Home > After 30 Years, Ozone is Recovering, Study Claims
A report shows the rate of ozone destruction declining
for the first time since CFCs were banned.
By Peter N. Spotts
Staff writer
The Christian Science Monitor
from the August 01, 2003 edition
For the first time, scientists have uncovered what they
see as unambiguous evidence that Earth’s sunscreen, a
tenuous shield of ozone in the stratosphere, is slowly
beginning to recover from nearly 30 years of human-
triggered loss.
In addition, new research is showing for the first time
that a worrisome latecomer to the list of compounds
threatening the ozone layer is vanishing worldwide from
the lower atmosphere.
The findings are good news on the environment that, for
some, underscore the effectiveness of global treaties
at prodding countries to curb pollution. In this case
the treaty in question is the 1987 Montreal Protocol
and its amendments, which are credited with triggering
these changes.
From a global standpoint, "this is the most significant
environmental success story of the 20th century," says
Michael Newchurch, an atmospheric chemist at the
University of Alabama at Huntsville.
The Montreal Protocol first limited, and then banned, a
group of chemicals known as chloroflurocarbons (CFCs).
It was later expanded to include a wider range of
ozone-threatening chemicals, including halons and, most
recently, methyl bromide. CFCs were widely used in a
range of products and technologies, while halons found
broad use as fire suppressants.
At first blush, Dr. Newchurch’s assessment may seem
like excessive praise for something that affects such a
tiny component of the atmosphere. On average, out of
every 1 million molecules of air, only a few will be
ozone. Some 10 percent of the atmosphere’s ozone
resides in the lower atmosphere, where a range of human
activities and natural processes can generate ozone
smog. High in the stratosphere, however, the remaining
90 percent of Earth’s ozone absorbs much of the
ultraviolet (UV) light coming from the Sun. Ozone is
particularly good at absorbing wavelengths of UV light
that can sever the chemical bonds of DNA, the
biological molecule that carries the genetic blueprint
for living organisms.
Encouraging new data
Until now, evidence that the Montreal Protocol is
having its desired effect has come from measurements
tracking the decline of ozone-destroying chemicals in
the lower atmosphere and in the stratosphere.
In the current case for the protocol’s impact, Exhibit
A comes from satellite measurements of ozone itself in
a region stretching from 22 to 28 miles above Earth’s
surface. Since 1979, instruments on a series of NASA
satellites have measured the concentrations of a range
of atmospheric gases by looking for their spectral
fingerprints as the satellites slip behind Earth and
catch sunlight passing through the thin veil of gases
enveloping the planet.
An initial study of ground-based data published last
year by Gregory Reinsel, a statistician at the
University of Wisconsin at Madison, suggested that
ozone destruction was declining.
When Dr. Newchurch’s team, which included Dr. Reinsel,
added the satellite data, "the evidence was so
compelling," Newchurch says. The study has been
accepted for publication in the Journal of Geophysical
Research.
Exhibit B comes from data that, for the first time,
show a global drop in bromine in the lower atmosphere.
While bromine appears in smaller concentrations than
CFCs and has a shorter residence time in the
atmosphere, researchers say, molecule for molecule it
packs more ozone-destroying punch than CFCs. The first
hints of the trend in bromine concentrations also came
last year, when Japanese researchers crunched the
numbers on data from one ground site and from aircraft
measurements.
This year, Stephen Montzka, an atmospheric chemist at
the National Oceanic and Atmospheric Administration’s
Climate Monitoring and Diagnostics Lab in Boulder,
Colo., looked at bromine data from its monitoring
stations around the world and found that bromine
concentrations have fallen nearly 5 percent since 1998.
The decline "is substantial," he notes, "considering
that bromine is 45 times more efficient" at depleting
ozone than the chlorine in CFCs. His team’s work is
slated to appear in an upcoming edition of Geophysical
Research Letters.
More progress needed
While this one-two punch of good news is encouraging,
the ozone layer is hardly home free, researchers say.
In the upper stratosphere, where Newchurch’s team saw
its trends, chemical reactions between chlorine and
ozone play the biggest role in determining how much
ozone there is.
In the lower stratosphere, where 80 to 90 percent of
the stratospheric ozone resides, factors such as
changing temperatures can affect ozone depletion and
restoration, says William Randel, an atmospheric
scientist at the National Center for Atmo spheric
Research in Boulder and a lead author of the United
Nations Environment Program’s "Scientific Assessment of
Ozone Depletion: 2002."
Those temperatures, he says, can be affected by
greenhouse gases - such as methane and water vapor -
that creep into the stratosphere and cool it. Strato
spheric cooling can slow the reactions that destroy
ozone at mid latitudes, which helps recovery. But the
cooling also has the perverse effect of accelerating
the destruction of ozone at the poles.
Until such conflicting forces can be sorted out and
correctly modeled, coming up with useful forecasts of
when the rate of ozone decline gives way to ozone
restoration will be difficult. Researchers say they
expect any recovery to take decades.
"We need to stay diligent" and keep to the protocol’s
provisions to have any hope of restoring the ozone
layer, says Elizabeth Weatherhead, an atmospheric
scientist at the University of Colorado at Boulder.
Still, she adds, Newchurch’s work represents "a valid
first indicator of recovery."