ESRL Quarterly Newsletter - Fall 2010

Antarctic Ozone Hole Persists, At Least for Awhile

Major success in reducing ozone-depleting substances may not pay off in the Antarctic for several more years

August in Antarctica means the Sun starts rising over the horizon again, following four months of darkness. For NOAA Corps officer Nick Morgan (GMD), stationed at the South Pole, the month also marks the moment when he begins measuring ozone in earnest.

For most of the year, Morgan and his colleagues launch giant plastic balloons into the air about weekly. Tethered to the balloons are instruments that take ozone readings up to about 18 miles high.

Then, in the Antarctic spring (August through October), sunlight-sparked chemical reactions begin eating away at ozone. Scientists start making measurements more often, and by October, Morgan or his colleagues are outside in minus 80°F temperatures about every other day. Morgan and other scientists around the world are watching those data carefully, looking for evidence that the Antarctic ozone hole is beginning to heal after decades of hurt.

There’s scant evidence yet, from the balloon-borne instrumnets or others on the ground and on satellites: At the end of September, total ozone was at its annual low of 122 Dobson units. Typical fall, winter, and summertime levels are 250-300 Dobson units. The worst-of-the-year ozone levels have averaged 108 during the last 24 years.

It will be difficult to establish a clear-cut recovery trend in Antarctic ozone levels because seasonal cycles and other variable natural factors—from the temperature of the atmosphere to the stability of atmospheric layers—can make ozone levels dip and soar from one day to another, says NOAA ESRL scientist Bryan Johnson. But the time is coming, probably within a few decades, when ozone depletion will no longer be observed each spring, Johnson said. 

“And within the next decade or so,” Johnson says, “observations are anticipated to begin showing reduced severity of the ozone hole.”

As soon as the Sun crosses the horizon again during the Antarctic spring, sunlight-triggered chemical reactions involving air pollutants begin destroying ozone in a region of the atmosphere called the stratosphere. The stratospheric ozone layer protects Earth from some damaging ultraviolet radiation, so an ozone hole means more of that radiation can hit the surface and trigger elevated rates of skin cancer and crop damage.

In the Antarctic, the ingredients for ozone depletion line up perfectly around September: Sunlight, low temperatures in the stratosphere, polar stratospheric clouds that help catalyze the destructive chemistry, and the continued presence of ozone-depleting chemicals, many of them released decades ago. Most years, those conditions ease by early December, and the hole closes.

“The ozone hole has taken somewhat of a back seat in the public eye,” Morgan wrote in a recent blog post from the South Pole.  “And maybe that is a sign of success.”

Levels of most ozone-depleting substances in the atmosphere have declined significantly since the 1987 Montreal Protocol was signed, he noted.

That international treaty initiated the phasing out of chemicals called chlorofluorocarbons (CFCs), then used widely in refrigeration, as solvents, and in aerosol spray cans. The chemicals were breaking down in the stratosphere, and reactive parts—chlorine and bromine atoms—triggered ozone destruction, when conditions are ripe (sunlight, polar stratospheric clouds, cold temperatures).

International scientists contributing to the quadrennial 2010 Ozone Assessment— including many NOAA scientists—have calculated that although global stratospheric ozone may recover by midcentury, the ozone hole in the Antarctic will likely persist longer.

More: http://esrl.noaa.gov/gmd/dv/spo_oz/ and http://icestories.exploratorium.edu/dispatches/south-pole-ozonesonde-launch (video).