Chlorine- and bromine-containing ozone-depleting substances (ODSs) are
controlled by the 1987 Montreal Protocol. In consequence, atmospheric
equivalent chlorine peaked in 1993 and has been declining slowly since
then. Consistent with this, models project a gradual increase in
stratospheric ozone with the Antarctic ozone hole expected to disappear
by ~2050. However, we show that by 2013 the Montreal Protocol had
already achieved significant benefits for the ozone layer.
Using a 3D
atmospheric chemistry transport model, we demonstrate that much larger
ozone depletion than observed has been avoided by the protocol, with
beneficial impacts on surface ultraviolet. A deep Arctic ozone hole,
with column values <120 DU, would have occurred given meteorological
conditions in 2011. The Antarctic ozone hole would have grown in size by
by 2013, with enhanced loss at subpolar
latitudes. The decline over northern hemisphere middle latitudes would
have continued, more than doubling to ~15%
Concern over the depletion of the ozone layer started in the 1970s
with the suggestion that chlorine from chlorofluorocarbons could reach
the stratosphere and cause ozone loss1.
Research activities intensified greatly in the mid-1980s following the
surprise discovery of significant ozone depletion over the Antarctic2,
the so-called ozone hole. This large decrease in ozone was explained by
chlorine- and bromine-catalysed loss acting in the particularly cold
conditions of the Antarctic polar lower stratosphere, which allow polar
stratospheric clouds to form (see, for example, Solomon3).
Ozone depletion was subsequently observed in the Arctic springtime
stratosphere, but the loss was much smaller than the Antarctic and more
variable from year to year4, 5. Cold years in the Arctic stratosphere, such as 1994/95, lead to an integrated column loss of around 120–140 DU (~35%), while warm years, without polar stratospheric cloud occurrence, produce essentially zero loss.
discovery of the Antarctic ozone hole helped stimulate the initial
signing in 1987 of the Montreal Protocol, an international treaty to
limit production of chlorine- and bromine-containing ozone-depleting
substances (ODSs). The Montreal Protocol has since been strengthened
greatly through subsequent amendments and adjustments, supported by
ongoing research, which has enhanced our understanding that ozone loss,
in the polar regions and globally, is driven by chemical processes
involving chlorine- and bromine-containing gases, arising mainly as
breakdown products of ODSs. With global compliance with Montreal
Protocol regulations, atmospheric chlorine peaked at ~3.6 parts per
billion by volume (p.p.b.v.) in 1993 at the surface and a few years
later in the stratosphere, and then began to decline. The current
tropospheric loading is 10% below the 1993 peak value.
Most of the major ODSs have long lifetimes (many decades) in the atmosphere6,
so a significant time delay is expected before the reduction in
atmospheric emissions of chlorine and bromine translate into an increase
in stratospheric ozone. Column ozone amounts in the northern middle
latitudes reached their minimum values in the mid-1990s at about 6–8% below the 1964–1980 mean7,
due to the combination of elevated stratospheric chlorine and bromine
with enhanced aerosol loading after the 1991 eruption of Mt Pinatubo.
Mean northern mid-latitude ozone values at the present day are still
around 4% below the long-term mean8.
The Antarctic ozone hole continues to re-appear each spring;
stratospheric chlorine is not expected to return to 1980 levels, when
the Antarctic ozone hole was first detectable, until about 2050.
Substantial Arctic ozone loss also continues to be observed in the late
winter/early spring of some years when polar stratospheric temperatures
are particularly low. The largest Arctic ozone loss observed to date
occurred in the recent cold winter of 2010/11 (ref. 9). By early April 2011, about 75%
of the ozone had been destroyed in a limited altitude region of the
polar lower stratosphere. Despite this relatively large local loss there
were substantial differences in the spatial extent and the degree of
local depletion between a typical Antarctic ozone hole and the 2011
Arctic loss. Near complete removal of ozone occurs annually in the
Antarctic lower stratosphere while the maximum loss in the 2011 in
Arctic is estimated at just over 80% at 18–20 km within a polar vortex covering a much smaller area than in the south. By late March, around 45%
of the Arctic polar vortex had column ozone values below 275 DU.
However, the observed column did not fall below the threshold for the
Antarctic ozone hole, taken as 220 DU. Therefore, by that definition an
‘Arctic ozone hole’ did not occur.
Several studies have investigated the benefits of the Montreal Protocol. Prather et al.10
first discussed the possible impacts if the protocol had not come into
force. Later studies have attempted to use atmospheric models to
quantify the benefits. These have focused on the avoided future impacts
later this century, assuming a continued strong growth in ODS emissions,
and stratospheric chlorine. Morgenstern et al.11
used a coupled chemistry-climate model (CCM) to compare the atmosphere
in 2030 with and without the effect of the Montreal Protocol. By 2030
their assumed stratospheric chlorine loading was 9 p.p.b.v. In addition
to increased stratospheric ozone loss, they found a feedback on
stratospheric dynamics and an impact on surface climate. Newman et al.12 used a similar CCM to investigate the impact on an uncontrolled 3%
per year growth in chlorine through 2065. As expected, they found very
large stratospheric ozone depletion and also large temperature changes
in response to the ozone loss. Garcia et al.13
used their CCM to study the chemical processes responsible for this
possible collapse of the ozone layer in the mid twenty-first century.
Finally, Egorova et al.14
also used a CCM to study the impact of the Montreal Protocol on the
ozone layer and again focused mainly on the large losses that would have
occurred by the end of this century. Their analysis did extend from
1987 to 2100 but, as they were using a free-running CCM like all of the
above studies, internal model variability prevented them from being able
to diagnose the impact of the Montreal Protocol during specific years
of the recent past.
These ‘World Avoided’ studies have thus mainly
looked far into the future under the assumption that no action would
have been taken to control ODS emissions, and compared the results with
those expected with full implementation of the Montreal Protocol. In the
face of the expected mounting evidence of an impact it seems quite
unlikely that ODS emissions would have continued to grow at the assumed
uncontrolled rates without some policy action by the middle of this
century. Furthermore, these studies have partly focused on the coupled
climate impact of large O3 loss, but climate models often
have temperature biases that make accurate quantitative modelling of the
strongly temperature-dependent polar ozone loss extremely challenging.
In contrast, off-line chemical transport models (CTMs) have been shown15, 16, 17
to give an accurate simulation of stratospheric ozone loss when they
use meteorological analyses—our best estimate of the ‘real’ meteorology.
CTMs have also been developed with a focus on accurate and complete
representations of chemical processes, although CCMs often use
computationally fast schemes in more complex climate models. Therefore, a
good CTM is likely to produce a more faithful simulation of the present
and past atmosphere than a good CCM.
Here we use CTM
calculations, covering the past two decades during which the growth of
atmospheric chlorine and bromine abundances first slowed and then
started to decline, to assess the benefits already achieved by the
Montreal Protocol. We show that much larger ozone depletion than
observed has been avoided by the protocol. A deep Arctic ozone hole,
with column values <120 DU, would have occurred given the
meteorological conditions in 2011. The Antarctic ozone hole would have
grown in size by 40% by 2013, with enhanced loss
at subpolar latitudes. The decline over northern hemisphere middle
latitudes would have continued, more than doubling to ~15% by 2013. This ozone loss would have led to increases in surface ultraviolet of up to 8–12% in Australia and New Zealand and 14% in the United Kingdom, and consequent increases in skin cancer.