Ozone benefits from treaty

Scientists say they have produced the first conclusive evidence that the ozone layer is being restored to health.

Ozone layer will not be completely healed for at least 50 years

A team of US researchers has found that the rate at which the layer is being destroyed has markedly slowed down.

However, they say it will not be completely healed for at least 50 years.

They believe the recovery is due to the success of an international ban on damaging chemicals.

Protocol success

The scientists analysed data collected over the last 20 years by three satellites and three instruments based on the ground.

They found that in the upper stratosphere - where the topmost slice of the ozone layer resides - the rate at which ozone is disappearing has slowed down markedly.

We fully expect it to stop declining entirely in the next few years Professor Michael Newchurch

"It has been declining at about 8% per decade for a couple of decades, and now it's only about 4% per decade," says the scientist who led the research, Professor Michael Newchurch from the University of Alabama.

"We fully expect it to stop declining entirely in the next few years."

The recovery is due to the success of one of the first global environmental treaties, the Montreal Protocol.

Established in 1987, it banned the chemicals responsible for ozone damage, notably CFCs, used predominantly in aerosols and refrigerators.

Long time frame

Although the upper reaches of the ozone layer have taken their first steps towards regaining full health, the situation at lower levels is more complex.

Here, climate warming is changing temperatures and wind patterns, which is delaying recovery.

Altogether Professor Newchurch's team believes it will be the second half of this century before the ozone can return to a proper balance.

Newchurch: Recovery more problematic at lower altitudes

But they say the success of the Montreal Protocol shows what could be done with other international treaties for other even more serious issues such as climate change.

"The lesson is that with these large, global environmental problems - it is very important to take the first geopolitical steps even if it is not the full scientific solution," Professor Newchurch told the BBC.

"Science works - peer-reviewed science works, because scientists are driven to the right answers," he added.

"We continually challenge each other to present the evidence before we accept the results. And just as we have come to understand the ozone problem, so we will come to understand the global warming problem and the air and ocean pollution problems, which in many ways are more serious and more difficult than the ozone issue."

Ozone is a molecule that is composed of three oxygen atoms. It is responsible for filtering out harmful ultra-violet radiation (less than 290 nanometres) from the Sun.

Ozone is constantly being made and destroyed in the stratosphere, about 30 km above the Earth. In an unpolluted atmosphere, this cycle of production and decomposition is in equilibrium.

But CFCs will rise into the stratosphere where they are broken down by the Sun's rays. Chlorine atoms released from the man-made products then act as catalysts to decompose ozone.

Complex chemistry

This is most pronounced at high latitudes.

The whole process requires complex meteorological conditions that are peculiar to the polar stratosphere during the long, dark winter months.

It is only when these conditions have been established and the sunlight returns in the polar spring that the infamous Antarctic ozone hole, for example, starts to appear.

Ozone thinning above the Artic is not as extensive as in the southern polar region.

Different atmospheric conditions are also in play which means the patterns of ozone depletion are dissimilar as well.

Details about the latest research were released at the US Government meeting on Earth observation, and will be published in the American Geophysical Union's Journal of Geophysical Research - Atmospheres.

Dr Ross Salawitch, an atmospheric research scientist with the US space agency, commented: "This research provides further demonstration that the connection between the release of CFCs at the ground and depletion of upper stratospheric ozone is occurring in a manner consistent with our fundamental understanding of the underlying chemistry, and that the worldwide ban on the production of CFCs and other halogenated halocarbons that was initiated by the Montreal Protocol is having the desired, positive effect on Earth's ozone layer."