|
2007 ozone hole
‘smaller than usual’
3 October 2007 European Space
Agency
The ozone hole over Antarctica
has shrunk 30 percent as compared to last year's record size. According to
measurements made by ESA’s Envisat satellite, this year’s ozone loss peaked
at 27.7 million tonnes, compared to the 2006 record ozone loss of 40 million
tonnes. Ozone loss is derived by measuring the area and the depth of the ozone
hole. The area of this year’s ozone hole – where the ozone measures less
than 220 Dobson Units – is 24.7 million sq km, roughly the size of North
America, and the minimum value of the ozone layer is around 120 Dobson Units. A
Dobson Unit is a unit of measurement that describes the thickness of the ozone
layer in a column directly above the location being measured. For instance, if
an ozone column of 300 Dobson Units is compressed to 0º C and 1 atmosphere (the
pressure at the Earth’s surface) and spread out evenly over the area, it would
form a slab of ozone approximately 3mm thick.
Time
series of the average ozone loss in the period 21-30 September 2007.
Credits: KNMI - ESA
Scientists
say this year’s smaller hole – a thinning in the ozone layer over the South
Pole – is due to natural variations in temperature and atmospheric dynamics and is not indicative of a
long-term trend.
"Although the hole is
somewhat smaller than usual, we cannot conclude from this that the ozone layer
is recovering already,” Ronald van der A, a senior project scientist at Royal
Dutch Meteorological Institute (KNMI), said.
"This year's ozone hole was
less centred on the South Pole as in other years, which allowed it to mix with
warmer air, reducing the growth of the hole because ozone is depleted at
temperatures less than -78 degrees Celsius."
During the
southern hemisphere winter, the atmospheric mass above the Antarctic continent
is kept cut off from exchanges with mid-latitude air by prevailing winds known
as the polar vortex. This leads to very low temperatures, and in the cold and
continuous darkness of this season, polar stratospheric clouds are formed that
contain chlorine.
Ozone
hole monitoring in terms of ozone loss over the last 10 years. Credits: KNMI -
ESA
As the polar spring arrives, the
combination of returning sunlight and the presence of polar stratospheric clouds
leads to splitting of chlorine compounds into highly ozone-reactive 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.
The ozone hole, first recognised
in 1985, typically persists until November or December, when the winds
surrounding the South Pole (polar vortex) weaken, and ozone-poor air inside the
vortex is mixed with ozone-rich air outside it.
KNMI uses data from Envisat's
Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY)
instrument to generate daily global ozone analyses and nine-day ozone forecasts.
Ozone is a
protective layer found about 25 km above us mostly in the stratospheric stratum
of the atmosphere that acts as a sunlight filter shielding life on Earth from
harmful ultraviolet rays. Over the last decade the ozone layer has thinned by
about 0.3% per year on a global scale, increasing the risk of skin cancer,
cataracts and harm to marine life.
The thinning of the ozone is
caused by the presence of ozone destructing gases in the atmosphere such as
chlorine and bromine, originating from man-made products like
chlorofluorocarbons (CFCs), which have still not vanished from the air but are
on the decline as they are banned under the Montreal Protocol, which was signed
on 16 September 1987.
Envisat can localise ozone
depletion and track its changes, enabling the rapid estimation of UV radiation
as well as providing forecasting. The three atmospheric instruments aboard
Envisat are SCIAMACHY, the global ozone monitoring by occultation of stars (GOMOS)
sensor and the Michelson interferometer for passive atmospheric sounding (MIPAS).
ESA data
form the basis of an operational near-real time ozone monitoring and forecasting
service forming part of the PROMOTE (PROtocol MOniToring for the GMES (Global
Monitoring for Environment and Security) Service Element) consortium, made up of
more than 30 partners from 11 countries, including KNMI.
As part of the PROMOTE and TEMIS
service, the satellite results are combined with meteorological data and wind
field models so that robust ozone and UV index forecasts can be made.
GMES responds to Europe’s needs
for geo-spatial information services by bringing together the capacity of Europe
to collect and manage data and information on the environment and civil
security, for the benefit of European citizens.
The GMES Service Element (GSE)
has been preparing user organisations in Europe and worldwide for GMES by
enabling them to receive and evaluate information services derived from existing
Earth Observation satellites since 2002.
|
