The Antarctic ozone layer is recovering, but The Independent claim fails to grasp significance of study it relies on

REVIEW

CLAIM: The ozone layer is continuing to heal and has the potential to fully recover, according to a new study

SUMMARY

The claim that “The ozone layer is continuing to heal and has the potential to fully recover” appeared in an article published by The Independent in March 2020, and has been shared on Facebook more than 11,000 times. The claim relies on a study recently published in Nature by Banerjee et al.^[1]. Previous studies have demonstrated that global total ozone is recovering and has the potential to fully recover by the end of the century^[2,3,4], providing support for this claim. However, scientists emphasize that the article misrepresents the focus of the Banerjee paper, as described in their comments below.

The claim and study it is based on focus on the Antarctic ozone layer, which has shown patterns of healing in response to reductions in ozone-depleting substances, such as chlorofluorocarbons (CFCs)^[1-4]. But rather than showing that the Antarctic ozone is healing, Banerjee et al. (2020) demonstrate that atmospheric circulation trends that were driven by ozone depletion have paused or reversed in the Southern Hemisphere^[1].

SCIENTISTS’ FEEDBACK

William Seviour, Vice Chancellor's Research Fellow, University of Bristol:
The claim that “the ozone layer is healing and has the potential to fully recover” I believe is accurate. I’ll break this claim into its two parts. First, whether the ozone layer is healing: The focus of the article and paper is on Antarctic ozone, and there have now been some studies suggesting we are starting to see a healing signal there. This is quite tricky to detect because there is a lot of natural variability from year-to-year. Solomon et al. (2016) was probably one of the first^[2]. Trends are a bit clearer looking at ozone over the whole globe (not just Antarctica). The SPARC LOTUS report is a good reference here—Fig. 1 of the Executive summary shows increasing ozone trends after 1997 in most places^[3].

Figure—Satellite and ground-based records show ozone trends (% change in ozone amount) at different levels in the stratosphere before 1997 (top row) compared to post-2000 (bottom row). Columns represent different latitudes around the world: southern latitudes (left), tropical latitudes (middle), and northern latitudes (right). Note the trends in ozone concentrations are negative pre-1997 (upper panel), but neutral and positive post-2000 (bottom panel). From the SPARC LOTUS report (2019)^[3].

Second, the potential to fully recover: The authoritative reference on this subject is the latest (2018) WMO/UNEP Scientific Assessment of Ozone Depletion—e.g. see Figure ES-1 of the Executive Summary^[4]. This shows that ozone concentrations (both global and Antarctic) are projected to recover to 1960 levels by the end of this century.

Figure—Comparisons of total ozone observations (red points and lines) and chemistry climate models (black lines with grey regions showing uncertainty) for the globe (top) and Antarctic (bottom). Annual global total ozone is averaged over 60°N to 60°S latitudes, whereas Antarctic total ozone is averaged over 60°S to 80°S latitudes. Models project that ozone concentrations will return to levels observed in the 1960s by 2100, assuming future compliance with the Montreal Protocol, which is an international treaty designed to limit ozone-depleting substances, and an increase in greenhouse gas emissions following the RCP-6.0 scenario^@. Black lines with arrows indicate the years that ozone abundances are projected to return to values observed in 1980. Adapted from World Meteorological Organization (WMO) Executive Summary: Scientific Assessment of Ozone Depletion (2018)^[4].

The evidence linking ozone recovery to reductions in CFCs is very strong, so this claim is accurate. There have been a series of papers, such as Morgenstern et al. (2008), looking at “world avoided” scenarios without the Montreal Protocol and they all show much larger ozone depletion^[5]. There has been some recent evidence of violations in the Montreal Protocol with production of a particular CFC (CFC-11) linked to China^[6]. Dhomse et al. (2019) has suggested that this violation might delay ozone recovery if it continues^[7].

My main criticism of the article is that it largely misses the point of the paper. The paper is not directly about whether the ozone layer is healing, but whether atmospheric circulation changes that have been attributed to ozone depletion are pausing or reversing. Plenty of previous studies (linked above) have looked at recovery of ozone trends, but looking at the circulation trends is new. The fact that the paper shows they are reversing is good news because these changes have been linked to droughts over Australia for instance. I think this Guardian article does a better job at focusing on the winds, not just the ozone.

In summary, I think the article is accurate but does not do a very good job of focusing on the results of the paper.

Paul Young, Lecturer, Lancaster University:
The claim that “the ozone layer is healing and has the potential to fully recover” is broadly accurate, backed up by several studies. In general, the best sources of information for those interested in the state of the ozone layer, and the levels and emissions of the synthetic substances that can harm it, are the 4-yearly scientific reports produced under the auspices of the World Meteorological Organisation (WMO) and United Nations Environment Program (UNEP). These reports are written by scientists from all over the world, who assess and review the body of scientific evidence in the peer-reviewed literature. The most recent of these reports was published in 2018^[4].

For the specific case of the Antarctic ozone hole, the WMO/UNEP report concluded that there is evidence to show that the hole is beginning to shrink in size, all as a result of the actions of the 1987 Montreal Protocol (and its amendments), which has strong controls on the emissions of halogen-containing ozone depleting substances. However, it is important to note that the ozone hole still continues to appear in southern hemisphere spring every year, and will likely continue to do so beyond the middle of this century (e.g., see Dhomse et al., 2019)^[8]. This is because of the long lifetime of many ozone depleting substances in the atmosphere, meaning that they persist even after emissions are essentially zero (e.g. chlorofluorocarbons, CFCs).

As the Banerjee et al. Nature paper discusses, the Antarctic ozone hole has also been an important driver of southern hemisphere (SH) climate change, sometimes in a way that is opposite to the effects of the most important human-emitted greenhouse gas, CO₂^[4]. One of the most important ways in which the ozone hole has impacted SH climate is through pulling the SH jet stream more towards the poles. Among other effects, this has led to a poleward (=southward) trend in rainfall, since the jet stream is basically synonymous with where storms happen. Consistent with the emergence of healing of the ozone hole, their work demonstrates that the ozone hole-driven climate effects are beginning to reverse.

In terms of recovery of the world-wide ozone layer, the story is a little more nuanced. As the UNEP/WMO report notes (chapter 3), there is no statistically significant recovery of the total amount of ozone^[9]. Detection of recovery is hampered by large, natural year-to-year variability in the atmosphere (“the weather”), which in turn drives large ups and downs in the ozone layer. Nevertheless, one place where the ozone layer is showing clear signs of recovery is the upper stratosphere, which is 35-45km, or ~20-30 miles, up. The recovery here is being driven both by the gradual decreases in ozone depleting substances, but also a slow down in the chemical reactions that destroy ozone. This slowdown is because the upper atmosphere is cooling due to the increases in greenhouse gases (e.g. Thompson et al. (2012); see Box 5-1 of the WMO/UNEP report for an explanation)^[10,11].

A. R. Ravi Ravishankara, Distinguished Professor, Colorado State University:
The findings of the paper are not unexpected since this is what should happen as the ozone hole is starting to heal. The indicators that the ozone hole is showing signs of healing has been known for a few years. This paper shows that the trends in tropospheric circulation due to the ozone hole not getting bigger and showing some healing is detectable. This is great.

I want to change the phraseology of the paper. The ozone hole is not really “healing,” it is not getting worse and has stayed just as bad for the past few years as the equivalent effective stratospheric chlorine (EESC) has been nearly constant.

I hasten to add that the ozone hole is one indicator of the ozone layer depletion. It is just that the ozone hole is a large signal that is periodic. So, the consequences of its changes are much larger than the global ozone layer depletion starting its course towards its pre-ozone depleting gases state. So, one should expect to detect these changes due to ozone hole changes (but it is really not getting better—just not getting worse!)

The Ozone Layer Assessments have clearly indicated that the ozone hole and its “recovery” is least influenced by climate changes.  It has also shown that the EESC controls the ozone hole well into the future, unlike the global ozone layer changes that are more influenced by climate change (especially stratospheric cooling and the chemical activity of “climate gases” such as methane and N2O). So, I wonder how circulation changes play out in the future globally.

This paper has mixed up the issues between the global ozone layer and the ozone hole.  So, I hasten to add that there are no indicators of the changes in tropospheric climate from the global ozone layer depletion or its changes.

One should ask what is happening in the Arctic ozone changes (larger depletions than globally but less than the ozone hole) in the Northern winter/spring. That would be nice to know.

There has been much written about the “world avoided” by not continuing to increase ODS and the influence of the Montreal Protocol. So, one should also ask what major tropospheric climate changes have been avoided by the Montreal Protocol. I was surprised to not see anything about it.

Also, it is important to know that the ozone hole is still pretty large. It is to be expected to continue for decades. The EESC is not changing much. Their Figure 1a shows the small change in EESC. The responses shown in the paper should be viewed as responses due to the absence of continued increases in the ozone hole.

I wonder what the circulation changes signals would be when there are sudden stratospheric warmings and hence much smaller ozone holes (i.e., if we focused only on the sudden warming years).

Lastly, I want to note that chemically, the ozone hole is not linearly dependent on EESC.  That is why it appeared only in the 1980s and one would expect the changes to be small as long as the EESC is still high.

Antara Banerjee, CIRES Visiting Postdoctoral Fellow, University of Colorado Boulder:

Indeed, our study supports this claim. There already exists observational evidence that the actions of the Montreal Protocol in 1987 have led to declines in atmospheric concentrations of ozone-depleting substances (the CFCs). The production and consumption of these substances was banned by the treaty in an effort to prevent further destruction of the ozone layer. This seems to have been effective: there have been signs that the ozone layer is recovering^[2,4]. Measurements show that ozone in the upper stratosphere outside of the polar regions has been increasing by 1-3% per decade since around 2000. The Antarctic ozone hole – the most severe example of ozone depletion – has also started to shrink since around 2000, even though the hole is still occurring every year.

In the last few decades of the 20th century, there have been some striking changes in the large-scale wind patterns of the Southern Hemisphere during summertime. Notably, the midlatitude jet stream has been shifting towards the South Pole and the tropics have been widening^[12]. Antarctic ozone depletion has been shown to be the main driver of these changes^[11,13].

The new finding in our paper is that these trends in the Southern Hemisphere circulation have stopped, and might even be reversing, since around the year 2000. Crucially, we use model simulations to attribute this lack of trends to the reduction in ozone-depleting substances due to the Montreal Protocol and the healing of the Antarctic ozone hole. This is therefore yet another sign, this time in the wind patterns, of the effects of the Montreal Protocol.

NOTES

@: A Representative Concentration Pathway (RCP) is a greenhouse gas concentration trajectory used by the IPCC. RCP-6.0 is an intermediate scenario in which radiative forcing stabilizes at 6.0 W m-2 after 2100.

REFERENCES

• 1 – Banerjee et al. (2020) A pause in Southern Hemisphere circulation trends due to the Montreal Protocol. Nature. 
• 2 – Solomon et al. (2016) Emergence of healing in the Antarctic ozone layer. Science. 
• 3 – SPARC/IO3C/GAW (2019) SPARC/IO3C/GAW Report on long-term ozone trends and uncertainties in the stratosphere. I. Petropavlovskikh, S. Godin-Beekmann, D. Hubert, R. Damadeo, B. Hassler, V. Sofieva (Eds.), SPARC Report No. 9, GAW Report No. 241, WCRP-17/2018.
• 4 – World Meteorological Organization (2018) Executive Summary: Scientific Assessment of Ozone Depletion: 2018. Global Ozone Research and Monitoring Project–Report No. 58.
• 5 – Morgenstern et al. (2008) The world avoided by the Montreal Protocol. Geophysical Research Letters. 
• 6 – Rigby et al. (2019) Increase in CFC-11 emissions from eastern China based on atmospheric observations. Nature.
• 7 – Dhomse et al. (2019) Delay in recovery of the Antarctic ozone hole from unexpected CFC-11 emissions. Nature Communications.
• 8 – Dhomse et al. (2019) Estimates of ozone return dates from Chemistry-Climate Model Initiative simulations. Atmospheric Chemistry and Physics.
• 9 – World Meteorological Organization (2018) Chapter 3: Update on global ozone: Past, present, and future. Global Ozone Research and Monitoring Project–Report No. 58.
• 10 – Thompson et al. (2012) The mystery of recent stratospheric temperature trends. Nature. 
• 11 – World Meteorological Organization (2018) Chapter 5: Stratospheric ozone changes and climate. Global Ozone Research and Monitoring Project–Report No. 58.
• 12 – Thompson & Solomon (2002) Interpretation of recent Southern Hemisphere climate change. Science. 
• 13 – Polvani et al. (2011) Stratospheric ozone depletion: The main driver of twentieth-century atmospheric circulation changes in the Southern Hemisphere. American Meteorological Society.

UPDATES

• 8 April 2020: This post was updated to include a comment by Antara Banerjee.