The Montreal Protocol has been a great success at banning
the production of ozone-depleting substances.
And we know this because we've been measuring those substances at the Earth's surface since
the 1980s or even before. in some cases. So before the Montreal Protocol.
ozone-depleting substances at the surface were going up rapidly.
Once the Protocol was signed and the regulations went into effect.
we saw at the surface. levels of ozone-depleting substances going down.
And so that's great. But those substances have to
get high up into the stratosphere before they can destroy ozone. and they have to break down
high up in the stratosphere. and the chlorine that gets released from the chlorofluorocarbons
that's what actually destroys ozone. What we haven't had up until
now is any measurement inside the Antarctic ozone hole
that the chlorine levels there is actually going down. What I've shown
is that if you're very careful about when you measure
hydrogen chloride. that's HCl. in the atmosphere. and you measure it over time.
you can see that HCl. so that reactive chlorine that destroys
ozone. those levels are actually going down inside the Antarctic
ozone hole. So that's great. so that's part of saying.
"Hey. the ozone hole's recovering. it's getting smaller and it's because of declining chlorine."
But the other part is to also look at how much ozone depletion
is going on at the same time. because ozone levels can vary
for a lot of different reasons. mostly because of temperature. If one year it's warm.
you don't have as much ozone depletion. One year's really cold. you have more ozone depletion.
So it makes it really hard to see the signal that the atmosphere's showing
us. is the ozone hole really recovering? So what we did
in this study. we were able to look at ozone changes during
a period of time. in the winter. when most of that ozone change
is coming just from chemical changes. so that temperatures aren't really affecting it very much.
And we're able to do this because of measurements from the NASA Aura
instrument called the Microwave Limb Sounder. so what we've seen by using the MLS
data is that ozone depletion has declined.
It does vary a lot still. but it declines and it's declining
sort of in step with the chlorine changes. and so that's what we're
excited about. is that we see. for the first time. chlorine levels are definitely
going down. and ozone levels are responding to it.