What's up with the Ozone Layer?

[This is a transcript of the video embedded below.]

Without the ozone layer, life, as we know it, would not exist. Scientists therefore closely monitor how the ozone layer is doing. In the past years, two new developments have attracted their attention and concern. What have they found and what does it mean? That’s what we’ll talk about today.

 
First things first, ozone is a molecule made of three oxygen atoms. It’s unstable, and on the surface of Earth it decays quickly, on the average within a day or so. For this reason, there’s very little ozone around us, and that’s good, because breathing in ozone is really unhealthy even in small doses.

But ozone is produced when sunlight hits the upper atmosphere, and accumulates far up there in a region called the “stratosphere”. This “ozone layer” then absorbs much of the sun’s ultraviolet light. The protection we get from the ozone layer is super-important, because the energy of ultraviolet light is high enough to break molecular bonds. Ultra-violet light, therefore, can damage cells or their genetic code. This means, with exposure to ultraviolet light, the risk of cancer and other mutations increases significantly. I have explained radiation risk in more detail in an earlier video, so check this out for more.

You have probably all heard of the ozone “hole” that was first discovered in the 1980s. This ozone hole is still with us today. It was caused by human emissions of ozone-depleting substances, notably chlorofluorocarbons – CFCs for short – that were used, among other things, in refrigerators and spray cans. CFCs have since been banned, but it will take at least several more decades for the ozone layer to completely recover. With that background knowledge, let’s now look at the two new developments.

What’s new?

The first news is that last year we have seen a large and pronounced ozone hole over the North Pole, in addition to the “usual” one over the South Pole. This has happened before, but it’s still an unusual event. That’s because the creation of an ozone hole is driven by supercooled droplets of water and nitric acid which are present in polar stratospheric clouds, so clouds that you find on the poles in the stratosphere. But these clouds can only form if it’s cold enough, and I mean really cold, below about −108 °F or −78 °C. Therefore, the major reason that ozone holes form more readily over the South pole than over the North Pole is quite simply that the South Pole is, on average, colder.

Why is the South Pole colder? Loosely speaking it’s because there are fewer high mountains in the Southern hemisphere than in the Northern hemisphere. And because of this, wind circulations around the South Pole tend to be more stable; they can lock in air, which then cools over the dark polar winter months. Air over the North Pole, in contrast, mixes more efficiently with warmer air from the mid latitudes.

On occasion, however, cold air gets locked in over the North Pole as well, which creates conditions similar to those at the South Pole. This is what happened in the Spring of 2020. For five weeks in March and early April, the North Pole saw the biggest arctic ozone hole on record, surrounded by a stable wind circulation called a polar vortex.

Now, we have all witnessed in the past decade that climate change alters wind patterns in the Northern Hemisphere, which gives rise to longer heat waves in the summer. This brings up the question whether climate change was one of the factors contributing to the northern ozone hole and whether we, therefore, must expect it to become a recurring event.

This question was studied in a recent paper by Martin Dameris and coauthors, for the full reference, please check the info below the video. Their conclusion is that, so far, observations of the northern ozone hole are consistent with it just being a coincidence. However, if coincidences pile upon coincidences, they make a trend. And so, researchers are now waiting to see whether the hole will return in the Spring of 2021 or in the coming years.

The second new development is that the ozone layer over the equator isn’t recovering as quickly as scientists expected. Indeed, above the equator, the amount of ozone in the lower parts of the stratosphere seems to be declining, though that trend is, for now, offset by the recovery of ozone in the upper parts of the stratosphere, which proceeds as anticipated.

The scientists who work on this previously considered various possible reasons, from data problems to illegal emissions of ozone-depleting substances. But the data have held up, and while we now know illegal emissions are indeed happening, these do not suffice to explain the observations.

Instead, further analysis indicates that the depletion of ozone in the lower stratosphere over the equator seems to be driven, again, by wind patterns. Earth’s ozone is itself created by sunlight, which is why most of it forms over the equator where sunlight is the most intensive. The ozone is then transported from the equatorial regions towards the poles by a wind cycle – called the “Brewer-Dobson circulation” – in which air rises over the equator and comes down again in mid to high latitude. With global warming, that circulation may become more intense, so that more ozone is redistributed from the equator to higher latitudes.

Again, though, the strength of this circulation also changes just by random chance. It’s therefore presently unclear whether the observations merely show a temporary fluctuation or are indicative of a trend. However, a recent analysis of different climate-chemistry models by Simone Dietmüller et al shows that human-caused carbon dioxide emissions contribute to the trend of less ozone over the equator and more ozone in the mid-latitudes, and the trend is therefore likely to continue. I have to warn you though that this paper has not yet passed peer review.

Before we talk about what this all means, I want to thank my tier four supporters on Patreon. Your help is greatly appreciated. And you, too, can help us produce videos by supporting us on Patreon. Now let’s talk about what these news from the ozone layer mean.

You may say, ah, so what. Tell the people in the tropics to put on more sun-lotion and those in Europe to take more vitamin D. This is a science channel, and I’ll not tell anyone what they should or shouldn’t worry about, that’s your personal business. But to help you gauge the present situation, let me tell you an interesting bit of history.

The Montreal protocol from 1987, which regulates the phasing out of ozone depleting substances, was passed quickly after the discovery of the first ozone hole. It is often praised as a milestone of environmental protection, the prime example that everyone points to for how to do it right. But I think the Montreal Protocol teaches us a very different lesson.

That’s because scientists knew already in the 1970s, long before the first ozone hole was discovered, that chlorofluorocarbons would deplete the ozone layer. But they thought the effect would be slow and global. When the ozone hole over the South Pole was discovered by the British Antarctic Survey in 1985, that came as a complete surprise.

Indeed, fun fact, it later turned out that American satellites had measured the ozone hole years before the British Survey did, but since the data were so far off the expected value, they were automatically overwritten by software.

The issue was that at the time the effects of polar stratospheric clouds on the ozone layer were poorly understood, and the real situation turned out to be far worse than scientists thought.

So, for me, the lesson from the Montreal Protocol is that we’d be fools to think that we now have all pieces in place to understand our planet’s climate system. We know we’re pushing the planet into regimes that scientists poorly understand and chances are that this will bring more unpleasant surprises.

So what do those changes in the ozone layer mean? They mean we have to pay close attention to what’s happening.