the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
The return to 1980 stratospheric halogen levels: A moving target in ozone assessments from 2006 to 2022
Abstract. The international scientific assessment of ozone depletion is prepared every four years to support decisions made by the Parties to the Montreal Protocol. In each assessment an outlook of ozone recovery time is provided. The year when equivalent effective stratospheric chlorine (EESC) returns to the level found in 1980 is an important metric for the recovery of the ozone layer. Over the past five assessments, the expected date for the return of EESC to the 1980 level, for mid-latitudes, has been delayed, from year 2049 in the 2006 assessment to 2066 in the 2022 assessment, which represents a delay of 17 years over a 16-year assessment period. Here, we quantify the primary drivers that have delayed the expected EESC recovery date between each of these assessments. We find that by using identical EESC formulations the delay between the 2006 and 2022 assessment’s expected return of EESC to 1980 levels is shortened to 12.6 years. Of this delay, bank calculation methods account for ~4 years, changes in the assumed atmospheric lifetime for certain ODSs account for ~3.5 years, an under-estimate of the emission of CCl4 accounts for ~3 years, and updated historical mole fraction estimates of ODSs account for ~1 year. Since some of the underlying causes of these delays are amenable to future controls (e.g. capture of ODSs from banks and limitations on future feedstock emissions), it is important to understand the reasons for the delays in expected recovery date of stratospheric halogens.
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RC1: 'Comment on egusphere-2024-1289', Andreas Engel, 13 Jun 2024
The manuscript by Lickley et al. discusses the projected return date of EESC to 1980 levels, which is generally taken as a proxy for an expected recovery of the ozone layer. Most importantly, this measure has been used regularly in the WMO SAOD reports. The paper is well written, scientifically sound and will be an important contribution to explain why EESC recovery dates have constantly shifted further into the future. This is a very important contribution and clearly differentiates the different drivers. The approach to use a consistent EESC formulation is a very important and I think that the manuscript is nearly ready for publication. There are some minor points which I would like the authors to consider.
Minor comments:
l. 58: variable or changing atmospheric transport has also been discussed as a possible reason for the delayed decrease of CFC-11 mole fractions.
l. 124: rho is the in this case the mole fraction which would be expected in absence of chemical loss.
l. 133: if a is the fractional release factor of CFC-11, then the fi values in (1) must be relative values to CFC-11 (see formula (1) in box 8.1. of WMO 2006.
l. 158: modeled concentrations (not concentrations modeled). And actually these are mole fractions, rather than concentrations.
l. 187: form many species modelled lifetimes were used in the SPARC 2013 assessment.
l. 248: I could not find a shaded region in the plot in my print-out.
l. 353: as above: if relative fractional release to the fractional release of CFC-11 is used, then the whole calculation should be multiplied by the frf of CFC-11 (again, see box 8.1. in WMO 2006). It seems to me that the calculation in WMO 2006 and in the plots in chapter 8 of WMO 2006 may actually be missing this. Maybe it might be useful to just apply this factor here, so that the calculation is consistent with other reports?
l. 357: you may want to specify that this is the mid latitude return date.
L 401: you may want to add that this large difference is mainly due to a different value of EESC derived for 1980 with my formulation, which takes into account that the released fraction has on average experienced longer transit times, thus that the inorganic chlorine is dominated by an input which is “older” than the mean age value.
l. 415 (Figure 4): maybe you could add a line going upwards from 1980 and then parallel in order to visualize the calculation of return date.
l. 427: I’m not sure I understand this. EESC is (as shown in your formula (1) a sum, so the effect of individual compound should add up linearly. Or have I misunderstood the statement?
l. 435: are these increases or slower than expected decreases?
l. 447: I find the term bank emissions difficult; maybe use emissions from banks?
l. 498 and following: I’m not sure if the discussion of EECL is very important here or adds much more to the points you make.
l. 524: I do not think that they appear as an outlier. The difference has been made clear in my 2018 paper already and was thus clearly to be expected.Citation: https://doi.org/10.5194/egusphere-2024-1289-RC1 -
RC2: 'Comment on egusphere-2024-1289', Anonymous Referee #2, 14 Jun 2024
Dear author team,
I very much enjoyed reading this well written and significant work and would recommend its publication after the following points have been addressed:
l39-41 This is very brief introduction of a term that is central to this manuscript. There is a more comprehensive one later in the
manuscript, which would be nice to link to.l42-45 As the authors state, the MP entered into force in 1989. Why are they then only considering the last 16 years/5 assessments
for their analysis?l45-48 Very long sentence. Consider splitting it in two.
l47 Please make it clearer to the reader, which one is the newest SAOD, and which one first adopted the new EESC definition.
l51 Consider changing this to "global atmospheric lifetime".
l73-74 This is partly an odd choice of references for proving the point of a "consistent underestimate of the global production
of ODSs". The Benish et al., paper is not global at all nor does it estimate production or emissions, and none of the cited works
focuses on the fourth primary gas listed later in this paragraph, i.e., halon-1301.l83-84 I encourage the author to consistently use either common names or chemical formulas. It would also be recommendable to
give both in at least one place, e.g., Table S2.l84 Looking at Table 2, there are other ODSs that appear to make significant contributions (e.g., HCFC-22, halon-2402). The changes
in these gases just changed signs during the various assessments, resulting in a comparably small net change between the 2006 and
2022 SAOD. While I do not agree with this approach, I can certainly see how it simplifies the analysis. However, I would recommend to
at least explain this simplification to the reader upfront.l112-115 How small exactly? And how does this link to the effect from "updated historical mole fraction estimates of ODSs"
mentioned in the abstract?l141: "age-of-air" is not defined here nor in Table 1. There is, at this stage of the manuscript, only a brief explanation of
the "age spectrum" in l134-135 and it is not made clear to the reader how this links to "age-of-air".l141-142 This is not correct - see, e.g., Box 1-4, Figure 1-18, and Table 1-7 in the 2018 SAOD. Only for projections (different chapter)
was the Newman et al. method preferred.l147-148 As far as I understand the Engel et al. approach it leads to lower effective mean ages for chemical tracers as compared
to inert ones.l184-187 This is a rather confusing mixture of examples. I recommend revising the statement to better distinguish between largely
satellite-based methods focusing on the determination the stratospheric burden (1st example), complete atmospheric burden and loss rate
modeling approaches with input from near-ground observations (2nd example) and outdated early estimation methods of the stratospheric
loss rate (3rd example). Perhaps it is also worth clarifying that the loss for CFCs and halons occurs pretty much entirely in the
stratosphere.l203 Coming back to my earlier point (l42-45), why was, e.g., the 2002 SAOD cited here not included in the analysis?
l248-249 Amazing that the shaded region is so narrow. Does this create a conflict with the earlier statement of the large uncertainties
in bank estimates? Also, presumably lifetime uncertainties are not included here?l253 Please consider an opening statement detailing why CCl4, and CCl4 only, gets an extra section here. It is also unclear why this
rather lengthy section repeats the well-known CCl4 story, which has received much attention in the past including an entire SPARC
report. Is this perhaps meant as an illustrative example of how and why bank estimates and projections can change over time?l261 Please add "CCl4".
l267 These should probably be negative percentages. Alternatively, you could add "decreases of", although "decrease" is already used in
the following sentence.l268 "near zero" in terms of absolute emissions?
l277-278 Perhaps worth explaining what "feedstock" means in this context. Also, there is only one PCE.
l339-341 Consider shifting this sentence to the beginning of "Step 6" for narrative reasons.
L348 The title of this section is somewhat inappropriate as it contains quite a bit of discussion.
l358-361 Again, I don't think this is the correct explanation. Have a look at Figure 1-18 in the 2018 SAOD: If it were a simple shift,
then the two different EESCs should not be virtually identical at polar winter conditions.l386 Consider giving the WMO 2022 return date in the caption as it is not included in the table.
l404-405 It might be worth adding that the main reason behind the differences in polar and midlatitude EESC is not merely the longer
residence time: For the former air predominantly arrives via the upper branch of the Brewer-Dobson circulation and has therefore been
exposed to much more tropical UV photolysis, whereas the latter is influenced more by the lower branch.L485 In the version I downloaded the medium and light purple color bars appeared to be missing. Also, why is WMO 2022 not shown?
l491 Two consecutive sentences starting with "The next update corresponds to...".
l510 Other ODSs such as HCFC-141b or CH3CCl3 also have their most of their loss in the troposphere. The main reason for HCFC-22 being
much less influential in the EESC is its low FRF. Consider adding this to the discussion.l511 The resolution of this figure appears to be lower than that of others in the manuscript.
l530-532 This statement is a bit misleading as the main changes to lifetime estimates did not occur in the 2022 SAOD.
l537 Again (see l84 comment), it would be good to state explicitly that this is only the NET change between the 2006 and the 2022 SAOD.
l556-559 I think this might be a bit of an overstatement. Quantitatively, how high might the "portion of the slower than expected
decline of EESC" that is "caused by inadvertent atmospheric releases of CCl4 from a wide range of industrial activities" actually be?l577 From a brief look at Bednarz et al., I think this should be the lower rather than the upper stratosphere here.
l583 I think the EECl rather than the EESC is used to estimate "radiative forcing of climate by ODSs"?
l589 It's not necessarily the EESC return dates that are changing. To a large degree it is merely the estimates/projections of these.
l594-596 Given that this very work demonstrates that a substantial part of the accumulated delay stems from an improved formulation of
EESC, it might be worth tempering this statement a bit. In addition, it might be worth at least mentioning the potential impact of
anthropgenically driven global warming, which is expected to lead to changes to, e.g., stratospheric circulation therefore inducing
further "atmospheric uncertainties".l596-598 A very nice closing statement. Given that it is not certain that the EESC delay trend is going to continue in the future, the
use of the subjunctive might be more appropriate, though.Citation: https://doi.org/10.5194/egusphere-2024-1289-RC2
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