Tag: Nature Geoscience

Image by PiccoloNamek (English wikipedia) [GFDL (www.gnu.org/copyleft/fdl.html), via Wikimedia Commons

You may have noticed a story reported on widely recently on the discovery of 4 ‘new’ man-made ozone-depleting gases. This follows the publication of a study in the journal Nature Geoscience on the first measurements of these gases, their abundances in the atmosphere and estimated global emission rates. Responses to the reporting of this publication have ranged from the Daily Mail’s “Ozone Crisis” to the inevitable internet-based diatribe of “any research from UEA is clearly made up” in various comment sections. So just how concerned should we be about the emissions of these four gases?

The reason we care about CFCs is because they deplete ozone high up in the atmosphere, potentially exposing humans to harmful UV rays. Oh, they also happen to be extremely potent greenhouse gases, with each molecule of a CFC being equivalent to 1000s of molecules of CO₂, and they sit around in the atmosphere for 10s or 100s of years before being removed. Basically they’re pretty bad, and sure they might have been great refrigerants and aerosol propellants but at what cost?

The production of CFCs has now to all intents and purposes ceased, although that doesn’t mean that emissions have completely stopped; various banks of these gases exist in fridges for example. These might leak during use or when destroyed. So it’s not entirely surprising to read that this study has found that various CFCs are still being released.

In fact the reason this paper is important is more to do with the fact that these gases have never before been measured.  Many of the media articles seem to lead with the fact these are ‘new’ ozone-depleting gases, which is a little misleading. They’re not new; they’ve been around for decades, only nobody has been able to measure them in the atmosphere before. Why’s that you might ask? Well much of it is to do with just how small their concentrations are in the atmosphere.

The fact of the matter is that the concentrations of these gases (CFC-112, CFC-112a, CFC-113a, HCFC-133a) are tiny. All four have atmospheric mixing ratios of less than 1 part per trillion (ppt). In other words, if you could isolate a trillion molecules of air (1 x 10¹²) then not even one of them would be one of these ’new’ CFCs. By contrast CO₂ in the atmosphere has a mixing ratio of hundreds of parts per million.

Compare these newly measured gases to the major CFCs (CFC-11, CFC-12, CFC-113) whose current atmospheric concentrations are hundreds if not thousands of times greater. Even though emissions of these major CFCs are now close to zero they will still be around in the atmosphere at these elevated concentrations for decades to come. This is shown in the plot below taken from the AGAGE network measurements of CFC-12. Although the concentration has reached a peak it will take at least one hundred years for levels to get back down to pre-1980 levels, with the current mixing ratio still over 500 ppt.

Plot taken from the AGAGE network measurements of CFC-12

So emissions of these newly measured gases would have to really pick up for a sustained period of time to add significantly to the ozone-depleting effect of what is already in the atmosphere. To say the measurement of these compounds has created some sort of ozone crisis is therefore a gross exaggeration. That’s not to say that this work was a waste of time; it’s vital that we know about these compounds and their atmospheric abundance so we can ensure their contribution to ozone depletion remains negligible.

There are other potentially more important causes for concern as well. Hydrochlorofluorocarbons (HCFCs) were introduced as replacements for CFCs but also contribute to ozone depletion, albeit in a less effective way. Although these are also being phased out many of these will have a greater impact on the recovery of the ozone ‘hole’ than these newly measured species. Just a few months ago the United Nations Environment Programme (UNEP) released a report saying another gas, Nitrous Oxide (N₂O), is now considered to be the biggest threat to the ozone layer over the next 50 years. Not to mention that one of the impacts of a rise in global surface temperatures could be a slowing in ozone hole recovery. There’s a genuinely interesting (honest!) explanation for why that is which I will cover in another blog.

The point is that there are lots of factors which affect the Earth’s ozone layer. Studies like the one recently published in Nature Geoscience are vital for our understanding of what the recent and current atmospheric composition is like. It might not be a problem now, but surely the key to looking after our planet, and ourselves, is to prevent things from becoming problematic in future. If we can take steps to find out where these emissions are coming from and why some of them are increasing then measures could be put in place to limit their future influence on ozone recovery.

This blog is written by Mark Lunt, Atmospheric Chemistry Reseach Group, Cabot Institute, University of Bristol, .