A study that explored the prospects for stratospheric ozone under different greenhouse-gas emissions scenarios found that the continued emission of carbon dioxide could over-compensate for lost stratospheric ozone towards the end of this century, leading to less transmission of ultraviolet (UV) radiation.

The consequences are unknown, but could affect ecosystems and levels of vitamin D uptake in humans, the scientists say.

"Stratospheric ozone depletion is generally considered to pose more problems than benefits for human health," said Amy Butler of the US National Oceanic and Atmospheric Administration. "But having higher stratospheric ozone levels than historically experienced also has implications for surface UV."

Most policy makers consider stratospheric ozone a problem solved thanks to the 1989 Montreal Protocol, which phased out the production of chlorofluorocarbons and other ozone-depleting substances. Scientists expect that such substances will return to near historic levels by the second half of this century, and that as a result ozone loss will be gradually remedied.

But the generation of ozone is also dependent on gases that are usually discussed in the context of climate change. Nitrous oxide destroys ozone, while carbon dioxide and methane promote its formation. As Butler and colleagues point out, this means that decisions regarding the emission of these gases could not only affect climate change but also levels of stratospheric ozone.

The researchers performed climate simulations to find out how stratospheric ozone responds to different carbon dioxide, methane and nitrous oxide concentrations for the periods 1955–1975 and 2075–2095. They discovered that a scenario involving little increase in greenhouse gases leads to some stratospheric ozone depletion at most latitudes late this century.

But a strong rise in greenhouse gases would lead to substantial stratospheric ozone increases at high latitudes – what the researchers call a "super recovery" of ozone. In this latter scenario, further mitigation of nitrous oxide would boost ozone even more beyond historic levels.

"Reducing nitrous oxide may be a ‘win–win’ solution in the immediate future while ozone remains depleted, because it’s both a significant ozone-depletor and a potent greenhouse gas," Butler explained. "But if carbon dioxide in particular continues to increase, by the end of the century its effects on ozone will dominate the effects by nitrous oxide. In that scenario, we may see large increases in extratropical ozone above historical levels, and further reductions of nitrous oxide would just exacerbate already high ozone amounts."

Butler believes that interdisciplinary teams should determine the positive and negative effects on ecosystems and human health of different levels of UV exposure. She also reckons there’s a need to account better for changes in aerosols and clouds, which can also affect UV transmission.

Now, Butler and her colleagues want to explore changes in ozone in more detail. "We are planning to consider how circulation changes, along with changes in stratospheric ozone, may affect tropospheric ozone in the future," she said.

The study is published in Environmental Research Letters (ERL).

• This article was amended on Sept 30 to correct an error.

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