There's more evidence that the hole in the ozone layer over Antarctica is recovering and that humans' efforts are making a difference. At the same time, however, a new study suggests the ozone layer is surprisingly thin at lower latitudes, where solar radiation is stronger and billions of humans live.

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 in January. The study, conducted by Strahan and colleague Anne R. Douglass, was published Jan. 4 in the journal Geophysical Research Letters.

Thinning ozone layer

Earth's atmosphere The ozone layer is located within Earth's stratosphere. (Photo: Shutterstock)

On the other hand, a study published Feb. 6 raises concerns about the health of the broader ozone layer, especially at lower latitudes. Although the greatest losses occurred in the ozone hole over Antarctica, which seems to be recovering, the new study indicates the layer is thinning in the lower stratosphere over non-polar areas.

And that's a particularly bad place for the ozone layer to weaken, since lower latitudes receive stronger radiation from the sun — and are home to billions of humans. It's not clear yet why this is happening, researchers report in the journal Atmospheric Chemistry and Physics, and models so far don't reproduce this trend.

They do have some suspicions, though, noting that climate change is altering the pattern of atmospheric circulation, which causes more ozone to be carried away from the tropics. Another possibility is that chemicals known as very short-lived substances (VSLSs) — which contain chlorine and bromine — could be destroying ozone in the lower stratosphere. VSLSs include chemicals used as solvents, paint strippers and degreasing agents, and even one used as an ozone-friendly alternative to CFCs.

"The finding of declining low-latitude ozone is surprising, since our current best atmospheric circulation models do not predict this effect," says lead author William Ball, of ETH Zürich and the Physical Meteorological Observatory in Davos, in a statement. "Very short-lived substances could be the missing factor in these models."

VSLSs were thought to be too short-lived to reach the stratosphere and affect the ozone layer, the researchers note, but more research may be needed.

Phasing out CFCs

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 created the ozone hole.

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's 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.

Recovering ozone hole

Aura satellite, NASA Launched in 2004, the Aura satellite studies Earth's ozone layer, air quality and climate. (Photo: NASA)

Strahan and Douglass used the Microwave Limb Sounder (MLS) aboard the Aura satellite to collect their measurements, a 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 in a number of molecules. After chlorine is finished destroying the available ozone, however, 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 total chlorine levels were declining on average by about 0.8 percent annually, or a roughly 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.

It will still take decades to decrease the ozone hole, 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."

Global problem, global response

As for ozone depletion at lower latitudes, Ball and his colleagues note that it's not as extreme as what was happening above Antarctica a few decades ago, but the effects could still be more severe due to conditions closer to the equator.

"The potential for harm in lower latitudes may actually be worse than at the poles," says co-author Joanna Haigh, co-director of the Grantham Institute for Climate Change and the Environment at Imperial College London. "The decreases in ozone are less than we saw at the poles before the Montreal Protocol was enacted, but UV radiation is more intense in these regions and more people live there."

The Montreal Protocol is working for the ozone hole over Antarctica, the study's authors write, although its efficacy may start to be questioned if the thinning trend continues elsewhere. They argue these findings illustrate the value of how closely we've learned to study the ozone layer since the 1980s, as well as the need for ongoing research to reveal what exactly is going on at lower latitudes.

"The study is an example of the concerted international effort to monitor and understand what is happening with the ozone layer," Ball says. "Many people and organizations prepared the underlying data, without which the analysis would not have been possible."

Editor's note: This story has been updated with new information since it was originally published in January 2018.