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The Montreal Protocol has been a great success at banning

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the production of ozone-depleting substances. 

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And we know this because we've been measuring those substances at the Earth's surface since 

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the 1980s or even before, in some cases. So before the Montreal Protocol, 

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ozone-depleting substances at the surface were going up rapidly.

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Once the Protocol was signed and the regulations went into effect, 

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we saw at the surface, levels of ozone-depleting substances going down.

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And so that's great. But those substances have to

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get high up into the stratosphere before they can destroy ozone, and they have to break down

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high up in the stratosphere, and the chlorine that gets released from the chlorofluorocarbons

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that's what actually destroys ozone. What we haven't had up until 

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now is any measurement inside the Antarctic ozone hole 

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that the chlorine levels there is actually going down. What I've shown

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is that if you're very careful about when you measure 

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hydrogen chloride, that's HCl, in the atmosphere, and you measure it over time, 

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you can see that HCl, so that reactive chlorine that destroys

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ozone, those levels are actually going down inside the Antarctic 

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ozone hole. So that's great, so that's part of saying,

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"Hey, the ozone hole's recovering, it's getting smaller and it's because of declining chlorine."

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But the other part is to also look at how much ozone depletion

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is going on at the same time, because ozone levels can vary 

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for a lot of different reasons, mostly because of temperature. If one year it's warm, 

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you don't have as much ozone depletion. One year's really cold, you have more ozone depletion.

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So it makes it really hard to see the signal that the atmosphere's showing 

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us, is the ozone hole really recovering? So what we did 

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in this study, we were able to look at ozone changes during 

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a period of time, in the winter, when most of that ozone change 

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is coming just from chemical changes, so that temperatures aren't really affecting it very much.

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And we're able to do this because of measurements from the NASA Aura

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instrument called the Microwave Limb Sounder, so what we've seen by using the MLS

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data is that ozone depletion has declined.

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It does vary a lot still, but it declines and it's declining 

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sort of in step with the chlorine changes, and so that's what we're 

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excited about, is that we see, for the first time, chlorine levels are definitely 

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going down, and ozone levels are responding to it.

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