The international treaty to protect the ozone layer is working and it's likely to fully recover during the second half of this century.

That is the message coming from NIWA researchers who have made a prominent contribution to new research.

The announcement comes despite the discovery of increased levels of ozone-depleting hydrogen chloride in the Northern Hemisphere.

Measurements made at NIWA's atmospheric research laboratory at Lauder in Central Otago form the basis of an article in scientific journal Nature.

The international team, including NIWA researchers Dan Smale and John Robinson, based their findings on measurements from a network of stations across the globe, which was backed up by satellite observations and model simulations.

An unexpected increase in hydrogen chloride detected in the Northern Hemisphere since 2007 contradicts the success of the Montreal Protocol, introduced in 1987 to protect the ozone layer.

The study shows the increase is a result of a temporary but prolonged anomaly in atmospheric circulation and confirms that the ozone layer is likely to fully recover during the second half of this century.

'NIWA's role in this research is to provide high quality, continuous measurements in a data sparse region of the globe,” says Dan.

'We had to make sure our data was bomb proof.”

One of the study authors adds: 'Our observations do not challenge the general view that the Montreal Protocol is working.

'They rather show that atmospheric variability and perhaps climate change can significantly modify the path towards full recovery. It will be a bumpy ride rather than a smooth evolution”.

This is the second time research conducted by NIWA researchers has featured in Nature in the past six months.

In May, Alison Kohout, Mike Williams and Sam Dean reported on a breakthrough in understanding one of the key processes driving changes in sea ice.