Researchers have revealed that human-caused ozone depletion likely became detectable as early as the late 1950s in the tropical upper stratosphere, primarily due to industrial use of carbon tetrachloride decades before widespread CFC emissions.
Study: The emergence of human influence on the ozone layer by the 1960s. Image Credit: Bilanol/Shutterstock.com
Historical Ozone Depletion
The stratospheric ozone layer plays a critical role in protecting life on Earth from harmful ultraviolet radiation. Since the mid-20th century, human activities have released ozone-depleting substances into the atmosphere, triggering chemical reactions that degrade the ozone.
While the discovery of the Antarctic ozone hole in 1985 led to widespread recognition of the problem and regulatory action, the precise timeline of the earliest detectable anthropogenic influence on ozone has remained unclear. Prior studies focused on depletion post-1980, often centered around chlorofluorocarbons (CFCs) commonly used in refrigeration and aerosols from the 1950s onward.
This study aims to identify the “when,” “where,” and “why” of the earliest signs of human-induced ozone depletion by exploiting modern high-precision satellite-equivalent observations simulated backward in time, thereby uncovering the initial stages of atmospheric chemical impacts from industrial solvents and greenhouse gases.
Detection Framework and Models
The researchers conducted a thought experiment assuming the availability of modern satellite-level observational capabilities extending back to 1950. They employed large initial-condition ensemble simulations using the Community Earth System Model coupled with the Whole Atmosphere Community Climate Model (CESM-WACCM6).
This model integrates fully coupled ocean-atmosphere chemistry-climate processes, capturing anthropogenic forcings such as ozone-depleting substances (ODS), including chlorofluorocarbons and carbon tetrachloride (CCl4), greenhouse gases, and natural variability such as volcanic aerosols and solar cycles. Sixteen ensemble members were run from differing starting conditions to distinguish forced ozone changes (“signal”) from natural fluctuations (“noise”).
The analysis focused on altitude-resolved ozone trends in three stratospheric layers: upper, middle, and lower, across latitudinal bands, evaluating spatial trends and signal-to-noise ratios over multi-decadal periods.
Equivalent effective stratospheric chlorine (EESC) metrics were used to quantify the halogen loading impacting ozone chemistry, emphasizing the contribution of CCl4 relative to other ODS. Comparisons with actual Microwave Limb Sounder satellite data between 2005 and 2019 validated the model's realism in reproducing ozone patterns and variability.
Emergence of Early Ozone Signal
The study’s simulations revealed that anthropogenic ozone depletion likely began in the tropical upper stratosphere as early as 1957, nearly three decades before the 1985 discovery of the Antarctic ozone hole. Despite smaller absolute ozone losses in the tropics compared to polar regions, the tropical upper stratosphere’s minimal internal variability enabled earlier detection of the human influence.
This environment exhibits strong chemical sensitivity to halogen-induced catalytic destruction of ozone with less dynamical masking effects. The effective halogen loading analysis demonstrated that carbon tetrachloride, an industrial solvent widely used since the 1930s, accounted for approximately 69% of anthropogenic-equivalent effective stratospheric chlorine in 1950 and remained dominant until about 1960, well before large-scale chlorofluorocarbon emissions increased.
Greenhouse gases exerted a secondary impact, promoting modest increases in ozone in the upper and middle stratosphere, partially offsetting the negative trend induced by halogens and thus slightly delaying detectable depletion.
Volcanoes introduced episodic perturbations that enhanced or reduced ozone through heterogeneous chemistry on stratospheric aerosols, with effects varying over time. Importantly, the study highlights that traditional total column ozone measurements, often confounded by opposing trends in different layers due to dynamical and chemical interactions, obscure early depletion signals.
The research also highlights methodological advances through the use of large ensemble simulations, enabling rigorous detection of subtle anthropogenic signals amid natural variability. Signal-to-noise ratio analyses confirm that even when ozone loss signals were weak, low-variability areas, such as the tropical upper stratosphere, enabled early, confident attribution.
Observationally, this suggests that modern satellite instruments, if available in the mid-20th century, could have identified human fingerprints decades before policy interventions.
The authors discuss the implication that the earliest disturbances to ozone chemistry were associated with chemicals outside the commonly highlighted refrigerants and aerosol propellants. Early industrial solvents like CCl4, whose emissions peaked before regulatory attention to CFCs, played a foundational role in ozone depletion.
This reinforces the importance of broad chemical awareness and monitoring in environmental policy and industrial regulation. Furthermore, the study’s results caution against simplistic attributions of ozone trends to single substances or drivers without considering the balance between chemical loading, atmospheric dynamics, and observational limitations.
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Implications of Early Human Impact
This research demonstrates that anthropogenic influences on the stratospheric ozone layer were detectable as early as the late 1950s, led predominantly by carbon tetrachloride emissions before widespread use of chlorofluorocarbons.
The tropical upper stratosphere emerged as the optimal region for early detection due to its low internal variability and chemical sensitivity, highlighting the importance of altitude-resolved ozone observations.
These insights emphasize the critical need for comprehensive monitoring of a broad spectrum of industrial chemicals to anticipate and mitigate environmental harm. Looking ahead, maintaining and enhancing global stratospheric monitoring capabilities is essential to inform effective environmental stewardship and guide policy decisions addressing atmospheric pollutants.
Journal Reference
Guan J., Santer B. D., et al. (2026). The emergence of human influence on the ozone layer by the 1960s. Proceedings of the National Academy of Sciences of the United States of America (PNAS) 123(28):e2608286123. DOI: 10.1073/pnas.2608286123, https://www.pnas.org/doi/10.1073/pnas.2608286123