Ozone (trioxygen, or O3) present in the Earth's stratosphere, known as the ozone layer, is the primary shield that life has from the harmful ultraviolet radiation produced by the Sun. Ozone is also a greenhouse gas, warming not just from its absorption of ultraviolet radiation but also because it absorbs the infrared radiation emitted from the Earth's surface. It's therefore a critical component of the Earth's atmosphere – and one that researchers are keen to understand in detail.

The early 1980s saw a reduction in the thickness of stratospheric ozone so marked that popular accounts told of ozone "holes". That reduction halted after about two decades, largely thanks to the 1989 Montreal Protocol, which regulated the use of chlorofluorocarbons and other substances known to destroy ozone. But ozone can also be affected indirectly by substances outside of the limits of the Montreal Protocol – specifically carbon dioxide and nitrous oxide. Nitrous oxide leads to the production of nitrogen oxides, which react directly with ozone to destroy it, while carbon dioxide cools the stratosphere, boosting ozone in two ways: by depleting nitrogen oxides and by cutting the rate of photochemical reactions that attack ozone.

Now, Darryn Waugh and Richard Stolarski at John Hopkins University, US, together with colleagues from NASA Goddard Space Flight Center, also in the US, have run a computer simulation to find out what effect carbon dioxide and nitrous oxide will have on stratospheric ozone in years to come. The simulation is 2D, with one dimension for altitude and one for latitude – a simplification that reflects the smoothing-out of longitudinal variations at stratospheric altitudes – and is based on factors known to affect ozone. These include the presence of various molecules whose atmospheric concentrations are given in different future climate scenarios; "raining out" of water-soluble molecules; and solar radiation.

The researchers found that, regardless of the future scenario of carbon dioxide and nitrous oxide employed, there would be a thickening of the ozone layer. That will be due mostly to the predicted increase in carbon-dioxide emissions, although for all scenarios the thickening is neither as large nor as rapid as the thinning witnessed in the 1980s.

The 1980s thinning caused global concern, but Stolarski points out there is no reason to believe a thickening is necessarily a good thing. "One interesting question that is raised by this research is, what is the optimal thickness of the ozone layer?" he said, adding that finding out will entail further investigation of ozone history. "Now that we think that we understand the sensitivity of the ozone layer to many potential perturbations, perhaps we can go back and determine what kind of excursions the thickness of the ozone layer may have exhibited in the past."

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

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