We used NASA satellite data to show the Antarctic Ozone Hole is recovering because of the ban on chlorofluorocarbons (CFCs). We are Susan Strahan and Anne Douglass from the NASA Goddard Space Flight Center - ask us anything about the ozone layer, past, present, or future!


We are NASA Earth Scientists Susan Strahan and Anne Douglass and we recently published an article using satellite measurements to show that the Antarctic Ozone Hole is beginning to recover! An international treaty signed over 30 years ago banning CFCs – the Montreal Protocol – is the reason for today’s good news. We’re here to talk about the issues that affect the ozone layer and how NASA data and computer models are used to understand today’s ozone and how it will change in the future.

You can see a NASA video on our recent paper or read about our work in a NASA Earth Observatory article.

How’s is what you’ve done different from past studies that have argued the ozone hole is recovering due to CFC bans (e.g. Solomon 2016, Science)?


Thanks for this question. What’s similar about our work and Solomon’s is that we both agree that the ozone hole is recovering due to the CFC ban! What’s different is that we were able to determine this directly from observations of Antarctic ozone made during winter so we could calculate ozone change (i.e., the depletion). Solomon’s paper looked at total column O3 levels in September, and that can vary for several reasons other than a decline in CFCs. We were also able to directly infer the total amount of chlorine – and that controls how much depletion you can have.

To sum up, we quantified ozone depletion over the past decade, determined it was declining, and determined that the decline was in response to chlorine change. By doing this we were able to attribute the Ozone Hole ‘improvement’ to the ban on ozone depleting substances.

Why does the ozone layer have a hole at all, rather than just be thinner all around?


Good question. ‘Hole’ is a term that was popularized in the mid 1980s when the extreme thinning of Antarctic ozone was first discovered. To answer your question we have to explain that there are 2 different ways that chlorine and bromine (the ozone depleting substances) destroy ozone. One of those ways occurs in the upper stratosphere (about 40 km above the surface), it affects ozone chemistry year-round, and it happens at nearly all latitudes. This depletion results in a relatively small (few percentage) depletion occurring year-round at most latitudes. The other way is the depletion that occurs over the poles in winter (in the lower stratosphere about 12-22 km above the surface), and mostly just over Antarctica. This depletion occurs on ice particles that naturally occur at the low temperatures found in polar winter, and this depletion has a much greater impact on ozone than the upper stratospheric loss. Over Antarctica this can cause rougly half the ozone layer to be destroyed – but it’s a SEASONAL effect. Depletion is very large in September and October, but as the polar region warms in spring (that’s Sep-Dec in the southern hemisphere) the depletion ends and some chemistry occurs that restores much (not all) of the loss. Some of this loss does get mixed out to the rest of the world.

To summarize: Antarctic ozone depletion is large (~50%) but occurs only in southern spring and its impact is mostly between 60-90S. The rest of the Earth experiences ozone loss occurring via a different set of reactions and losses are a few percent (less than 10).

What more can I do to help improve the condition of the ozone layer?


International treaties have banned the production and use of ozone depleting substances. That means that as an individual there aren’t many choices you can make – for example, changing what products you buy – that affect ozone. But as climate warms there will be additional natural emissions of ozone depleting gases from the oceans as they warm, so you can help the ozone layer somewhat by reducing your carbon footprint to try to lessen your impact on climate and on ocean warming.

What more can I do to help improve the condition of the ozone layer?


International treaties have banned the production and use of ozone depleting substances. That means that as an individual there aren’t many choices you can make – for example, changing what products you buy – that affect ozone. But as climate warms there will be additional natural emissions of ozone depleting gases from the oceans as they warm, so you can help the ozone layer somewhat by reducing your carbon footprint to try to lessen your impact on climate and on ocean warming.

When can we expect the damage we have done to be completely recovered? And how do we prevent it from happening again?


We expect ozone depletion over Antarctica to be fully recovered by about 2070, give or take 10 years. Over the middle latitudes where much of the Earth’s population lives, we expect recovery to occur sooner, by roughly 2040. Depletion over the middle latitudes is far less several than over Antarctica – just a few percent. There is actually very little depletion over the tropics.

To prevent ozone depletion in the future it’s critical that all the nations of the world continue to comply with international treaties that ban ozone depleting substances! Without complete compliance the problem will persist.

Is there anything someone can do around the home to help in preserving the atmosphere, or is it all up to badgering our leaders to enforce cleaner energy, manufacture, transport etc?


This is a similar question to one already answers, but I'll expand on it.

There are still sources of CFCs and HCFCs (hydrofluorochlorocarbons that initially replaced the CFCs) in use today. These are older refrigerators and air conditioners. If you have one and decide to get rid of it, be sure to dispose of it properly so that the CFCs and HCFCs inside can be reclaimed. I suspect that appliance dealers (and the internet) can tell you how to dispose properly. (Sorry, I can’t.)

I think badgering may be a good idea! But also it’s important to urge our elected leaders to continue to fund atmospheric measurement programs (for example, programs at NASA, NOAA, and universities). The only way to be certain that ozone is on the road to recovery - and that there aren’t new threats - is to continue to measure ozone and trace gas concentrations in the atmosphere. This is how ozone depletion was discovered in the first place and it is how we know whether the threats to ozone are diminishing and ozone is recovering. Scientists continue to measure CFC levels around the globe, and this is critically important in order to know whether the nations of the world are complying with treaties banning CFCs.

The Montreal Protocol is seen as a huge win for environmentalists. Was there as much widespread denial for ozone degradation as there is for climate change now?

Would further restoration of the ozone layer help reign in increasing temperatures on Earth?


The science that shows that man-made chlorofluorocarbons (CFCs) could damage the ozone layer has a long history (e.g., Stolarski and Cicerone (Canadian Journal of Chemistry, 1974), Rowland and Molina (Nature, 1974). At this time scientists did not have the global database necessary to observe CFC in the stratosphere, to confirm their power to destroy ozone, or to project how severe a decline might be. There were unknown chemical processes, especially chemical reactions that could take place on the surfaces of stratospheric clouds – these clouds were known to exist, but there was no idea of their importance. Stratospheric chemistry is complex, involving between 100 and 200 photochemical reactions. Ground-based measurements showed that the ozone maxima were near polar latitudes, so we knew that ozone had to be transported from its production region. Scientists knew that ozone absorbed ultra violet radiation at all latitudes, heating the stratosphere – this heat affects the winds that control the transport. To sum up, scientists knew that we would have to measure ozone globally for more than a decade to detect a chemical loss because ozone varies naturally from year to year. We needed a huge database of reaction rates for the many photochemical reactions that control ozone. Scientists also knew that we needed much better computational tools to simulate the interactions between ozone, radiation, and winds.

The scientific basis exploded over the next decade, the Nimbus-7 Total Ozone Mapping Spectrometer began making measurements in late 1978, and negotiations to reach an environmental treaty limiting CFC production began. The discovery of the ozone hole in ground-based measurements by Farman et al. (Nature, 1985) took the scientific community by surprise. Aircraft campaigns showed that chlorine was the cause for the ozone hole, that the chlorine came from man-made compounds, and that reactions on cloud particles were the big piece of the puzzle that were not included in calculations up to that time.

While all this was going on, of course there were people who thought that chlorofluorocarbons were not causing anyone any trouble. Merchants of Doubt (Oreskes and Conway) discusses some of the opposition to the Montreal Protocol and its amendments. Certainly the discovery of the ozone hole and its intensification in the following years showed one of the key elements to developing an environmental treaty where all nations have agreed – that it IS possible for humankind to have a global impact.

There are some key similarities and differences between the people who thought CFCs would not be damaging to the ozone layer and the climate deniers of today. When CFCs were first thought to be important for ozone destruction, there was a huge dearth of scientific information. That is similar to the case for CO2, noted to be increasing even before the CFC issue was identified. The ozone responded to CFC increase much more rapidly than climate is changing, the Chemical Manufacturers Association (also sponsored research on the impact of CFCs on ozone) developed replacement compounds, so people didn’t notice so much when CFCs were banned (new cars cost more because of the replacements in the air conditioner, but new cars cost more anyway). Now our computer capability is much improved, our data base for the earth system is much broader, and the scientific basis is much more firmly established. The shift needed in energy sources is complicated, much more expensive that adopting the CFC replacement compounds, and with so much more to lose the naysayers have much louder voices.

The CFCs and replacement compounds are also green-house gases, so regulation of these gases will help control surface climate. Ozone's role in global warming is more complicated - while the surface warms, the upper stratosphere cools, and ozone is projected to increase because photochemical reactions will slow down. In the lower stratosphere, the changes in the circulation are projected to change the ozone distribution and its radiative impact. Fixing the ozone layer won't have 'fix' surface climate.

What percentage has recovered?


It’s not possible to give an exact percentage but I can give you an idea of where we are at now. Our recent paper on the Antarctic showed that depletion between 2005 and 2016 decreased by about 20%, but the amount of depletion each year strongly depends on Antarctic temperatures. Ozone depletion will get less over time but it will be a bumpy road! That is, some years the depletion will be a little more than the previous year, sometimes less, but overall the road will take the Antarctic to a full recovery in about 2070, give or take a decade. The peak ozone loss over Antarctica occurred around 2000.

In the middle latitudes where many of us live, depletion at its worse was about 6%. This depletion also appears to be going down, but there is a lot of year to year dynamical variability here too, and the size of that variability is usually greater than the amount of recovery we expect to see by now. In other words, the signal in ozone recovery due to declining chlorine is much smaller than the observed meteorological variability so improvement is difficult to observe let alone quantify. Our chemistry climate models - which as pretty good - tell us we can expect recovery in middle latitudes around 2040 or 2050.

Finally, as ozone recovers, it’s recovering in an environment of higher CO2, CH4, and N2O – these are greenhouse gases and they change stratospheric temperatures, and this changes ozone chemistry. Because of this we may see ozone levels in the late 21st century that as somewhat higher than they were in 1980.

How would you ELI5 the importance of the ozone to people who have never heard of it?


When I explain the ozone layer to non-scientists, I try to relate to people’s common experiences. I usually start with the spectrum of light and the oxygen molecule. Most people already know that you can use a prism to break visible light into colors – we connect that to wavelength and energy. The uv light you can’t see is higher energy than the colored light you can see, and needed by plants to grow (hence ‘grow lights’). Most people have been sunburned at least once so they understand what too much uv can do. Then we talk about the oxygen molecule – so stable – we know that, we breathe it! Then – what does it take to break oxygen molecules apart – very high UV. What happens to oxygen atoms – they are so unhappy to be unbonded, they bond with anything available. What is available – oxygen molecules! So they bond and make an ozone – not a strong bond like oxygen molecules, but strong enough to be broken apart by ultraviolet radiation. So keeping the surface of the earth safe is a two-step process – first the high high energy light is taken out by oxygen molecules, then the high energy light is taken out by ozone – just enough so that we can still grow plants.

Then – depending on the person asking – build complexity or leave it at that.

You can find lots of information about this on the internet, for example NASA’s OzoneWatch page https://ozonewatch.gsfc.nasa.gov/.

Has the ozone layer been permanently damaged by CFCs or can there be a full recovery eventually?


No permanent damage! CFCs have long atmospheric lifetimes so they will be around for many decades – and over a century for some of the CFCs – but once they are gone from the atmosphere they will no longer affect ozone chemistry. There will be a full recovery provided that all nations continue complete compliance with international treaties. The stratosphere is cooling due to climate change and this may lead to slightly higher ozone by the end of the 21st century than was around in 1980.

Did you learn anything surprising/unexpected?


One of the science questions for the Earth Observing Platform Aura (launched in 2004) was ‘is the ozone layer recovering as expected?’ That is such a funny question, because it really depends on what you expected, and whether your expectation was realistic or not. Now that we have trusted multi-decadal records for many gases, if your expectation is well grounded in past observations, you are unlikely to be surprised. For the ozone hole – we have the long record of measurements over Antarctica, where the ozone hole started, then was weaker, stronger, weaker, stronger for a few years while CFCs continued to grow, and then stayed pretty large for a long time (with variations). CFCs are long-lived in the atmosphere (50 – 100 year lifetimes) and are decreasing much more slowly than they rose. Our personal expectation is that the signs of recovery will take time to develop – it took more than a decade of observations to identify ozone loss outside the ozone hole, so it will take many decades of observations to identify recovery. I think people were surprised that the ground-based measurements of the total column of HCl (one of the main chlorine containing gases in the stratosphere) would show year over year increases between 2005 and 2010 when we knew for sure that the CFCs were decreasing at the ground – but on reflection we already knew in the 1990s that dynamical variations on multi-year time scales made it hard to see ozone trends, so if we were surprised probably we should not have been.

What challenges did you face with setting up and executing the project?


The first challenge here is putting it all together. Way back in the about 1990, a theorist published a paper that showed that when Antarctic ozone at a particular level reaches very low values (< 0.5 ppbv), the concentration of chlorine atoms would grow (since there is ‘no’ ozone for them to react with) until the normally slow reaction of chlorine with methane forming hydrochloric acid (HCl) would become fast, and HCl would rapidly fill the vortex. We had evidence that this was correct – the HALogen Occulation Experiment (HALOE) on the Upper Atmosphere Research Satellite had measured a few profiles with very high HCl in the lower stratosphere in 1992, but the orbit pattern never returned to the Antarctic vortex during spring. Some aircraft measurements showed high HCl – very limited in space and time. We had to wait for the Microwave Limb Sounder (MLS) on Aura to see the HCl fill the vortex as ozone reached low values (check out this video! https://svs.gsfc.nasa.gov/30548\). Then some years go by, and we have the inspiration that this chemistry is giving us a measure of total chlorine inside the vortex – and that we may be able to detect a trend.

Later challenges are to actually analyze the data, use another constituent to account for known sources of interannual variability, figure out how to quantify the ozone loss dependence on chlorine after accounting for variability in temperature, and then convince your co-author that you have done all of this correctly. Last challenge is to answer questions from the reviewers, revise the paper, and convince the editor.

Actually, this evolution is classic!

What's it like working at a space flight center?


The best part of working at Goddard Space Flight Center is being part of a group of people with expertise in many areas of Earth Science. Earth Science is different from other areas of scientific inquiry in that we don’t conduct controlled experiments on the Earth – we learn by combining measurements, lab experiments and simulations. In the Atmospheric Chemistry and Dynamics Laboratory, we have experts in meteorology, in radiation, and in chemical processes, and study the atmosphere from the near surface through the troposphere, stratosphere and above. We have remote sensing experts, who develop the satellite data sets that are necessary to understand the strengths and limitations of our simulations. We have scientists developing instruments to measure key species from aircraft and from satellite. Our strong relationship with measurements assists us in developing world-class predictive models. We have the opportunity to interact with scientists who use data from NASA satellite platforms worldwide. We share our science broadly, with citizens, with schools (career days, science fair judges, etc.).

Our leadership is committed to making this an environment where we thrive as scientists and through collaborations. As a woman scientist, I valued the efforts of our leadership to make it possible for me to maintain a reasonable work-life balance. GSFC has an excellent on-site daycare facility for toddlers through kindergarten. We have flexible hours that make it possible to accommodate the unexpected (childhood illness) or the fun aspects of raising children (a game, a play, or chaperoning a school activity). Our leadership is committed to equal opportunity, and it shows – for example, women have served as Project Scientist for major Earth Observing Platforms including Aqua, Aura and the Global Precipitation Measurement. Of course, things like daycare and flextime are important to the fathers of the current generation, but when I started (1981) they were much more much more important to the mothers.

Are there any differences in other thin spots/holes in the ozone compared to the Antarctic's hole?


Ozone thinning is relatively minor (a few percent) over most of the global outside of the Antarctic region. In some years the Arctic gets very cold and there is depletion on the orders of tens of percent at high northern latitudes (above the Arctic circle). But as in the Antarctic, this depletion is seasonal. By the end of Arctic winter, most of the loss recovers as the sun returns and the chemistry changes. Some of that loss mixes out to the middle latitudes (like over the US, Europe, and Asia) but the effects is a few percent.

How does the process of access to the data to the data work and in what form do you get the data? Do you need to wade through a database for the relevant tables? Is the satellite set up to send specific information for your specific study? Do you specify what data set you need and somebody set up a database for you?


Satellite instruments relay their data back to Earth where the measurements are processed and archived. The archives are public and information on obtaining the data can be found in our publication in Geophysical Research Letters. See https://giovanni.gsfc.nasa.gov/giovanni/ to get an idea of all the NASA data available. NASA data is free for anyone in the US or abroad who wants to use it.

As a scientist I decide which of the measurements would be useful in an analysis, then it is up to me to obtain the data.

Do you have to go to Antarctica to collect some of your data? If so what sort of challenges do you face as a result of this? I imagine that sort of environment could be quite harsh on scientific equipment and scientists alike.


There are ground-based measurements on the Antarctic continent. These measurements were used in the first published report of the Antarctic ozone hole (Farman et al, Nature, 1985). Those measurements continue today, but we get far more data from satellite instruments. We used O3 and HCl measurements from the Microwave Limb Sounder instrument on the NASA Aura satellite in our study. In addition to this data set there a number of other satellite instruments that measure total column O3 globally, although many of those instruments can't make measurements during polar night. (So they can't see inside the ozone hole while it is growing in late winter.) Ground-based measurements are also important because they are used to 'verify' or corroborate what satellite instruments measure.

Satellite data are also incredibly useful because they give us near-global information, unlike the ground-based instruments that look up from the station where they are located. Occasionally there are aircraft missions that fly through the ozone hole making measurements. NASA polar aircraft missions in the late 1980s (AAOE and AASE) were critical for identifying the cause of polar ozone depletion.

All of these different data sets working together give us a consistent picture of Antarctic ozone and how it has changed over the past 50-60 years (that's how far back some of the ground-based measurements go.)

Thank you for doing this AMA!


You're welcome! We hope you found it interesting.

USA blew up one of the biggest nuclear bombs in history over the north pole somewhere up near the ozone.

Do you think this may have contributed to the hole?


There is no 'Arctic ozone hole', only a seasonal thinning of Antarctic ozone. Nuclear explosions are not responsible for ozone depletion.

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