Atmospheric Oxygen Program.  This is a global warming study measuring oxygen:nitrogen ratios.  The processes of photosynthesis, respiration and fossil fuel combustion change both the atmospheric oxygen content (and hence the oxygen:nitrogen  ratio), and the concentration of atmospheric carbon dioxide.  However, the atmospheric oxygen content is much less influenced by air-sea exchange than is that of carbon dioxide.  Thus changes in the oxygen content indicate carbon uptake and release by photosynthesis, respiration and fossil fuel combustion much better than changes in atmospheric carbon dioxide content itself. 

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