Why did it take so long to notice that the ozone layer had holes in it? Which satellite provided the data?

Question

Wikipedia says:

The discovery of the annual depletion of ozone above the Antarctic was first announced by Joe Farman, Brian Gardiner and Jonathan Shanklin, in a paper which appeared in Nature on May 16, 1985. (paywalled)

I believe the discovery was made by orbiting satellite, but I'm not sure which one.

Questions:

1. Which Earth-observation satellite(s) first provided the data that alerted scientists that the ozone layer was disappearing near the poles?
2. What took so long? Why this (these) satellites and not any of the previous Earth-observation?
3. What exactly are the enabling technologies that made this first possible?

Answer 1

I believe the discovery was made by orbiting satellite, but I'm not sure which one.

That is not the case.

Look at the author affiliation for the article to which you linked. The three authors of that paper were from the British Antarctic Survey. These scientists were part of a larger expedition to Antarctica. They pointed a cheap instrument (extremely cheap compared to a satellite-borne instrument, and rather cheap compared to the cost of sending scientists to Antarctica) called a Dobsonmeter at the sky and found rapidly declining ozone levels at the end of winter / early spring in 1984.

The ozone hole could have discovered by a pair of instruments on the Nimbus 7 satellite, the Solar Backscatter UltraViolet (SBUV) and Total Ozone Mapping Spectrometer (TOMS). That satellite was launched in 1978, but those instruments did not detect the ozone hole because the ground software that processed the data from those devices marked the relevant data as missing rather than as very low.

There were solid reasons for doing so. Remote sensing works best when the readings are corroborated by more direct techniques, in this case by rocketsondes and balloonsondes that can directly measure the constituents of the atmosphere as a function of altitude. (The suffix "sonde" is a fancy way of saying "probe", or more precisely, a sounding probe.)

Readings as low as those implied by SBUV/TOMS had never been observed by these direct techniques, or even by ground-based remote sensing devices such as a Dobsonmeter (at least not until 1984). There was no validation for those very low measurements. Moreover, SBUV/TOMS worked by measuring ultraviolet light that was backscattered by the atmosphere. This technique was deemed to be perhaps suspect when the Sun was very low on the horizon, which was exactly the case in late winter / early spring in polar regions.

The SBUV/TOMS ground processing software, which had to process lots and lots of readings and which had to run unattended, simply rejected readings that were either overly low or overly high and when the Sun was low on the horizon. I wrote the software that made those checks, per requirements made by the team scientists. Even though I left the Ozone Processing Team in early 1980, I received a number of phone calls from NASA in late 1984 / early 1985 because "your name is all over the software." Fortunately, it was easy to fix: Simply relax those constraints and rerun the software on the historical data. NASA soon found that the ozone hole went all the way back to 1978, when the satellite was launched.

How it works

Oversimplifying, the underlying science is the same reason the sky is blue, which is Rayleigh scattering. The sky is blue because the atmosphere scatters blue light much more than it scatters red light. Since Rayleigh scattering is inversely proportional to wavelength raised to the fourth power, the atmosphere scatters ultraviolet light even more strongly than it scatters blue light. While some of this light is scattered toward the surface of the Earth, some of it is scattered out into space. Looking at the frequency distribution of this backscattered light gives insight into how much ultraviolet light is being absorbed by ozone in the stratosphere, and this into how much ozone is in the stratosphere.

That was oversimplified. There are other types of scattering involved (e.g., Raman scattering and Mie scattering). Clouds reflect rather than scatter. Aerosols change the Rayleigh coefficient and other scattering coefficients. The angle at which sunlight hits the atmosphere has a significant effect. These and other effects are taken into account, and the algorithms have been tweaked repeatedly so as to better comport with direct measurements.

Answer 2

To quickly summarise the answer:

1. Nimbus 7 was the satellite involved - but it wasn't first.
2. The ozone hole did not substantially materialise before the early-1980s - in retrospect the decline was visible, as this graph shows, but the catastrophic drop hadn't happened yet. Nimbus 7 was the first satellite (I think?) to carry an ozone spectrophotometer, which came online just as the rapid decline was beginning circa 1980.

The Farman, Gardiner & Shanklin work that you cite was purely ground-based - the three researchers named were from the British Antarctic Survey. The data was taken from a long-running time series using a Dobson spectrophotometer, recorded at Argentine Islands (65 S) and Halley Bay (76 S) since the International Geophysical Year in 1957. (By happy chance, Halley is perfectly positioned for observing the hole.) As I understand it, the Dobson in use in the 1980s was essentially unchanged from the one installed in the 1950s; there was no significant technological change.

However, the satellites saw it as well. In retrospect, it turned out that Nimbus 7 had also identified dropping ozone levels at around this time. When the US satellite data for late 1983 was processed in mid-1984, the low ozone counts were flagged up as an anomaly, so they were compared to "ground truth" ground-based counts at the South Pole. Unfortunately, those counts were actually in error, unknown to everyone involved, and the British data from Halley was not yet fully available so there was nothing else to compare them to. The US team eventually decided in late 1984 that the satellite data probably showed a real phenomenon, and planned to announce it at a conference in August 1985. The British paper, however, came out in May...

(As a bonus: definitely read the comments on that post, as it contains some footnotes from Jon Shanklin.)

Answer 3

Ozone depletion first came up as a publicly visible issue around 1976. Up to that time, almost nobody was aware that ozone was an issue at all, so there hadn't been a big push to build satellites to observe it.

Even when ozone depletion did become known, some greeted it with skepticism (especially people making money off of selling aerosols, of course). For example, here are a couple of extracts from a New York Times story from 1977¹:

One industry spokesman, who requested anonymity, said, “All the scientific theories against fluorocarbons are just that—theories, not facts. What we need is more research before there are any more bans or badmouthing. We don't want another false scare.”
[ ...]
Indeed, the industry is confident that the ozone depletion theories wild be overturned. “It is possible that consumers are starting to regard Rowland's assumptions as the nonsense I think they are,” Mr. Abplanalp said.

TOMS was launched in 1978, only two years after the ozone layer and ozone depletion had entered the public eye as issues.

But, as @David Hammen has already alluded, at the time we lacked the experience necessary to process the data as well as possible, so at first, the data indicating an actual hole in the ozone layer was rejected as anomalous. So, that led to a delay of another five years (or so) before the actual hole (as such) was recognized.

Much the same applied to earlier measurements as well. For example, Nimbus-4 carried instruments for both backscatter UV and measuring ozone at around the 40 kilometer level, but²:

This instrument had occasionally measured very low total ozone values over Antarctica, lower than values measured anywhere else in the world. However, at the time it was not clear whether such values were caused by measurement error or some real geophysical effect.

I'd also note that especially at the time, the ozone hole was interesting primarily as an indication/demonstration of the depletion of the ozone layer in general. It's only rather more recently that evidence has indicated that the ozone hole is something of an issue in itself. It apparently leads to localized cooling of the stratosphere near the pole, which in turn leads to higher winds, and changes atmospheric circulation³.

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1. https://www.nytimes.com/1975/06/22/archives/aerosol-feels-the-ozone-effect.html
2. https://www.sciencedirect.com/science/article/pii/S1631071318301196?via%3Dihub
3. https://climate.nasa.gov/faq/15/is-the-ozone-hole-causing-climate-change/

Answer 4

The holes in the Ozone layer were known as far back as 1957. Scientists just forgot about the earlier report because no one was interested in the issue at the time of discovery.

I encountered this fact while reviewing events recorded in either the NY Times or Omaha Herald (via microfiche) that occurred in my year of birth while in college.

I have not found any web articles that confirm this but a NASA article does mention that the British measure ozone back in 1956/1957: https://ozonewatch.gsfc.nasa.gov/facts/history_SH.html