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A new study led by NASA researchers finds a tightly controlled greenhouse gas and ozone-depleting substance is making its way into the atmosphere.

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Carbon tetrachloride, or carbon tet for short, is an invisible gas that falls under regulation of the Montreal Protocol.

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The international treaty, adopted in 1987, sets limits on substances that cause damage to the ozone layer.

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Today, emissions of this chemical have been banned globally.

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And the United Nations Environment Program says there were no emissions from 2007 to 2012.

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But scientists now estimate that over the same time period, 39,000 metric tons were released into the atmosphere each year.

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Right now the countries of the world report that there are zero emissions of carbon tetrachloride.

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What we're seeing in our study is no that's incorrect. In fact, there's still carbon tetrachloride being released into the atmosphere.

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So the Montreal Protocol community has made a huge effort to reduce these ozone-depleting substances, and for some reason, for some unknown reason, there's still some being produced.

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Based on the slow decline of observed levels, scientists also believe the chemical lifetime, or time it takes for the gas to break down, may be greater than previously thought.

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The current lifetime expectancy is 25 years.

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According to this study, 35 years is a more accurate estimate.

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Carbon tet accounts for about 11 percent of the total chlorine that destroys stratospheric ozone, so it's a pretty major compound.

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The longer lifetime means the amount of carbon tet accumulated in the atmosphere in the past decades will be removed from the atmosphere slower than the originally expected rate.

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The source of emissions remains a mystery and scientists say more research is needed.

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