The hole in the ozone layer is now steadily closing, but its repair could actually increase warming in the southern hemisphere, according to scientists at the University of Leeds.

The Antarctic ozone hole was once regarded as one of the biggest environmental threats, but the discovery of a previously undiscovered feedback shows that it has instead helped to shield this region from carbon-induced warming over the past two decades.

High-speed winds in the area beneath the hole have led to the formation of brighter summertime clouds, which reflect more of the sun’s powerful rays.

“These clouds have acted like a mirror to the sun’s rays, reflecting the sun’s heat away from the surface to the extent that warming from rising carbon emissions has effectively been cancelled out in this region during the summertime,” said Professor Ken Carslaw, a teacher of atmospheric science in the School of Earth and Environment, University of Leeds, who co-authored the research.

“If, as seems likely, these winds die down, rising CO2 emissions could then cause the warming of the southern hemisphere to accelerate, which would have an impact on future climate predictions,” he added.

The key to this newly-discovered feedback is aerosol – tiny reflective particles suspended within the air that are known by experts to have a huge impact on climate.

Greenhouses gases absorb infrared radiation from the Earth and release it back into the atmosphere as heat, causing the planet to warm up over time. Aerosol works against this by reflecting heat from the sun back into space, cooling the planet as it does so.

Beneath the Antarctic ozone hole, high-speed winds whip up large amounts of sea spray, which contains millions of tiny salt particles. This spray then forms droplets and eventually clouds, and the increased spray over the last two decades has made these clouds brighter and more reflective.

As the ozone layer recovers it is believed that this feedback mechanism could decline in effectiveness, or even be reversed, leading to accelerated warming in the southern hemisphere.

“Our research highlights the value of today’s state-of- the-art models and long-term datasets that enable such unexpected and complex climate feedbacks to be detected and accounted for in our future predictions,” added Professor Carslaw.

The Leeds team made their prediction using a state-of-the-art global model of aerosols and two decades of meteorological data. The research was funded by the Natural Environment Research Council’s Surface Ocean-Lower Atmosphere Study (UK SOLAS) and the Academy of Finland Centre of Excellence Programme.

A copy of the paper: Aerosol climate feedback due to decadal increases in southern hemisphere wind speeds will be published in Geophysical Research Letters tomorrow, Wednesday 27th January.

The ozone hole
The discovery by the British Antarctic Survey of the Antarctic ozone hole provided an early warning of the dangerous thinning of the ozone layer worldwide, and spurred international efforts to curb the production of CFCs.

The provisions of the Montreal Protocol of 1987 on Substances that Deplete the Ozone Layer have been revised and strengthened and are being followed by virtually all UN Member states. There is a reasonable prospect that the Antarctic ozone hole will permanently repair itself, but not until around 2070.

British scientists began their measurements of Antarctic ozone at Halley in 1956. The aim was to understand the important role that ozone plays through absorbing solar energy, in determining the temperature profile of the stratosphere and its wind circulation.

The amount of ozone overhead Halley station follows a regular seasonal pattern. In the spring, ozone amounts begin to fall and reach a minimum in early October - this is the peak of the Antarctic ozone hole. In late spring ozone amounts rise to a maximum and then slowly decline.

The spring-time ozone hole is the result of emissions, mainly in the northern hemisphere, of chlorofluorocarbons (CFCs) and halons. These gases were in widespread use in refrigeration, industrial solvents and fire control, but are now regulated by the Montreal Protocol.

The gasses are broken down into their constituents over the tropics, and circulate towards both poles. Only over the Antarctic during winter is it cold enough for clouds to form in the ozone layer, and here chemical reactions on the cloud surfaces convert chlorine into an active form. When the sunlight comes back in the spring, this activated chlorine destroys ozone at about 1% per day leading to the ozone hole. As the atmosphere warms, the clouds disappear and the ozone hole fills in.

Posted on Environment Times online on 26th January 2010.