There's more evidence that the hole in the ozone layer over Antarctica is recovering and that humans' efforts are making a difference.
Thanks to a satellite instrument built by NASA's Jet Propulsion Laboratory, scientists were able to accurately measure the levels of chlorine molecules, which deplete the ozone layer after they break off from human-made chlorofluorocarbons (CFCs). The result is a 20 percent reduction in ozone depletion than there was in 2005, the first year that NASA made measurements of the ozone hole using the Aura satellite.
"We see very clearly that chlorine from CFCs is going down in the ozone hole, and that less ozone depletion is occurring because of it," Susan Strahan, an atmospheric scientist from NASA's Goddard Space Flight Center said in a statement.
Strahan and her colleague Anne R. Douglass' findings were published in the Geophysical Research Letters.
A recovery 30 years in the making
CFCs — a molecule comprised of chlorine, fluorine and carbon — were used to create all sorts of products, including aerosol sprays, packing materials and refrigerants. But once these molecules were exposed to the UV rays of the sun, the chlorine would break off and destroy ozone molecules, which is what led to the reduction of the ozone layer.
We used CFCs for a number of years, but after the discovery of the hole in the ozone layer, we took action. In 1987, nations signed the Montreal Protocol on Substances that Deplete the Ozone Layer, an international treaty that regulated ozone-depleting compounds, CFCs among them. Later amendments to the Montreal Protocol phased out the use of CFCs entirely.
These efforts have led to a steady decline in the ozone layer's hole, but previous studies have relied largely on analyzing the size of the hole to make their claims. Strahan and Douglass' study was the first to measure the chemical composition of the hole to not only determine if it was decreasing, but also to know that a reduction in CFCs was responsible for the decrease.
Satellite's sensor view of the ozone layer
Strahan and Douglass used the Microwave Limb Sounder (MLS) aboard the Aura satellite to collect their measurements, a useful sensor that can measure trace atmospheric gases without the aid of sunlight, a useful feature for studying the ozone layer when there's limited sunlight available. Ozone levels over the Antarctic change starting at the end of the Antarctic winter, around early July to mid-September.
"During this period, Antarctic temperatures are always very low, so the rate of ozone destruction depends mostly on how much chlorine there is," Strahan said. "This is when we want to measure ozone loss."
Chlorine can be tricky to monitor since it's found a number of molecules. However, after chlorine is done destroying the available ozone, it begins to react with methane, and that forms hydrochloric acid; the gas formed by that reaction can be measured by MLS. In addition, this long-lived gas behaves like CFCs do in the atmosphere, so if CFCs were declining overall, there would be less chlorine available to form hydrochloric acid — evidence that the phasing out of CFCs was successful.
"By around mid-October, all the chlorine compounds are conveniently converted into one gas, so by measuring hydrochloric acid, we have a good measurement of the total chlorine," Strahan said
Using hydrochloric acid data collected between 2005 and 2016, Strahan and Douglass determined that total chlorine levels were declining on average by about 0.8 percent annually, or about a 20 percent reduction in ozone depletion over the course of the data set.
"This is very close to what our model predicts we should see for this amount of chlorine decline," Strahan said. "This gives us confidence that the decrease in ozone depletion through mid-September shown by MLS data is due to declining levels of chlorine coming from CFCs.
Before you break out the bubbly, we still have a ways to go. It will takes decades to decrease the ozone hold, according to Douglass, because CFCs linger in the atmosphere for up to 100 years.
"As far as the ozone hole being gone, we're looking at 2060 or 2080. And even then there might still be a small hole," she said.
At least we'll have something to look forward to.