The Antarctic ozone hole was discovered in 1985, when scientists observed a severe depletion in the Earth's protective layer of stratospheric ozone. Industrial chemicals known as chlorofluorocarbons (CFCs), then widely used as refrigerants, propellants, foam-blowing agents, and solvents, were at the root of the ozone depletion. After concerted global effort to phase out the use of CFCs, ozone today is recovering, especially in the Antarctic.
The discovery of the ozone hole was possible thanks, in part, to the measurement tools that were available at the time. Advances in those tools, along with satellites and other monitoring technologies, have since allowed scientists to track ozone's recovery.
But what if today's tech was available much earlier? Would scientists have been able to spot even earlier signs of human-induced ozone depletion? And if so, when would those first signs have popped up, and where?
MIT scientists now have some answers. The team, led by atmospheric chemist Susan Solomon, has carried out a thought experiment in which they consider a hypothetical world where today's atmospheric monitoring capabilities were available throughout the last century. In this scenario, they simulated the atmosphere's chemistry through history and discovered not only when the earliest sign of ozone depletion would have been detectable, but also where, and why.
In a study appearing today in the Proceedings of the National Academy of Sciences, the scientists suggest that the first signs of ozone depletion appeared as early as 1957 — about 30 years before the ozone hole was discovered. And, this first signal of ozone loss popped up not in the Antarctic, but in the upper stratosphere of the tropics. What's more, the cause of this early depletion was not due to CFCs, but to another industrial chemical: carbon tetrachloride.
"What we've learned from textbooks is that CFCs result in ozone depletion," says the study's first author, Jian Guan, a graduate student in MIT's Department of Earth, Atmospheric and Planetary Sciences (EAPS). "It turns out there was another compound that caused ozone depletion much earlier than CFCs. This was a big surprise."
For Solomon, who was an early pioneer in the study of ozone's effects on the atmosphere, and who was the first to show that CFCs were the main agent eroding Antarctic ozone, the new results were a complete shock.
"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," says Solomon, the Lee and Geraldine Martin Professor of Environmental Studies and Chemistry at MIT. "This study shows it's really important to keep monitoring so that we can fully understand how the atmosphere responds and recovers."
The study's MIT co-authors include Peidong Wang, Yaowei Li, and Kane Stone; along with Benjamin Santer of the University of East Anglia; Qiang Fu of the University of Washington; Rolando Garcia, Douglas Kinnison, and Jun Zhang of the National Center for Atmospheric Research; Jean-Francois Lamarque of Climate Modeling and Analysis LLC; and Gabriel Chiodo of the Spanish National Research Council.
Chlorine connection
Ozone is a highly reactive molecule, made from three oxygen atoms, that exists naturally in the upper layers of the atmosphere. In the stratosphere, ozone acts as a shield, absorbing the sun's rays and reducing the harmful ultraviolet radiation that can reach the Earth's surface.
In the late 1980s, after scientists first observed signs of ozone depletion in the Antarctic, Solomon led expeditions to the region to measure the stratosphere's composition. Those measurements confirmed that ozone's agent of destruction was CFCs — the chemicals which were used globally in refrigeration, air conditioning, and aerosol propellants, among other uses.
Specifically, Solomon measured higher-than-expected levels of chlorine dioxide in the Antarctic stratosphere. The presence of this molecule, in the same place where ozone depletion was observed, had only one chemical explanation: Ozone was being broken apart by rogue atoms of chlorine. At the time, chlorine-heavy CFCs were in wide use, and MIT chemist Mario Molina proposed that if CFCs drifted up to the stratosphere, photons from the sun could break apart the molecules and release atoms of chlorine, which would then be free to break apart ozone's oxygen atoms.
Molina's work, and Solomon's measurements, were key in showing that CFCs could deplete ozone — a discovery that earned Molina a share of the 1995 Nobel Prize in Chemistry. Soon after, nearly every country in the world signed the Montreal Protocol, which ultimately led to the successful phase-out of CFCs and other ozone-depleting substances. In recent years, as a result of that global cooperation, scientists have observed initial signs of ozone recovery.
"We know what we have now, and ozone is starting to recover," Solomon says. "But no one has ever really documented where and when and why the first ozone depletion would have happened."
Signal over noise
For their new study, Solomon, Guan, and their colleagues took a "what-if" approach, posing the question: What if the past had the monitoring capabilities of the present? When would we have been able to detect the earliest sign of human-induced ozone depletion?
Today's monitoring tools are sensitive to a certain signal to noise, meaning they can identify patterns of ozone loss that are more likely a "signal" of human-induced depletion (such as from CFCs), versus ozone loss that is due to "noise," such as random fluctuations from weather and natural phenomena.
With this in mind, the team looked to reproduce the chemistry of the atmosphere over the last century to see whether they could see a signal over the noise, based on the sensitivity of today's monitoring tools.
The team used 16 different model runs, each of which simulates varying conditions and dynamics of the atmosphere at various latitudes and altitudes, as well as the concentrations and interactions of ozone and other molecules. Ozone is affected by not only human-caused chemicals but also natural phenomena such as volcanic eruptions and El Niño weather patterns. Each model run simulates ozone's response to these natural phenomena, which the team combined to establish a range of "noise," or ozone depletion that likely is due to natural variability.
They added to each model the various industrial chemicals that were known to have been produced at various times over the last century.
"Year by year, we have estimates from industry of how much of these chemicals were made and sold globally, and the emissions of all these chemicals, which the models include," Solomon explains. "And in the case of carbon tetrachloride, the really cool thing is, we also have ice core data."
Ice cores are drilled-out cylinders of deeply buried ice, that had formed in the Antarctic and Arctic from the falling and layering of snow over hundreds of years. Ice cores contain the remnants of snow, as well as whatever trace chemicals in the atmosphere the snow originally fell through. Scientists can therefore use ice cores to estimate the composition of the atmosphere through history.
"We actually see in the ice cores that carbon tetrachloride starts increasing already by the 1940s," Solomon notes.
The team incorporated industrial and ice core data into their models, then looked to see whether a signal of human-induced ozone loss stood out from the noise of natural fluctuations. Their analysis revealed that a signal did appear, as early as 1957. Not only did they see when the signal appeared, but also where: in the tropics, rather than the Antarctic.
The researchers say that human-induced ozone loss was likely occurring globally, but was easier to spot in the tropical upper stratosphere, since that is the region where the range of natural fluctuations is the smallest, and therefore where a signal can stand out better.
Finally, the analysis indicated that carbon tetrachloride, and not CFCs, was the cause of the earliest ozone depletion.
"That's the only ozone-depleting substance that was increasing that early," Solomon says. "We started using carbon tetrachloride in the 1930s as a dry-cleaning agent, and as a degreasing solvent. We didn't start using CFCs until quite a bit later."
Carbon tetrachloride has since been phased out of use in most of the world, initially due to its health concerns; the chemical can cause nervous system disorders with prolonged exposure and is a suspected carcinogen. Since the Montreal Protocol began to tightly limit its use in the 1990s, the molecule's concentrations in the atmosphere have been on a decline. Still, Solomon says the new study highlights the need for vigilance in monitoring carbon tetrachloride, CFCs, and other ozone-depleting substances that may have been phased out but can still linger for decades.
"We've gone through a big effort to get rid of these chemicals," Solomon says. "Don't we have an obligation to keep monitoring to make sure the atmosphere responds the way we think it should?"
This research was supported, in part, by the National Science Foundation, the National Oceanic and Atmospheric Administration, and the European Commission.
