A new MIT study reveals that Earth's protective ozone layer was already thinning in the 1950s, triggered by a chemical most people have never heard of: carbon tetrachloride.
The Antarctic ozone hole was discovered in 1985, and the world responded with the Montreal Protocol, a landmark agreement that phased out chlorofluorocarbons (CFCs) and set the ozone layer on a path to recovery. But what if the first signs of human-induced ozone depletion appeared decades earlier — and had nothing to do with CFCs at all?
MIT atmospheric scientists led by Professor Susan Solomon — the same researcher who first confirmed CFCs were destroying Antarctic ozone — have now answered that question. Using modern atmospheric simulation tools, they turned back the clock and asked: if today's monitoring technology had been available in the last century, when would the earliest signs of ozone depletion have appeared?
The answer, published in the Proceedings of the National Academy of Sciences, is 1957 — nearly 30 years before the ozone hole was actually discovered. And the first signal did not appear over Antarctica, but in the upper stratosphere above the tropics.
The chemical responsible was not a CFC, but carbon tetrachloride, a compound widely used from the 1930s onward as a dry-cleaning solvent and degreasing agent. Like CFCs, carbon tetrachloride releases chlorine atoms when broken apart by sunlight in the stratosphere. Those chlorine atoms then catalyze the destruction of ozone molecules — a single chlorine atom can destroy thousands of ozone molecules before it is neutralized.
"What we've learned from textbooks is that CFCs result in ozone depletion," said Jian Guan, the study's first author and a graduate student at MIT. "It turns out there was another compound that caused ozone depletion much earlier than CFCs. This was a big surprise."
For Solomon, who led expeditions to Antarctica in the 1980s to measure ozone depletion firsthand, the results were a shock even after decades in the field. "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," she said.
Ozone is a highly reactive molecule made of three oxygen atoms. In the stratosphere, it acts as Earth's sunscreen, absorbing harmful ultraviolet radiation from the Sun. When chlorine atoms from industrial chemicals reach the stratosphere, they break apart ozone molecules in a catalytic chain reaction — each chlorine atom can destroy tens of thousands of ozone molecules before it is removed from the stratosphere.
The study found that carbon tetrachloride emissions, which peaked in the 1940s and 1950s before being gradually replaced by CFCs, were sufficient to cause measurable ozone depletion decades before the more famous CFC-driven hole appeared over Antarctica.
The study's authors emphasize that the findings do not diminish the importance of the Montreal Protocol, which remains one of the most successful environmental treaties in history. Rather, they highlight how much we still do not know about the long-term behavior of the atmosphere, and how even well-studied environmental problems can hold surprises.
"This study shows it's really important to keep monitoring so that we can fully understand how the atmosphere responds and recovers," Solomon said. "The atmosphere is a complex system, and we need to stay vigilant."