appearing today in the Proceedings of the National Academy of Sciences<\/a>, the scientists suggest that the first signs of ozone depletion appeared as early as 1957 \u2014 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\u2019s more, the cause of this early depletion was not due to CFCs, but to another industrial chemical: carbon tetrachloride.\u00a0<\/p>\n\u201cWhat we\u2019ve learned from textbooks is that CFCs result in ozone depletion,\u201d says the study\u2019s first author, Jian Guan, a graduate student in MIT\u2019s Department of Earth, Atmospheric and Planetary Sciences (EAPS). \u201cIt turns out there was another compound that caused ozone depletion much earlier than CFCs. This was a big surprise.\u201d<\/p>\n
For Solomon, who was an early pioneer in the study of ozone\u2019s 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.\u00a0<\/p>\n
\u201cThe 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,\u201d says Solomon, the Lee and Geraldine Martin Professor of Environmental Studies and Chemistry at MIT. \u201cThis study shows it\u2019s really important to keep monitoring so that we can fully understand how the atmosphere responds and recovers.\u201d<\/p>\n
The study\u2019s 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.\u00a0<\/p>\n
Chlorine connection<\/p>\n
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\u2019s rays and reducing the harmful ultraviolet radiation that can reach the Earth\u2019s surface.\u00a0<\/p>\n
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\u2019s composition. Those measurements confirmed that ozone\u2019s agent of destruction was CFCs \u2014 the chemicals which were used globally in refrigeration, air conditioning, and aerosol propellants, among other uses.\u00a0<\/p>\n
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\u2019s oxygen atoms.\u00a0<\/p>\n
Molina\u2019s work, and Solomon\u2019s measurements, were key in showing that CFCs could deplete ozone \u2014 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.<\/p>\n
\u201cWe know what we have now, and ozone is starting to recover,\u201d Solomon says. \u201cBut no one has ever really documented where and when and why the first ozone depletion would have happened.\u201d<\/p>\n
Signal over noise<\/p>\n
For their new study, Solomon, Guan, and their colleagues took a \u201cwhat-if\u201d 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?\u00a0<\/p>\n
Today\u2019s monitoring tools are sensitive to a certain signal to noise, meaning they can identify patterns of ozone loss that are more likely a \u201csignal\u201d of human-induced depletion (such as from CFCs), versus ozone loss that is due to \u201cnoise,\u201d such as random fluctuations from weather and natural phenomena.\u00a0<\/p>\n
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\u2019s monitoring tools.\u00a0<\/p>\n
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 Ni\u00f1o weather patterns. Each model run simulates ozone\u2019s response to these natural phenomena, which the team combined to establish a range of \u201cnoise,\u201d or ozone depletion that likely is due to natural variability.<\/p>\n
They added to each model the various industrial chemicals that were known to have been produced at various times over the last century.\u00a0<\/p>\n
\u201cYear 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,\u201d Solomon explains. \u201cAnd in the case of carbon tetrachloride, the really cool thing is, we also have ice core data.\u201d<\/p>\n
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.\u00a0<\/p>\n
\u201cWe actually see in the ice cores that carbon tetrachloride starts increasing already by the 1940s,\u201d Solomon notes.\u00a0<\/p>\n
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.\u00a0<\/p>\n
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.<\/p>\n
Finally, the analysis indicated that carbon tetrachloride, and not CFCs, was the cause of the earliest ozone depletion.\u00a0<\/p>\n
\u201cThat\u2019s the only ozone-depleting substance that was increasing that early,\u201d Solomon says. \u201cWe started using carbon tetrachloride in the 1930s as a dry-cleaning agent, and as a degreasing solvent. We didn\u2019t start using CFCs until quite a bit later.\u201d<\/p>\n
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\u2019s 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.<\/p>\n
\u201cWe\u2019ve gone through a big effort to get rid of these chemicals,\u201d Solomon says. \u201cDon\u2019t we have an obligation to keep monitoring to make sure the atmosphere responds the way we think it should?\u201d<\/p>\n
This research was supported, in part, by the National Science Foundation, the National Oceanic and Atmospheric Administration, and the European Commission.<\/p>\n","protected":false},"excerpt":{"rendered":"The Antarctic ozone hole was discovered in 1985, when scientists observed a severe depletion in the Earth\u2019s protective…\n","protected":false},"author":2,"featured_media":502065,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[68126,250631,250627,250625,43782,111,139,69,250629,250628,250630,147,250626],"class_list":["post-502064","post","type-post","status-publish","format-standard","has-post-thumbnail","category-science","tag-atmospheric-chemistry","tag-carbon-tetrachloride","tag-jian-guan","tag-mit-chemistry","tag-mit-eaps","tag-new-zealand","tag-newzealand","tag-nz","tag-ozone-depletion","tag-ozone-hole","tag-ozone-recovery","tag-science","tag-susan-solomon"],"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/posts\/502064","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/comments?post=502064"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/posts\/502064\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/media\/502065"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/media?parent=502064"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/categories?post=502064"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/nz\/wp-json\/wp\/v2\/tags?post=502064"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}