February 3, 2024: Ozone Layer

Good evening. Most readers who are at least ~40 years old remember when the ozone layer was the dominant environmental topic, much as climate change is today. So how serious is ozone depletion, why did the issue seemingly go away, and is it really a solved problem?

Let’s start with the basics, which are better reviewed at Our World in Data than I will attempt here. In short, ozone is an unstable (easily formed and easily destroyed) molecule O3—three oxygen atoms. Ozone can form near the surface, in which case it is considered a form of pollution, and that’s a whole other subject. In the stratosphere, ozone is formed by the impact of ultraviolet light on oxygen that is released from photosynthesis. Ozone, in turn, absorbs short-wave ultraviolet radiation; without this effect, life on the surface would be exposed to a harmful level of UV radiation, causing skin cancer among other nasty effects. Although stratospheric ozone exists from around 10-50 kilometers above the Earth’s surface, the layer is very thin; if a column of stratospheric ozone was concentrated, it would be three millimeters thick, or about the thickness of two pennies.

As said above, ozone is an unstable, easily destroyed molecule, and the substances of greatest concern are chlorine (element 17 on the periodic table), bromine (element 35), and compounds that contain these elements. When chlorine or bromine atoms enter the stratosphere, they can destroy thousands of ozone molecules before falling back to the surface. For complex reasons of atmospheric chemistry that are explained here, ozone-destroying reactions occur most readily under the weather conditions seen at the poles, and that is why the ozone holes are most severe at the poles, especially Antarctica.

For much of the 20th century, chlorofluorocarbons (CFCs) were used as refrigerants, aerosols, and for other purposes. In the 1970s, Frank Rowland and Mario Molina discovered that these compounds would have long residency times in the stratosphere and contribute greatly to ozone depletion. Concerns about ozone depletion led to the Montreal Protocol in 1987, which banned most ozone-depleting substances. Because CFCs have a long residency time in the stratosphere, the peak of ozone depletion did not occur until around 2000, but since then the ozone layer has generally been recovering.

The ozone layer has become a template for a successful environmental campaign. It was only about 15 years from Rowland and Molina’s paper, which first brought the problem to public attention, and the Montreal Protocol. Unlike any previous United Nations treaty, the Montreal Protocol was ratified unanimously by all UN member states. The provisions within the protocol more or less solved the problem, albeit with some course corrections required later on. None of these things can be said about any international efforts to deal with climate change. Hannah Ritchie tells the story in greater detail.

The United Nations Environment Programme estimates that the ozone layer will recover to 1980 values around 2045 globally, 2060 in the Arctic, and in Antartica, where the hole is the most severe, around 2066. To try to put a dollar value on it, in 2012, UNEP estimated that the value of the Montreal Protocol—based on what is has done to the emissions of ozone-depleting substances versus what would have happened under a business-as-usual trajectory—is $3.9 trillion (2022 dollars). This value comes in the form of reduced cases of skin cancer and avoided damage to agriculture, fisheries, and materials. By comparison, this is equal to about one to two years of damages from CO2 emissions at a mainstream carbon valuation of $50 to $100 per ton.

There are a few wrinkles in the case of ozone, though. Like all laws, the Montreal Protocol is not followed perfectly, and there are some contravening sources of emissions of ozone-depleting substances. However, these emissions are a small fraction of the legal sources of emissions before Montreal. After unexplained emissions were flagged from China, a more recent report suggests that those emissions may be declining.

Rocket fuel has always been a source of ozone depletion due to the exhaust in some kinds of fuel, much of which is emitted in the stratosphere, but so far the size of global space launch has been too small to worry much about this. As far what impact rocket fuel has on stratospheric ozone, estimates vary widely, and here are a few of them. By way of comparison, unabated CFC emissions, absent any agreement like the Montreal Protocol, would have resulted in the depletion of most stratospheric ozone in the 21st century.

This study models what were then 87 launches per year (this was in the 1990s) and finds that the resulting ozone depletion is 0.025%. This means that if the cadence of 87 flights per year would have continued indefinitely, the equilibrium stratospheric ozone concentration would have been 0.025% less than it otherwise would have been. This old study finds 0.25% depletion from 15 annual flights. Here’s a more recent paper that models several scenarios for a greatly expanded commercial spaceflight industry. They model 10,000 flights per year and find 0.2% depletion; a model of 100,000 flights per year gives 0.4-1.5% depletion; a model of 300,000 flights per year finds 3.5-3.9% depletion, and a model of 1,000,000 flights per year finds 11% depletion. This paper models 1000 flights per year and finds a depletion of 1%. This paper looks at all the launches in 2018, of which there were 114, and finds 0.01-0.1% depletion. If you look at all these estimates and ranges, the depletion per annual flight ranges from 0.000004% (five zeros) to 0.017%.

While we’re on the subject of the impact of space activities on the ozone layer, there are also concerns about the impact of satellites re-entering the atmosphere and burning up. There are an estimated 5200 tons of micrometeorites that fall to Earth every year. Here’s a marketing report that estimates about 1460 tons of satellites to be launched per year over the next 10 years; most of that mass would presumably be disposed of through reentry. However, beyond sheer mass, satellites contained more alumina, which is of concern from an ozone-depleting perspective, than meteoroids. As far as I can, there is no good estimate of the magnitude of this problem.

In the 1990s, NASA engaged in what they called the High Speed Research program, which sought to bring supersonic commercial flight beyond the Concorde and toward something that would have been more sustainable financially. Among other goals, this program attempted to solve two environmental problems associated with supersonic flight: ozone depletion and sonic booms. By all counts, the program was successful from a technical perspective, but commercial demand for supersonic flight was deemed to be lacking, and the High Speed Research program was cancelled before it was meant to be completed. Commercial supersonic flight would be a good subject for a later time. Anyway, despite the commercial failure, this program has been cited as a good model for a government/industry consortium to develop low-depletion rocket fuel for a possible large commercial spaceflight industry. Hard data on the depletion potential of various types of fuels is limited, but generally solid fuels perform much worse than liquid fuels.

A final thought on this subject is whether it would be feasible to take active measures to restore the ozone layer. Almost all discussion of stratospheric ozone argues that the best approach is to minimize emissions and, with the passage of time, let Mother Nature bring the ozone layer back to its natural level, which is the approach embodied in Montreal. But the amount of stratospheric ozone is such a small fraction of the total atmosphere that maybe it would be possible to generate it with an ozone generator, lifted on balloons because we want the ozone in the stratosphere and not near the ground. Here’s a crude attempt to cost that out.

Total mass of stratospheric ozone is ~3 billion tons and about 300 Dobson units at full health (1 DU is a thickness of 0.01mm if compressed). According to a 2018 study of the state of ozone, the shortfall was 7 DU between 60 °S and 60 °N, 50 DU above 60 °N, and 100 DU below 60 °S. Using a dark art called math, that’s about 176 million tons of shortfall.

Ozone generators are common, well-established technology. Using the specs of the aforementioned industrial generator, it should cost $51 billion to generate 176 million tons of ozone using this device. Using an electricity price of 22.1¢/kWh, which seems high but is reasonable for a distributed (as opposed to grid-fed) system, we get a cost of $386 billion, which is the largest of the costs. The cost of lifting might be around $7 billion, based on a similar cost analysis of solar radiation management. The price of the oxygen is estimated at $28 billion, based on what NASA spends on oxygen for rocket fuel. We get a total cost of $473 billion (numbers don’t add up exactly due to rounding).

Now, when we estimate the benefit of this, I noted above an estimate of $3.9 trillion for the Montreal Protocol, though had the Protocol not happened, the loss would have been far greater than the 176 million ton shortfall we see today. Another way to look at it is that the social cost of CFC-11 (a typical benchmark for costing ozone-depletion) is, CPI-adjusted to 2020, $618 per ton. Based on ozone-depleting potentials of various substances from NASA and current emissions as reported in the 2018 analysis—which, I remind you, are far less than legacy emissions—we have annual damages from $86-199 billion per year.

As far as this idea of lofting ozone generators to repair the damage, I may very well have made an error in my calculations somewhere, or I might be overlooking some important logistical details. But the idea hardly seems outlandish. And I wonder, if it turns out that there are irresolvable emissions associated with commercial spaceflight, if this kind of clean-up is the best solution to ozone depletion. We have many precedents, such as reclamation bonds for mining. And we know from common sense, the solution to dirty dishes is to run the dishwasher, not to stop eating.

Quick Hits

It has been brought to my attention that antirobust, a Substack blogger focuses on collections of links, included a link to my October post on deep ecology.

Israeli intelligence has asserted that several employees of United Nations Relief and Works Agency for Palestine Refugees in the Near East (UNRWA) have taken a direct role in the October 7 attack which precipitated the current war. Israel also accuses 190, out of 13,000 UNRWA employees, of currently being Hamas or Islamic Jihad militants at the time they were employed by UNRWA, and as many as a tenth have prior associations with militant groups. Many international donors of UNRWA have cut funding, with the shortfall amounting to 82% of UNRWA’s total budget, according to Wikipedia. Needless to say, these are very serious accusations, but I don’t have a sense of how deep this scandal runs. How UNRWA, and the United Nations more broadly, respond to this will be critical.

Last week we commemorated the anniversary of the Space Shuttle Challenger explosion, which occurred on January 28, 1986, and of the Space Shuttle Columbia explosion, which occurred on February 1, 2003. Out of the Challenger disaster came the Rogers report, which found a culture of inattention to safety, particularly among the non-engineering management at NASA. A similar report after the Columbia disaster found that the safety problems at NASA that were identified earlier were not fixed.