Arctic ozone: ‘Hole’ or just not whole?

This past spring, the Arctic stratosphere’s ozone layer suffered unprecedented depletion. But whether the record loss constituted a “hole” depends on which experts you consult.

In a Nature paper published online earlier this week, Gloria Manney of NASA’s Jet Propulsion Laboratory in Pasadena and more than two dozen coauthors describe the 2011 loss as “an Arctic ozone hole.” Other renowned scientists have been weighing in — and some argue that as dramatic as this year’s thinning was, a hole it wasn’t.

Reports of a putative hole in the far North’s ozone are far from new. A quarter century ago to this day, Science News ran a story noting that “while everyone’s attention has been riveted on the atmosphere above Antarctica, a NASA researcher has discovered what he believes is another ozone cavity that forms each [winter] on the other side of the world. . . . This Arctic ozone hole is ‘not as large in magnitude, but it’s unquestionably there.'”

Since then, descriptions of the recurrent depletion of Arctic ozone have been scaled back to more of just a demonstrable thinning. There’s been little question that its triggers, however, are identical to those that seasonably eat away huge portions of Antarctica’s stratospheric ozone.

What made 2011 different — and a watershed — argues Michelle Santee (a JPL colleague of Manney’s and coauthor on the new paper), is that at long last, “the magnitude of the [Arctic] loss is comparable to that in the early Antarctic ozone holes in the mid 1980s.”

Santee observes that “the actual definition of an ozone hole has never been codified, 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, and 220 is about the minimum needed to constitute a hole.

Ordinarily, Santee says, 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, like 250 or 260 Dobson units [for a hole].” Still, this atmospheric chemist cautions, “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, 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, Braathen points out, the aerial expanse and depth of the Antarctic hole greatly dwarfs the Arctic region that experienced substantial thinning earlier this year.

“Going into this Arctic spring, many of us — myself included — really thought this might be the year that we would see a real Arctic ozone hole,” observed Susan Solomon, of the University of Colorado, Boulder, at the recent American Chemical Society meeting in Denver. “But in the end,” she says, “I think it’s fair to say that we didn’t.”

It may be a matter of semantics, she concedes, but there was a rapid resupply of ozone from outside the Arctic vortex (that swirling wall of winds in the stratosphere that largely corrals a patch of atmosphere, rendering it vulnerable to ozone-destroying chemical reactions). Such a resupply does not occur in the Antarctic vortex, she notes; and that’s what permits its stratospheric ozone concentrations to plummet to zero over a several-kilometer height.

So, although the new paper clearly demonstrates that at some altitudes Arctic ozone was efficiently destroyed, Solomon says, “I wouldn’t call this an ozone hole.” 

Whatever you call 2011’s Arctic ozone depletion, “I consider this to be a ‘big deal,’” says Ross Salawitch of the University of Maryland in College Park. Moreover, there’s no reason to suspect that in some future years, the losses won’t be substantially worse, he says. 

Warming surface temperatures can cool the stratosphere, promoting conditions that accelerate ozone depletion, he notes. It takes prolonged cold temperatures in the winter and spring stratosphere to maximize ozone losses. Although such conditions have not been recurring annually in the Arctic, they have returned at three- to four-year intervals. And each new cold snap has been more extreme than the last, Salawitch points out.

Reinforcing concerns about future ozone depletion, Braathen says, is recognition that for many years to come there will be more than enough chlorine- and bromine-based pollutants in the stratosphere to allow for the possibility of “complete destruction of ozone — even in the Arctic — if it’s cold enough.”