Ozone hole during 7 October 2008
as measured by the Scanning Imaging Absorption Spectrometer for
Atmospheric Cartography (SCIAMACHY) atmospheric sensor onboard ESA’s
Envisat.
Credits: KNMI/ESA
7
October 2008 European Space Agency
The 2008 ozone hole – a thinning in the ozone layer over Antarctica – is
larger both in size and ozone loss than 2007 but is not as large as
2006.
Ozone is a protective atmospheric layer found in about 25 kilometres
altitude that acts as a sunlight filter shielding life on Earth from
harmful ultraviolet rays, which can increase the risk of skin cancer and
cataracts and harm marine life.
This year the area of the
thinned ozone layer over the South Pole reached about 27 million square
kilometres, compared to 25 million square kilometres in 2007 and a
record ozone hole extension of 29 million square kilometres in 2006,
which is about the size of the North American continent.
The depletion of ozone is
caused by extreme cold temperatures at high altitude and the presence of
ozone-destructing gases in the atmosphere such as chlorine and bromine,
originating from man-made products like chlorofluorocarbons (CFCs),
which were phased out under the 1987 Montreal Protocol but continue to
linger in the atmosphere.
Depending on the weather
conditions, the size the Antarctic ozone hole varies every year. During
the southern hemisphere winter, the atmosphere above the Antarctic
continent is kept cut off from exchanges with mid-latitude air by
prevailing winds known as the polar vortex – the area in which the main
chemical ozone destruction occurs. The polar vortex is characterized by
very low temperatures leading to the presence of so-called stratospheric
clouds (PSCs).
As the polar spring
arrives in September or October, the combination of returning sunlight
and the presence of PSCs leads to a release of highly ozone-reactive
chlorine radicals that break ozone down into individual oxygen
molecules. A single molecule of chlorine has the potential to break down
thousands of molecules of ozone.
Julian Meyer-Arnek of the
German Aerospace Centre (DLR), which monitors the hole annually,
explained the impact of regional meteorological conditions on the time
and range of the ozone hole by comparing 2007 with 2008.
“In 2007 a weaker
meridional heat transport was responsible for colder temperatures in the
stratosphere over the Antarctic, leading to an intensified formation of
PSCs in the stratosphere,” Meyer-Arnek said. “Therefore, we saw a fast
ozone hole formation in the beginning of September 2007.”
“In 2008 a
stronger-than-usual meridional heat transport caused warmer temperatures
in the Antarctic stratosphere than usual, reducing the formation of
PSCs. Consequently, the conversion of chemically inactive halogens into
ozone-destroying substances was reduced. As a result in the beginning of
September 2008, the ozone hole area was slightly smaller than average,”
he continued.
Since the polar vortex
remained undisturbed for a long period, the 2008 ozone hole became one
of the largest ever observed.”
Minimum values of the
ozone layer of about 120 Dobson Units are observed this year compared to
around 100 Dobson Units in 2006. A Dobson Unit is a unit of measurement
that describes the thickness of the ozone layer in a column directly
above the location of measurement.
DLR’s analysis is based
upon the Scanning Imaging Absorption Spectrometer for Atmospheric
Cartography (SCIAMACHY) atmospheric sensor onboard ESA’s Envisat, the
Global Ozone Monitoring Experiment (GOME) aboard ESA’s ERS-2 and its
follow-on instrument GOME-2 aboard EUMETSAT’s MetOp.
Scientists say that since
the size and precise time of the ozone hole is dependent on the
year-to-year variability in temperature and atmospheric dynamics, the
detection of signs of ozone recovery is difficult.
“In order to detect these
signs of recovery, a continuous monitoring of the global ozone layer and
in particular of the Antarctic ozone hole is crucial,” Meyer-Arnek said.
In order to train the
next generation of atmospheric scientists to continue the monitoring,
students at ESA’s Advanced Atmospheric Training Course, held 15–20
September at University of Oxford, UK, were given the task of analysing
this year’s ozone hole with Envisat sensors.
Studying the Envisat
data, the students’ findings were in line with atmospheric scientists
that the south polar vortex was more concentric in 2008 than in 2007,
leading to a relatively late onset of ozone depletion, and that the size
of this year’s hole is similar to previous years.
“This exercise led us to
realise that although many questions have been answered and much has
been learned about the stratospheric chemistry and atmospheric dynamics
driving ozone hole behaviour, many new questions must be raised
especially concerning ozone hole recovery,” said Deborah C Stein Zweers,
a post-doc satellite researcher from the Royal Netherlands
Meteorological Institute (KNMI) who attended the course.
“We want to know when the
ozone hole will recover, how its recovery will be complicated by an
environment with increasing greenhouse gases and how atmospheric
dynamics will shape future ozone holes. These and many other questions
will attract the attention of our generation of scientists for the next
several decades.”
