MIT study finds human-caused ozone depletion detectable by 1957, decades before Antarctic hole discovery

Scientists at the Massachusetts Institute of Technology have determined that human-caused depletion of the stratospheric ozone layer became detectable as early as 1957—roughly three decades before the 1985 discovery of the Antarctic ozone hole. The early signal appeared in the upper stratosphere over the tropics and was driven mainly by emissions of carbon tetrachloride, an industrial solvent, rather than the chlorofluorocarbons (CFCs) later identified as the primary cause of the polar ozone hole, according to a study published in the Proceedings of the National Academy of Sciences (pnas.org).

The research team, led by atmospheric chemist Susan Solomon and graduate student Jian Guan, conducted a thought experiment. They asked when the first signs of anthropogenic ozone loss would have been identifiable if today’s satellite-based monitoring capabilities and analysis methods had been available throughout the 20th century. Using a large ensemble of 16 simulations from the Community Earth System Model–Whole Atmosphere Community Climate Model (CESM-WACCM), the researchers separated the “signal” of human-forced ozone changes from the “noise” of natural variability caused by factors such as volcanic eruptions, the solar cycle, and weather patterns (news.mit.edu).

Their analysis showed that a statistically detectable signal of ozone depletion emerged around 1957 in the tropical upper stratosphere (roughly 1–5 millibars). This region exhibits relatively low natural variability, allowing a smaller forced signal to stand out more clearly than in higher-latitude or lower-altitude regions where internal fluctuations are larger. While ozone losses later became more severe at higher latitudes, the tropics provided the earliest window for detection under modern observational standards (pnas.org).

The models incorporated historical emissions estimates of ozone-depleting substances, including data constrained by ice-core and firn-air measurements. These records show that atmospheric concentrations of carbon tetrachloride (CCl₄) began rising substantially in the 1930s and 1940s as the chemical was widely used as a dry-cleaning agent and degreasing solvent. By contrast, the main CFCs (CFC-11 and CFC-12) did not become widespread until later decades. Equivalent effective stratospheric chlorine calculations indicated that carbon tetrachloride dominated the early anthropogenic halogen loading, accounting for the majority of the excess chlorine available to destroy ozone before 1960 (news.mit.edu).

“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,” Solomon said in a statement released with the paper. Guan noted that textbooks typically emphasize CFCs, yet another compound caused depletion much earlier. Carbon tetrachloride has since been phased out under the Montreal Protocol and related regulations, primarily for health reasons as well as its ozone-depleting potential; its atmospheric concentrations have declined, though the chemical can persist for decades (iflscience.com).

The Antarctic ozone hole, discovered in 1985 by British Antarctic Survey scientists, formed through a combination of high chlorine loading from CFCs and unique polar conditions involving polar stratospheric clouds and springtime sunlight. That discovery, together with earlier theoretical work by Mario Molina and F. Sherwood Rowland, led to the 1987 Montreal Protocol, widely regarded as a highly successful international environmental agreement. Ozone levels have since begun to recover as concentrations of major ozone-depleting substances have fallen. The new study does not alter the established science of the polar hole or the effectiveness of the Protocol, but it extends the documented timeline of human influence on the ozone layer back by decades (nature.com).

The authors emphasize the value of continued atmospheric monitoring to verify that recovery proceeds as expected and to detect any emerging threats from new substances or activities. The work was supported in part by the National Science Foundation, the National Oceanic and Atmospheric Administration, and the European Commission.