Ozone Layer
© NASA/Science Photo Library
A coloured satellite map of atmospheric ozone in the southern hemisphere between mid-August and early October 1998. An ozone "hole" is seen over Antarctica.
Pointing to the recovery of the ozone layer as humanity's one great triumph of environmental remediation may have been premature, a new report warns.

A team led by Joanna Haigh of the Grantham Institute at Imperial College London, UK, has discovered that while ozone density is indeed improving at the poles, it is not doing so at lower latitudes, roughly between 60 degrees north and 60 degrees south.

That encompasses everywhere on the planet between the Shetland Islands off the north coast of Scotland to south of Tierra del Fuego at the foot of South America.

The researchers found that although the decrease in ozone concentration is not as great as that seen at the poles before the banning of ozone-depleting chlorofluorocarbons (CFCs) in 1987, the effects may be worse, because ultraviolet radiation is stronger in the region, and it contains most of the world's population.

Ozone is an inorganic molecule also known as trioxygen, or O3. It is present in low concentrations throughout the Earth's atmosphere, but is found in much larger levels in the stratosphere, about 20 to 30 kilometres above the planet's surface, where it is formed by the interaction of O2 with ultraviolet light from the sun.

The ozone layer absorbs most of the ultraviolet radiation beaming towards the planet, and thus protects the biosphere from its harmful effects.

In the 1970s scientists realised that the layer was being destroyed by the use of CFCs, which were present in a wide range of commodities, including refrigerators and aerosol products such as hairspray and deodorant. The effect was most severe above the poles.

Concern led to the creation of the Montréal Protocol on Substances That Deplete the Ozone Layer, in 1987, a treaty binding nations to phasing out the use of 96 chemicals used in 240 industrial sectors.

The agreement was amended and strengthened several times in subsequent years, with additional chemicals added to the banned list.

In September 2016, a paper in the journal Science reported the first signs that the treaty was having an effect. A team led by Susan Solomon of the Massachusetts Institute of Technology in the US reported that the "onset of healing of Antarctic ozone loss" had begun.

The hole in the ozone layer above the South Pole, the team found, had shrunk by more than four million square kilometres since the year 2000.

The latest report, however, published in the journal Atmospheric Chemistry and Physics, indicates that the subsequent jubilation may not have been fully warranted. Haigh and her 21 co-authors found that there was compelling evidence, drawn from several different analytical approaches, to find that ozone concentration across the Earth's mid-latitudes has continued to decrease since 1998.

The result is surprising, because none of the existing models established to predict atmospheric behaviour has produced figures to show ozone depletion. The reason for the downturn is unknown. The scientists offer a couple of possible explanations, including increasing levels of other types of human-made substances pumped into the air, a spike in naturally occurring compounds known as very-short-lived substances (VSLSs) containing chlorine or bromine, or a change to air current behaviour caused by the accumulation of greenhouse gases.

These suggestions, however, are acknowledged as no more than informed speculation. The scientists stress that the "causes now urgently need to be established".

Reactions to the findings among the science community have been swift.

Paul Read from the Melbourne Sustainable Society Institute at the University of Melbourne in Australia says the results suggested that existing mitigation regulations are not enough.

"Although the authors suggest otherwise, this paper could be a challenge to the effectiveness of the Montreal Protocol," he says.

"Ozone, like atmospheric carbon, is critical to maintaining the survivability of Earth's solar budget - all of life depends on them being maintained in a tight range. Ozone we need; too much atmospheric carbon we don't."

Ian Lowe, emeritus professor of science at Griffith University in Australia, adds that although significant progress has been made in eliminating the use of CFCs and similar compounds, there is still a way to go.

"The amount of ozone-depleting substances has been reduced dramatically, from about 1.5 million tonnes CFC-equivalent to 0.3 million," he says.

"It has not been reduced to zero. The reduction in the rate of release of these chemicals has halted the worsening of ozone depletion, but we are not yet seeing significant repair.

"This paper also shows that the system is complicated and there are still aspects we do not understand well enough to model the observed data. It should be another urgent reminder that we must scale back our assault on natural systems if we are to achieve our stated goal of living sustainably."

And ecologist Bill Laurence from James Cook University, also in Australia, puts a very pointed and practical spin on the findings.

"This study is scary," he says. "Until we understand what's really happening you'd be silly to sun yourself, except in polar regions. The era of suntanning could be over; we might be entering the age of the unfailing sunburn."