The ozone layer high above Antarctica might be mending nicely, but the rest of the world tells a different story.
A long-term overview of satellite data shows that ozone levels are actually dropping in the lower stratosphere: the layer of the atmosphere about 10 to 20 kilometres above Earth's surface.
The effect was seen across most of the world, too: as far north as the Scottish highlands and as far south as the southernmost tip of Chile.
And while atmospheric chemists can't yet put their finger on the ozone-draining culprit, global warming is likely playing a leading role, according to atmospheric chemist Stephen Wilson from University of Wollongong.
"If you look at predictions of change due to climate change on ozone, it's exactly the pattern you'd expect to see," said Dr Wilson, who was not involved in the research.
Overall, the total amount of ozone in the entire atmosphere appears to be holding steady, but that's because ozone levels in the troposphere — the lower part of the atmosphere, where we live — are rising.
And that's not good news.
Ozone doesn't belong down here and the increase in tropospheric ozone is mostly due to air pollution, commented Robyn Schofield, an atmospheric chemist from the University of Melbourne.
"[Burning fossil fuels] produces nitrogen oxides and they go on to produce ozone," she said.
Breathing ozone not only damages our lungs, it's bad for crops too, she added.
"Ozone is very effective at destroying plants. We have good reasons for controlling tropospheric ozone," Dr Schofield said.
What's up with ozone?
Without ozone, life on Earth would really struggle. The gas forms a shield against harmful UV light.
It's formed when sunlight hits and busts apart oxygen molecules (O2) in the stratosphere, creating two freewheeling oxygen atoms.
These free atoms bind with other oxygen molecules, creating ozone molecules (O3) which help block UV radiation before it can reach the planet's surface.
Most ozone is made in the sun-drenched tropics and circulated towards the poles through the atmosphere.
But ozone is unstable and will happily react with other molecules — particularly chlorine and bromine, which are found in chlorofluorocarbons (CFCs) and other ozone-depleting compounds.
So in 1987, 27 nations signed the Montreal Protocol, which aimed to phase out these ozone destroyers.
And in the 30 years since, it looks like the hole in the ozone layer above Antarctica is finally bouncing back — but it doesn't appear to be the case all over the world.
Data 'daisy chain'
Tracking ozone over a long time is not an easy task, said William Ball, an atmospheric scientist at PMOD World Radiation Centre in Switzerland and lead author of the study, published today in Atmospheric Chemistry and Physics.
Many satellites have taken ozone readings over the years, but they differ in the way they make the measurements, their altitude and so on.
"What you have is 20 years of data from one satellite, 10 years from another, five from another and so forth," Dr Ball said.
"They all have slightly different properties that make merging them together into a long time-series pretty tricky."
So he and his colleagues developed an algorithm that let them "daisy chain" multiple satellite missions together to get an idea of ozone levels over decades.
"We found a nice recovery in the upper stratosphere in line with the Montreal Protocol, as expected, but then we saw, in the lower part, this downward trend," Dr Ball explained.
Climate change, volcanoes, short-lived compounds
So what's going on?
One idea is that the troposphere and lower stratosphere are mixing more than usual.
The boundary between the layers is called the tropopause. Think of it as a leaky lid, Dr Wilson said, where air from the two layers swirls together.
Add heat from global warming and the lid becomes leakier. Air from the stratosphere, where the bulk of the ozone sits, mingles with the troposphere below — and ozone-poor air is stirred up into the stratosphere.
"You can imagine a saucepan. If you put more energy into a saucepan, the lid rattles more," Dr Wilson said.
So-called "very short-lived substances" might also be a player in the lower stratosphere, Dr Schofield said.
"They're important because they release [ozone-destroying] halogens … much lower in the stratosphere."
Another potential driver, she added, is volcanic activity around the tropics, which spews huge amounts of sulphur into the atmosphere.
"The lower stratosphere will destroy ozone preferentially on volcanic aerosols," Dr Schofield said.
Ideally, sending a weather balloon up to through the atmosphere to take more measurements would be helpful, Dr Ball said.
"What we're seeing here is something we don't understand yet," he said.