Despite the Arctic ozone
layer's record low this year,
the ozone loss is still
much less than the annual
ozone "hole" that occurs
over Antarctica in
September and October
during the Southern
Hemisphere spring.
Ozone levels
over Antarctica
typically drop to
120 Dobson units.
Ozone levels
above the Arctic
reached a record low
on March 12 at
205 Dobson units.
Similar low
ozone levels
occurred in
the upper
atmosphere,
or stratosphere,
in 1997 and 2011.
The lowest March
ozone value observed
in the Arctic is usually
around 240 Dobson units.
"This year's low Arctic ozone
happens about once per decade,"
said Paul Newman,
chief scientist for
Earth Sciences at NASA's
Goddard Space Flight Center
in Greenbelt, Maryland.
"For the overall health
of the ozone layer,
this is concerning since
Arctic ozone levels
are typically high
during March and April."
The stratospheric
ozone layer,
roughly 7 to 25 miles
above Earth's surface,
is a sunscreen,
absorbing harmful
ultraviolet radiation.
That radiation
can damage
plants and animals
and can affect people
by causing cataracts,
skin cancer and
suppressed immune
systems.
The March Arctic
ozone depletion
was caused by
a combination of factors
due to unusually weak
upper atmospheric
"wave" events from
December through
March.
These waves drive
movements of air
through the upper
atmosphere, like
weather systems
in the lower atmosphere,
but much bigger in scale.
In December 2019 and
January through March
of 2020, the stratospheric
wave events were weak
and did not disrupt
the polar winds.
The winds acted
like a barrier,
preventing ozone
from other parts
of the atmosphere
from replenishing
the low ozone levels
over the Arctic.
"We don't know
what caused the
wave dynamics
to be weak
this year,"
Newman said.
"But we do know
that if we hadn't
stopped putting
chlorofluorocarbons
into the atmosphere
because of the
Montreal Protocol,
the Arctic depletion
this year would have
been much worse."
Since 2000, levels of
chlorofluorocarbons
and other man-made
ozone-depleting
substances
have decreased
in the atmosphere
and continue to do so.
Chlorofluorocarbons
are long-lived
compounds
that take decades
to break down, and
scientists expect
stratospheric ozone
levels to recover
to 1980 levels
by mid-century.