On the road back to normalcy. 20% healing of the Antarctic Ozone Hole during

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1 On the road back to normalcy. 20% healing of the Antarctic Ozone Hole during By Jos de Laat, Michiel van Weele, and Ronald van der A October, 2017 De Bilt, The Netherlands In recent years it has been reported that the ozone layer, globally thinning since the late 20 th century, is on track to full recovery somewhere halfway the 21 st century. Observations unequivocally show that the amount of ozone depleting substances in the ozone layer is already decreasing, and that the ozone levels are increasing. Both trends are the atmospheric response to the international policy measures taken since the 1980s. However, until now it has been unclear to what extent the annual springtime ozone hole over Antarctica is healing in a similar fashion as the ozone layer is healing globally. In this blog we try to shed some light on the closing of the ozone hole so far (until 2017), largely based on our 2017 KNMI-paper in the Journal of Geophysical Research. During the summer of 2018 the next WMO/UNEP Ozone Assessment Report will be released. This report issued every four years summarizes the current status of the ozone layer and the state of ozone layer science. The reports are written since the 1990s in support of the Montreal Protocol (1987) on Substances that Deplete the Ozone Layer. Next to policy measures the protocol asks for regular scientific updates. An important question within the upcoming report will be whether the Antarctic Ozone Hole is healing, and whether any healing can be attributed to decreasing Ozone Depleting Substances (ODSs). The latter would mark an important milestone and demonstrate that the international measures for the protection of the ozone layer are also effective for the healing of the Antarctic Ozone Hole.

2 Figure 1. The Antarctic OMD on 14 September for each year between 1979 and 2017 based on MSR assimilated total ozone column data. Years with enhanced wave activity (so-called PSClimited years ) and consequently reduced Antarctic ozone depletion shown in red. The lower plots show total ozone columns over Antarctica on that particular day for a number of selected years. What is new in the KNMI study? During recent years several scientific papers have been published that report signs of increasing ozone in the Antarctic Ozone Hole. However, because in every paper different methodological approach and/or use different datasets are used, the results and the overall conclusions vary significantly. Until recently it has been unclear what would be the preferred method and data sets to use to unequivocally demonstrate increasing ozone levels in the ozone hole season attributed to a decreasing amount of ODSs. In our paper in the Journal of Geophysical Research we discuss the various choices to make with regard to observational data and metrics, as well as some of the unavoidable methodological choices. The most important conclusion of the paper is probably that with well-chosen data sets and methods, the observations show that the amount of ozone in the Antarctic Ozone Hole over the period has increased steadily with approximately 20%. And, secondly, that this observed healing is consistent with what model simulations suggest should have happened in the ozone hole given the decrease in ODSs in the polar stratosphere since the year 2000 [Chipperfield et al., 2017].

3 Some facts about the Antarctic Ozone Hole International attention for stratospheric ozone first emerged in 1985, when scientists reported a drastic decrease in springtime stratospheric ozone over Antarctica. In the ensuing decade, the Antarctic Ozone Hole grew considerably and it reached maximum ozone depletion around the year The Antarctic Ozone Hole appears every year during local spring (September). Halogens like chlorine and bromine in combination with sunlight cause rapid catalytic destruction of ozone, mostly in the month of September. The ozone depletion occurs within the Antarctic stratospheric vortex, a region in the stratosphere over Antarctica which is bordered by stratospheric air masses encircling Antarctica at high wind speeds that isolate the air masses inside the vortex from the air masses outside of the vortex. The strong vortex allows stratospheric temperatures to become very low (down to -80 Celsius and colder) which causes the formation of Polar Stratospheric Clouds (PSCs). These PSCs are crucial for the occurrence of rapid catalytic ozone destruction inside the vortex between km altitude during local springtime (August-September). These layers become completely devoid of ozone by the end of September. By early October the PSCs evaporate due to rising temperatures, and the ozone depletion ceases. However, the vortex air masses depleted of ozone may survive mixing with ozone rich midlatitude air masses until late November or early December. Stages of ozone recovery and recovery detection After the discovery of the Antarctic Ozone Hole, within a few years it was internationally agreed that the emissions of industrial ODSs should be discontinued by the mid-1990s. This indeed has happened, and since approximately the year 2000, stratospheric concentrations of ODSs are on the decline by approximately 1-1.5%/year. Note that it takes a few years for at surface-level emitted ODSs to reach the higher Antarctic stratosphere. An important question for both the scientific community and the signatories of the international agreements is whether and how fast stratospheric ozone levels increase in response to the decrease in ODSs. To identify success, different stages of recovery have been defined: first, stratospheric ozone should stop decreasing. Then, stratospheric ozone should start to increase and the increase should be attributable to the decrease in ODSs. The increase should also be statistically significant, meaning that the increase should clearly persist over a longer period of time. Finally, for full recovery stratospheric ozone levels should return to their approximate 1960 levels. However, it is neither straightforward to identify ozone recovery unequivocally, nor to attribute ozone changes to a decrease in the amount of ODSs. While ozone vertical profile observations do provide best insights in the specific chemical processes at hand (De Laat and van Weele, Scientific Reports, 2010), a complete coverage of ozone in the whole atmosphere is best provided by total ozone column

4 observations in combination with data assimilation (van der A et al., Atmos. Meas. Tech., 2015). For our detection of ozone recovery we use the extended MSR-2 total ozone column climate data record. First, concerning the data analysis method, there is no consensus on what actually is the best metric for studying Antarctic ozone depletion. In the past, different studies have used different metrics based on the total ozone column observations: the ozone hole area, the average total column ozone over a predefined region, or e.g. the daily ozone mass deficit, the amount of ozone that would be needed to increase the total ozone column to 220 DU over the southern hemisphere high-latitudes on any day during spring. Secondly, the Antarctic Ozone Hole is subject to large interannual variability. During certain years, natural variability causes the Antarctic vortex to become what is called disturbed or PSC-limited, meaning it is less stable which causes temperatures within the vortex to rise and which affects the effectiveness of the catalytic ozone destruction. A little bit of warming can cause a reduction in ozone depletion of tens of percent. The year 2017 is an example of a PSC-limited year, but in the past there have been several other PSC-limited years with a disturbed vortex. Thirdly, for the data analysis method, a choice has to be made over which (part of the) season the ozone depletion is calculated. Then, also the relevant area must be defined. However, which time period and which area is chosen is to some extent arbitrary and not well constrained due to large interannual variability in the ozone hole. Consequently, a wide variety of time periods and areas has been used in previous scientific publications on the ozone hole. The known presence of naturally varying processes affecting Antarctic ozone depletion implies that associated variations in ozone depletion must be taken into account. A common used method is to try to account for natural variations statistically by means of a multivariate regression (MVR), but in de Laat et al. [2015] it was shown that the MVR approach comes with new methodological choices that need to be motivated and introduce new uncertainties in the analyses. Finally, to complicate matters even more, natural variability does not cause only natural variations in Antarctic ozone, but it also changes the effectiveness of the ozone depletion caused by the ODSs. Discussion of methods Given all these complications, it is maybe not surprising that results from different studies using different methods and different datasets yield different results. This is where our new paper comes in. First, we analyze and discuss the three most common metrics used for the Antarctic Ozone Hole: area, average ozone amount, and ozone mass deficit (OMD). The OMD is considered the preferred metric, mainly because the number of choices to make using OMD as metric is smaller than using the area or the average ozone, thus reducing uncertainties intrinsically. Also, the part of springtime over which to calculate the OMD is discussed at length. E.g. a calendar month is a rather arbitrary time period. The sensitivity of (not-) finding ozone recovery as a function of the chosen time period is explained.

5 Further, we argue that it may be better to step away from the above-mentioned MVR time series analyses, which requires to make additional and possibly subjective choices. As it is well established that it is the natural variability that is the most important obfuscating process, we propose to objectively identify years that are disturbed, and remove these years from the long-term data record. Identification of such years can be physically based by looking at stratospheric temperatures. Importantly, we argue that the removal of certain years from the record can be done objectively, while many assumptions and choices made in other methods, such as using MVR time series analyses, are generally not well justified. Figure 2. Correlation (R2) between average daily OMD and EESC (upper panels A and C) and post year-2000 trend regression (lower panels B and D), excluding PSC-limited years (left panels A and B) and including PSC-limited years (right panels C and D), as a function of start date and length of the time period over which the average daily OMD is calculated. The full time period considered is 1 August to 31 October. The black dot denotes the preferential time period DOY discussed in section 3.1. Dotted lines are added for visual guidance. OLR = Ordinary Linear Regression. Healing of the ozone hole When using the OMD to study the recovery of the Antarctic Ozone Hole, the authors find that in particular for a mid-september period, the OMD has decreased since the year The decrease is

6 statistically significant (2-4σ) even though natural variations are large. By the objective and physicallybased removal of a couple of disturbed years, the statistical significance of the decrease in OMD increases dramatically (6-10σ). In essence, we find a steady decrease in OMD of about 20%, consistent with what numerical models have calculated that should have happened since the year 2000 [Chipperfield et al., 2017]. The steady decrease in OMD emerges most prominently for the mid-september period. If the period starts too early and/or ends too early, or starts too late and/or ends too late, the steady decrease in OMD vanishes as other (natural) processes start affecting OMD values. This finding is consistent with recent papers, which note that recovery detection appears to be better in September than in October, which had been a favorite month before. Figure 2 shows the correlation between the OMD and the total amount of ODSs (here as the effective chlorine, EESC, which stands for Equivalent Effective Stratospheric Chlorine) as a function of the length of the time period over which the OMD is calculated (y-axis) and the start of the period (x-axis) in panels A and C. The statistical significance of the post year-2000 trend in OMD is provided in panels B and D. In panels A and B a total of 6 disturbed - PSC-limited - years are removed from the full 37-year record for the period (1986, 1988, 2002, 2004, 2010, 2012). In panels C and D all 37 years are included. The comparison of panels A/B with panels C/D shows testifies the effect the PSC-limited years have on the trend significance. Panel A shows that the OMD correlates well with EESC only when at least part of September is included in the time period over which is averaged, while there may not be much need to average over long periods of time. Panel B shows that the post year-2000 trend in OMD decreases strongly if the period does not cover mid-september (either finishing too late, starting too early, or being too long). This occurs because outside of the mid-september period, natural variability has a strong impact on year-toyear variations in the OMD. Combining panels A+B, we find using a period including mid-september the OMD that (i) the post year trend in average OMD is statistically highly significant (trend significance up to almost 10σ) and (ii) that the change in OMD correlates very well with the change in EESC. Combining both findings provides support for the argument that the observed recovery is consistent with decreasing ODSs. Figure 3 shows OMD time series for particular choices of the time period over which the OMD is calculated. Clearly, the post year-2000 change in OMD strongly depends on the chosen time period. The figure also shows how certain years (red) have anomalously small OMD values, in particular for the favored time periods during September. Figure 3 highlights how conclusions about post year-2000 healing of the Antarctic Ozone Hole can vary depending on available choices.

7 Figure 3. Examples of long term changes in OMD for different days (middle panels) and time periods over which OMD is averaged. Figure not in de Laat et al. [2017]. Final remarks Our analysis implies that with a change of 20% in OMD over the period , the recovery of the Antarctic Ozone Hole is well underway, and that this development is according to recent expectations from models. Whether or not this paper formally addresses the attribution question, i.e. can the observed change in ozone/omd solely be caused by the change in ODSs remains open for discussion. This would probably require quantification of all (dynamical) processes not related to ODSs. However, some of these non-ods effects may be not so easy to isolate. This could formal attribution of recovery of the Antarctic Ozone Hole for years to come, and may even never be completely solved. The larger picture. This paper provide arguments as to why the Ozone Mass Deficit appears to be a preferable metric to monitor the Antarctic Ozone Hole, rather than average ozone or the ozone hole area. Further, it appears that a period around mid-september is the preferred period to consider for recovery detection, rather than October or longer time periods. It is proposed for recovery detection and trend calculations to better objectively remove a couple of perturbed years from the long record rather than trying to

8 account for them using statistical methods. By doing so, the authors find a 20% recovery of ozone In the Antarctic Ozone Hole, consistent with the estimated decrease on ODSs as well as model simulations of recovery. Clearly the healing of the Antarctic Ozone Hole is not yet a settled issue, but we hope that the paper will spur some further debate about recovery detection methods. The results of the paper and their assessment in the upcoming WMO 2018 report may provoke further expert elicitation on the preferred metrics, data sets, and methods, for which international scientific advisory boards and committees could take the lead. The path to full Antarctic Ozone Hole recovery is still long, likely beyond Continued monitoring is warranted, as worries about (new) industrial activities and emissions that may slow the recovery regularly appear. Stratospheric changes caused by climate change in the 21 st century adds to the uncertainties for the longer term. The recently launched TROPOMI satellite instrument is meant to contribute to the maintenance of our satellite ozone layer monitoring capacity up to standards for the next decade. References Chipperfield, M. P., Bekki, S., Dhomse, S., Harris, N. R., Hassler, B., Hossaini, R.,... & Weber, M. (2017). Detecting recovery of the stratospheric ozone layer. Nature, 549(7671), , de Laat, A. T. J., van Weele, M. & A, R. J. (2017). Onset of Stratospheric Ozone Recovery in the Antarctic ozone hole in assimilated daily total ozone columns. Journal of Geophysical Research: Atmospheres,