NASA Sees First Direct Proof of Ozone Hole Recovery
For the first time, direct satellite observations of the ozone hole have shown that ozone-destroying chlorine levels are declining, resulting in less ozone depletion.
For the first time,
scientists have shown through direct observations of the ozone hole by a
satellite instrument, built by NASA's Jet Propulsion Laboratory in Pasadena,
California, that levels of ozone-destroying chlorine are declining, resulting
in less ozone depletion.Measurements
show that the decline in chlorine, resulting from an international ban on
chlorine-containing human-produce chemicals called chlorofluorocarbons (CFCs),
has resulted in about 20 percent less ozone depletion during the Antarctic
winter than there was in 2005 -- the first year that measurements of chlorine
and ozone during the Antarctic winter were made by NASA's Aura satellite."We see very clearly that chlorine from
CFCs is going down in the ozone hole, and that less ozone depletion is occurring
because of it," said lead author Susan
Strahan, an atmospheric scientist from NASA's Goddard Space Flight Center in
Greenbelt, Maryland.CFCs are
long-lived chemical compounds that eventually rise into the stratosphere, where
they are broken apart by the Sun's ultraviolet radiation, releasing chlorine
atoms that go on to destroy ozone molecules. Stratospheric ozone protects life
on the planet by absorbing potentially harmful ultraviolet radiation that can
cause skin cancer and cataracts, suppress immune systems and damage plant life.Two
years after the discovery of the Antarctic ozone hole in 1985, nations of the
world signed the Montreal Protocol on Substances that Deplete the Ozone Layer,
which regulated ozone-depleting compounds. Later amendments to the Montreal
Protocol completely phased out production of CFCs.Past
studies have used statistical analyses of changes in the ozone hole's size to
argue that ozone depletion is decreasing. This study is the first to use
measurements of the chemical composition inside the ozone hole to confirm that
not only is ozone depletion decreasing, but that the decrease is caused by the
decline in CFCs.The study was
published Jan. 4 in the journal Geophysical Research Letters.The Antarctic ozone hole forms during September in the
Southern Hemisphere's winter as the returning Sun's rays catalyze ozone
destruction cycles involving chlorine and bromine that come primarily from
CFCs.To determine how ozone
and other chemicals have changed year to year, scientists used data from JPL's Microwave
Limb Sounder (MLS) aboard the Aura satellite, which has been making
measurements continuously around the globe since mid-2004. While many satellite
instruments require sunlight to measure atmospheric trace gases, MLS measures
microwave emissions and, as a result, can measure trace gases over Antarctica
during the key time of year: the dark southern winter, when the stratospheric
weather is quiet and temperatures are low and stable.The change in
ozone levels above Antarctica from the beginning to the end of southern winter
-- early July to mid-September -- was computed daily from MLS measurements
every year from 2005 to 2016. "During
this period, Antarctic temperatures are always very low, so the rate of ozone
destruction depends mostly on how much chlorine there is," Strahan said. "This
is when we want to measure ozone loss."They found that
ozone loss is decreasing, but they needed to know whether a decrease in CFCs
was responsible. When ozone destruction is ongoing, chlorine is found in many
molecular forms, most of which are not measured. But after chlorine has
destroyed nearly all the available ozone, it reacts instead with methane to
form hydrochloric acid, a gas measured by MLS. "By
around mid-October, all the chlorine compounds are conveniently converted into
one gas, so by measuring hydrochloric acid we have a good measurement of the
total chlorine," Strahan said.Nitrous oxide
is a long-lived gas that behaves just like CFCs in much of the stratosphere.
The CFCs are declining at the surface but nitrous oxide is not. If CFCs in the
stratosphere are decreasing, then over time, less chlorine should be measured
for a given value of nitrous oxide. By comparing MLS measurements of
hydrochloric acid and nitrous oxide each year, they determined that the total
chlorine levels were declining on average by about 0.8 percent annually.The 20 percent
decrease in ozone depletion during the winter months from 2005 to 2016 as
determined from MLS ozone measurements was expected. "This is very close to what our model predicts we
should see for this amount of chlorine decline,"
Strahan said. "This gives us confidence that the decrease in
ozone depletion through mid-September shown by MLS data is due to declining
levels of chlorine coming from CFCs. But we're not yet seeing a clear decrease
in the size of the ozone hole because that's controlled mainly by temperature
after mid-September, which varies a lot from year to year."Looking
forward, the Antarctic ozone hole should continue to recover gradually as CFCs
leave the atmosphere, but complete recovery will take decades. "CFCs have lifetimes from 50 to 100 years, so
they linger in the atmosphere for a very long time,"
said Anne Douglass, a fellow atmospheric scientist at Goddard and the study's
co-author. "As far as the ozone hole
being gone, we're looking at 2060 or 2080. And even then there might still be a
small hole."To read the
study, visit:http://onlinelibrary.wiley.com/doi/10.1002/2017GL074830/abstractFor more on
MLS, visit:https://mls.jpl.nasa.gov/index-eos-mls.php