The ozone hole that opens every year over the Antarctic is on course to be the smallest in three decades, scientists have said.
Researchers say the hole is also a particularly unusual shape this year, being heavily skewed towards South America instead of centring on the South Pole.
The rare shape – never before observed – indicates a significant distortion to the usual polar vortex, which maintains low temperatures in the stratosphere.
The hole is currently well under half the area that usually opens up by mid-September, and may have already reached its maximum size, a little smaller in area than the Antarctic continent.
Ozone is a gas formed in the upper atmosphere when electrical charges in the atmosphere combine with ultraviolet (UV) rays from the sun. The ozone layer absorbs the majority of the sun's harmful UV radiation.
The hole disappears and reforms every year over the Antarctic due to the unique weather patterns that create incredibly cold circling winds above the pole.
According to the British Antarctic Survey, the hole had reached an area of 11 million square kilometres in early September, when the annual "spring warming" event kicked in much earlier than usual.
In 2018, the hole reached a maximum area of 22.9 million square kilometres.
In 2002, the polar vortex split in two, creating two separate ozone holes over the continent.
Anna Jones a scientist at the British Antarctic Survey told The Independent the shape of the hole this year was "very unusual".
She said: "Very occasionally in the Antarctic you will get disturbances. And I don't think anybody knows what's triggered it this year. This is a pretty new phenomenon. It could be a multiple collection of factors."
Satellite records for the polar vortex in Antarctica go back to the late 1970s, but the 2002 split vortex is the only other major change.
"Scientists haven't quite figured it out yet ... there are various things they can look at, but it's certainly very unusual," Dr Jones said.
Explaining how the polar vortex normally works, she said: "Over Antarctica you have a very strong westerly wind system which sets up over the winter. You get very cold temperatures over the Antarctic continent and the winds are very fast, which holds it in place."
"The winds are driven by the temperature difference between equatorial temperatures and polar temperatures. This is what you expect over the Antarctic."
"In Antarctica you have a continent surrounded by a flat ocean. You don't have a lot of stuff at ground level, so the air can flow around without anything getting in its way."
"In the Arctic, it's the opposite. You have an ocean surrounded by landmasses with mountains. So you don't get the strength of the vortex you get in the Antarctic. "
Dr Jones said the key to reducing the size of the ozone hole in the long term was continuing to clampdown on the illegal man-made chemicals causing the destruction. She also warned people living in South America could now be at risk from dangerous UV radiation as the ozone has thinned.
"The key thing is that the gases that destroy ozone are still around, still in the atmosphere," she said.
But a warming planet due to climate change is also bad news for the ozone layer. Higher atmospheric temperatures could also degrade the conditions required to restore the ozone layer.
"If we warm the lower levels of the atmosphere, we actually cool the stratosphere," Dr Jones said. "This could extend the lifetime of the ozone hole."
She added: "This is an unusual feature resulting from unusual dynamics, but the chemistry which destroys ozone is still there.
"I doubt we will see this again next year. We can't be complacent, it absolutely doesn't mean the ozone hole is never going to be seen again."