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EVEN as the ozone holes over the poles heal, ozone levels in mid-latitudes, where the majority of the world’s population lives, are set to decline later this century. To make matters worse, the ozone layer will be at its thinnest in late summer, the height of the holiday season. The result could be soaring death rates from skin cancers.

Until now, the main cause of the thinning ozone layer and seasonal ozone holes has been a build-up of chlorine and bromine compounds in the stratosphere. Thanks to the 1987 Montreal Protocol, emissions of these compounds are falling fast, and the holes should be repaired by mid-century. Scientists assumed that the ozone layer away from the poles would recover at the same time.

But a modelling analysis by the Australian research agency CSIRO casts doubt on this. Ian Plumb and colleagues calculate that rising emissions of another ozone-destroying chemical, nitrous oxide, will soon put the improvement into reverse. Nitrous oxide is released by nitrogen fertilisers, some industrial processes and the burning of fossil fuels. The UN’s Intergovernmental Panel on Climate Change predicts that its concentration in the air will rise 45 per cent by 2100.

Ozone levels in mid-latitudes are currently an average of 6 per cent lower than they were in 1980. Plumb says they will recover a little, but start falling again after 2040. They could be 9 per cent lower by the end of the century, and falling fast.

Ozone destruction involving chlorine and bromine compounds occurs fastest when spring sunlight first strikes freezing polar air, leading to the ozone holes. But nitrous oxide destroys ozone fastest in summer, making the ozone layer over Europe and North America thinnest at a time when intense ultraviolet radiation is hitting the atmosphere.