A new study led by NASA researchers finds a tightly controlled greenhouse gas and ozone-depleting substance is making its way into the atmosphere.
Carbon tetrachloride, or carbon tet for short, is an invisible gas that falls under regulation of the Montreal Protocol.
The international treaty, adopted in 1987, sets limits on substances that cause damage to the ozone layer.
Today, emissions of this chemical have been banned globally.
And the United Nations Environment Program says there were no emissions from 2007 to 2012.
But scientists now estimate that over the same time period, 39,000 metric tons were released into the atmosphere each year.
Right now the countries of the world report that there are zero emissions of carbon tetrachloride.
What we're seeing in our study is no that's incorrect. In fact, there's still carbon tetrachloride being released into the atmosphere.
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.
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.
The current lifetime expectancy is 25 years.
According to this study, 35 years is a more accurate estimate.
Carbon tet accounts for about 11 percent of the total chlorine that destroys stratospheric ozone, so it's a pretty major compound.
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.
The source of emissions remains a mystery and scientists say more research is needed.