Ozone loss was a thing even before CFCs were widely used

The ban on ozone-depleting substances that successfully reversed the growth of the hole in the ozone layer isn’t seen as a missed opportunity. On the contrary, the quick global response is one of the best cases of common-sense environmental action. But what if it could have been done even earlier? The fact that chlorofluorocarbons (CFCs)—chemicals once common in aerosol cans and refrigerant loops—could destroy ozone in the atmosphere was discovered in 1974. Within just a few years, bans on CFCs began to roll out based on the projected consequences. The seasonal ozone “hole” discovered over Antarctica in 1985 pushed things along even faster, and in 1987 an international agreement was signed to phase out CFCs everywhere. A new study led by Jian Guan at MIT asks an interesting what-if question: Would it have been possible to detect this problem even sooner with today’s scientific tools? The use of CFCs started in the 1950s and ramped up through the 1960s, but they weren’t the first ozone killer to enter the picture. The industrial solvent carbon tetrachloride had been around for several decades before that. Not only do we have estimates of how much was produced for use, but records from the dense snow atop ice cores can confirm how much was in the atmosphere. This data shows that in 1950, carbon tetrachloride was about 3–4 times as prevalent as initial CFC levels. This would have had some impact on ozone, but detecting that effect could be difficult given that ozone levels vary naturally for several reasons. The formation of ozone (O3) is driven by the interaction of sunlight and oxygen gas (O2), it’s sensitive to the 11-year cycle in solar activity, for example. Emissions from volcanic eruptions can also cause some chemical chaos in this system. And because these processes can vary at different altitudes, just examining the total amount of ozone in a column through the atmosphere can obscure a depletion trend at a specific altitude. Current satellite data measures ozone separately in the lower, middle, and upper stratosphere, and model simulations help scientists work out the causes of any changes in these layers. This is the capability we’re imagining adding to the world of the 1950s. The researchers ran a climate model that includes ozone chemistry, feeding it the history of greenhouse gas emissions, ozone-depleting pollution, and natural events like volcanic eruptions. After setting the background with a few simulations starting in 1850, they ran many simulations from 1950 onward with slightly different starting atmospheric conditions to generate a range of realizations. Detecting a trend of declining ozone depends both on how strong the trend is and on how strong the noise is. The lower and middle portions of the stratosphere respond much more strongly to things like volcanic eruptions—and we have the 1963 eruption of Mount Agung to contend with. Ozone in the upper stratosphere is much less variable, and also quite sensitive to ozone-depleting pollutants. While the effects of these pollutants are strongest at middle to high latitudes, variability is lowest near the tropics. In the model, this is actually where the ozone depletion trend emerged first. If we switched on our modern scientific infrastructure in 1950, ozone depletion would first be detectable (clearing the 95 percent statistical confidence bar) in the upper stratosphere over the tropics around 1957. At this point, one-half to two-thirds of the ozone-eating chlorine up there was still carbon tetrachloride rather than CFCs. Elsewhere, it would have taken a bit longer. By 1976, it would have been detectable in the lower stratosphere—including over Antarctica, where the ozone hole wasn’t actually discovered until another decade had passed. So it seems that ozone depletion was technically detectable significantly earlier than its discovery, meaning we might even have intervened sooner and better prevented ozone loss. However, the researchers also point out that this kind of monitoring is currently at risk. The satellite currently measuring ozone at multiple heights in the stratosphere has been in orbit since 2004 and is well past its intended expiration date. (Last year’s White House budget proposal called for shutting it down, in fact.) Without a replacement, it will become much harder to detect future changes while they’re still small. PNAS, 2026. DOI: 10.1073/pnas.2608286123 (About DOIs).