone. The stratosphere needs to remain relatively unmixed over the course of the winter (something that happens every year over Antarctica), and it has to get cold enough for ice crystals to form at the low pressures present there. The surface of that ice is necessary for the reactions that produce ozone-destroying CFC derivatives to form, and the return of sunlight in the spring enhances these reactions. This year, those conditions prevailed over the Arctic, dropping stratospheric ozone concentrations by 40 percent; the previous record for this hemisphere was 30 percent.

(This winter, the Arctic surface was actually unusually warm, even thought the stratosphere was colder than usual. An enhanced greenhouse effect, which traps heat in the lower atmosphere and prevents it from reaching the stratosphere, may have contributed to this difference.)

The WMO's announcement is intended to serve as a warning for those in Scandinavia and Central Europe: their expectations for how long they'd need to be out in the sun before getting a burn may not match reality this spring. Over time, this year's hole will dissipate as the stratosphere mixes, but we're probably still over a decade away from the point where problems like this will no longer be a concern.