NASA scientists have revealed the inner workings of the ozone hole
that forms annually over Antarctica and found that declining chlorine in
the stratosphere has not yet caused a recovery of the ozone hole.
More
than 20 years after the Montreal Protocol agreement limited human
emissions of ozone-depleting substances, satellites have monitored the
area of the annual ozone hole and watched it essentially stabilize,
ceasing to grow substantially larger. However, two new studies show that
signs of recovery are not yet present, and that temperature and winds
are still driving any annual changes in ozone hole size.
"Ozone holes with smaller areas and a larger total amount of ozone
are not necessarily evidence of recovery attributable to the expected
chlorine decline," said Susan Strahan of NASA's Goddard Space Flight
Center in Greenbelt, Md. "That assumption is like trying to understand
what's wrong with your car's engine without lifting the hood."
To
find out what's been happening under the ozone hole's hood, Strahan and
Natalya Kramarova, also of NASA Goddard, used satellite data to peer
inside the hole. The research was presented Wednesday at the annual
meeting of the American Geophysical Union in San Francisco.
Kramarova
tackled the 2012 ozone hole, the second-smallest hole since the mid
1980s. To find out what caused the hole's diminutive area, she turned to
data from the NASA-NOAA Suomi National Polar-orbiting Partnership
satellite, and gained the first look inside the hole with the
satellite's Ozone Mapper and Profiler Suite's Limb Profiler. Next, data
were converted into a map that shows how the amount of ozone differed
with altitude throughout the stratosphere in the center of the hole
during the 2012 season, from September through November.
The map revealed that the 2012 ozone hole was more complex than
previously thought. Increases of ozone at upper altitudes in early
October, carried there by winds, occurred above the ozone destruction in
the lower stratosphere.
"Our work shows that the classic metrics
based on the total ozone values have limitations – they don't tell us
the whole story," Kramarova said.
(A look inside the 2012 ozone hole with the Ozone Mapper and Profiler
Suite shows how the build-up of ozone (parts per million by volume) in
the middle stratosphere masks the ozone loss in the lower stratosphere.)
The classic metrics create the impression that the ozone hole has
improved as a result of the Montreal protocol. In reality, meteorology
was responsible for the increased ozone and resulting smaller hole, as
ozone-depleting substances that year were still elevated. The study has
been submitted to the journal of Atmospheric Chemistry and Physics.
Separate
research led by Strahan tackled the holes of 2006 and 2011 – two of the
largest and deepest holes in the past decade. Despite their similar
area, however, Strahan shows that they became that way for very
different reasons.
Strahan used data from the NASA Aura
satellite's Microwave Limb Sounder to track the amount of nitrous oxide,
a tracer gas inversely related to the amount of ozone depleting
chlorine. The researchers were surprised to find that the holes of 2006
and 2011 contained different amounts of ozone-depleting chlorine. Given
that fact, how could the two holes be equally severe?
The researchers next used a model to simulate the chemistry and winds
of the atmosphere. Then they re-ran the simulation with the
ozone-destroying reactions turned off to understand the role that the
winds played in bringing ozone to the Antarctic. Results showed that in
2011, there was less ozone destruction than in 2006 because the winds
transported less ozone to the Antarctic – so there was less ozone to
lose. This was a meteorological, not chemical effect. In contrast, wind
blew more ozone to the Antarctic in 2006 and thus there was more ozone
destruction. The research has been submitted to the journal Geophysical
Research Letters.
This work shows that the severity of the ozone
hole as measured by the classic total column measurements does not
reveal the significant year-to-year variations in the two factors that
control ozone: the winds that bring ozone to the Antarctic and the
chemical loss due to chlorine.
Until chlorine levels in the lower
stratosphere decline below the early 1990s level – expected sometime
after 2015 but likely by 2030 – temperature and winds will continue to
dictate the variable area of the hole in any given year. Not until after
the mid 2030s will the decline stratospheric chlorine be the primary
factor in the decline of ozone hole area.
"We are still in the period where small changes in chlorine do not
affect the area of the ozone hole, which is why it's too soon to say the
ozone hole is recovering," Strahan said. "We're going into a period of
large variability and there will be bumps in the road before we can
identify a clear recovery."
