Ozone Depletion and Climate Change

“Global warming…doesn’t that have something to do with the ozone?” Well, no. Environmental issues are not all the same. It’s common for people to confuse climate change and ozone depletion, but they are separate issues – although they are indirectly connected in some interesting ways.

Ozone, which is made of three oxygen atoms stuck together (instead of two, which is what normal oxygen gas is made of), is vital to life on Earth. It forms a layer in the stratosphere, the second layer up in the atmosphere, that is very good at absorbing ultraviolet (UV) radiation from the Sun. UV radiation severely damages organisms if enough of it reaches the surface. The 3% or less that gets through the ozone already gives us sunburns and skin cancer, so you can imagine what the situation would be like if the ozone layer wasn’t there at all.

In the middle of the 20th century, synthetic gases known as chlorofluorocarbons (CFCs) became popular for use in refrigerators and aerosol products, among other applications. They were non-toxic, and did not react easily with other substances, so they were used widely. However, their chemical stability allowed them to last long enough to drift into the stratosphere after they were emitted.

Once in the stratosphere, the CFCs were exposed to UV radiation, which was able to break them down. Free chlorine atoms (Cl) were liberated, a substance that is very reactive indeed. In fact, Cl acts as a catalyst in the decomposition of ozone, allowing two ozone molecules to become three oxygen molecules, losing their UV absorbing power in the process. Since catalysts are not used up in a reaction, the same Cl radical can continue to destroy ozone until it reacts with something else in the atmosphere and is removed.

Over the poles, the stratosphere is cold enough for polar stratospheric clouds (PSCs) to form. These PSCs provided optimum conditions for the most reactive chlorine gas of all to form: ClO (chlorine monoxide). Now there wasn’t just a catalytic cycle of free Cl radicals depleting the ozone, there was also a cycle of ClO. It turns out that Antarctica was more favourable for ozone depletion than the Arctic, both because its temperatures were lower and because its system of wind currents prevented the ozone-depleting substances from drifting out of the area.

Before long, there was a hole in the ozone layer over Antarctica (due to the PSCs), and concentrations were declining in other locations too (due to the basic Cl reactions). The issue became a frontier for scientific research, and scientists Crutzen, Rowland, and Molina won the 1995 Nobel Prize in Chemistry for their work with atmospheric ozone.

In 1987, politicians worldwide decided to ban CFCs under the Montreal Protocol. This movement was largely successful, and the use of CFCs has become nearly negligible, especially in developed nations. They have been replaced with gases that safely decompose before they reach the stratosphere, so they don’t interfere with ozone. The regulations are working: the ozone hole in Antarctica has stabilized, and global stratospheric ozone concentrations have been on the rise since 1993.

In contrast, climate change is a product of greenhouse gases such as carbon dioxide. Unlike CFCs, most of them are not synthetic, and they are released from the burning of fossil fuels (coal, oil, and natural gas), not specific products such as refrigerators. Rather than destroying a natural process, like CFCs do, they strengthen one to the point of harm: the greenhouse effect. This phenomenon, which traps heat in the atmosphere, is absolutely vital, as the Earth would be too cold to support life without it. Increasing the concentrations of greenhouse gases with fossil fuels becomes too much of a good thing, though, as the greenhouse effect traps more heat, warming the planet up.

Just a few degrees Celsius of warming can cause major problems, as agricultural zones, wind and ocean currents, and precipitation patterns shift. The sea level rises, submerging coastal cities. Many species go extinct, as the climate changes faster than they can adapt. Basically, the definition of “normal” in which our civilization has developed and thrived is changing, and we can’t count on that stability any more.

Unlike the Montreal Protocol, efforts to reduce greenhouse gas emissions have more or less failed. Fossil fuels permeate every part of our lives, and until we shift the economy to run on clean energy instead, convincing governments to commit to reductions will be difficult at best. It remains to be seen whether or not we can successfully address this problem, like we did with ozone depletion.

Although these two issues are separate, they have some interesting connections. For example, PSCs form in cold areas of the stratosphere. That’s why the ozone hole is over Antarctica, and not somewhere else. Unfortunately, global warming is, paradoxically, cooling the stratosphere, as a stronger greenhouse effect means that less heat reaches the stratosphere. Therefore, as climate change progresses, it will make it easier for the ozone depletion reactions to occur, even though there are fewer CFCs.

Additionally, CFCs are very strong greenhouse gases, but their use has drastically reduced so their radiative effects are of lesser concern to us. However, some of their replacements, HFCs, are greenhouse gases of similar strength. They don’t deplete the ozone, but, per molecule, they can be thousands of times stronger than carbon dioxide at trapping heat. Currently, their atmospheric concentrations are low enough that they contribute far less forcing than carbon dioxide, but it wouldn’t take a large increase in HFCs to put us in a bad situation, simply because they are so potent.

Finally, these two issues are similar in that ozone depletion provides a smaller-scale analogue for the kinds of political and economic changes we will have to make to address climate change:

  1. Unintended chemical side effects of our economy posed a serious threat to all species, including our own.
  2. Industry representatives and free-market fundamentalists fought tooth and nail against conclusive scientific findings, and the public became bewildered in a sea of misinformation.
  3. Governments worked together to find sensible alternatives and more or less solved the problem.

We’ve already seen the first two events happen with climate change. Will we see the third as well?


23 thoughts on “Ozone Depletion and Climate Change

  1. Me, I wasn’t aware of the HFCs problem. I had thought that the Montreal Protocol was a resounding success, and a good example how international cooperation can successfully address global issues.

    If I read this right, a measure introduced to solve one problem (ozone depletion) — ie HFCs replacing CFCs — has now come full circle: HFCs increase global warming, and global warming introduces a threat to the ozone layer… oh, this just gets better and better.

    An epitaph for humans: “Not so smart after all.”

    P.S. I got a bit lost at the end. Will we see the third what?

  2. “…as a stronger greenhouse effect near the Earth’s surface means that less heat escapes up to the stratosphere. ”

    The greenhouse effect has only a tiny effect on the amount of heat escaping upwards from the troposphere (0.9 W/m² out of about 250 W/m²) and this is not a significant cause of the cooling of the stratosphere. I admit it is difficult to give a more realistic explanation succinctly (anybody?) but I don’t think that justifies saying something misleading. You’ve already said it’s paradoxical so maybe just something like “…as more CO₂ helps heat escape from the stratosphere” would be sufficient.

    • Ed, ozone is produced only during the sunlight months. During the long winter darkness, temps drop and CFCs build up in the PSCs. When spring comes and sunlight resumes, there is overwhelming destruction of the ozone.
      This is why the ozone hole appears in the spring. Some scientists suggest ozone depletion is the dominant driver of lowered stratospheric temperatures.

  3. The “ozone hole” mechanism is very different from that which weakens the ozone layer at mid-latitudes. The Stratospheric Ozone Textbook is maybe the best place to look it all up, but Robert Parson’s FAQ, although older, is perhaps a better read. The mechanism you describe is appropriate to the mid-latitude catalytic destruction.

    The replacements are mostly HCFC (hydrochlorofluorocarbons) and HFCs (hydrofluorocarbons) where the strategy is more to produce a refrigerant that can react with OH radicals in the troposphere and thus never get up to the ozone layer. Early replacements for CFCs used to clean machined components of oil (a major use of CFCs) were FCs (fluorocarbons) which are terrible for climate change because they essentially never degrade in the atmosphere. They have now, in turn, been replaced by degradable molecules

    Thanks for the corrections, Eli. I didn’t know there were two separate processes at work. I’ll take a look at those links and see how I could make the piece more accurate while remaining accessible. -Kate

  4. China and India are somewhat exempt from the Montreal Protocol, and production of the CFCs there, particularly HCFC22 produces HFC23, and they are making money to destroy this waste product, under offsets in various’ countries CO2 reduction efforts. This money incentive causes them to produce more of the product.

  5. And how do you propose closing the loophole, where China and India agree to give away their exemption and lose the money? I would think you have a better chance getting Nigeria to go aggressively fight the spammers in their country, who just happen to be bringing in millions of dollars to the country.

  6. Kate, a trivial suggestion for what is looking like an excellent blog. For much of the US “Celsius” and other metric terms are incomprehensible jargon. While it will frequently be necessary for scientific articles to include specific units, in this particular article you just dropping the word Celsius would make it more widely understood without harming accuracy at all.

    Thanks for the suggestion. Why is the US still using imperial, anyway? American physics textbooks sort of make me shudder, calories and pounds and miles per hour…Personally I think we should also adopt metric time, with 100 seconds per minute, so everything would be in base 10. -Kate

    • My tuppence:

      It might make sense to drop the reference to the units being used (Celcius, in this case) if the audience could be guaranteed to all be using the same units of measurement; unfortunately, this article is posted on the world wide web, not the USAn web.

      Dropping ‘Celcius’ would simply add to the confusion, as it does in many other instances (eg “is the rise in global temperature a fraction of a degree, or more than one degree?” — this depends upon the unit chosen, giving rise to totally irrelevant arguments).

    • Please continue to directly reference and use metric units. I live in the US and am trying my best to unlearn the outmoded imperial measurements. Fight the anachronism! =)

    • There is something a bit bizarre in the use of “Imperial” units as that empire largely abandoned them a long time ago.

      FWIW I have purchased American physics textbooks since the 1960s and all of them have been in SI units. They were all at Undergrad/Grad level, are you saying that kids are taught science in Imperial and then they have to change over if they wish to continue their studies at advanced levels?


  7. And how do you propose closing the loophole, where China and India agree to give away their exemption and lose the money?

    I expect that the answer involves a thing called diplomacy.

  8. Mike, it would be useful if you looked up how much CFC was produced by China and India after the Montreal Protocols went into effect compared to what was being produced in the US and Europe before and what the current amount China and India produce is (zilch)

  9. Great post – just a couple of points to clarify.
    1) It is chlorine radicals not chloride ions that are responsible for enhanced rates of ozone depletion. A chlorine atom is a radical because it has a single unpaired electron, but it does not have a negative charge like a chloride ion does.
    2) While it’s true that CFC replacements like HCFCs and HFCs have high global warming potentials, many CFCs have equally large or higher global warming potentials, especially over long time horizons. If the Montreal Protocol had not acted to phase out CFC use, there would likely be a much more significant positive radiative forcing right now.

    Another connection: In 2009, NOAA scientists published a paper showing that nitrous oxide is currently the single most important ozone-depleting substance on the basis of ozone depletion potential–weighted anthropogenic emissions. It is unregulated by the Montreal Protocol and also contributes significantly to the greenhouse effect.

    Thank you, Jennifer. I’ve made some corrections. -Kate

  10. Thank you so much for your article. I read a few NASA and NOAA FAQs on the subject and was still confused. I appreciate the time and energy you put into communicating a complex topic so clearly. I admire the passion you have for climate science and hope my own children will find a science topic that engages them so deeply.

  11. The question is, with ozone depletion that would allow extra UV and I gather heat in on the earths lower atmosphere making ozone depletion as the primary cause?

    Now if im correct would that not transmit extra heat onto the other green house gases, making these gases as a secondary effect of global warming?

    I know many say they are separate issuses but are?

    • Greenhouse gases are transparent to UV radiation, i.e. they don’t interact with it. They only interact with (absorb and re-emit) infrared radiation which has lower wavelengths than UV.

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