This numerical model study is motivated by the observed global
deviation from assumed emissions of chlorofluorocarbon-11 (CFC-11,
CFCl3) in recent years. Montzka et al. (2018) discussed a strong
deviation of the assumed emissions of CFC-11 over the past 15 years, which
indicates a violation of the Montreal Protocol for the protection of the
ozone layer. An investigation is performed which is based on chemistry–climate model
(CCM) simulations that analyze the consequences of an enhanced CFC-11
surface mixing ratio. In comparison to a reference simulation (REF-C2),
where a decrease of the CFC-11 surface mixing ratio of about 50 % is
assumed from the early 2000s to the middle of the century (i.e., a mixing ratio
in full compliance with the Montreal Protocol agreement), two sensitivity
simulations are carried out. In the first simulation the
CFC-11 surface mixing ratio is kept constant after the year 2002 until 2050
(SEN-C2-fCFC11_2050); this allows a qualitative estimate of
possible consequences of a high-level stable CFC-11 surface mixing ratio on
the ozone layer. In the second sensitivity simulation, which is branched off
from the first sensitivity simulation, it is assumed that the Montreal Protocol is fully implemented again starting in the year
2020, which leads to a
delayed decrease of CFC-11 in this simulation (SEN-C2-fCFC11_2020) compared with the reference simulation; this enables a rough and most
likely upper-limit assessment of how much the unexpected CFC-11 emissions to
date have already affected ozone. In all three simulations, the climate evolves
under the same greenhouse gas scenario (i.e., RCP6.0) and all other ozone-depleting substances decline (according to this scenario). Differences
between the reference (REF-C2) and the two sensitivity simulations
(SEN-C2-fCFC11_2050 and SEN-C2-fCFC11_2020)
are discussed. In the SEN-C2-fCFC11_2050 simulation, the total
column ozone (TCO) in the 2040s (i.e., the years 2041–2050) is particularly
affected in both polar regions in winter and spring. Maximum discrepancies
in the TCO values are identified with reduced ozone values of up to around 30 Dobson
units in the Southern Hemisphere (SH) polar region during SH spring (in the
order of 15 %). An analysis of the respective partial column ozone (PCO)
for the stratosphere indicates that the strongest ozone changes are calculated
for the polar lower stratosphere, where they are mainly driven by the
enhanced stratospheric chlorine content and associated heterogeneous
chemical processes. Furthermore, it was found that the calculated ozone
changes, especially in the upper stratosphere, are surprisingly small. For
the first time in such a scenario, we perform a complete ozone budget
analysis regarding the production and loss cycles. In the upper stratosphere, the budget analysis shows
that the additional ozone depletion due to the
catalysis by reactive chlorine is partly compensated for by other processes
related to enhanced ozone production or reduced ozone loss, for instance
from nitrous oxide (NOx). Based on the analysis of the
SEN-C2-fCFC11_2020 simulation, it was found that no major
ozone changes can be expected after the year 2050, and that these changes are related to the
enhanced CFC-11 emissions in recent years.
Introduction
The estimation of the future evolution of the ozone layer is a central part
of the UNEP/WMO Scientific Assessment of Ozone Depletion. For this reason,
chemistry–climate models (CCM) are employed to carry out long-term simulations (for
several decades). These models are used to perform comprehensive numerical
simulations under well-defined boundary conditions that prescribe
possible future changes of ozone-depleting substances (ODSs), particularly
those related to changes in chlorofluorocarbon (CFC) concentrations. In
recent years, model guidelines have been defined to facilitate the
intercomparison of CCM results from different modeling groups worldwide.
For instance, in 2012, the Chemistry–Climate Model Initiative (CCMI), under
the umbrella of IGAC/SPARC, defined the boundary conditions for the next
round of coordinated reference (REF) and sensitivity (SEN) simulations
(Eyring et al., 2013). The boundary conditions for the CCM simulations
consider not only the expected changes in ODSs according to the regulations
of the Montreal Protocol and its amendments, but also the influence of
different climate change scenarios. Here, the greenhouse gas concentrations
for the Representative Concentration Pathways (RCPs) adopted by the IPCC for
its Fifth Assessment Report (AR5) in 2014 (van Vuuren et al., 2011) were
recommended. The suggested reference simulation for the future (REF-C2)
assumes full compliance with the Montreal Protocol, expecting more or less
no further production of CFCs.
More recently the respective CCM results have been presented and discussed in
several scientific papers, for instance Dhomse et al. (2018). Among other
topics, the question of ozone recovery has been investigated, including how the speed of recovery and the return date of ozone are affected by the
expected decrease in ODSs and by climate change. The results of the CCM
simulations were taken into account as the foundation for the latest WMO
ozone report (WMO, 2018).
During the preparation phase of WMO (2018) a paper by Montzka et al. (2018)
was published (hereafter referred to as M18), which indicated a clear deviation of the expected surface
concentration of chlorofluorocarbon-11 (CFC-11, CFCl3) during the past 15 years. Observational datasets discussed by M18 showed the following:
(1) in the last 10 years (until 2017) the decline of CFC-11 surface
mixing ratios has been much slower than expected (see Fig. 1a in
M18); and (2) the decline of CFC-11 surface mixing ratios was nearly constant
from 2002 to 2012, whereas in the following years the decrease of CFC-11
surface mixing ratios decelerated (here until 2017; see Fig. 1b in M18).
The measurements indicated increased CFC-11 emissions after 2012 (see Fig. 2a in M18; see also Figure ES-2 in WMO, 2018). M18 proposed that these observations imply “a gap in our understanding of
CFC-11 sources and sinks since the early 2000s”.
Schematic of the EMAC model simulations performed: a reference
simulation (REF-C2) and two sensitivity simulations
(SEN-C2-fCFC11_2020 and SEN-C2-fCFC11_2050)
enabling an assessment of enhanced CFC-11 surface mixing ratios on the ozone
layer. The prescribed CFC-11 surface mixing ratios are given on the
nonlinear vertical axis. The prescribed CFC-11 surface mixing ratios are
based on Table 5A-3 in Daniel et al. (2011).
Based on these findings a significant impact on the recovery of the ozone
layer seems to be possible, in particular, if CFC-11 emissions do not
decline as previously anticipated (e.g., Daniel et al., 2011;
Carpenter et al., 2014). Therefore, the assumption of a decreasing
CFC-11 surface mixing ratio in the future by CCMI is somewhat questionable.
Currently, the future evolution of CFC-11 emissions is uncertain (Harris et
al., 2019). Therefore, for this numerical study, we cannot decide which
future scenario is most likely; the possible ranges of future CFC-11
emission changes are also difficult to estimate. Our approach is to employ a high-emission scenario of CFC-11 in a sensitivity simulation
(SEN-C2-fCFC11_2050) by imposing constant surface mixing
ratios of CFC-11 from 2002 onwards (Fig. 1). The sensitivity simulation
covers the period from 2002 to 2050. This sensitivity simulation should not
be considered as a specific future scenario that we deem likely. As previously stated,
this model setup can be viewed as a high-level scenario regarding CFC-11
background conditions. The reason for starting this sensitivity simulation
in the year 2002 is motivated by the findings of M18, who showed that the observed emissions started to diverge from the expected
emissions of CFC-11 from 2002 onward (e.g., Fig. 2a in M18). We note that stable CFC-11
emissions are not equal to stable CFC-11 surface concentrations; however, due to
the fact that the future evolution of CFC-11 emissions (and also the surface
mixing ratio) is uncertain, such a simplified (crude) assumption of a
constant surface mixing ratio is absolutely justified as a high-level
scenario dating back to the point when expected emissions and observations
started to diverge. An alternative sensitivity simulation could be created
assuming a later starting date (e.g., 2017) with a constant surface mixing
ratio on a lower level, i.e., prescribing a lower surface mixing ratio. We
expect that the results of such a sensitivity simulation after
∼50 years of simulation (e.g., 2065 for a start in 2017) would only
change slightly in a quantitative manner
compared with our SEN-C2-fCFC11_2050 (in 2050), but not qualitatively compared with the
reference simulation. Based on our understanding, the calculated ozone changes are
primarily affected by the prescribed CFC-11 differences (e.g., between REF-C2
and SEN-C2-fCFC11_2050) rather than by the CFC-11
background value.
Although M18 hinted that the additional CFC-11 emissions might be released
in eastern Asia (see also Rigby et al., 2019), we do not impose any specific
regional features due to the long lifetime of CFC-11 (e.g., Rigby et al.,
2013). In comparison with the reference simulation (REF-C2), the SEN-C2-fCFC11_2050 sensitivity
simulation allows us to investigate the
potential impact of previously unaccounted for CFC-11 emissions. It enables a
rough estimation of the additional possible ozone loss under a constant CFC-11
surface mixing ratio in the coming years and how it may impact the timing of
the full recovery of the ozone layer. In addition, the second sensitivity
simulation (SEN-C2-fCFC11_2020, see also Fig. 1) is carried
out to allow for an estimate of ozone changes in case of full recognition
and implementation of the Montreal Protocol again in the coming years. After
an 18-year period with a constant CFC-11 surface mixing ratio (in line with
SEN-C2-fCFC11_2050 from 2002 to 2019), the CFC-11 surface
mixing ratio is decreased in the following years (in parallel with REF-C2)
under the assumption that the recent additional CFC-11 emissions will drop
down to zero starting in the year 2020.
It is the aim of this study to show when and where ozone loss occurs due to the additional influx of CFC-11 into the atmosphere while other ODSs decline as
expected. Our scenario, described in SEN-C2-fCFC11_2050, could
be taken as “what should be avoided”, somewhat in the tradition of the
Newman et al. (2009) paper. Furthermore, SEN-C2-fCFC11_2020 provides an estimate of the impact of the temporary increase in CFC-11
emissions in recent years on the ozone layer and its recovery. Our analysis
presented here does not aim at specifically investigating the effects of the
discovered CFC-11 emissions and numerous “directly” related scenarios.
Instead, we want to assess the impact of an enhanced CFC-11 surface mixing ratio as
a sensitivity study. The foundation for our numerical exercise is the REF-C2
simulation, which was performed with our CCM EMAC (see description in
Sect. 2). The results of this reference simulation were one of our several
contributions to the Dhomse et al. (2018) study and WMO (2018). The results
of EMAC were checked against observations (for the past) in detail (e.g.,
Jöckel et al., 2016) and were also compared with results derived from
other CCMs. No obvious weaknesses or significant deficiencies could be
identified.
After a short description of the CCM used and the model simulations analyzed
(REF-C2, SEN-C2-fCFC11_2050, and SEN-C2-fCFC11_2020) in the next section (Sect. 2), the CCM results are presented in Sect. 3. With respect to the results, we focus on changes in total column ozone and partial
stratospheric columns in specific geographical regions and seasons. To the
best of our knowledge, a detailed ozone budget analysis for such sensitivity
simulations is performed and presented for the first time. Finally, the
discussion and conclusion are presented at the end of this paper.
Description of the model and simulations
In this study, version 2.52 of the CCM EMAC, which stands for “European Centre for
Medium-Range Weather Forecasts – Hamburg (ECHAM)/Modular Earth Submodel
System (MESSy) Atmospheric Chemistry model”, is used
and is operated in T42L90MA resolution, corresponding to a quadratic
Gaussian grid of approx. 2.8∘× 2.8∘ in latitude and longitude with
90 levels up to 0.01 hPa. More details are given in Jöckel et al. (2016).
The joint IGAC/SPARC Chemistry–Climate Model Initiative (CCMI) proposed
several reference and sensitivity simulations for CCM studies. The aim was
to support upcoming ozone and climate assessment reports. From this connection,
an internally consistent simulation from the past into the future between
1960 and 2100 has been suggested (Eyring et al., 2013). This transient
reference simulation, i.e., REF-C2, as used in this study, is forced by trace
gas projections as well as prescribed sea surface temperatures (SSTs) and sea ice
concentrations (SICs). The projection component of REF-C2 uses greenhouse
gas concentrations (i.e., CO2, CH4, and N2O) that follow the
Intergovernmental Panel on Climate Change (IPCC) Coupled Model
Intercomparison Project Phase 5 (CMIP5) Representative Concentration
Pathways 6.0 (RCP6.0) scenario (van Vuuren et al., 2011). Monthly mean
global SST and SIC data, which were simulated by the HadGEM2 climate model
with an interactive ocean (Hadley Centre Global Environmental Model version 2,
data used for the RCP6.0 scenario; see Jones et al., 2011), were used as
boundary conditions for the REF-C2 simulation.
For this study, in addition to the REF-C2 simulation (for more details see
Jöckel et al., 2016; here it is the “RC2-base-04” reference
simulation), two specific sensitivity simulations
(SEN-C2-fCFC11_2050 and SEN-C2-fCFC11_2020)
are designed to address the possible consequences of additional emissions of
CFC-11, which affect the chlorine content of the stratosphere after a period of years. In all EMAC simulations mixing ratios of ODSs (CFCs: CFCl3,
CF2Cl2, CH3CCl3, and CCl4; HCFCs: CH3Cl and
CH3Br; Halons: CF2ClBr and CF3Br) in the lowest model layer are
adapted by Newtonian relaxation to observed or projected surface mixing
ratios (Kerkweg et al., 2006). The applied tracer nudging procedure
diagnoses the emission flux of CFC-11, which is necessary to adjust to the
prescribed surface mixing ratio. In the REF-C2 simulation, the mean CFC-11
surface mixing ratio in the year 2002 is 258.3×10-12 mol mol-1 (see Table 5A-3 in Daniel et al., 2011), and it is significantly reduced by
more than 50 % (127.2×10-12 mol mol-1) in the year 2050 (i.e., the
mixing ratio of the baseline (A1) scenario; WMO, 2011; the respective values
are also presented in Fig. 1). The 2050 value is projected under the
assumption of full compliance with the Montreal Protocol. The cumulative
CFC-11 emissions in the REF-C2 simulation (from 2002 to 2050) result in
about 400 Gg. Apart from one point, the sensitivity simulation
(SEN-C2-fCFC11_2050) is identical to the reference simulation
(REF-C2): in the sensitivity simulation the CFC-11 mean surface mixing ratio
is kept constant at 258.3×10-12 mol mol-1 after the year 2002, whereas in REF-C2 it declines. The CFC-11 emissions required to achieve the
constant surface mixing ratio value in our model after the year 2002 in
SEN-C2-fCFC11_2050 is about 90 Gg yr-1 (e.g., for the year 2003 it
is 87 Gg yr-1; the emissions in our model simulation increase slightly
with time, which is likely due to the small reduction in the CFC-11 lifetime; see for
instance SPARC, 2013). The cumulative CFC-11 emissions (from 2002 to 2050)
result in about 4500 Gg (i.e., roughly 4100 Gg more than in REF-C2). The
emission values derived from observations given by Montzka et al. (2018) are
about 65 Gg yr-1 (mean) for 2002 to 2012 and 75 Gg yr-1 from 2014 to 2016
(see also Rigby et al., 2019). The figures presented by Rigby et al. (2019)
and Harris et al. (2019) with respect to the temporal evolution of CFC-11
emissions indicated a further increase after 2016. In the second sensitivity
simulation, SEN-C2-fCFC11_2020, full adherence
of the Montreal Protocol is assumed after the year 2020. The starting point of this simulation
is aligned with SEN-C2-fCFC11_2050, assuming the same constant
CFC-11 surface mixing ratio between 2002 and 2019 (i.e., 258.3×10-12 mol mol-1), but with decreasing mixing ratios after 2020 until 2050. The
cumulative CFC-11 emissions (from 2002 to 2050) result in about 2100 Gg
(i.e., roughly 1700 Gg more than in REF-C2). A schematic of our three model
simulations is presented in Fig. 1.
In both sensitivity simulations, we do not emphasize specific regions
regarding outstanding changes of the CFC-11 surface mixing ratio, e.g., in
eastern Asia (see discussion in Sect. 1). The modified CFC-11
boundary condition in the SEN-C2-fCFC11_2050 and
SEN-C2-fCFC11_2020 simulations should cause a change of the
stratospheric chlorine loading after about 10–15 years (e.g., Engel et al.,
2002).
Presentation of CCM resultsReactive chlorine
Based on the prescribed changes of the CFC-11 boundary conditions the
stratospheric content of reactive chlorine compounds (ClOx=Cl+ClO+OClO+2ClOOCl+2Cl2+HOCl) is expected to change. In
Fig. 2, the simulated change (i.e., SEN-C2-fCFC11_2050 and
SEN-C2-fCFC11_2020 minus REF-C2, respectively) of the reactive
chlorine mixing ratios with time is shown for the lower stratosphere (LS,
near 50 hPa) and the upper stratosphere (US, near 2 hPa). Because all model
simulations are operated in a “free-running-mode” (i.e., they do not have the
same meteorology), the year-to-year difference (thin red and blue) curves as
shown in Fig. 2 (and also in other figures shown afterwards)
indicate the possible range of interannual fluctuation. Obviously it
takes about 10–15 years (as expected) before the ClOx values of
the REF-C2 and SEN-C2-fCFC11_2050 simulations clearly diverge
from each other. At the end of the SEN-C2-fCFC11_2050
simulation (i.e., in 2050), the resulting absolute (mean) difference in EMAC
amounts to an approximate 6×10-12 mol mol-1 increase in the LS and
about a 50×10-12 mol mol-1 increase in the US compared with the REF-C2 simulation.
The vertical profile (structure) of the changes of the chlorine mixing ratios
qualitatively resembles the reference ClOx profile in EMAC (i.e., changes
are biggest where the ClOx mixing ratios are biggest), which displays a
distinct maximum at about 2–3 hPa (not shown). It turns out that the
amount of chemically active chlorine species (ClOx) in the
SEN-C2-fCFC11_2050 simulation is about 17 %
larger on average than in the REF-C2 simulation in the US
(above 30 km) in the 2040s (i.e., the time period from 2041 to 2050); in the LS (below 30 km) the respective
amount of ClOx is enhanced by about 30 %. As can be expected, the
respective ClOx differences between the SEN-C2-fCFC11_2020 and the REF-C2 simulations in the 2040s are clearly smaller,
i.e., about 2×10-12 mol mol-1 in the LS (about 14 %) and about
25×10-12 mol mol-1 in the US (about 9 %).
Temporal evolution of the annual global mean ClOx mixing
ratio differences (in mol mol-1) at around 2 hPa (US, a) and 50 hPa (LS,
b) between the SEN-C2-fCFC11_2050 and REF-C2 simulations (in red)
and between the SEN-C2-fCFC11_2020 and REF-C2 simulations (in blue). The
11-year solar cycle (smoothed with a 1-2-1 filter) has been removed from
both time series. The thicker curves in red and blue show the 5-year running
means. The red and blue lines show the linear regression estimate of the
unsmoothed time series.
Total and partial ozone columns
The impact of the enhanced atmospheric ClOx content due to the constant
CFC-11 surface mixing ratio after 2002 on total column ozone (TCO) is shown
in Fig. 3. It illustrates the differences in the mean annual cycle for the
last decade depending on the geographical latitude between the REF-C2 and
the SEN-C2-fCFC11_2050 simulations. The largest changes in the TCO values
are found in both polar regions: in particular in the Northern Hemisphere
during winter (December, January, and February) and in the Southern Hemisphere
in late winter (August) to early spring (September and October). In the
SEN-C2-fCFC11_2050 simulation, the TCO values are clearly
reduced by up to about 30 Dobson units (DU) (in the order of 15 % in the
Southern Hemisphere) in comparison with the REF-C2 simulation. During other
times of the year and in other latitudinal regions the identified TCO
changes are much smaller: they are mostly below ±5 DU.
A closer look at the near-global mean (60∘ S–60∘ N)
temporal behavior of the TCO (Fig. 4) indicates increasing ozone values in
both simulations (Fig. 4a, for both simulations the solar
cycle was removed, see figure caption). The results of the
SEN-C2-fCFC11_2050 simulation show slightly smaller
TCO values compared with REF-C2 and a slightly flatter slope of the
linear trend line (this regression accounts for possible autocorrelation at
lag 1, see, e.g., the method described in Tiao et al., 1990). The linear
regression based on the results of the REF-C2 simulation (here the black
line presented in Fig. 4a) indicates an increase of 1.7 DU per decade for the TCO (annual near-global mean), whereas the linear
regression based on the results of the SEN-C2-fCFC11_2050
(red line in Fig. 4a) shows a reduced increase of 1.3
DU per decade. This finding is supported by the TCO difference (Fig. 4b), which indicates a small reduction in the TCO (in
SEN-C2-fCFC11_2050) of up to about 2 DU (in the order of less
than 1 %) until 2050. The linear regression (again accounting for possible
lag 1 autocorrelation) gives -0.5 DU per decade (±0.25 DU per decade; given
by two times the estimated standard error, which roughly corresponds to the
95 % confidence interval and will be used throughout this paper as a
measure of uncertainty). This effect can be rated as negligible in
comparison with the expected annual fluctuations in this region. Therefore,
in the following, we focus on the analyses of the polar regions, in
particular on the Antarctic region in September. One reason for choosing the
month September for further analyses of Antarctic ozone chemistry is that we
found the most obvious ozone changes here. Another reason is that this month
is less noisy than October (Solomon et al., 2016).
Mean annual cycle of total column ozone (TCO) differences (in
Dobson units, DU) between SEN-C2-fCFC11_2050 and REF-C2 for
the 2040s (i.e., SEN minus REF).
(a) Temporal evolution of total column ozone (TCO; in DU) for the
annual near-global mean (60∘ S–60∘ N) in REF-C2
(black curves) and SEN-C2-fCFC11_2050 (red curves).
(b) TCO differences (in DU) between SEN-C2-fCFC11_2050
and REF-C2 (i.e., SEN minus REF). For the absolute TCO time series (a) the
11-year solar cycle (smoothed with a 1-2-1 filter) has been removed. Thicker
curves show the 5-year running means, respectively. The corresponding lines
(a, b) show the respective linear regression estimates based on
the unsmoothed data.
In the Southern Hemisphere polar region (70–90∘ S)
obvious ozone changes can be identified in September (Fig. 5). The mean
differences of the TCO between the 2000s and the end of the simulation
amount to about 20 DU, indicating that the mean September ozone values in
the SEN-C2-fCFC11_2050 simulation are about 10 % lower than
in the REF-C2 simulation. The trend estimate gives -4.1 DU per decade (±1.7 DU per decade). This trend estimate and the uncertainties were obtained
by multiple linear regression, which accounts for possible lag 1
autocorrelation and uses the difference of the temperature anomalies (at
100 hPa over 70–90∘ S) from the REF-C2 and SEN-C2-fCFC11_2050
simulations as a second independent variable in addition to the linear trend. It is found that much of the interannual variability can be explained by
including the difference of the polar temperature anomalies at 100 hPa in
the regression model. This agrees with Langematz et al. (2016), who
used temperature anomalies to regress polar TCO. The temporal evolution of
TCO differences and the size of the ozone disturbance found in the Northern
Hemisphere polar region in January have the same order of magnitude (not
shown), but the signal is more noisy because of the stronger dynamic
variability.
Temporal evolution of TCO differences (in DU) between
SEN-C2-fCFC11_2050 and REF-C2 (i.e., SEN minus REF) for the
Antarctic region (70–90∘ S) in September. The
thicker curve shows the 5-year running mean. The corresponding line shows
the trend estimate between the unsmoothed time series using a multiple
linear regression – including differences of temperature anomalies as the
dependent variable – which accounts for possible autocorrelation with lag 1
(see text for details).
Next, we look into stratospheric partial columns of
ozone (PCO) in more detail with respect to the upper stratosphere (US, above about 30 km, i.e., the 10 hPa pressure level) and the lower stratosphere (LS, between 100 and 10 hPa) for the Antarctic region in September. Figure 6 shows the PCO
differences for the US (panel a) and the LS (panel b) between the
SEN-C2-fCFC11_2050 and the REF-C2 simulations. Both
show the expected negative trend, indicating lower values at the end of
the SEN-C2-fCFC11_2050 simulation in the late 2040s. Again
the trends are obtained by the same regression model as for the TCO (see
the previous paragraph), but for the US the temperatures at 10 hPa were
used. The mean PCO changes for the US of about 2 DU are much smaller than
those calculated for the LS (about 20 DU). The temporal evolution of the PCO
differences in the LS show similar results as those found for the TCO differences
(Fig. 5): in the SEN-C2-fCFC11_2050 simulation the TCO is
reduced by about 20 DU until the year 2050. The strongest signature of ozone
change found in the polar LS points to the importance of heterogeneous
chemical processes. Viewing the vertical profile of the differences of
net ozone production rates of ClOx between the REF-C2 and the
SEN-C2-fCFC11_2050 simulations in the 2040s clearly indicates
an absolute minimum (i.e., less net ozone production in
SEN-C2-fCFC11_2050) at around 50 hPa and another relative
minimum at about 1.5 hPa in September (Fig. 8b; see explanation
in Sect. 3.3). Moreover, looking at the PCO changes in middle- and
lower-latitude regions (60∘ S–60∘ N, not shown), the
partial column differences clearly indicate that the small ozone differences
detected in the TCO (Fig. 4b) are affected by ozone
reductions of similar magnitudes in the US and the LS, displaying only small
contributions to the TCO.
Temporal evolution of partial column ozone (PCO) differences (in
DU) between SEN-C2-fCFC11_2050 and REF-C2 (i.e., SEN minus
REF) for the Antarctic region (70–90∘ S) in
September. Panel (a) shows PCO values for the US (above 30 km); panel (b) shows PCO values
for the LS (below 30 km). Red thicker curves show the respective 5-year running
means. The corresponding lines show the trend estimates for
the unsmoothed time series using a multiple linear regression – including
differences of temperature anomalies as the dependent variable – which accounts
for possible autocorrelation with lag 1 (see text for details).
In Fig. 7, we compare the calculated differences between the
SEN-C2-fCFC11_2050 and REF-C2 simulations as presented in
Figs. 4–6 with the corresponding results derived from
SEN-C2-fCFC11_2020 and REF-C2. In Fig. 7a–d,
it is obvious that an immediate re-implementation of the Montreal Protocol
eventually leads back to the direction of the expected ozone conditions
around the end of the 2040s, as they are calculated in our REF-C2
simulation. In the REF-C2 simulation the model-diagnosed CFC-11 emissions
are nearly zero after about 2030, whereas in the
SEN-C2-fCFC11_2020 simulations they steadily decrease
from higher values in 2020 down to zero around the year 2050. This is caused by
the prescribed CFC-11 surface mixing ratio as indicated in Fig. 1.
Stratospheric ozone budget
In the following, a detailed analysis of individual ozone production and
loss processes is carried out. This ozone budget analysis is used to
investigate the role of separate chemical cycles and reactions, which are
responsible for ozone production and loss in the stratosphere. For this
analysis the MESSy tool “strato3bud” (cf. Meul et al., 2014, based on
Jöckel et al., 2006) is employed. The respective reactions responsible
for stratospheric ozone production (attributed to photolysis hυ,
HO2, and CH3O2) and loss (attributed to Ox, NOx,
HOx, ClOx, and BrOx) are described by Meul et al. (2014, see
their Table 2). In Fig. 8, the results of this budget analysis are shown as
changes of the ozone production rate between the
SEN-C2-fCFC11_2050 and the REF-C2 simulations ΔPSEN-REFprc with respect to the total production rate in the
REF-C2 simulation. The explicit formula for calculating the changes of the
ozone production rate at a certain level is given as
ΔPSEN-REFprclev=∑lat∈RPSENprclat,lev-∑lat∈RPREFprclat,lev∑lev∑lat∈RPREFtotlat,lev.
Here, P denotes the temporal mean and zonally summed ozone production rate
(molecules s-1). The subscript denotes the respective simulations, and the
“prc” superscript denotes which process (hυ, HO2,
CH3O2, Ox, NOx, HOx, ClOx, and BrOx) is
analyzed. Loss cycles are regarded as negative production rates. Further,
the superscript “tot” denotes the sum of all positive production rates
(namely of hυ, HO2, and CH3O2) and the summation goes
over all latitudes, which lie in the respective latitudinal band R. Here we
show profiles of ΔPSEN-REF for the annual global mean and the
Southern Hemisphere polar region (70–90∘ S) during
September 2041–2050. In the Antarctic spring season, (Fig. 8b)
it is obvious that the enhanced content of reactive chlorine in the
SEN-C2-fCFC11_2050 simulation is responsible for the
intensified ozone loss in the LS (around 50 hPa) and the US (around 1.5 hPa). In
the LS, ozone loss through reactive bromine compounds (BrOx=Br+BrO+HOBr+BrCl+2Br2) is also enhanced, which is probably related to
the enhanced chlorine loading as a result of ozone loss due to the reaction of BrO with
ClO (see Table 2 in Meul et al., 2014, which specifies the reactions that
are considered in our analysis of ozone production and loss cycles). Conversely, as a consequence of more available chlorine, other loss cycles
or production processes show a tendency to compensate for the enhanced ozone
destruction by chlorine. For instance, the catalytic NOx cycle shows some balancing in the altitude region between about 19 km (50 hPa) and the
stratopause (about 0.7 hPa). This means that the ozone
depletion by NOx is clearly reduced (i.e., a relative ozone production)
in the SEN-C2-fCFC11_2050 compared with the REF-C2 simulation
above about 40 hPa (see the red line in Fig. 8, which indicates the change
of the net ozone production rate) in the 2040s. It is, however, not straightforward to
further disentangle the underlying processes of the most relevant chemical
cycles, as the underlying kinetic system is highly nonlinear. The
system in the SEN-C2-fCFC11_2050 simulation is heading
towards a different chemical equilibrium, due to the distribution of educts
and the temperature change. The results with respect to the global annual means
of ozone production and loss in the 2040s shown in Fig. 8a
indicate that below about 50 hPa no obvious changes are detected. Above 20 km the ozone loss is strongly affected by reactive chlorine, and, again, some
compensation effects due to other competing ozone loss cycles are
clearly identified in the US. The positive values with respect to the photolysis rates
indicate a slight downward shift of the ozone layer (ozone maximum) to lower
altitudes. This is probably due to enhanced ozone loss via chlorine at
higher altitudes, which allows more UV radiation to reach lower altitudes,
where this additional radiation then causes higher photolysis rates.
Overall, the red line in the Fig. 8a indicates that the change
in the net ozone production rate is nearly zero at all altitudes due to
these compensating effects.
Different temporal evolution of column ozone differences (in DU)
between the individual sensitivity simulations and the reference simulation
(in DU): SEN-C2-fCFC11_2050 minus REF-C2 values are indicated
in red, and SEN-C2-fCFC11_2020 minus REF-C2 values are shown in blue. (a) TCO for the annual near-global mean (60∘ S–60∘ N);
(b) TCO for the Antarctic (70–90∘ S) in
September; (c) PCO for the Antarctic (70–90∘ S) in the US in September; (d) PCO for the Antarctic
(70–90∘ S) in the LS in September. The red and
blue lines show the trend estimates for the unsmoothed time series using a
multiple linear regression – including differences in temperature anomalies
as the dependent variable – which accounts for possible autocorrelation with lag
1 (see text for details).
The relative change in ozone production rates (in %), which are
normalized to the total column production (through photolysis hυ,
HO2, and CH3O2) in the REF-C2 simulation. For the individual
ozone production and loss processes, mean differences are shown that
have been derived from the REF-C2 and SEN-C2-fCFC11_2050 (i.e., SEN minus REF) simulations for the 2040s (from 2041 to 2050). The
change in the net ozone production rate, which refers to the sum of
all changes, is indicated by the red line. Panel (a) shows the mean annual global
mean profiles, and panel (b) shows the values for the south polar region (70–90∘ S) in September. Negative values indicate an intensified
ozone loss or a decreased ozone production in the
SEN-C2-fCFC11_2050 simulation, whereas higher values indicate
more ozone production or less loss via a specific process. Thin
horizontal lines indicate the nearest pressure levels to the model
grid boxes.
The analogous ozone budget analysis is carried out for changes in the ozone
production rate between the SEN-C2-fCFC11_2020 and the REF-C2
simulations. As expected, the vertical dependence of the ozone production
rates in the 2040s looks similar to those in the analysis of the
SEN-C2-fCFC11_2050 simulation (not shown), namely (1) the global annual
means of the ozone production and loss below about 20 km (50 hPa) do not
indicate obvious changes. Higher up ozone loss is strongly affected by
reactive chlorine, and, again, some compensation effects in the US due to other
competing ozone loss cycles are clearly identified; however, all ozone production
rate changes are about half as strong as those found between the
SEN-C2-fCFC11_2050 and REF-C2 simulations (see Fig. 8a). (2) In the Antarctic region during September in the 2040s the
intensified ozone loss in the LS (around 50 hPa) via the ClOx and
BrOx cycles are again obvious, but the ozone destruction rates are only
one-third of the magnitude of that found between the
SEN-C2-fCFC11_2050 and REF-C2 simulations (see Fig. 8b).
Finally, to check the possible impact of temperature changes due to enhanced
CFC-11 concentrations on ozone chemistry, we analyze the overall temperature
trends in the US (near 1 hPa) and LS (near 50 hPa) as well as the differences
between REF-C2 and SEN-C2-fCFC11_2050. The global annual mean
long-term temperature behavior in the REF-C2 simulation indicates a
cooling of about 1 K in the LS and of about 3 K in the US from the early
2000s until to the year 2050. The temperature difference between REF-C2 and
SEN-C2-fCFC11_2050 amounts to an additional cooling of about
0.3 K in the US in the SEN-C2-fCFC11_2050 simulation, whereas
no obvious change in the long-term behavior can be identified in the LS (not
shown). It is difficult to separate the individual contributions of the
additional cooling in a coupled CCM simulation, i.e., radiative cooling due to
enhanced CFC-11 concentrations and due to less ozone in the stratosphere caused
by the enhanced chlorine loading without additional diagnostics. We assume
that both processes will contribute to the calculated additional cooling in
the SEN-C2-fCFC11_2050 simulation.
Taking a closer look at the Southern Hemisphere polar region in spring, the
REF-C2 simulation indicates a clear cooling trend of about 4 K in the US
(near 1 hPa) until 2050, whereas no obvious trend can be identified in the
LS (not shown). With respect to the temperature differences of
SEN-C2-fCFC11_2050 and REF-C2, the US does not show a clear
change, whereas in the LS the SEN-C2-fCFC11 simulation suggests some
additional cooling from the early 2000s until 2050 by about 2 K. However,
this difference is superposed by large interannual fluctuations.
With this in mind, we can try to evaluate the calculated ozone differences in
the sensitivity simulations in comparison to REF-C2. With respect to the global mean in the US,
on the one hand, enhanced chlorine mixing ratios lead to enhanced
ozone depletion via the catalytic ozone destruction cycle; on the other hand,
the extra cooling is known to create a reduction of the ozone depletion
rates by gas-phase chemistry (e.g., Haigh and Pyle, 1982). It was found that
the net effect here is slightly negative, i.e., there are indications that ozone
differences between REF-C2 and the sensitivity simulations in the US are
dominated by the enhanced chlorine content. With respect to the global mean in the LS, where no
clear cooling is simulated, the smaller ozone values are mainly
caused by the enhanced chlorine content.
Looking closer at the south polar region in spring, it is obvious that the enhanced chlorine content is again mostly responsible for the
slightly reduced PCO in the SEN-C2-fCFC11_2050 simulation in
the US
(Fig. 6a). In the LS, where heterogeneous chemical processes
are the most important drivers of ozone changes, the enhanced chlorine
mixing ratios intensify the ozone destruction. This leads to significantly
reduced PCO over the time (Fig. 6b), which eventually leads to
the indicated (slight) extra cooling of the polar lower stratosphere in
spring. However, a first analysis of polar stratospheric cloud (PSC) statistics for
the REF-C2 and SEN-C2-fCFC11_2050 simulations did not display a considerable trend in the PSC surface area (not shown).
Therefore, we cannot identify any hint of enhanced chlorine activation.
Discussions and conclusion
After the detection of an unexpected and persistent increase in global
emissions of CFC-11 (Montzka et al., 2018; see also update in Harris et al.,
2019), it is still unclear (i) how much these additional emissions have
already affected stratospheric ozone, (ii) how the CFC-11 emissions could
further develop in the coming years, and (iii) how large the potential for a
disturbance of the temporal evolution of the ozone layer is. The discussions
during the international CFC-11 Symposium in Vienna (March 2019) on the unexpected
increase of CFC-11 emissions came to the conclusion that a major problem
is the creation of a realistic assessment of future CFC-11 levels (Harris et al.,
2019). There are many factors that have significant uncertainties, such as the role of bank emissions or a possible co-production of CFC-12
(CF2Cl2) with CFC-11. There was a general acceptance that higher
CFC-11 emissions are creating enhanced ozone depletion, but so far the
corresponding magnitudes of ozone disturbances are uncertain. This study
aims to estimate possible ozone changes due to enhanced CFC-11 values in
recent and coming years, while the amount of other ODS are declining as
expected. For that reason, a simplified study based on CCM simulations is
conducted to (1) roughly estimate the implications of a constant mean
CFC-11 surface mixing ratio for ozone depletion instead of reducing CFC-11
on longer timescales, and (2) to establish approximately how strong the maximum ozone effect is
due to the additional CFC-11 emissions in recent years. To keep things as
simple as possible we do not consider regional differences with respect to
CFC-11 emissions in our sensitivity simulations. From our point of view,
considering regional differences would not have relevant effects on the
results presented due to the long lifetime of CFC-11 (e.g., Rigby et al.,
2013; Engel et al., 2018), which leads to global mixing (Hoffmann
et al., 2014). Our simplified approach assumes an extreme boundary
condition (i.e., constant CFC-11 surface mixing ratios until the year 2050 in
SEN-C2-fCFC11_2050 and until the year 2019 in
SEN-C2-fCFC11_2020), which is justified as a
more realistic approach with respect to future CFC-11 levels is currently not
available (Harris et al., 2019). Therefore, the results presented should not
at all be taken as a robust prediction of future conditions.
The results presented generally indicate that in particular the ozone layer over the Arctic
and Antarctic in late winter and spring is affected by the
prescribed CFC-11 surface mixing ratio change. In our case, at the end of
the SEN-C2-fCFC11_2050 simulation, the impact on the TCO
culminates in a maximum ozone decrease of up to 30 DU in both polar regions
(Fig. 3). The calculated ozone changes in midlatitude and tropical
regions are surprisingly small (less than ±5 DU) and are therefore
generally not significant in the sense that the range of variability is in the
same order of magnitude. An estimate of possible ozone changes in the late
2040s based on the perturbation of “equivalent effective stratospheric
chlorine” (EESC) may lead to similar results, but appropriate explanations
are lacking. Therefore, we perform a detailed ozone
budget analysis of such sensitivity simulations for the first time, showing interesting results
with regard to compensation and buffering effects associated with different
production and loss cycles. It can be seen from our results that the
strengthened ozone depletion by enhanced chlorine is partly compensated for by other ozone-depleting catalytic cycles (e.g., NOx) and other molecules
(e.g., HO2). With respect to the global mean picture, there is no large TCO difference
visible, as the effects of ozone production and loss processes are nearly
canceling. However, in the polar regions, although there are also
compensating effects, the signal is noteworthy in spring (e.g., about 20 DU
for the Antarctic region). We identify where (altitude) and when
the ozone amount is decreased in the SEN-C2-fCFC11_2050
simulation compared with REF-C2. The ozone response to CFC-11 changes looks
quasi-linear, but the processes in the background are obviously nonlinear.
Finally, based on the results of our SEN-C2-fCFC11_2050
simulation, we try to approximately estimate the possible shift of the
closure date of the ozone hole over Antarctica under the implied conditions
of this sensitivity. To do so, we look at the temporal evolution of
the total stratospheric ClOx loading in the REF-C2 (started in the
middle of the 20th century) and the SEN-C2-fCFC11_2050
simulations. In the REF-C2 simulation ClOx values are strongly
increasing from 1960 onwards and are highest at the end of the 1990s.
Starting in the 2000s the ClOx concentration is decreasing. The 1980
value of the REF-C2 simulation can be regarded as a reference for chlorine
conditions before the ozone hole appeared. This “pre-ozone-hole” chlorine
content is reached again around the year 2050 in our REF-C2 simulation,
which is some years earlier than the multi-model mean based on all REF-C2
simulations as calculated by Dhomse et al. (2018). By extrapolating the
linear regression line of the ClOx content (for 2002–2050) of the
SEN-C2-fCFC11_2050 simulation (not shown) into the future, we
estimate that a pre-ozone-hole, i.e., “1980”, ClOx loading is likely
to be reached before 2070. Therefore, we can roughly estimate a maximum
delay of the closure of the ozone hole of less than about 20 years
when keeping the CFC-11 surface mixing ratio at a 2002 level versus a decline in the
CFC-11 surface mixing ratio which would occur due to adherence to the
Montreal Protocol. Considering that Dhomse et al. (2018) determined that the
closure of the ozone hole would occur by the year 2060 and that the 1σ
standard deviation was in the range of about ±5 years (see also Fig. 4.22 by Langematz et al., 2018), this indicates that the
calculated effects of a constant CFC-11 surface mixing ratio could have a
non-negligible effect on the closure date of the ozone hole. This finding is
in line with other model results mentioned by Harris et al. (2019), who stated that the
closure of the ozone hole and ozone recovery as a whole will be delayed
depending on the CFC-11 emission levels. A first estimate presented in WMO (2018) showed that if total CFC-11 emissions were to continue at the levels
experienced from 2002 to 2016 (67 Gg yr-1), the return of midlatitude and
polar EESC to the 1980 value would be delayed by about 7 years and 20 years,
respectively. For the Arctic region, enhanced stratospheric chlorine content
means that there is the possibility of stronger ozone depletion under
specific dynamic conditions (i.e., a stable and cold polar vortex until
March) for a slightly longer time period (e.g., Dameris et
al., 2014).
The results presented do not show dramatic consequences for the global mean
ozone layer due to an enhanced CFC-11 surface mixing ratio for the coming years,
but indicate relevant changes in the polar regions in winter and spring. In
light of our results regarding chemical feedback processes, which
dilute the effects due to enhanced CFC-11 levels in parts, compliance
with the guidelines of the Montreal Protocol is absolutely necessary. Without
a continued strong regulation of CFC-11 and other ODS emissions (e.g.,
Laube et al., 2014; Hossani et al., 2017), the recovery of the ozone layer could be significantly affected – including the timescale of
ozone hole closure – and this should be avoided!
Code and data availability
The Modular Earth Submodel System (MESSy) is continuously developed and
applied by a consortium of institutions. Use of MESSy and access to
the source code is licensed to all affiliates of institutions that are
members of the MESSy Consortium. Institutions can become a member of the
MESSy Consortium by signing the MESSy Memorandum of Understanding. More
information can be found on the MESSy Consortium website
(http://www.messy-interface.org, last access: 12 November 2019).
Author contributions
Both sensitivity simulations were set up and carried out by PJ with
support from MD. MD structured and composed the paper. All authors
analyzed the model data and compiled the results, and all three
authors contributed to the paper.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
The authors would like to thank Heidi Huntrieser for her internal review and her
useful suggestions that improved the paper. The authors are also grateful to the two
anonymous referees for their helpful comments on the paper. The work
described in this paper has received funding from the Initiative and
Networking Fund of the Helmholtz Association through the “Advanced
Earth System Modelling Capacity (ESM)” project. The authors also
acknowledge financial support from the DLR internal project KliSAW
(Klimarelevanz von atmosphärischen Spurengasen, Aerosolen und Wolken)
and the Research Unit SHARP of the Deutsche Forschungsgemeinschaft (DFG).
The model simulations were performed at the German Climate Computing
Center (DKRZ) supported by the Bundesministerium für Bildung und Forschung
(BMBF). The NCAR Command Language (NCL, 2018) was used for data analysis and
to create some of the figures in this study. NCL is developed by
UCAR/NCAR/CISL/TDD and is available online at
10.5065/D6WD3XH5. CDO (climate data operators;
Schulzweida, 2019) was employed for processing the data. The authors wish to
thank all contributors to the ESCiMo (Earth System Chemistry
integrated Modelling) project, which provided the model configuration and initial
conditions for this study. The World Climate Research Programme's Working
Group on Coupled Modelling, which is responsible for CMIP, is also acknowledged, and the authors are thankful to the
HadGEM climate modeling group for producing their model output and making it
available for use. With respect to CMIP, the United States Department of Energy's Program for Climate
Model Diagnosis and Intercomparison provides coordinating support and led
development of the software infrastructure in partnership with the Global
Organization for Earth System Science Portals.
Financial support
This research has been supported by the Deutsche Forschungsgemeinschaft (DFG; Forschergruppe SHARP), the Helmholtz-Gemeinschaft (Advanced Earth System Modelling Capacity, ESM), and the Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR; KliSAW project).The article processing charges for this open-access publication were covered by a Research Centre of the Helmholtz Association.
Review statement
This paper was edited by Jens-Uwe Grooß and reviewed by two anonymous referees.
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