The ozone hole over Antarctica is finally "healing" almost 30 years after the world banned the chemicals responsible for its creation, researchers say.

Key points: The ozone hole fluctuates from year to year due to natural factors

The ozone hole fluctuates from year to year due to natural factors Last year volcanic eruptions increased the hole to its largest size ever

Last year volcanic eruptions increased the hole to its largest size ever But now there is evidence it is finally on a downward trend

But now there is evidence it is finally on a downward trend This is 30 years after the decision to ban the chemicals that created it

According to the latest measurements, the ozone hole above the Antarctic is now smaller than it was around the year 2000, by about 4 million square kilometres.

However, renowned ozone hole expert Professor Susan Solomon, from the Massachusetts Institute of Technology, said the hole still averages about 17 million square kilometres in size.

"It isn't completely healed, but it's better than the 21 million we had around 2000," she said.

"It's a long road to a full recovery, but it's on the mend now."

The surprise finding comes just a year after scientists reported the ozone hole was the biggest it had ever been.

However, Professor Solomon and colleagues have pinpointed the growth in the hole in 2015 to the eruption of the Calbuco volcano in Chile, which increased particles in the Antarctic stratosphere.

"The reason we have an ozone hole is because Antarctica is so cold that clouds form in the Antarctic stratosphere, and chlorine can react on the surfaces of those cloud particles," Professor Solomon said.

"Volcanic particles are one thing that can serve as the 'seed corn' for those clouds, so a volcanic eruption will increase the clouds, and slow down the healing."

A map of the ozone hole in 2015 when it was the biggest it has ever been ( NASA )

Legacy of past pollution

The ozone layer plays a critical role in protecting life on Earth by absorbing ultra-violet radiation from the sun. UV radiation is linked to skin cancer, genetic damage and immune system suppression in living organisms.

It is also linked to reduced productivity in agricultural crops and the food chain.

In 1987 there was an international decision to phase out the use of chlorine-containing gases called chlorofluorocarbons (CFCs), which were identified as being the main cause of depletion of the ozone layer.

But Professor Solomon said the chemicals that had led to the ozone hole had a life span of between 50 to 100 years in the atmosphere.

"These molecules have long lifetimes in our atmosphere so even though we aren't making them anymore there is still a lot in the atmosphere," she said.

"It will be many years before the hole closes completely, but we can now see signs that it is not only not getting worse, but actually starting to get better."

Professor Solomon said the discovery, published today in Science, lent hope for the fight against climate change.

"The ozone example shows that when people engage with environmental problems, policymakers have a basis for making choices," she said.

"The world decided to take an action on these chemicals, and the planet is responding as we expected. People can take heart by seeing that our choices can help the environment."

More than just natural variability

For the study the research team used already available satellite data that showed how big and deep the ozone hole was, and how fast it formed; along with balloon measurements that showed what altitudes were affected.

"There are two stations, South Pole, and Syowa, where people have been measuring ozone by balloons for many decades, and these were key in allowing us to show that the altitudes where the ozone is now coming back are exactly where we expect them to be based on the known chemistry," Professor Solomon said.

She said one of the issues in tracking the health of the ozone layer was annual variability due to natural events such as cold temperatures, or changes in winds, or volcanic particles.

Professor Solomon said the latest study explained how these processes contributed to the year-to-year variability.

"That allows us to be confident that we understand that the trend [down] is really due to chlorine, not just these ups and downs," she said.

End of an argument?

"For a few years, scientists have been arguing whether ozone recovery is now evident or not," said Dr Olaf Morgenstern of NIWA in New Zealand.

"This new research is consistent ... with model projections which indicate a gradual healing of the ozone hole and its disappearance in the second half of this century."

"However," added Dr Joseph Lane, of the University of Waikato, "there remains some uncertainty in the rate of recovery, with atmospheric emissions of nitrous oxide (an unregulated ozone depleting substance) continuing to climb."