The hidden threat to the ozone layer

If you were around in the 1980s you probably remember the concern about the hole in the ozone layer that protects the Earth from harmful UV rays. This was caused by man-made chemicals known as chloro-fluoro-carbons, or CFCs, escaping from the Earth's surface and heading up into the stratosphere where they reacted with the ozone layer and depleted it. An international effort led to a worldwide ban on CFCs, and the ozone layer began to recover. But now researchers led by Martyn Chipperfield at the University of Leeds, have shown that other, similar chemicals with shorter lifespans are still getting up into the atmosphere and could be having an impact on ozone. He spoke to Kat Arney about this chemical threat, and where it comes from...

Martyn - These so-called shorter-lived gases come from both natural and anthropogenic sources.

 Kat - So, that's human sources.

 Martyn - Yes, that's right. The natural sources for the compounds that contain bromine are related to ocean processes. They're released by seaweed and by phytoplankton in the ocean. And then more recently, it's become apparent that human activity is making a contribution through mainly chlorine compounds. What is known is compounds such a dichloromethane. They're used as feedstocks in the production of other chemicals such as hydrofluorocarbons. Ironically, these are the compounds which were supposedly ozone-friendly replacements for the chlorofluorocarbons. In our paper, we showed some results that show quite large recent increases in some of these chlorinated compounds, but in fact although we can see them in the atmosphere and we see regions where they're very elevated such as South East Asia, around China, India, the exact sources of these compounds is not known. That's a topic for future research, now we've detected that they're present.

 Kat - So, now that we know that they're present, what are they doing? Is there evidence that they are causing a significant impact on the ozone layer?

 Martyn - Yes, there is. I mean, certainly for the natural brominated compounds we've known for a few years that these compounds reach the stratosphere and they make about 25% contribution to the bromine amount in the stratosphere, and therefore a similar proportion to the ozone depletion caused by those overall bromine compounds. For the anthropogenic chlorine compounds, we can see that they do reach the stratosphere. Their contribution is smaller than the chlorine that comes from CFCs, but as CFCs are now controlled and their abundance is decreasing and, as far as we can see, the contribution of these short-lived compounds is increasing, we expect that contribution to become more important in the future.

 Kat - Why haven't they been banned if they're causing these problems?

 Martyn - The treaty that protects the ozone layer, called the Montreal Protocol, has been very successful. It's one of these sort of - maybe the prime example of a - global, international treaty to address an environmental problem. So, why it's been very effective at controlling gases such as chlorofluorocarbons, it hasn't paid attention to these so-called short-lived compounds. That's partly because scientists thought that the shorter lived species wouldn't be around in the atmosphere long enough to actually reach the stratosphere and their abundance had not been detected to appreciable extents in the atmosphere.

 Kat - A lot of scientists around the world are making various models of how our climate is changing, how our activity and natural activities are affecting our climate. Is this something now that needs to be added into the mix of these climate models?

 Martyn - Well already, these kind of models do include chemistry. in our group, we are doing lots of work which includes adding detailed chemical processes into these models because climate change is not just about CO2 which is relatively inert. It's about other gases as well such as methane and CFCs, ozone. So, we're building those models. In fact, the coming state of the art is to work towards what's called "Earth system models" that include a whole range of processes from land surface processes, oceanographic processes that can really get feedbacks of how changes to chemistry in the atmosphere is impacting climate change.

 Kat - Martyn Chipperfield there from the University of Leeds.