Ozone Depletion First Became Detectable in 1957 — and the Culprit Wasn't CFCs

For decades, the story of ozone depletion had a clear villain—chlorofluorocarbons (CFCs)—and a clear starting point: the discovery of the Antarctic ozone hole in 1985. A new MIT-led study published June 28, 2026, in the Proceedings of the National Academy of Sciences (PNAS) rewrites that timeline dramatically .

The study finds that human-caused ozone depletion first became detectable as early as 1957 — nearly 30 years before scientists spotted the hole over Antarctica. More surprisingly, the earliest signal was driven not by CFCs but by carbon tetrachloride (CCl₄), a chemical widely used as a dry-cleaning and degreasing agent starting in the 1930s. And it appeared not over the poles but in the upper stratosphere of the tropics .

"The fact that ozone depletion would have happened as early as the late 1950s, which is much earlier than I would have thought, just absolutely blew my mind," said Professor Susan Solomon, an atmospheric chemistry pioneer and co-author of the study .

Here's what the research reveals about when, where, how — and why it matters for the future of atmospheric monitoring.

When It Began: A Thought Experiment with Modern Tools

The scientists conducted a kind of time-travel exercise. They asked: if the sophisticated satellite instruments and detection algorithms available today had been operating in the mid-20th century, when would they have first detected a clear human-caused signal in the ozone layer?

Using historical atmospheric data and modern modeling techniques, they found that the first statistically unambiguous fingerprint of ozone depletion would have been observed in 1957. This is roughly 30 years before the Antarctic ozone hole was first reported in 1985 .

The Surprising Culprit: Carbon Tetrachloride, Not CFCs

Textbooks have long blamed chlorofluorocarbons (CFCs) — used in refrigerants, propellants, and foam-blowing agents — as the primary cause of ozone loss . The new study reveals a different starting culprit.

The earliest detectable depletion was caused by carbon tetrachloride (CCl₄) , a chemical compound used extensively in dry cleaning and industrial degreasing from the 1930s onward. This was a significant surprise to the research team .
While CFCs did eventually become the dominant ozone-depleting substances, carbon tetrachloride appears to have been the first to accumulate in sufficient concentrations in the upper atmosphere to cause measurable damage .

Where It Started: Not Antarctica, but the Tropics

The first clear "fingerprint" of human-caused ozone loss did not appear over the poles. Instead, it was found in the upper stratosphere of the tropics . This region lies between about 30 and 50 kilometers above the Earth's surface, near the equator. The finding upends the conventional geographical narrative of ozone depletion, which has always centered on the Antarctic .

Implications: The Case for Vigilant Monitoring

Beyond correcting the historical record, the study carries a forward-looking message about atmospheric science and policy.

It demonstrates that the atmosphere can show subtle, early warning signs of damage long before a dramatic effect—like the Antarctic ozone hole—becomes visible. It also shows that the chemicals responsible can be unexpected, and not necessarily the ones that eventually get the most attention .

Solomon emphasized the urgency of this lesson: "It's really important to keep monitoring so that we can fully understand how the atmosphere responds and recovers" .

The paper serves as a reminder that the Montreal Protocol, while a resounding success that has set the ozone layer on a path to healing , should not lull the world into complacency. The same kind of subtle, early-phase damage could be happening today with other chemicals if monitoring is not sustained and advanced.