Saturday, June 21, 2014

Antarctica, the Ozone Hole, and Global Warming


Mount Erebus, Ross Island, Antarctica from above the clouds.
Photo © Bruce Luyendyk

Are Global Warming and the Antarctic Ozone Hole related? Yes. Before I explain that, some basic facts. 


We live in the lower part of the atmosphere called the troposphere where Global Warming is taking place. Above the troposphere is the stratosphere where the ozone hole develops over Antarctica in Southern Hemisphere Spring. Most of us have been near the top of the troposphere – that is where transcontinental jet aircraft fly – about 11 kilometers up (35,000 feet). Another fact – ozone (a molecule of three oxygen atoms - O3) in the stratosphere is good. It shields us from the Sun’s damaging ultraviolet radiation that causes skin cancer. Ozone in the troposphere where we live is bad – a health hazard and greenhouse gas. It is generated there by industrial activity.

So what’s happening and what’s the connection? 
First, the ozone hole is not really a hole but rather a seasonal decrease in ozone concentration over the Antarctic continent. Chlorine compounds that are attached to icy polar stratospheric clouds destroy ozone in the stratosphere. Ultraviolet radiation from the Sun breaks up the compounds letting loose chlorine atoms. Chlorine atoms (Cl) break ozone (O3) into ordinary oxygen (O2) and capture the freed atomic oxygen atom (O) to make ClO. The ClO attacks another ozone molecule, breaks it into two O2 molecules and a free Cl atom. Then the process starts all over again. A chain reaction is set up. Chlorine is the catalyst in the reaction. It is supplied by chlorofluorocarbons (CFCs; aerosols) that we once used in spray cans and air conditioners. Some CFCs in the troposphere eventually make their way up to the stratosphere. Nobel Prizes were awarded to those who figured this out.

In 1990 an international treaty known as the Montreal Protocol banned the use of CFCs to prevent the further destruction of ozone in the stratosphere. Is it working? It appears that the decrease in ozone has bottomed out (http://ozonewatch.gsfc.nasa.gov) – chlorine concentrations are diminishing – but it will take decades to clear out all CFCs. At the same time more carbon dioxide (CO2) has been pumped into the troposphere below, mostly from burning fossil fuels. The greenhouse gases in the troposphere prevent radiation from the Earth’s surface to reach the stratosphere – this is Global Warming. Consequently the stratosphere cools and more icy clouds form over Antarctica. This increases ozone depletion - contributes to the hole. But ozone depletion in the stratosphere means it cools down more, and this causes a cooling effect on the troposphere below, but not enough to reverse Global Warming.
  
Bottom line, Global Warming contributes to ozone depletion and that depletion tends to offset a small amount of Global Warming.


1 IPCC/TEAP Special Report on Safeguarding the Ozone Layer and the Global Climate System: Issues Related to Hydrofluorocarbons and Perfluorocarbons (summary for policy makers) (PDF). International Panel on Climate Change and Technology and Economic Assessment Panel. 2005. Archived from the original on February 21, 2007. Retrieved 2007-03-04.

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