loss is comparable to that in the early Antarctic ozone holes in the mid 1980s,
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.”

Santee observes that “the actual definition of an ozone hole has never been codified,
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, even for the Antarctic.” Over the past several decades, however, a de facto rule of thumb has developed. It's measured in terms of the total ozone throughout a column of the atmosphere spanning from Earth’s surface up to satellite height. Such composite measurements are logged in Dobson units,
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, and 220 is about the minimum needed to constitute a hole.

Ordinarily, Santee says,
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, 450 Dobson units “is sort of the canonical value for the Northern polar region’s ozone — what would be your sort of basal level.” This year, she notes, total-column Arctic ozone values “were below 250 Dobson units for nearly a month — and reached 220 to 230 for about a week.”

Geir Braathen, senior scientific officer with the World Meteorological Organization in Geneva, concurs that “scientists have not agreed on any threshold ozone loss,
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, like 250 or 260 Dobson units [for a hole].” Still, this atmospheric chemist cautions,
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, “I would be careful about calling the Arctic depletion an ozone hole” because it might lead people to think it's comparable to what emerges in the Antarctic. And it isn’t.

Antarctica's hole recurs annually,
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, whereas mega-thinning in Arctic ozone is novel.?Antarctica’s ozone also thins at some point to zero in a band many kilometers high. At no altitude has Arctic ozone ever fallen to zero — even in 2011. Finally,
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, Braathen points out, the aerial expanse and depth of the Antarctic hole

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