Ozone recovery could be hampered by climate change

Scientists monitoring the health of the ozone layer following the ban of chlorofluorocarbons are being urged to take more account of the impact of climate change by the author of a new NASA study, which suggests that by 2030, climate change effects will overtake CFCs as the main source of ozone depletion.

Drew Shindell, an atmospheric scientist from NASA’s Goddard Institute for Space Studies (GISS) and Columbia University, NY, has discovered that certain products of climate change, such as increases in water vapour, and temperature changes in the upper atmosphere, may delay future ozone recovery over heavily populated areas.

Ozone thinning can occur when increased emissions of methane get transformed into water in the stratosphere. At high altitudes, water vapor can be broken down into molecules that destroy ozone. The study examined the ozone layer over heavily populated areas around the equator and mid-latitudes where ozone thinning occurs, excluding the Polar regions, where ‘ozone holes’ form.

The greenhouse effect of heating air within the lowest layer of the atmosphere – the troposphere, can alter atmospheric circulation and make the air more moist, as warmer air holds more water. Shindell, points out that though this complex is not well understood, there is evidence that water vapour can get wafted from the troposphere into the stratosphere by shifting air currents caused by climate change.

Climate change from greenhouse gases can also affect ozone by heating the lower stratosphere where most of the ozone exists. When the lower stratosphere heats, chemical reactions speed up, and ozone gets depleted. The chemical and atmospheric processes in the lower stratosphere are complex, quite variable, and not well understood. Shindell’s conclusions about ozone in the lower stratosphere are inferred from his study on the upper stratosphere where processes are simpler and better understood.

Computer model simulations were used to separate the different factors that contribute to ozone changes. According to the models, which contain some uncertainty, ozone levels are expected to reach their lowest point in recorded history by around 2006. The current CFC ban is based on the expectation that it will enable ozone to eventually return to healthier levels, like those that existed prior to 1979.

One simulation isolated the impacts of CFCs on ozone, and showed that as CFCs decline, by the year 2040 overall ozone makes close to a full recovery from current low levels. When CFCs, water vapor and temperature changes were all combined in a computer model, by 2040, overall ozone levels recovered only slightly from their current low point. Overall, the simulations suggest that climate change from greenhouse gases may greatly slow any anticipated ozone recovery.

The new research contradicts another recent study by scientists in Japan that found that the ozone layer could recover by 2040 (see related story).

The latest paper appears in the latest issue of the Journal of Geophysical Research – Atmospheres

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