A Hidden Threat to the Ozone Layer: Persistent Carbon Tetrachloride Emissions from China Despite Global Controls
The elusive persistence of carbon tetrachloride (CCl4), a potent ozone-depleting substance once widespread in industrial applications, continues to challenge efforts aimed at healing the Earth’s ozone layer. While global regulations under the Montreal Protocol have drastically reduced CCl4 production and emissions, recent research reveals that emissions remain unexpectedly high, particularly originating from China. This revelation underscores a significant, previously underestimated source of atmospheric pollution, threatening to erode gains in ozone recovery made over the past decades.
Carbon tetrachloride is notorious for its capacity to catalytically destroy ozone molecules in the stratosphere, thereby weakening the protective shield that guards the planet from harmful ultraviolet radiation. Although phased out of intentional production and use in many countries since the 1990s, CCl4 lingers in the atmosphere due to long atmospheric lifetimes and undocumented emission sources. Direct measurements of atmospheric concentrations and sophisticated inverse modeling have consistently suggested that actual global emissions exceed known sources by approximately 30 to 40 percent, a discrepancy known as the “emissions gap.”
A groundbreaking study by An, M., Yao, B., Western, L.M., and colleagues delves into this unresolved puzzle by focusing on China’s emissions between 2011 and 2021. Utilizing an extensive network of atmospheric monitoring sites across China, combined with state-of-the-art top-down atmospheric inversion techniques, the research team unveiled that China’s annual emissions of CCl4 ranged from roughly 16 to 25 gigagrams (Gg) per year over the decade—dramatically surpassing prior bottom-up inventories that estimated emissions below 6 Gg/year since 2011.
This sharp contrast between observed emissions and previous inventories signals the presence of substantial unreported or underestimated emission sources within the country. The implications are profound: China alone accounted for approximately half of the global CCl4 emissions during this period, yet both the Chinese and global time series showed no significant declining trend, suggesting that these emissions persist unabated despite international efforts to curb them.
When translated into ozone depletion potential (ODP)-weighted terms, the scale of China’s CCl4 emissions matches or even rivals other ongoing atmospheric threats such as unexpected global releases of trichlorofluoromethane (CFC-11) observed between 2013 and 2018 or total global hydrochlorofluorocarbon (HCFC) emissions as recently as 2020. This equivalence highlights that the stubborn presence of CCl4 is far from a minor residual problem but rather a major and current obstacle to stratospheric ozone recovery.
Mechanistically, the researchers identified several plausible contributors to these elevated emissions. Substantial releases are likely associated with legal feedstock usage in chemical manufacturing, including during the unanticipated resurgence of CFC-11 production in the years 2013 through 2018, activities officially banned but apparently ongoing clandestinely. Additionally, inadvertent byproduct formation and venting during industrial processes may be significant emission pathways that have not been fully accounted for in prior emission inventories.
Nevertheless, after considering all understood sources, an estimated 4 to 15 Gg per year of emissions remain unaccounted for. This residual emission fraction forms a substantial portion—potentially over half—of the remaining global emissions gap. The persistent mystery of this unexplained component raises difficult questions about unidentified industrial practices, compliance with environmental legislation, and the robustness of existing monitoring frameworks in China and possibly other regions.
This research not only provides an important snapshot of the issue but also serves as a crucial call for enhanced atmospheric monitoring and improved transparency in industrial emission reporting. More rigorous enforcement of production bans, thorough audits of chemical manufacturing processes, and international cooperation will be critical to eliminating these unexpected emissions.
The long atmospheric lifetime of CCl4, approximately 26 years, means that the current elevated emissions will continue impacting the ozone layer and radiative forcing for decades to come. This lingering legacy extends the timeline for the ozone layer’s recovery, which has already been delayed by other emerging halogenated compounds and climatic complexities.
The study also exemplifies the power of top-down atmospheric observation networks paired with inverse modeling, a method that leverages measured atmospheric mole fractions and atmospheric transport models to pinpoint emission sources and strengths. This contrasts with traditional bottom-up methods, which calculate emissions based on reported industrial activity and known usage patterns, often vulnerable to gaps due to undisclosed sources or faulty inventories.
A notable strength of this work lies in its decade-spanning dataset, enabling the identification of temporal trends and the absence of meaningful emission declines over time. This persistence despite global and national policies underscores the challenge of fully phasing out ozone-depleting substances in practice—even when policies are in place.
Considering the global context, China’s significant contribution to persistent CCl4 emissions changes the narrative, highlighting that while many regions have successfully reduced emissions to near negligible levels, others remain problematic. The findings stress how regional discrepancies in industrial control and enforcement can influence global atmospheric chemistry and hinder global environmental goals.
The researchers emphasize the critical importance of closing the remaining emissions gap, not only for protecting the ozone layer but also because carbon tetrachloride is a potent greenhouse gas with a global warming potential far exceeding that of carbon dioxide on a per-molecule basis. Thus, addressing these emissions will also contribute to mitigating anthropogenic climate change.
This multifaceted challenge calls for renewed international attention towards chemical production oversight, technological development of better emission control technologies, and cross-boundary data sharing to enhance the effectiveness of global ozone layer protection strategies. Only through combined efforts spanning science, policy, and industry can the remaining sources of CCl4 emissions be identified and suppressed.
In conclusion, the persistent and substantial emissions of carbon tetrachloride from China, as illuminated in this study, reveal a critical and ongoing obstacle to the recovery of the Earth’s protective ozone layer. These findings demand urgent response measures, improved monitoring capabilities, and stronger enforcement of international agreements to prevent the backsliding of decades of environmental progress. Without decisive action, the lingering shadow of carbon tetrachloride will continue to erode the planet’s ozone shield with implications extending far beyond national borders.
Subject of Research: Persistent emissions of ozone-depleting carbon tetrachloride from China during 2011–2021
Article Title: Persistent emissions of ozone-depleting carbon tetrachloride from China during 2011–2021
Article References:
An, M., Yao, B., Western, L.M. et al. Persistent emissions of ozone-depleting carbon tetrachloride from China during 2011–2021.
Nat. Geosci. (2025). https://doi.org/10.1038/s41561-025-01721-4
Image Credits: AI Generated
