Almost 30 years after the world agreed to phase out chemical substances that deplete the ozone layer, the annual "hole" above Antarctica "appears to be on a healing path" according to a new study in the journal Science.
In the study, a team of scientists found that the September ozone hole has shrunk by more than 1.5 million square miles (4 million square kilometers) -- about half the area of the contiguous United States -- since 2000, when ozone depletion was at its peak. This is despite the fact that other factors, such as volcanic eruptions, have at times interfered to temporarily slow or even reverse that progress.
The ozone "hole" -- in reality an area of significantly diminished levels of stratospheric ozone -- was first noticed by scientists from the British Antarctic Survey in 1982. It is caused by the chlorine in chemicals such as chlorofluorocarbons (CFCs) -- which, among others things, were used as propellants in aerosol sprays. Over the course of the long and extremely cold Antarctic winter, this chlorine becomes attached to thin, wispy polar stratospheric clouds; the return of sunlight prompts a reaction in which the chlorine atoms attack ozone. That is why the hole first appears each year in late August, as Antarctica emerges from its dark winter shroud.
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Although the 1987 Montreal Protocol committed countries to phase out the use of CFCs, they did not disappear from use immediately; additionally, CFCs can take several years to reach the stratosphere, where the chlorine atoms that separate from them can persist for decades.
The hole is at its greatest in October each year; and, said the study's lead author Susan Solomon, the Ellen Swallow Richards professor of atmospheric chemistry and climate science at MIT, "I think people, myself included, had been too focused on October, because that's when the ozone hole is enormous, in its full glory. But October is also subject to the slings and arrows of other things that vary, like slight changes in meteorology."
As an extreme example, last October actually saw the hole reach a record size, which Solomon and colleagues determined was due primarily to the eruption of the Chilean volcano Calbuco. Volcanoes don't inject significant chlorine into the stratosphere but they do increase small particles, which in turn increase the amount of polar stratospheric clouds with which the human-made chlorine reacts.
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Solomon and colleagues elected to focus on measurements of the hole in September, when cold winter temperatures still prevail and the ozone hole is opening up. "September is a better time to look because chlorine chemistry is firmly in control of the rate at which the hole forms at that time of year," Solomon said. "That point hasn't really been made strongly in the past."
They analyzed ozone measurements taken from weather balloons and satellites, as well as satellite measurements of sulfur dioxide emitted by volcanoes. They also tracked meteorological changes, such as temperature and wind, which can shift the ozone hole back and forth.
They then compared their yearly September ozone measurements with model simulations that predict ozone levels based on the amount of chlorine that scientists have estimated to be present in the atmosphere from year to year. Not only did they find that the hole had declined in size from its 2000 peak, they further found that this decline matched the models' predictions, and that more than half the shrinkage was due solely to the reduction in atmospheric chlorine.
"We can now be confident that the things we've done have put the planet on a path to heal," Solomon said. "Which is pretty good for us, isn't it? Aren't we amazing humans, that we did something that created a situation that we decided collectively, as a world, 'Let's get rid of these molecules'? We got rid of them, and now we're seeing the planet respond."
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